Unit 2. Introduction to Aerospace and Safety

Transcription

1 Unit 2. Introduction to Aerospace and Safety 2 1. Introduction to Aerospace Space history Introduction to space lift vehicles Rocket science Orbital mechanics Introduction to Launch Systems Rocket structures Staging and separation Navigation, guidance, and control Solid, liquid, and gas propulsion systems Solid-, liquid-, and gas-propellant systems Payload electrical power sources T HIS unit covers a wide variety of core subjects in the aerospace work environment. The information provides the necessary technical foundation for successful technicians with the abilities to make sound maintenance decisions and ensure a safe working environment. The first section introduces multiple aerospace concepts, from key events in space history and a brief description of multinational rockets. Then section 2 2 will introduce propulsion and propellant systems to include orbital mechanics Introduction to Aerospace This portion of the unit provides key historical facts and a brief introduction to some of the rocket systems across the world Space history As far back as recorded history goes, the human race has looked to the night sky in wonder. Some even decided to try to reach out and touch the stars. Knowing the milestones provides a path to understanding the importance of the maintenance an aerospace technician accomplishes. Early rocketry Historians believe the first to start experimenting with the foundation of rocket-propelled devises were the Chinese. In 1232, the Chinese army used crude rocket-propelled devises called fire arrows. The fuel for the arrows consisted of saltpeter, sulfur, and charcoal. The Chinese had discovered a method to control the thrust of the burn. Similar weapons using solid fuel would continue to be used for warfare throughout history. Solid-propellant weapons were used during the crusades, the Battle for Orléans during the 100 Year s War, the War of 1812, and the Civil War. The demand for new weapons, capable of carrying larger payloads further and further, eventually sparked the imagination. The very force leading to our current space program. Development of modern rocketry An unimaginable number of individuals has contributed to the development of our current space program, and every day someone else contributes in some way. This lesson covers only a few of the more prominent ones. William Congreve ( ) An English artillery officer, William Congreve, saw the advantages of rockets in warfare, and by some historians was named the inventor of the military rocket. He focused his efforts on improving

2 2 2 solid-fuel motors and flight stability to improve accuracy. The rocket military units utilizing Congreve s designs saw action during the Napoleonic Wars and even the battle at Fort McHenry (near Baltimore) during the War of 1812 inspiring the phrase the rockets red glare in Francis Scott Key s The Star-Spangled Banner. William Hale ( ) In 1844 William Hale would continue to improve accuracy of the Congreve rocket by inventing a rocket that spins. Instead of using a stick to control stabilization, he removed the guide stick and directed part of the exhaust through holes causing it to rotate while in flight.. Wilhelm Theodore Unge ( ) A Swedish inventor, Wilhelm Unge, improved the mechanical strength of solid propellant and developed launcher-rotated rockets capable of traveling eight kilometers (km). Konstantin Tsiolkovsky ( ) In 1898, Russian schoolteacher and scientist, Konstantin Tsiolkovsky, proposed the idea of space exploration. His proposal included a design of a reaction thrust motor to get there and using Earth s own atmosphere as a braking force to return. Later in life, he published a report in 1903 stating the use of liquid propellants would achieve greater range and speed limited only by the exhaust velocity of the escaping gases. He would also go on to propose the concept of multistage rockets to increase the efficiency of rockets attempting to reach space. Additionally, he drafted his first liquid-oxygen and liquid-hydrogen-powered rocket demonstrating the necessary design to create the thrust required to push a rocket off the ground. Historians have labeled him as the father of modern astronautics for his ideas, research, and vision. Robert Goddard ( ) Robert Goddard, an American, was designing rockets while the Wright brothers were learning how to fly. His goal was to use rockets to explore the atmosphere and space. He also was one of the first to realize solid-fuel rockets lacked the power necessary to reach the upper atmosphere and liquid-fuel rockets could. He went on to publish A Method of Reaching Extreme Altitudes in 1919 that supported his realization but also suggested we could visit the moon with a rocket. He went on to launch the first liquid-fueled rocket and held 214 patents in rocketry. Hermann Oberth ( ) Oberth is known for his study of the necessity of staging rockets. He knew if a rocket had less weight than fuel, the rocket speed would increase. In order to maintain the imbalance between empty fuel compartments to rocket fuel, the stages would have to be discarded. His formulas led to the understanding of how a larger stage after jettison would increase the speed of a smaller one. Wernerher von Braun ( ) Von Braun was in charge of directing Germany s military rocket-development program during World War II (WWII). He and his team developed the first long-range ballistic missile, the A 4 rocket, or commonly known as the vengeance weapon number 2 (V2). On 3 October 1942 the rocket traveled 120 miles to hit its target. Not only was the A 4 a success, it also was considered by many historians to be the ancestor of many of today s rockets. Near the end of WWII, von Braun led his team of German scientists across Allied lines and defected to America to escape Nazi execution. The United States quickly put him and his team back to work developing rockets for experimental and weapons use. Working for the Army, his team would develop the Redstone Arsenal s intermediate-range ballistic missile named Jupiter. The Jupiter C version would launch the first American satellite, Explorer 1, on 31 January 1958.

3 2 3 Sergey Korolyov ( ) Korolyov s team launched the Soviet Union s first liquid-propellant rocket in During WWII, Korolyov was held under technical arrest but spent the years designing and testing liquid-fuel rocket boosters for military aircraft. After the war, he modified the German V2 missile, increasing its range to about 426 miles (685 km). In 1953 he began to develop the series of ballistic missiles that led to the Soviet Union s first intercontinental ballistic missile (ICBM). Korolyov was placed in charge of systems engineering for Soviet launch vehicles and spacecraft; he directed the design, testing, construction, and launching of the Vostok, Voskhod, and Soyuz manned spacecraft as well as of the unmanned spacecraft in the Cosmos, Molniya, and Zond series. He was the guiding genius behind the Soviet spaceflight program until his death. Important milestones in aerospace history The following table will provide some important events in twentieth-century aerospace history after WWII, which led to where we are today. Milestone 1947, breaking of the sound barrier 1951, multistage rockets studied 1955, Vanguard project approved 1957, first satellite Sputnik in space Additional Information Charles Yeager accomplished this in the X 1 plane. Sergey Korolyov team, while working within the Russian ballistic missile program, considered the possibilities to use the multistage rockets to place a satellite in orbit. US project to place a satellite into orbit. Placed in orbit by the Soviet Union. 1957, Sputnik 2 launched The craft carried a live dog named Laika into space. 1958, Explorer 1 launched The first American satellite. 1958, Vanguard reached orbit The first time a satellite used solar energy as a power source. 1958, Mercury project began Goals of this project were to place a manned spacecraft in orbital flight around Earth, to investigate man s performance capabilities and his ability to function in the environment of space, and lastly, to recover the man and the spacecraft safely. 1959, Luna 1 escaped orbit Leading to Luna 2 and 3, which were the first to impact and take pictures of the far side of the moon. Both missions were necessary milestones to land on the moon. 1960, Pioneer 5 launched The craft was the first to transmit a radio signal across 22.5 million miles. 1960, Vostok spacecraft returned to earth Carried two dogs, Belka and Strelka, into orbit and returned them to earth. 1961, first manned spaceflight Russian astronaut Yuri Gagarin made the trip onboard the Vostok spacecraft. 1961, first American to complete a suborbital flight Allen Sheppard accomplished this task onboard of a Mercury spacecraft. 1961, Venera 1 launched Probe launched towards Venus. 1961, Apollo project announced by Pres. John F. Kennedy The purpose was to establish the technology to meet other national interests in space with the follow intentions: achieve preeminence in space for the United States, carry out a program of scientific exploration of the moon, and develop man s capability to work in the lunar environment. It would take the success of the Mercury and Gemini projects to see this as a reality.

4 2 4 Milestone Additional Information 1962, Gemini project began The goals behind the project: subject man and equipment to space flight up to two weeks in duration; rendezvous and dock with orbiting vehicles and to maneuver the docked combination by using the target vehicle s propulsion system; and to perfect methods of entering the atmosphere and landing at a preselected point on land. Its goals were also met, with the exception of a land landing, which was cancelled in , Mariner 2 completed first Venus flyby 1962, first American to complete an orbital flight 1963, world s first woman went to space John Glenn accomplished this task onboard a Mercury-Atlas 6 spacecraft. Valentina Tereshkova completed an orbital flight onboard a Vostok spacecraft. 1965, first space walk Alexei Leonov had the pleasure during a 24-hour orbit Voskhod 2 mission. 1965, Mariner 4 did a flyby of Mars 1967, Venera 4 launched First probe to penetrate Venus atmosphere. 1969, Apollo 8 completed the first translunar flight and orbits the moon 1969, Apollo 11 manned spacecraft lands on the moon. 1971, Mars 3 lander lands on Mars 1973, Pioneer 10 completed flyby of Jupiter 1973, Skylab placed in space by a Saturn rocket 1974, Mariner 10 completed flyby of Mercury 1975, Venera 9 lands on Venus 1976, Viking spacecraft lands on Mars 1979, Pioneer 11 completed a flyby of Saturn 1981, shuttle Columbia launched 1986, Voyager 2 completed a flyby of Uranus 1989, Voyager 2 completed a flyby of Neptune 1995, Galileo entered orbit around Jupiter Carried three American astronauts. Neil Armstrong was the first man on the moon. Only a few seconds of data was transmitted before failure. Provided a long-term working environment in space. First images of Venus. First images of Mars. The shuttle became the first reusable spacecraft. The shuttle was used to repair, retrieve, and place satellites in orbit. It also carried the first American woman (Sally Ride) into space. First and only during the twentieth century.

5 2 5 Milestone 1997, pathfinder landed on Mars 1998, first component of the international space station placed in orbit. Deployed the first rover. Additional Information Marked the beginning of a multinational venture to create a living, working presence in space. 2004, SpaceShipOne launched First privately manned spacecraft in space piloted by 63-yearold Michael Melvill. Poor judgment can lead to catastrophic events In the table, multiple success stories were presented, but the reality is that they were not achieved without loss of life and equipment. An aerospace technician can directly affect the outcome of a project or mission. Faulty or half-hearted maintenance practices can and will result in tragedy maybe tragedies like the one that occurred on 27 January 1967, while on the launch pad during a preflight test for Apollo/Saturn 204 (AS 204, better known as Apollo 1), scheduled to be the first Apollo manned mission, and would have been launched on February 21, 1967, Astronauts Virgil Grissom, Edward White, and Roger Chaffee lost their lives when a fire swept through the command module (CM). In the investigation report, investigators could not conclusively identify the arcing source but did go on to identify the following as failures in the process. Those organizations responsible for the planning, conduct, and safety of this test failed to identify it as being hazardous. Contingency preparations to permit escape or rescue of the crew from an internal CM fire were not made. No procedures for this type of emergency had been established for either the crew or the spacecraft-pad work team. The emergency equipment located in the White Room and on the spacecraft work levels was not designed for the smoke condition resulting from a fire of this nature. Emergency fire, rescue, and medical teams were not in attendance. Both the spacecraft work levels and the umbilical tower access arm contain features such as steps, sliding doors, and sharp turns in the egress paths that hinder emergency operations. Knowing your aerospace history will help prevent repeat failures caused from lack of preparation maybe even save a few lives Introduction to space lift vehicles Most space lift vehicles were originally used by the Air Force as ICBMs. Through a series of improvements and system modernizations, these vehicles have transitioned into versatile and reliable space boosters. In the early years, American space-lift technology developed at an incredibly rapid rate from placing its first satellite in orbit on 31 January 1958 to the first landing on the moon on 20 July This section familiarizes you with the historical development, mission, and major components of the various space-lift vehicles we use today. Atlas The Atlas originally fielded as an ICBM in the early 1960s. The Air Force replaced the Atlas ICBMs with the faster, smaller Minuteman missiles and converted the leftover Atlas rockets into space lift vehicles beginning in the late 1960s. Figure 2 1a is an Atlas II. The Atlas V is the newest member of the Atlas rockets and is designed with multiple configurations depending on load weight and placement.

6 2 6 Figure 2 1a. Atlas II (expanded). On 20 February 1962, US astronaut John Glenn became the first US astronaut to orbit Earth. This first-manned orbital flight used an Atlas booster carrying the Friendship 7 Mercury capsule into orbit as part of the Project Mercury space program (fig 2 1b). Atlas rockets have been used to launch the unmanned Surveyor lunar probes, the interplanetary Mariner probes to Mars, Venus, and Mercury, as well as the Pioneer probes to Jupiter, Saturn, and Venus.

7 2 7 Figure 2 1b. Atlas with Mercury capsule. Delta The Delta IV is the latest generation of the Delta launch vehicle family. It is an expendable mediumlift launch vehicle used to launch NAVSTAR Global Positioning System (GPS) satellites into orbit (figs. 2 2a and 2 2b), providing navigational data to military and civilian users.

8 2 8 The Delta launch vehicle family originated in 1959 using components from the US Air Force s Thor ICBM program and the US Navy s Vanguard launch vehicle program. The first Delta was launched from Cape Canaveral, Florida, in The Delta has evolved to meet the ever-increasing demands of its payloads including weather, scientific, and communications satellites. Continued improvements over the years, and the USAF s decision to return to a mixed fleet of expendable launch vehicles (ELV) following the 28 January 1986 Challenger accident, led to the development of the Delta II medium lift vehicle and newest Delta IV. Figure 2 2a. Delta II (expanded).

9 2 9 Figure 2 2b. Delta II. Titan Our discussion of US ELVs continues with the two operational Titan lift vehicles, the Titan II and the Titan IV, our largest lift vehicle (figs. 2 3a, 2 3b and 2 3c). The Titan family of space lift vehicles resulted from the requirement for a two-stage ICBM.

10 2 10 Figure 2 3a. Titan space lift vehicle family (expanded). The Titan missile program began in 1955 with the development of the Titan I ICBM weapon system. Titan I was not well liked by their maintainers because their fuel and oxidizer were not capable of being stored for extended periods. Introduced in 1962 with storable propellants, the Titan IIs remained on alert for 25 years with only periodic servicing. As their name implies, the Titan was a huge rocket with tremendous range, capable of reaching Russia by flying south over Antarctica. As the Air Force replaced the Titans with smaller, faster Minuteman missiles, they converted the leftover Titan rockets into space lift vehicles and eventually the Titan IV. Titan IVs were originally intended as a backup for the space shuttle to be used only as conditions warranted. The loss of shuttle Challenger on 28 January 1986 dramatically changed the face of the Titan IV program. After the loss and subsequent stand-down of the space shuttle fleet, USAF faced an emergency need for a reliable launch vehicle to carry their large military satellites. The Titan IV is the largest unmanned space booster used by the Air Force. The vehicle is designed to carry large payloads, equivalent to the size and weight of those carried on the space shuttle. Titan IV is capable of placing our largest satellites like MILSTAR and Defense Support Program (DSP) into geosynchronous orbit. It is also use for launching interplanetary research satellites. The last of the Titians was launched in 2005.

11 2 11 Figure 2 3b. Titan II. Figure 2 3c. Titan IV. Falcon Falcon is a two-stage, liquid oxygen (LOX) and rocket-grade kerosene (RP 1), semireusable powered launch vehicle manufactured by SpaceX (Space Exploration Technologies Corporation). It is designed from the ground up for cost-efficient and reliable transport of satellites to low Earth orbits (LEO). Minotaur This is a combination launch vehicle of Minuteman or Peacekeeper ICBM rocket motors as the vehicle s first and second stages with third and fourth stages, structure and payload fairing, derived from a Pegasus XL rocket. Minotaur made its inaugural flight in January 2000, successfully delivering a number of small military and university satellites into orbit and marking the first-ever use of residual US government Minuteman boosters in a space launch vehicle (SLV). Minotaur is manufactured by Orbital Sciences Corporation.

12 2 12 Pegasus This Orbital Sciences Corporation rocket was launched for the first time on 5 April 1990 from underneath a NASA B 52 carrier aircraft. For current missions, Pegasus is carried aloft by an L 1011 Stargazer aircraft to approximately 40,000 feet over open ocean, where it is then released to free-fall into a horizontal position for five seconds before igniting its first-stage rocket motor. With the aerodynamic lift generated by its unique delta-shaped wing, Pegasus typically delivers satellites into orbit in a little over 10 minutes. This patented air-launch system reduces cost and provides the flexibility to operate from virtually anywhere on Earth with minimal ground-support requirements. The three-stage Pegasus is used by commercial, government, and international customers to deploy small satellites into LEO. Taurus This is a ground-based variant of a Peacekeeper ICBM first stage and a second and third stage based on the two stages of a Pegasus. Taurus debuted in 1994 and offers an affordable, reliable means of launching small satellites into LEO. Orbital Sciences Corporation manufactures Taurus launch vehicles. Evolved Expendable Launch Vehicle Program Evolved Expendable Launch Vehicle (EELV) program is a government-procured commercial launch service. Currently the program uses the Delta IV and Atlas V rockets. Information on both is owned by the individual manufactures; therefore, training for these systems is provided by the contractor once Air Force personnel are assigned duties associated with systems. The contractors assume the risk of development, and perform all aspects of manufacture, processing, and flight. The contractors retain ownership of the booster, ground support equipment (GSE), and facilities. However, this does not mean the government has no say in EELV processing and launch activities. Governmental control is maintained through the use of an Operational Safety, Suitability, and Effectiveness Assurance Plan. AFPD 63 12, Assurance of Operational Safety, Suitability, and Effectiveness (OSS&E). The space shuttle The space transportation systems (STS), more commonly known as the space shuttle, was first conceived in 1969 shortly after the first moon landing of the Apollo program when NASA embarked upon a program to develop a new STS based on a reusable manned spacecraft. First launched in 1981, it has served as NASA s primary launch vehicle, placing scores of scientific and military satellites into orbit. The facts that the shuttle is a manned vehicle and that it has the ability to return from space for reuse separate it from all other space lift vehicles. The shuttle provides NASA and the military with the flexibility to deploy payloads into orbit, retrieve payloads from orbit, service satellites in orbit, and conduct experiments in a free-fall (an artificially produced zero-gravity effect) environment. The shuttle is capable of placing payloads of up to 55,000 pounds (lb.) into LEO. If mission needs require it, either the payload assist module (PAM) or the inertial upper stage (IUS) can be employed with shuttle payloads. Although plans for launching the shuttle into polar and retrograde orbits from Vandenberg AFB were pursued aggressively in the mid 1980s, they were scrapped due to the Challenger accident. As a result, the shuttle is only launched into equatorial and prograde orbits from Kennedy Space Center. Russia Russia has historically been the leader in SLVs. Even today some of their rocket technology has been adopted for use by American launch companies. The Soyuz rockets are probably the best known of all the Russian rockets.

13 2 13 Soyuz The Soyuz is a medium-lift rocket. The Soyuz series (fig. 2 4) is the same family of SLVs that launched Sputnik (Russian for traveler), Laika (the first dog in space) and Yuri Gagarin (the first man in space). Over 1,700 launches later, the Soyuz vehicle is still used to carry cosmonauts, photoreconnaissance satellites, and earth resource satellites. Capable of carrying payloads of 7.5 tons to LEO, Soyuz launches most medium-sized Russian satellites. Figure 2 4. Soyuz rocket. The Russian Space Agency has upgraded the payload lifting capability of Soyuz, allowing Russia to launch Soyuz capsules from Plesetsk, rather than being dependent on the Baikonur (Tyuratam) launch facilities in Kazakhstan. The largest Russian heavy-lift launch vehicle in regular use is the Proton.

14 2 14 Proton The Proton was originally introduced in 1965 as a booster for heavy military payloads and for space stations (fig. 2 5). The Proton is among the most reliable heavy-lift launch vehicles in operation, with a reliability rating of about 98 percent. Capable of carrying payloads of 12 tons to LEO and 6 tons to geosynchronous Earth orbit (GEO), the Proton is being marketed in the west by International Launch Services, a joint venture between Krunichev and Lockheed Martin. Figure 2 5. Proton rocket. Proton is the most capable, commercial, expendable launch vehicle presently in operational service. It offers larger payload masses to orbit than any other commercial launch system. Proton s three-stage configuration is used primarily to launch large payloads into LEO, while the four-stage configuration is used to launch spacecraft into geosynchronous or interplanetary trajectories. All Russian geosynchronous and interplanetary missions are launched on Proton. Approximately 90 percent of all Proton launches have been the four-stage version. Proton plays an important role in launching space

15 2 15 station components. The Proton has flown over 200 missions and has been used to put the Mir and International Space Station modules into orbit. France France is a key member of the Europe and Space Transportation Systems that introduced the European-built Ariane launch vehicle in By the end of 2001 the organization had conducted over 800 missions using the Ariane family of launch vehicles (fig2 6a), almost all for purely commercial purposes. The Ariane 4 is their most successful launcher. Ariane 4 The Ariane 4 series (fig. 2 6b) holds the largest market share in the international commercial launch market. The vehicles are launched from Kourou, French Guiana, in South America. Ariane 4 variants can lift payloads of 10.5 tons to LEO and 4.5 tons to GEO. Figure 2 6a. Ariane family (photo: ESA, Ariane family). The Ariane 4 program is managed and launch services are marketed by Ariane space. In all, more than three dozen European companies provide significant services in the design, manufacture, and operation of the Ariane 4. The substantially larger Ariane 5 launch vehicle is used to lift heavier payloads. Ariane 5 The Ariane 5 (fig. 2 7) is the newest SLV in the French Space Agency. As Europe s premier heavylift vehicle, it is an all-new design. The Ariane 5 is designed to launch multiple large communications satellites for a lower cost than previous versions. The Ariane 5 had a disastrous blow-up on its first

16 2 16 flight and a second-stage failure on its second flight. After refinement, it is now successful and lifted Europe s heaviest payload into geosynchronous orbit in Satellites have continued to grow since the Ariane program was started over 10 years ago. Therefore, European Space Agency approved a roughly $1 2 billion Ariane 5 Evolution project to increase GEO payload to over 7 tons. Figure 2 6b. Ariane 4 booster (photo: ESA, Ariane 4).

17 2 17 Figure 2 7. Ariane 5 booster (photo: ESA, Ariane 5).

18 2 18 Figure 2 8. Chinese Long March CZ 3B space lift vehicle. Figure 2 9. Japanese H 2 launcher.

19 2 19 China The Chang Zheng (Long March) or CZ family launch vehicles include 11 different operational vehicles from the small CZ 1D to the CZ 3B heavy GEO launcher. They are used both for national programs and for international commercial launches. While Long March vehicles are restricted from undercutting western prices by more than 15 percent, they have been attractive to many satellite owners in Asia. CZ launch vehicles are capable of carrying payloads of 10 tons to LEO and 3.5 tons to GEO. The CZ 2E has suffered two poorly explained failures while carrying Hughes communication spacecraft. One of the Hughes satellites was recovered by using its orbital insertion motor to slingshot it around the moon in order to get it into a workable geosynchronous orbit. With three active launch sites, China has become a preeminent space launch provider. Several Long March vehicles were used to launch Iridium spacecraft in 1999 (fig. 2 8). Japan Japan came of age in space in 1994 with two launches of its powerful new H 2 heavy-lift launch vehicle. The launches took place at the new Yoshinobu Launch Complex on Tanegashima Island. Unlike its predecessor vehicles which were based on versions of the US Delta rocket, the H 2 was designed and developed entirely by Japanese technology. The H 2 first, or core, stage is powered by a rocket engine similar to the US space shuttle main engine (SSME) and is assisted by a pair of solid rocket boosters (SRB) strapped to its sides (fig. 2 9). The H 2 is the cornerstone of Japan s plans for increasing activities in space, including eventual human missions. The H 2 is designed to carry heavy payloads to orbit. Additionally it is capable of carrying payloads of 11 tons to LEO and 2.4 tons to higher orbit. With a price tag of $ million, the H 2 is currently not competitive with the roughly $60 80 million for an Atlas 2 or Ariane 4. Japan hopes to cut costs by as much as 50 percent by the turn of the century, in part by simplifying the design and including some foreign components. India India s first (albeit unsuccessful) orbital launch was in 1979, with the SLV capable of carrying 40 kilograms (kg) to orbit. Despite a very small budget and technical difficulties (early launches occurred only once every few years with a 33 percent success rate), India has continued to build a strong space program. All Indian space launches are conducted from the Sriharikota Island off the East Coast of India in the Bay of Bengal. The advanced SLV was used to orbit small experimental satellites. The newer polar SLV (fig. 2 10) is being used to orbit indigenously built, infrared remote-sensing satellites. A geosynchronous SLV is projected to come online in 2005 to launch India s communications satellites Rocket science Every satellite in space needs a launch vehicle to put it up there. A basic understanding of the subsystems and operation of space lift vehicles can provide technicians with the ability to understand how their actions impact the overall system. This unit will familiarize you with the fundamentals of launch vehicle subsystem design. Figure Indian Polar SLV.

20 2 20 Thrust Launch vehicle engines develop a reaction force called thrust by expelling particles at high velocities from their exhaust nozzles. The force of thrust acts in the opposite direction of the exhaust and propels the launch vehicle forward. Although we will not be conducting mathematical calculations to find exact velocities, use figures 2 11a and 2 11b as the following two main components of thrust are discussed. Momentum thrust Pressure thrust Total Thrust = Momentum Thrust + Pressure Thrust Momentum Thrust = propellant mass flow rate exhaust gas velocity T m = F m V g Pressure Thrust = (exhaust pressure - atmospheric pressure) cross-sectional area of nozzle exit plane T p = (P e - P a ) A e Figure 2 11a. Momentum thrust. Figure 2 11b. Pressure thrust. Momentum thrust is, by far, the biggest component of a launch vehicle s total thrust. Throughout the course of the entire burn, this thrust accounts for 80 percent of the vehicle s total thrust. Pressure thrust is negligible at the beginning of vehicle flight. As the launch vehicle gains altitude and the atmospheric pressure decreases, the pressure thrust increases. For example, ambient pressure is 14.7 pounds per square inch (psi), but as the rocket gains altitude, the pressure drops because of less atmosphere. Therefore, pressure thrust is always changing as the vehicle is in flight. Pressure thrust is an imbalance of the pressure of the outgoing exhaust and the outside pressure. If the exhaust pressure equals the atmospheric pressure, maximum efficiency is achieved. However, as a rocket goes up in

21 2 21 altitude, the atmospheric pressure is decreasing without any changes in the nozzle, and the pressure thrust will therefore increase. Other factors engineers and scientists must consider are expansion ratios, mass ratios, and specific impulse. We will briefly review what each component is and why it must be considered for any launch vehicle operation. Expansion ratios In discussing expansion ratios, it is important that we also discuss what affects this ratio: rocket nozzles. From the combustion chamber, the gases are constricted at the throat; that is, the narrowest point, where they reach Mach 1 (the speed of sound). Then, the nozzle expands along a carefully controlled contour where the gases gain speed and lose pressure. The larger the nozzle s cross section at the exit, the faster the gas can get, and the lower pressure the gas will be as it exits. The expansion ratio is a comparison of the cross-sectional area of the nozzle exit plane to the cross-sectional area of the nozzle throat or: Area of Exit Plane Expansion Ratio = Area of Throat We will cover the three types of expansion and what comprises each case. 1. Optimum. 2. Over. 3. Under. Optimum expansion The gas pressure at the nozzle exit should be exactly equal to the outside air pressure. In the vacuum of space, this is obviously impossible, and the bigger the exit area, the closer you get to optimum thrust (although at some point, the added thrust isn t worth the added mass to make the nozzle bigger). In the atmosphere it is difficult to get exactly optimum performance because the air pressure changes with the temperature and altitude. The expansion ratio of a nozzle dictates the altitude where the engine will produce maximum thrust. You can spot this on the ground when you see a plume that looks like figure Figure Optimally expanded nozzle. Underexpansion Most rockets, and all of the space launch rockets, climb in altitude during operation; therefore, the outside pressure at the nozzle exit changes in flight, and some average air pressure is chosen to optimize performance for that rocket. When a nozzle ends before the gas reaches the pressure of the outside air, it is called an underexpanded nozzle. In the underexpanded case, the rocket design is not getting all the thrust that it can from the engine. Thus, the expansion ratio is too low, and the exhaust gas pressure is greater than the atmospheric pressure, which increases the pressure thrust of the launch vehicle. However, momentum thrust is degraded. An example of this condition is shown in figure 2 13.

22 2 22 Figure Underexpanded nozzle. Overexpansion Overexpansion occurs when the expansion ratio is too high, and the exhaust gas pressure is less than the atmospheric pressure. Since the cross-sectional area of the exit plane is too large, the exhaust gases completely expand before they reach the exit plane of the nozzle. An overexpanded condition increases momentum thrust because of an increase in exhaust particle velocity. But the decrease in pressure thrust is greater than the increase in momentum thrust, resulting in a decrease in total thrust. Performance from an overexpanded condition is worse than from an underexpanded one because the drag from an overexpanded nozzle is so large that the rocket loses a lot of thrust. It also causes a lot of stress on the nozzle. You can spot an overexpanded nozzle when you see a plume that looks like figure Figure Overexpanded nozzle. Since a particular expansion ratio will only allow optimum expansion at a specific altitude, one must select an expansion ratio that gives the best average performance during powered flight. The two methods for optimizing expansion ratios at different altitudes are staging (discussed later in this unit) and extendible nozzles (not discussed in this course). Mass Ratios The mass ratio of a launch vehicle is its initial mass divided by its final mass. Initial Mass Mass Ratio = Final Mass = Mass at Engine Start Mass at Engine Shutdown For example, if a launch vehicle s mass is 45,351 kg at engine start and 4,535 kg at engine shutdown, its mass ratio is 10:1 or 10 to one. Mass ratio is a major launch vehicle design parameter. Launch vehicles with high-mass ratios perform better than those with low-mass ratios. Multistage launch vehicles have a great mass ratio advantage over single-stage launch vehicles. This is because the overall mass ratio of a multistage launch vehicle is the product of the individual mass ratio of each stage. If each stage of a three-stage launch vehicle has a mass ratio of 3:1, the overall mass ratio is 3:1 3:1 3:1, or 27:1. Heavy-lift space launch vehicles attain mass ratios of over 40:1.

23 2 23 For example, the shuttle s initial mass is approximately 2,045,500 kg, and its final mass is approximately 324,950 kg. The mass ratio of the shuttle s solid rocket boosters (SRB) is about 6.75:1, while the mass ratio of the orbiter and tank is about 9:1. The product of these gives the entire shuttle system a mass ratio of about 60:1. Specific impulse Specific impulse (ISP) is determined by the rocket s propellant. It is the amount of thrust produced by one unit of propellant in a single second. This is a measure of propellant efficiency. This figure is determined by factors such as a propellant s type or mixture ratio, combustion pressure, and the structure of the nozzle. For instance, if a propellant has a thrust of 500 tons and consumes 2 tons every second, its specific impulse figure is 250 (500 tons divided by 2 tons/second equals 250 seconds). The higher the ISP figure of the propellant, the better functions a rocket will have. For solid-fueled rockets, the ISP figure ranges from 250 seconds to 280 seconds, while liquid-fueled rockets are usually 300 seconds. For example, when liquid oxygen and liquid hydrogen are used as propellants, the ISP can be as high as 450 seconds. Even if a rocket is designed with a high ISP, designers have to take into account other factors such as wind resistance and gravity, to name a few. These two factors alone account for approximately a 20 percent decrease in a rocket s final speed. Specific impulse is expressed mathematically as follows: Thrust ISP = or Weight Per Second of Propellant Pounds Pounds/Second In this expression, pounds-thrust and pounds-weight cancel each other out, and you re left with just seconds. This is why ISP is always expressed in seconds. Specific impulse is also dependent upon combustion temperature and propellant molecular weight in the following way: ISP Temperature Molecular Weight This relationship tells you that launch vehicles will operate most efficiently when they consume propellants of low molecular weight, which burn at high temperatures. The most efficient liquid launch vehicle engine to date is the space shuttle main engine. Its fuel is hydrogen, the element with the lowest molecular weight. ISP ranges for various fuel types are show in the table below. Type of Fuel Typical ISP Range Liquid seconds Solid seconds Hybrid under development Nuclear 900 5,000 seconds Rocket equation A big question in rocketry is how many kilograms of propellant are required to produce a given delta V for our spacecraft. Delta V is the change in velocity from one state to another. Intuitively, the answer should depend on the efficiency of the rocket and the mass of the spacecraft. The key link between delta V and kilograms of propellant is the rocket equation. Δ V = I sp g o m ln( m initial final )

24 2 24 The change in velocity is directly proportional to the ISP, the acceleration of gravity, and the natural logarithm of the initial mass divided by the final mass, mass ratio. Figure 2 15 shows a graph of the rocket equation. This graph quickly shows us why it is so hard to get to orbit with a single-stage vehicle. We need V=9.3 km/sec to get to Earth orbit; with ISP = 450 seconds (the best current liquid-fuel engine), 88 percent of the mass of a single-stage vehicle on the pad must be propellant. This also can be considered the mass fraction (propellant fraction) of the space vehicle determined by dividing the mass of the propellant by the total mass of the vehicle. This is why we use staging, which we will learn more about later. Figure Propellant fraction. Rocket propellants Rockets typically come in two basic flavors, thermodynamic and electrodynamics. The distinguishing feature is the kind of energy used to accelerate the propellant. Thermodynamic uses heat or pressure energy while the electrodynamics uses electromagnetic energy. Thermodynamic Thermodynamic breaks can be broken down into two parts. Thermo, meaning temperature or energy in transit and dynamics relates to movement ; thus, in essence thermodynamics is the movement of energy and how energy movement can be used to move objects. Cold gas The first and most simple-rocket type is a cold gas system. It consists of a compressed gas cylinder connected to a valve and a nozzle. Like a balloon, when we open the valve and let out some of the gas in one direction, we get a force in the opposite direction. This kind of system is simple and reliable. There are no moving parts. It can be turned on and off repeatedly. However, it provides very low thrust and low ISP. This type of motor is typically used for satellite control and small maneuvers. One of the most visible uses of these thrusters is the Manned Maneuvering Unit for the space shuttle astronauts (fig.2 16). These units can be seen in use when astronauts work outside of the shuttle to accomplish maintenance or during space walks to propel them from one spot to another.

25 2 25 Figure Manned Maneuvering Unit. Monopropellant A monopropellant is a chemical propulsion fuel that does not require a separate oxidizer. A rocket engine that is based on a monopropellant requires only one fuel line instead of a fuel and oxidizer line. The mono in monopropellant means singular a fuel that can function alone. A monopropellant burns by itself because the oxidizer is bound into the molecule itself. This makes the rocket-engine lighter less expensive and more reliable. It also provides lower temperatures that reduce chamber and nozzle problems. A monopropellant is a single liquid propellant that produces thrust by decomposing over a catalyst bed. It is used for low-thrust applications such as attitude control thrusters for satellites, interplanetary probes, and upper stages. These types of systems have low ISP and can have high toxicity. The two most common monopropellant/catalyst combinations are as follows: 1. Hydrazine with an iridium catalyst bed. 2. Hydrogen peroxide with a platinum catalyst bed.

26 2 26 Bipropellants One of the most common types of rocket engines is the liquid bipropellant. This type of system combines two types of fluids, a fuel and an oxidizer, and uses a chemical reaction burning to generate heat. A bipropellant system is a combination of two liquids, a fuel and an oxidizer, that are stored separately, injected, and burned in the combustion chamber. The maximum theoretical ISP for bipropellant engines is 480 seconds. Bipropellants are divided into three categories: 1. Cryogenics. 2. Storables. 3. Hypergolics. Cryogenic propellants are liquefied gases that must be kept at very low temperatures to prevent boiloff. Cryogenics yield the highest specific impulse of all liquid propellants, with a maximum of 455 seconds for the space shuttle main engines. Delta stage 1, Atlas boosters, and most Russian and Japanese boosters use liquid oxygen as an oxidizer, with RP 1 as fuel. Some vehicles that use the LH 2 and LOX combination include the space shuttle, Centaur upper stage, Russian Energiya second stage, Japanese H 1 second stage, Ariane 5, and the Chinese Long March 3. These normally gaseous propellants are liquefied to reduce the volume and size of their respective tanks. If the space shuttle were to carry its propellants in gaseous form, its external tank would have to be 22.4 times larger than its present dimensions (154.2 ft ft.). Cryogenically fueled vehicles could not leave the ground if they used gaseous propellants. Storable propellants may be left in a launch vehicle for years at normal temperatures and pressures. In fact, storables were developed to replace early cryogenic ICBM systems to increase strategic readiness. The Delta second stage yields the highest specific impulse for storable propellants, seconds. Storable propellants include hydrocarbon-based liquids such as RP 1 (kerosene) and hypergolics. Hypergolic propellants are storable fuel/oxidizer combinations that ignite on contact and therefore don t require an igniter. The most common hypergolic combinations use a hydrazine (N2H4) based fuel and nitrogen tetroxide (N 2 O 4 ) oxidizer. However, as the table shows, there are many hypergolic fuel/oxidizer combinations. Fuel Oxidizer Vehicle Aerozine 50 N2O4 Titan Aerozine 50 N2O4 Delta 2nd Stage UH25 N2O4 Ariane 4 1st Stage UDMH N2O4 Long March 2 UDMH Nitric Acid Russian SL 8 1st Stage Bipropellant liquid systems are used extensively for heavy-lift applications. Most frequently, they are the core vehicles to which solid or liquid strap-on boosters are attached. Liquid systems are very versatile because they provide respectable thrust, long burn times, throttling capability, and restart capability. However, they are expensive, complex, heavy, and can have some very stringent storage requirements dependent on the type used. Solid propellants Solid propellants contain everything needed to sustain combustion and burn on their exposed surfaces to produce thrust. They have very high thrust. Unlike liquid systems, they can t be throttled and they don t have restart capability. It is also difficult to vector the thrust of solid rockets. Because the fuel and oxidizer are combined, they can be an explosive hazard. However, solid fuels are very stable and therefore easily stored for long periods of time. This was a major factor in the decision to convert our

27 2 27 ICBM forces from liquid to solid fuels like the Minuteman missile. Additionally, solids are simple and therefore inexpensive. Various composite propellants power military, scientific, and commercial launch vehicles that employ solid motors. Generally, composite propellants are a heterogeneous mixture of ammonium perchlorate oxidizer, aluminum powder fuel, and other additives held together in a synthetic rubber or plastic binder. The mixture that makes up the shaped mass of the composite propellant is called the grain. The binder in composite propellants also acts as a fuel. In fact, some systems use only the binder as fuel. Fuel The most prominent solid fuel is powdered aluminum. Other light metals have been researched but have been dismissed because of excessively high melting points, short shelf life, toxicity, and difficult manufacturing. The fuel makes up percent of the grain. Oxidizer Ammonium perchlorate dominates as a solid oxidizer because of its high-oxidizing potential and its compatibility with other propellant ingredients. It is supplied as a fine white powder and makes up percent of the propellant grain. Binder The binder holds all the ingredients in the grain together and has a great effect on motor reliability. Binders are plastic or rubbery materials that can also act as a fuel. Most binder material is polybutadiene-based. Additives Additives are added to fulfill many functions in the propellant grain. Some reasons include: Vary the curing time (catalyst). Improve casting properties. Add opaqueness. Tailor burning rate with inhibitors. Improve intergrain and grain-case bonding. Improve moisture resistance. Factors that affect solid rocket thrust Solid rocket motor thrust and burn rate can be tailored by adjusting three variables. Fuel-oxidizer ratio To an extent, an increase in the oxidizer content of the solid rocket grain will also increase the thrust and burn rate. However, an overabundance of oxidizer would be wasteful if all the fuel was consumed and unreacted oxidizer still remained (excess weight). Inhibitor content Inhibitors can be mixed throughout the grain to restrict the burn rate and lengthen the burn time. Additionally, they are applied to the motor casing wall where they act as liners or insulators. Even though inhibitors allow the motor to burn longer, they also reduce the thrust. Grain configuration Variations in the composition and shape of the grain change the burn rate and the thrust of a solid rocket motor. Grain composition can be designed to vary the oxidizer and inhibitor content at different points in the grain depending on the desired thrust-time profile. The geometric shape of the grain will change the surface of the burn area, thereby changing the burn rate and thrust. In general, a

28 2 28 larger burn area will garner greater thrust and shorter burn time. Conversely, a small burn area will produce less thrust but net a longer burn time. This is determined by the rockets grain configuration and burn rates and, of course, mission application. The following three types of burn rates are governed by the grain configuration: 1. Progressive burn. 2. Neutral burn. 3. Regressive burn. In a progressive burn, burn area and thrust increase with time as shown in figure Figure Progressive burn. In a neutral burn, burn area and thrust remain constant with time as shown in figure Figure Neutral burn.

29 2 29 In a regressive burn, burn area and thrust decrease with time as shown in figure Frequently, several grain shapes and compositions may be incorporated into one solid rocket motor. For instance, the uppermost grain in the shuttle solid rocket motors has an 11-point star geometry that slowly transitions aft to a smooth tapered center bore. Hybrid propellants Hybrid rockets attempt to get the best of both worlds. Hybrid propellant systems use a combination of a liquid oxidizer and a solid fuel. The liquid is injected into the solid, whose fuel reservoir also serves as the combustion chamber. The main advantage of such Figure Regressive burn. engines is that they have high performance, similar to that of solid propellants, but the combustion can be moderated, stopped, or even restarted. Technology has not caught up to make efficient use of this concept for very large thrusts, and thus, hybrid propellant engines are rarely built. Safety Hybrids can t detonate since the fuel contains no oxidizer. This also reduces cost in manufacture, transportation, and launch operations. Throttling Hybrids can be throttled by varying the oxidizer flow. This aids in acceleration control for fragile payloads and in trajectory shaping. Restart Stop and restart is accomplished by turning the oxidizer flow off and on, assuming the igniter has restart capability. Overall, the hybrid propellants have more moving parts than solids and lower specific impulse than the liquids. Nuclear thermal Nuclear propulsion attempts to maximize the temperature in the combustion chamber to increase both the specific impulse and the exhaust velocity of the lightweight hydrogen propellant being ejected. Nuclear thermal engines employ a very compact mass of nuclear fuel to release tremendous amounts of energy. That energy is used to heat lightweight hydrogen gas and shoot it through a nozzle to get thrust. The nuclear reaction heats the hydrogen molecules to much higher velocities than chemical combustion can. Additionally, you still have the capability to restart and throttle. Despite the high ISP and high thrust offered by nuclear rockets, the necessary shielding and the nuclear reactor are very heavy, giving a poor thrust-to-weight ratio. Additionally, there is potential for nuclear contamination. This is why they are not used for space lift. They are, however, ideal for interplanetary travel. Arcjet thruster An arcjet thruster also tries to heat the fuel to extreme temperatures to increase efficiency. The arcjet accomplishes this by passing the fuel through an electric arc to heat it, rather than a nuclear core. This arc can generate an electromagnetic pulse that can cause interference problems. Again, you get high ISP and a moderate thrust. However, because it requires a power source, it has a limited space life and a high power requirement to generate the arc.

30 2 30 Resistojet thruster Resistojet thrusters are similar to arcjets, but they use an electric filament to heat the fuel instead of an electric arc. These are less efficient than arcjets but are more reliable. They are used for station keeping and attitude control. They are simple and have a long lifetime. Just as arcjets, they have a high-power requirement. Additionally, they have a low specific impulse. Electrodynamics These engines rely on the forces between atoms to create energy for thrust (i.e., magnetism or electrical bonds). These engines are still in the developmental stages. This portion of the lesson will address two of the more commonly discussed electrodynamics engines. Ion thruster An ion engine relies on electrically charged atoms, or ions, to generate thrust. Xenon, an inert, noncombustible gas, is electrically charged, and the ions are accelerated to a speed of about 62,900 miles per hour using an anode-cathode generated electric field. The ions are emitted as exhaust from the thruster, creating a force that propels the spacecraft in the opposite direction. The big advantage to the ion thruster is efficiency. The thruster pushes its exhaust about 10 times faster than chemical rocket exhaust. With xenon, it is possible to reduce propellant mass onboard a spacecraft by up to 90 percent. The advantages of having less onboard propellant include a lighter spacecraft, and, since launch costs are set based on spacecraft weight, reduced launch cost. It gives a very high specific impulse, very long lifetime, and unlimited restart capability. However, it provides very low thrust and has a high-power requirement. Plasma rocket High-exhaust velocity can be achieved by the use of plasma, where the atoms of the gas have been stripped of some of their electrons, making it a mixture of charged particles. Superconducting magnets work a bit like a microwave oven by stimulating the hydrogen molecules, thus heating them. The magnets generate a field that corrals the plasma. With the plasma rocket, you get relatively high thrust and very high ISP. However, you have a limited life span, high-power requirements, and a limited ability to restart the system Orbital mechanics Bodies remain in space according to defined physical laws. Orbital motion in space is different from motion on the surface of Earth. However, many terms and concepts are transferable, and we apply similar logic to both cases. Orbital geometry Most satellite orbits take the form of an ellipse. The ellipse is only one of a family of curves called conic sections. Before discussing satellite orbits, it is essential to know a few facts about conic sections in general. Conic sections Any free-flight trajectory can be represented by a conic Figure Conic sections. section. Simply stated, a conic section is a curve formed when a plane cuts through a circular cone at any point except at the vertex or center (fig. 2 20). If the

31 2 31 plane cuts both sides of one nappe (either of the two cones which are divided by the vertex) of the cone, the section is an ellipse. The circle is a special case of the ellipse. If the plane cuts the cone in such a way that it is parallel to one of the sides of the cone, the section is called a parabola. If the plane cuts both nappes of the cone, the section is a hyperbola with two branches. If these sections are laid out on a piece of paper and their outlines traced, a series of curves results (fig. 2 21). These curves can represent all of the possible kinds of space-flight trajectories. All conic sections can be defined in terms of their eccentricity (E). Eccentricity is an indication or measure of the relative shape of the conic section. If the eccentricity is zero, the conic section is a circle. If the eccentricity is greater than zero but less than one, the conic section is an ellipse. If the eccentricity is equal to one, the conic section is a parabola. If the eccentricity is greater than one, the conic section is a hyperbola. Figure Trajectories from conic sections. Figure Elements of an ellipse. The simple ellipse Before learning Kepler s laws of planetary motion, it is necessary to understand a key geometric figure called an ellipse. Figure 2 22 depicts the key terms of an ellipse. An ellipse is simply the set of points whose distances to two fixed points (foci) are a constant sum. The easiest way to draw an ellipse is to use a loop of string around two pushpins. As a pencil draws the string taught, it will scribe out a curve, which keeps a constant total distance to the two pins. As the foci move closer together, the ellipse becomes more and more circular (its eccentricity decreases). A circle is simply an

32 2 32 ellipse whose foci lie one on top of the other (zero eccentricity). One thing to note at this point is that a satellite s orbit will never be a truly perfect circle or perfect ellipse. A satellite orbit is simply an ellipse with a large gravitational body occupying one of its foci. Such an orbit has several important locations and parameters (fig. 2 23). Figure Elements of an orbit. Occupied/primary focus. The occupied or primary focus is the gravitational center of the body about which the satellite orbits. For satellites in orbit, the occupied focus is Earth. Unoccupied/secondary focus. The unoccupied or secondary focus is defined as the geometric point equidistant from the ellipse s geometric center as the occupied focus. Both foci are on the major axis. Perigee. Perigee is the point on the ellipse where the satellite is the nearest to the occupied focus. Apogee. Apogee is the point on the ellipse where the satellite is the farthest away from the occupied focus. Major axis. The major axis is the largest diameter of an ellipse. Semimajor axis. The semimajor axis is half the length of the major axis, or the longest radius of an ellipse. Minor axis. The minor axis is the shortest diameter of an ellipse. The minor axis is perpendicular to the major axis. Semiminor axis. The semiminor axis is half the minor axis or the shortest radius of an ellipse. Geometric center. The geometric center is the center of the ellipse or the intersection between the major and minor axis. Focal length. The focal length is the distance from the geometric center to a focal point. Apogee height. The distance from the surface of Earth to the apogee. Radius of apogee. The distance from the center of Earth to the apogee. In Earth orbit, the radius and height of apogee differ by 3,964 mi., a significant distance in performing orbital calculations. Perigee height. The distance from the surface of Earth to the perigee. Radius of perigee. The distance from the center of Earth to the perigee.

33 2 33 The elliptical parameters just discussed are common to all ellipses. Some of these parameters define the size and shape of an ellipse while others describe distances of measurement or points within an elliptical orbit. The laws of orbital mechanics Johannes Kepler ( ), astronomer, astrologer, mathematician, and scientist, was a Copernican (believed in a sun-centered universe) to the core. Born in 1571, the offspring of an unhappy marriage and usually suffering from some illness, Johannes was blessed with superior intelligence, which was recognized even when he was a young child. With the help of generous scholarships, Kepler was able to attend a university where he studied astronomy. After completing his formal education, Kepler began to teach mathematics when his obsession with understanding celestial motion became a life-long ambition. Kepler sent a copy of his mathematical calculations and observations to Tycho Brahe, a Danish aristocrat who was also interested in astronomy and astrology. It was not until Brahe died that Kepler gained access to Brahe s astronomical observations and began what he called a war on Mars. Kepler s tireless efforts to fit the orbit of Mars to Brahe s observations in every possible combination of circular orbits repeatedly failed until Kepler finally discovered the true solution. Mars revolves in an elliptical orbit with the sun occupying one of its two foci. Not only did Mars travel in an elliptical path around the sun, so did all other planets. Kepler was the first to use the word satellite in Based on the results of his research and analysis, Kepler, developed his three laws of planetary motion. Kepler s first law (law of ellipses) The orbit of each planet is an ellipse, with the sun at one focus. For Earth satellites, orbits are ellipses with the center of Earth at one focus (fig. 2 24). Figure Law of ellipses. Though Kepler s first law addressed the motion of planets around the sun, it is applied to describe the closed orbital paths of any object as an ellipse, such as a satellite around Earth. Kepler s second law (law of areas) The line joining the planet to the sun sweeps out equal areas in equal time intervals. Kepler knew that the planets did not maintain constant speeds as they orbited the sun. They appeared to move slowly near apogee and quickly near perigee. Kepler lacked Newton s laws of gravitation to

34 2 34 explain the relationship between distance and velocity and could only describe the motion. He chose to do so in terms of the area swept out by a line joining the planets to the sun (fig. 2 25). Kepler s second law related the varying velocity of an orbiting body to the distance to its occupied focus by comparing the areas swept out by a line joining the two. Near perigee, the length of the line decreases and velocity increases. The opposite occurs at apogee. The effects balance, and the joining line sweeps out equal areas in equal times. Thus, the short line to a planet near perigee must sweep more quickly than the longer line to a planet near apogee. For Earth satellites, the line joining the satellite to the center of Earth sweeps out equal areas in equal time intervals. Kepler s third law (law of periods) Kepler further noted that larger orbits took longer to complete. He compared the size of an orbit (characterized by its semimajor axis,) to its orbital period. Before Figure Law of areas. developing this law, Kepler assumed an orbit, twice as large as another, would only take twice as long to complete. The true relationship was nonlinear (i.e., an orbit that was twice as large as another took more than twice as long to complete). His determination of the relationship between the period and semimajor axis became his third law. The squares of the sidereal periods of the planets are in direct proportion to the cubes of the semimajor axes of their orbits. Kepler s third law describes the relationship between orbit size (semimajor axis) and period. To apply Kepler s third law to an earth orbital system, we restate the law as: The squares of the periods of the orbits of two satellites are proportional to each other as the cubes of their mean distances from the center of Earth. The value of this law, if you know the satellite s distance, is that you can calculate its period. Inversely, if you know the satellites period, you are able to figure out its distance. While Kepler was working out his three laws of planetary motion, Galileo Galilei, a great Italian physicist and astronomer, was studying the effects of gravity on falling bodies in Pisa, a city in Tuscany, Italy. Kepler told us how orbital motion acts, but he did not tell us why. Sir Isaac Newton ( ) figured it out. No single person has had as great an impact on science as Isaac Newton. His numerous discoveries and fundamental breakthroughs, such as inventing calculus, inventing the reflecting telescope, and defining gravity, are some of his many accomplishments. Newton was a remarkable man for his time. He took the works of Kepler and Galileo and formulated the laws of motion, which have become the foundation for all physical science. For our purposes, we will see that the study of orbits (astrodynamics) builds on Newton s laws: three of motion and one describing gravity. Newton s first law (law of inertia) Everyone continues in a state of rest or of uniform motion in a straight line unless it is compelled to change that state by a force imposed upon it. In other words, a body at rest tends to remain at rest, and a body in motion tends to remain in motion unless an outside force acts upon it. We sometimes call this law the law of inertia.

35 2 35 Newton s second law (law of acceleration) When a force is applied to a body, the time rate change of momentum is proportional to and in the direction of the applied force. In other words, a body acted upon by a constant force will move with a constant acceleration in the direction of the force. This acceleration will be directly proportional to the acting force and inversely proportional to the mass of the body, F = MA. Newton s third law (law of action and reaction) Newton s third law states For every action there is a reaction equal in magnitude but opposite in direction to the action or in other words, For every action there is an equal and opposite reaction. For example, if body A exerts a force on body B, then body B exerts an equal force in the opposite direction on body A. These three laws are called Newton s Laws of Motion and determine mechanically how a spacecraft s motion begins and changes. Newton s fourth law is one of the most important because it relates how the force of gravity works. The law of universal attraction The force of gravity is directly proportional to the product of the masses of two bodies and is indirectly proportional to the square of the distance between them. Simply stated, the more massive or closer a body, the greater its force of gravity. The seven laws stated above comprise the bulk of the principles that govern space flight and the calculation of orbits. How satellites stay aloft Artificial satellites remain aloft for the same reason the moon does. The force of Earth s gravity and the velocity of the body in motion are nicely balanced. Like any body in motion, the moon s natural tendency is to travel a straight-line course that would carry it off into space. Newton s first law of motion applies here: A body tends to remain at rest or in motion unless acted upon by an outside force. The tendency of gravity is to pull any unsupported object to Earth. The moon or an artificial satellite has too much velocity to fall earthward and not enough to break away from Earth s gravitational field, so it keeps circling Earth in orbit. Similarly, Earth and the other planets of the solar system are held in their orbits by the sun s powerful gravity. In theory, an artificial satellite could continue to orbit Earth indefinitely. A satellite may fail to do this due to errors in either launch velocity or direction. A booster might not accelerate a satellite to the minimum speed necessary to counteract Earth s gravity (at an altitude of 300 miles, the required speed is about 18,000 miles per hour). Another possibility is it might launch a satellite at such an angle that it swings too close to Earth s atmosphere. In time, the slightest atmospheric friction can rob a satellite of momentum and start it spiraling back to Earth. Most artificial satellite orbits are elliptical (fig. 2 26) rather Figure Apogee and perigee. than circular. In an elliptical orbit, a satellite s speed varies continuously. When launched into orbit at perigee, we give

36 2 36 the satellite more than enough velocity to balance gravity. The satellite doesn t follow a path around Earth at a fixed altitude. The satellite gradually curves away from Earth. The satellite s surplus kinetic energy is expended as it pulls away from Earth, fighting gravity. Its velocity slowly decreases and reaches a minimum at apogee; the pull of gravity now becomes dominant. Throughout the second half of the revolution around Earth, the satellite drops through a long curve, picking up the velocity it lost on its outward swing toward apogee. At perigee, moving at maximum speed, the satellite has built up enough velocity to overcome gravity and starts shooting off into space again. However, as the satellite climbs back to apogee, it once more loses momentum, steadily relinquishing the advantage it gained in falling to perigee. We would repeat the cycle endlessly if no atmospheric resistance was met. Launch characteristics An important concept to understand is the launch window and how it relates to the launch site and the performance of the booster vehicle. Another significant launch concept is the relationship between Earth s rotational speed, the latitude of the launch site, and the launch azimuth. Launch window In space planning, planners choose the launch vehicle based on its performance, the payload weight, and the launch window. The launch window (fig. 2 27) is a term used for a period of time associated with a specific launch site and desired orbit. The launch window defines a time to conduct the most fuel-efficient launch. Two factors affect launch windows: the performance of the booster vehicle and the latitude of the launch site. The length of a specific window is dependent on the launch vehicle s performance. If there is more excess performance available from the launch vehicle, then there is more steering capability available and, consequently, the larger the launch window. Figure Launch window. Launch site The latitude of a launch site imposes a restriction on the initial inclination (angle of ascent or decent) of a spacecraft s orbit. Specifically, the latitude of the launch site is the lowest inclination possible for all direct launches. A launch azimuth (the measurement of the angle in reference to the launch pad) of

37 determines the minimum inclination of the orbit; any other azimuth results in an inclination greater than the latitude of the launch site. Because of this restriction, the launch site determines the number of direct-insertion launch opportunities. If the latitude of the launch site is lower than the inclination, then there will be two launch opportunities per day. For instance, if a network of satellites was to be placed in a 45 degree latitude orbit and launched from Cape Canaveral there are two launch opportunities. If the latitude of the launch site equals the inclination, then there is one launch opportunity per day. If the latitude of the launch site is greater than the inclination of the desired orbit, then there are no opportunities for direct insertion into the desired orbit. It is impossible to directly launch a spacecraft into an orbit whose inclination is less than the latitude of the launch site. To achieve an inclination less than the latitude of the launch site requires an orbital maneuver. This requires more propellant or a smaller payload, or an increase in the size of the launch vehicle. Earth s rotational speed As we mentioned earlier in the Space lift section, another significant impact to launch is the speed of Earth s rotation. There is a direct correlation between the latitude of the launch site and the speed of rotation (fig. 2 28). The rotational speed increases as latitude decreases. Therefore, rotational speed ranges from 0 mph at the poles, to 1,039 mph at the equator. Earth rotates from west to east; consequently, any launch in a prograde direction receives an advantage from the speed of rotation. Figure Earth s rotational speed. The greatest advantage is launching from the equator with a launch azimuth of 90 (directly east). Therefore, the closer the launch site is to the equator, the more efficient the launch operation for prograde orbits. Self-Test Questions After you complete these questions, you may check your answers at the end of the unit Space History 1. Who were the first to use devices similar to rockets?

38 Match each historical figure in column A with his accomplishment in column B. The answers can only be used once. Column A (1) Konstantin Tsiolkovsky (2) Robert Goddard (3) William Congreve (4) Wernerher von Braun (5) Sergey Korolyov (6) Herman Oberth 3. How does knowing your history make you a better technician? Column B a. Launched the first liquid-fuel rocket. b. Directed design, testing, and construction and launching of the Vostok manned spacecraft. c. Led development of the vengeance weapon number 2. d. Work increased the understanding of importance of fuel-to-weight ratio and its effects in multistaging. e. Russian school teacher considered the father of modern astronautics. f. Noted for developing the laws of motion. g. Considered the inventor of military rockets Introduction to space lift vehicles 1. Match each rocket in column A with the rocket s accomplishment in column B. The answers can only be used once. Column A Column B (1) Atlas (2) Delta (3) Titian a. Launched the unmanned Surveyor lunar probes; the interplanetary Mariner probes to Mars, Venus, and Mercury; as well as the Pioneer probes to Jupiter, Saturn, and Venus. b. Originally intended as a backup for the space shuttle to be used only as conditions warranted. c. Originated in 1959 using components from the US Air Force s Thor ICBM program and the US Navy s Vanguard launch vehicle program. 2. Which Russian space launch vehicle is being marketed in the West by a joint venture between Krunichev and Lockheed Martin? 3. Which space launch vehicle series holds the largest market share in the international commercial launch market? 4. Which space launch vehicle series of launch vehicles include 11 different operational vehicles? 5. Which space launch vehicle is the cornerstone of Japan s plans for increasing activities in space, including eventual manned missions?

39 Rocket science 1. What are the two different types of thrust? 2. What are the two methods for optimizing expansion ratios at different altitudes? 3. What is a rocket s specific impulse? 4. What are the two kinds of energy used by rockets to burn propellant? 5. What are the three burn rates of solid propellants governed by grain design? 6. What does an ion thruster use to create thrust? 016. Orbital Mechanics 1. Which conic section has an eccentricity of zero? 2. Which conic section has an eccentricity greater than zero but less than one? 3. At what point in an elliptical orbit is the satellite nearest Earth? 4. At what point in an elliptical orbit is the satellite farthest from Earth? 5. What is Kepler s first law of motion also known as? 6. How do we apply Kepler s laws to man-made objects? 7. What are the two forces that, when balanced, allow a satellite to remain in orbit?

40 At what point in an elliptical orbit is the velocity of a satellite at a minimum? 2 2. Introduction to Launch Systems This section will provide aerospace technicians with a solid foundation on launch systems and provide general knowledge on many different subsystems. Knowing how the system works when in operation is one of the first steps in a technician being able to perform analytical troubleshooting and the why behind maintenance procedures Rocket structures There are several different types of rocket designs, but the majority are composed of the same major systems discussed in this lesson. Each system may vary from designer to designer, but they share the same overall concepts. Liquid-propellant engine structures The six major components of a liquid-propellant engine are as follows: 1. Propellant tanks. 2. Feed system. 3. Injector system. 4. Combustion chamber. 5. Ignition system. 6. Nozzle. Figure Examples of typical tank configurations. you usually don t see them as main booster tanks. Propellant tanks Propellant tanks may be cylindrical or spherical. Cylindrical tanks are overwhelmingly used in the lower stages of launch vehicles. They are arranged in tandem configuration, one atop the other, figure 2 29, to give the lowest cross-sectional area for total tank volume. Tandem tanks may employ common bulkheads and internal piping. Propellant-tank structure size also depends on types of fuel. In a liquid-hydrogen and liquid-oxygen propellant system, the tank for the hydrogen requires less volume than the oxygen. Hydrogen molecular mass is smaller than that of oxygen. Spherical tanks are used as liquid-propellant tanks on upper stages, earth satellites, and interplanetary probes. Small spheres are also used as pressurant tanks throughout the launch vehicle. Spheres provide the largest volume for the smallest surface area that equates to lower weight. However, they are very space inefficient when stacked in tandem fashion. This is why Spherical pressurant tanks store inert gases (nitrogen or helium) under high pressure (to over 4,400 psi) for propellant-tank pressurization, thrust vector control, attitude control jets, and stage separation systems.

41 2 41 Feed system Liquid-propellant-feed systems transfer propellants from the tanks to the combustion chamber. For optimum performance feed systems are calibrated to deliver fuel and oxidize at the correct rate and ratio. The latest feed systems also have throttling and stop/restart capability. Liquid-propellant-feed systems are divided into pressure-feed and pump-feed systems as depicted in figure Figure Feed systems. Pressure-feed systems force propellants out of their respective tanks by displacing them with a highor low-pressure inert gas, such as helium or nitrogen. For example pressure will be maintained on a bladder system causing the bladder to collapse, forcing the fuel out and into the combustion chamber. Otherwise, in a 0 g condition, fuel would not flow. These systems are ideal for satellite low-thrust attitude control because of their simplicity and reliability. However, pressure-feed systems have several drawbacks that exclude them from heavy lift operations. They can t deliver enough propellant for high-thrust applications. In addition, the pressurant tanks and the propellant tanks must be very heavy to withstand the high internal pressures. Pump-feed systems deliver propellants from the tanks to the combustion chamber by means of highvolume, high-pressure-driven centrifugal pumps called turbopumps. Only pump-feed systems can deliver enough propellant for high-thrust applications. Injector system The liquid propellants enter the combustion chamber through the injector system. The injectors meter, atomize, and mix the propellants to ensure efficient combustion (fig. 2 31). Injectors are arranged in rings on an injector plate situated at the top of the combustion chamber. For instance, the 22-inchdiameter injector plate on the space shuttle main engine has 600 injectors. The injector plate on the Russian SL 6 contains 10 rings that hold 337 injectors.

42 2 42 Figure Some typical injector configurations. Combustion chamber The combustion chamber is the part of the launch vehicle engine where the propellants are burned at high pressure with low velocity and, in turn, create high-temperature gases. The combustion chamber consists of three major parts: the chamber, spark igniter and injectors. Several considerations are essential for a high-performance combustion chamber. Its volume must be sufficient for adequate mixing and complete combustion. Its cooling system must ensure wall integrity. The chamber diameter and weight will determine the vehicle dimensions and combustion chamber materials must withstand enormously high temperatures and pressures. The Atlas sustainer engine operates at 735 psi and 6,000 F. The highest thrust liquid-propellant engine ever flown is the Russian RD 170, which operates at the highest-known combustion-chamber pressure of 3,675 psi. Ignition system The ignition system provides the energy to start the combustion of the propellants. Once ignition has begun, the flame is self-supporting. The igniter must be placed sufficiently near the injectors, but it must not interfere with steady state combustion. We will look at five types of ignition systems: 1. Spark plug. 2. Precombustion chamber. 3. Auxiliary fluid injection. 4. Pyrotechnic charge. 5. Hypergolic fluid cartridge. Spark plug Spark plug ignition is primarily used on LH 2 /LOX systems, particularly those that require multiple restarts (space shuttle main engines, Centaur upper stage, etc.). The spark plug is usually built into the injector and is powered by onboard batteries. Precombustion chamber The precombustion chamber is analogous to a pilot light on a gas heater. A small amount of fuel and oxidizer maintains a torch-like flame that enters the combustion chamber and ignites the main propellant flow. A liquid-fueled launch vehicle equipped with a precombustion chamber has restart capability. Auxiliary fluid injection Auxiliary fluid injection supplies a fluid that forms a hypergolic mixture with one of the propellants to start the main combustion process. For instance, nitrogen tetroxide is hypergolic with many fuels. It can be the auxiliary fluid in a kerosene/lox system where it will ignite with the kerosene to start the main combustion process. This system has restart capability until the fluid reserve is exhausted.

43 2 43 Pyrotechnic charge Pyrotechnic ignition uses a small amount of electrically ignited solid propellant that burns within the combustion chamber for a few seconds. These are primarily used in hydrocarbon-based (RP 1) and solid-fueled launch vehicles. The charge can be built into the injector or held from the outside through the nozzle. Pyrotechnic ignition is used on the Delta launchers and does not allow restart capability. Hypergolic fluid cartridge Hypergolic fluid cartridges separate two fluids with burst diaphragms. When the diaphragms are broken with a small initiator charge, the hypergolic fluids mix and ignite. This process supplies the ignition source for the main propellant flow, but it does not allow for restart capability. Atlas boosters use this type of ignition. Nozzle The launch vehicle nozzle converts the random thermal and kinetic energy in the combustion chamber to coherent, directed kinetic energy necessary to propel the launch vehicle forward. The converging throat is generally about 1/4 the cross-sectional area of the combustion chamber, and it forces the combustion gases to accelerate into the diverging portion of the nozzle and extension. As the gases leave the nozzle, they expand and accelerate even more. Launch vehicle engines and nozzles operate at tremendous pressures and temperatures. While engines and nozzles only need to last for short amounts of time, they do need to maintain their structural integrity for the duration of the propellant burn. Cooling of the nozzle can be accomplished using cryogenic propellant to dissipate heat. Engine mount The engine mount connects a liquid-propellant engine to the main launch vehicle structure. It evenly distributes the force of thrust around the periphery of the rocket body and provides the mounting points for the Gimballing actuators. The engine mount is also called the engine truss or thrust cone. Solid rocket motor components Solid rocket motor components differ from their liquid counterparts in many ways. For example, solid rocket motors do not require the use of LOX and OX tanks because the solid-rocket fuel is bound tightly. Solids reduce the number of pumps and feed systems required on liquid systems, thus reducing weight. However, there are many advantages and disadvantages related to both liquid and solid systems, which we will discuss in detail later. We will briefly discuss the three main components of a solid rocket motor, which are the combustion chamber, igniter, and nozzle. Combustion chamber In the case of a solid-propellant system, the combustion chamber also constitutes the primary structure of the vehicle. It is the propellant tank, combustion chamber, launch vehicle body, and the attachment point for the nozzle, igniter, blowout ports, and handling hooks. Solid-propellant combustion chambers must also withstand enormous temperatures and pressures. Solid rocket motor casings are manufactured from high-strength steels, titanium alloys, and filamentwound organic fibers in epoxy resins. Igniter The igniter in a solid rocket motor generates the heat required to start the main propellant burn. Today s solid rocket motors commonly use pyrotechnic igniters, which are placed in the forward dome of the motor. The heat-releasing compounds in these igniters can be black powder or conventional solid-rocket propellant.

44 2 44 Nozzle The launch vehicle nozzle in solid systems also converts the random thermal and kinetic energy in the combustion chamber to coherent, directed kinetic energy. It also rapidly expands and accelerates the exhaust gases to enhance thrust. Solid rocket motor nozzles can be cooled by the same methods used by liquid systems, except the regenerative method. Rocket bodies The rocket body is the main vehicle structure. The rocket body, as shown in figure 2 32, houses all the fundamental internal launch vehicle structures, such as the propellant tanks, guidance and control equipment, and payload compartment. It is also the attachment point for the launch vehicle engine and strap-on boosters when they are used. In some instances, the rocket-body skin doubles as the propellant tank walls (Titan II, Atlas). Solid-propellant rocket bodies are also the propellant tank and the combustion chamber. The two rocket body configurations we will discuss are monocoque and semimonocoque. Figure Rocket body. Monocoque A monocoque rocket body carries all the launch stresses on the skin of the launch vehicle and has no internal framing. Because monocoques have no internal framing, they leave more room for equipment inside the rocket body. A monocoque may have formers around the inside periphery of the skin, but no longitudinal stringers. A pure monocoque is a metal or fiber-wound cylinder, such as a solidpropellant motor casing. Two different variations on the monocoque configuration include the balloon tank and the isogrid designs. In the balloon-tank configuration, the propellant tanks also serve as the rocket body and provide the structure that supports the entire launch vehicle and payload. The lightweight propellant tanks must be pressurized at all times for structural integrity. An isogrid is an integral cylindrical latticework structure with a skin. Isogrids are machined out of a solid slab of aluminum to leave the internal latticework and a thin external skin. They allow the propellant tank wall to function as the rocket body skin. Semimonocoque A semimonocoque rocket body employs longitudinal members called skin stringers, as well as formers or bulkheads to carry the stresses and reinforce the skin, figure The classic semimonocoque structure is a tubular-metal frame to which all the launch vehicle structures (tanks, engines, and payload) and skin are attached. The classic semimonocoque configuration is the heaviest of all and is presently only used on the Ariane 4 launch system. However, a variation of the semimonocoque is used extensively on American, Chinese, and Russian launch systems. It is known as the skin-stringer configuration. Its manufacturing method is similar to the isogrid rocket body. Small I-beam stringers are machined into a solid slab of aluminum to leave a

45 2 45 thin skin on the outside and the stringers on the inside. The circular formers and bulkheads are riveted to the I-beams without impinging on the skin. The skin-stringer configuration also allows the propellant tanks to double as the launch vehicle skin since the skin and stringers are one coherent structure. The Titan vehicles are a good example of the skin-stringer configuration. Figure Skin-stringer construction. Intertank The intertank is the mechanical connection between propellant tanks within one stage. The voids within the intertank serve as a compartment for instrumentation, figure The intertank is sometimes called the center body section. Figure Intertank. Interstage The interstage is the mechanical connection between stages in a multistage vehicle and is the adaptive interface between stages of different diameters. It is also the protective compartment for instruments, electronics, and upper-stage engine nozzles. Some space launch systems (Titan, Proton) have open or partially open interstages to allow the upper stages to ignite before the stages separate.

46 2 46 Payload fairing/shroud The payload fairing is the uppermost portion of the launch vehicle, which protects the payload from aerodynamic loading and heating during the ascent through the atmosphere. It also gives the vehicle a streamlined shape to reduce drag. Wiring tunnel The launch vehicle s wiring harness can t run through the interior of the rocket body since it would compromise the propellant tanks. Therefore, the wiring tunnel protects the wires that are mounted on the exterior of the rocket body. Boosters Many systems use boosters to assist in breaking Earth s gravity. Boosters allow multiple engines to fire at the same time and reduce the size of the core vehicle. Many of today s boosters are modified ICBMs. One modification is a small hole placed near the top but below the nose cone to help relieve internal air pressure as the booster gains altitude. The expansion caused by interior pressure can cause early separation of the booster Staging and separation Vehicles traveling into space are often designed to discard significant sections no longer needed for the journey. These sections are most often used to support fuel storage. When the space vehicle discards these fuelstorage sections of the rocket, it is referred to as staging and separation. Staging Staging involves a vehicle with two or more rocket units called stages that fire after the ones below them have consumed their fuel, figure The expended lower stages are jettisoned, allowing the upper stages to continue. The upper stages of multistage launch vehicles have higher expansion ratios in order to operate at higher altitudes. Purpose of staging Staging is the combination of two or more independent Figure Staging example. launch vehicle propulsion units to create a vehicle of increased performance. After propellants within a stage are consumed, the empty stage separates from the vehicle, and the operation continues with subsequent stages. Staging allows you to use the appropriate nozzle for different atmospheric levels. It is also an efficient method of increasing mass ratio and final velocity. Types of staging The two primary types of staging in use today are as follows: 1. Tandem. 2. Parallel.

47 2 47 Tandem Tandem staging involves each stage firing one after the other and stacking the propulsion units, one atop the other. Tandem staging allows us to increase the vehicle mass ratio and to optimize the nozzle expansion ratios for various altitudes. Additionally, tandem staging allows designers to easily adapt a vehicle to various upper stages for differing missions. However, total system reliability decreases due to the complexity of a tandem-staged vehicle. Parallel Parallel staging supplements the core engine thrust by having several stages fire at the same time usually with strap-on boosters. When the strap-ons are expended, they separate from the core vehicle that continues its flight. Engine clustering One way of improving safety, redundancy, and reducing the number of stages in a launch vehicle is engine clustering or using two or more engine or motor assemblies within one stage or strap-on. The big difference between engine clustering and parallel staging in the engine is not dropped off like in staging. Clustering improves safety through redundancy. However, one drawback to clustering is increased complexity, compared to single-engine configurations. The US has flown several clustered systems: 1. Shuttle: 3 Rocketdyne space shuttle main engines. 2. Saturn V: 5 F1s. 3. Saturn 1B: 8 Rocketdyne H1s. The king of cluster is still the Soviet N 1 moon rocket that was developed to support the Sovietmanned lunar program. This monster used 30 engines in its first stage to produce a thrust of 10.1 million pounds. It was tested (unsuccessfully) four times before the program was terminated in Separation methods There are several methods of stage separation: 1. Explosive bolts. 2. Rods and springs. 3. Separation rockets. 4. Pneumatic separation. Explosive bolts Explosive bolts are used to join launch vehicle components that are meant to eventually come apart, figure On command, they blow apart and allow the components to separate. Explosive bolts secure stages and fairing shells to each other. They also activate the latches, which tie the vehicle to the pad. Explosive bolts are often used in conjunction with rods and springs and separation rockets. Figure Explosive bolt.

48 2 48 Rods and springs Rods and springs are a low-stress, safe method of separating stages from each other or satellites from upper stages, figure A springloaded rod pushes the components apart as soon as explosive bolts have released them. Rods and springs separate the solid rocket boosters from the Delta and H1 vehicles. Satellites are frequently deployed from the shuttle cargo bay using rods and springs. Figure Rod and spring. Separation rockets Separation rockets immediately accelerate launch vehicle stages away from each other or solid rocket boosters away from the core vehicle, figures 2 38a and 2 38b. They are used where safety depends on rapid separation. The Ariane 4 uses eight rockets to separate the first and second stages. Separation rockets also push the solid rocket boosters away from the Titan and shuttle vehicles. In the case of the shuttle, eight 22,000 lb. thrust rockets separate the solids from the main tank. Figure 2 38a. Shuttle separation rockets (forward).

49 2 49 Pneumatic separation Pneumatic separation involves a compressed gas, which actuates a piston that directly pushes components apart or triggers a latch mechanism. There are two types of pneumatic systems: 1. Hot gas. 2. Cold gas. Hot Gas Hot gas systems employ a gas generator that provides the highpressure gas for the actuators. Cold Gas Cold gas systems store an inert pressurant (helium, nitrogen) in a spherical tank that supplies multiple actuators. Cold gas systems are capable of long-term storage and don t need to be preloaded on the pad (fig. 2 39) Navigation, guidance, and control Figure 2 38b. Shuttle separation rockets (aft). Guidance and navigation functions can originate from a ground-based system or from an independent system within the vehicle called an inertial guidance system. Early space launch systems and ICBMs used a ground-based system that required tracking radar to determine the velocity and position of the vehicle. This accomplished the navigation function. A ground-based computer, used to determine the steering commands and thus perform the guidance function, processed the navigational data. These commands were finally transmitted from the ground to the vehicle to accomplish the control function. Ground-based systems were very limited in that the ground station had to be within line of sight of the vehicle. In the case of ICBMs, one missile tied up a tracking radar and transmitter site until engine burnout. To have a credible retaliatory strike capability, each missile would have required a dedicated radar and ground transmitter. Additionally, the control function of a ground-based system is susceptible to jamming. The navigation function tells us where the vehicle is and how fast it is going. Now, how do we simultaneously get the vehicle to the proper position at burnout (r bo, lat bo, long bo ) and velocity at burnout (v bo ) to successfully place a satellite into orbit? This job is performed by the guidance function. Velocity at burnout and position at burnout are stored in the processors in the Inertial Navigation Unit (INU). This data is Figure Cold gas separation. compared to the actual velocity and position. The guidance system then sends commands to the control system until the preprogrammed and actual values for velocity and position match. When this happens, the inertial guidance system shuts down the launch vehicle engine and deploys the satellite. Today s ICBMs and space launch systems exclusively employ inertial guidance systems. This section will focus exclusively on inertial navigation, guidance, and control.

50 2 50 Inertial navigation Inertial navigation is the process of determining a launch vehicle s velocity and position without the aid of externally transmitted data. Once the vehicle senses acceleration on any axis, it can use computer processors and mathematics to find velocity and position (integrate once to find velocity and twice to find position). Inertial Navigation Unit The INU is a self-contained navigation system that determines a vehicle s position in inertial space through the integration of the output of accelerometers. Inertial navigation is based on the concept that you can use computer processors to calculate the velocity and position once you know the acceleration. Before the inertial guidance Figure 2 40a. Mechanical Inertial Navigation Unit. system can accomplish this, it must adhere to four basic principles: 1. It must carry and maintain its own reference frame from which acceleration can be measured. 2. It must be able to accurately sense acceleration. 3. It must have a preprogrammed knowledge of Earth s gravitational field distribution. 4. It must perform the calculations to find velocity and position. The reference frame from which acceleration is measured is called the stabilized platform. It is a gyroscopic device that maintains a preset reference plane regardless of the vehicle s movement. The INU accelerometers are mounted on the stabilized platform. Stabilized platforms employ either mechanical gyroscopes or ring-laser gyroscopes to maintain their orientation. A mechanical gyroscope (fig 2 40a) uses a gimballed spinning rotor to sense the change in angular momentum of the vehicle with respect to the spin axis of the gyroscope. Ring-laser gyroscopes spin two counter-rotating laser light beams within a triangular or square glass block (fig. 2 40b). They determine the change in angular momentum by measuring the frequency difference between the two Figure 2 40b. Ring-laser Inertial Navigation Unit. beams. Even though ring-laser gyroscopes cost about as much as mechanical gyroscopes, they have several advantages over them. Since they have almost no moving parts, they are more reliable. In addition, ring-laser gyroscopes have instant turn-on capability, while

51 2 51 mechanical gyroscopes take a while to spin up. They also are much lighter and more compact than mechanical gyroscopes. An accelerometer is a transducer (signal-sending unit) that measures acceleration. Although the accelerometer comes in many configurations, we classically describe it as a seismic mass suspended within a supporting frame by upper and lower springs. The displacement of this mass with respect to the supporting frame is used to measure acceleration. The INU uses three mutually perpendicular accelerometers mounted on a stabilized platform to measure acceleration on all three axes. Frequently, INUs for launch vehicles will have two accelerometers in the thrust axis for redundancy. Accelerometers can t measure acceleration due to Earth s gravitational field. They only measure nongravitational acceleration due to thrust, drag, and other contact forces such as wind. Therefore, the acceleration due to Earth s gravity must be added to the acceleration sensed by the accelerometers. The INU has a preprogrammed knowledge of Earth s gravitational field distribution at all relevant altitudes. As the INU determines the vehicle s altitude, it corrects for acceleration due to gravity and correctly determines acceleration. Once the INU has correctly determined the vehicle s acceleration, the onboard computer processors can calculate velocity and position. Guidance Guidance means to cause a rocket to follow a determined course and schedule. It is the process of comparing the vehicle s actual velocity and position to the predetermined flight regime and generating corrective steering commands based on the comparison. We will discuss three types of guidance systems in use today and their generalities. 1. Programmed guidance: the flight course stored in memory. 2. Radio guidance: dependence on radar or other transmitted signals. 3. Inertial guidance: use of onboard gyroscopes for course correction. Programmed guidance The course is put into a computer s memory before the flight to ensure that the rocket stays on a predetermined course. Timers and other instruments are used to send out control signals at regular intervals to guide the rocket. One drawback with this method is that it is difficult to compensate for such in-flight effects as strong wind. Radio guidance Radar and other signals relayed from Earth direct the course of the rocket while in flight. Should the rocket stray from its course, the amount of deviation is calculated, and signal commands are sent to the rocket to correct its course. Although this method allows remote control from Earth, it is not a very precise guidance method, and control is limited by how well you can observe the rocket from Earth. Inertial guidance Gyroscopes on board the rocket sense the attitude of the rocket, while data provided by accelerometers are used to calculate the speed and position. Such data are compared to scheduled course data stored in the computer to automatically keep the rocket on the right course. Since rocket speed is calculated using the law of inertia, this method of guidance is known as inertial guidance. It is far more accurate than programmed guidance, and for this reason, it is the most commonly used guidance method today. Control Control denotes the operations used to achieve these objectives. To launch a satellite and place it in a desired orbit, the direction and speed of the rocket must be accurately guided and controlled throughout the flight. It is the process of maintaining the vehicle on the correct trajectory so that it will achieve the nominal velocity and position for a successful payload insertion. The control system

52 2 52 executes the steering and thrusting commands received from the guidance system. The actuators that execute these commands can be electromechanical, pneumatic, and/or hydraulic. While the goal of the INU is to get the vehicle to the proper velocity and position at burnout, the guidance system continually sends commands to the control system in order for the launch vehicle to reach the point of successfully launching a satellite. The control system executes the steering and thrusting commands, along the control axes pitch, yaw and roll, to keep the vehicle stable and on the correct trajectory. The command signals from the guidance system actuate the thrust vector control systems. This system also helps provide the stability required for flight. We will define and discuss the three types of stability required for the successful execution of a launch. Stability Stability is the property of a rocket to maintain its attitude, to resist displacement, and, if it is displaced, to restore itself to its original condition. A rocket must be able to maintain its attitude with respect to roll, pitch, and yaw. There are three types of stability that are frequently used in combination: 1. Aerodynamic stability. 2. Spin stability. 3. Control stability. Aerodynamic stability Aerodynamic stability naturally keeps a launch vehicle oriented correctly and on its flight path if its center of mass is ahead of its center of pressure, figure The center of mass is the point at which the force of thrust and all rotational motions appear to act or where all the forces of drag appear to be concentrated. The center of mass (what we often also call the center of gravity, or CG) of an object is an imaginary point around which the mass of the object is balanced. This is where you could literally balance the rocket on your outstretched finger. Gravity affects the object the same way it would affect a single point mass at this same location, which simplifies quite a few physics problems. Another feature of the CG is that, if an unsupported object were to rotate, it would do so around the CG. Figure Aerodynamic stability. As you can see, a nose-heavy launch vehicle will be aerodynamically stable. The thrust vector that originates from the center of mass is pulling the launch vehicle forward. Whereas the drag vector (a retarding force), which originates at the center of pressure (CP), is pulling the launch vehicle

53 2 53 backward. These opposing forces are maintaining the launch vehicle in a stretched condition that is inherently stable. If the center of mass and the center of pressure were reversed, the launch vehicle would tumble. Frequently, fins are attached to the bottom of a launch vehicle to increase drag forces if aerodynamic stability needs to be enhanced. Similarly, you can think of the CP as the imaginary point at which the sum of all of the aerodynamic forces on the object are balanced. Spin stability A launch vehicle is spin stabilized by gyroscopic forces, which result from spinning the launch vehicle about its axis of symmetry. Spirally thrown footballs and bullets are spin stabilized. Only very small launch vehicles and upper stages are spin stabilized, usually just before ejecting a satellite. The Long March third stage is spun up to 180 RPM via cold gas nitrogen thrusters. Miniaturized rocket motors begin spinning the fourth stage of the Scout, six seconds before fourth-stage ignition. Control stability Control stability is the active control of the launch vehicle by controlling the thrust vector or by moving aerodynamic fins. Methods of controlling thrust A launch vehicle that can vary its thrust vector angle with respect to the main axis of the vehicle has thrust vector control capability. The types of thrust vector control methods we will look at are as follows: 1. Gimballing. 2. Jet vanes. 3. Verniers. 4. Rotating nozzles. 5. Secondary fluid injection. Gimballing Gimballing is the most prominent form of thrust vector control since it incurs almost no thrust loss. Gimballing is accomplished on liquid engines by pivoting the entire engine on a universal bearing, which interfaces between the rocket body and the engine mount. When necessary, hydraulic or pneumatic cylinders deflect the engine to achieve the desired thrust vector. Solid-propellant systems only gimbal the nozzle since the entire solid rocket motor can t be pivoted. A single-gimballed engine can provide pitch and yaw control but not roll control. Roll control for single-engine systems must be provided by small auxiliary rockets. A launch vehicle must contain at least a two-engine gimballed cluster to provide pitch, yaw, and roll control. Gimballed liquid engines require flexible propellant piping for the propellant to flow to a movable engine. Solid motors use a flexible bearing between the motor casing and the nozzle to allow articulation without losing pressure. Jet vanes Jet vanes are movable fins or airfoils placed in the launch vehicle exhaust flow behind the nozzle. By redirecting the exhaust flow, jet vanes can control the direction and attitude of the launch vehicle. They are frequently used in conjunction with movable aerodynamic fins, which are effective while the launch vehicle is still in the atmosphere. Verniers Verniers are low-thrust rocket engines, which are used to make fine adjustments in the core vehicle s velocity and attitude, figure The Russian SL 3, 4, and 6, use verniers for roll, pitch, and yaw

54 2 54 control with stationary main engines. Again, the Atlas system uses two bipropellant verniers for roll control. Gas generator exhaust will power the roll control verniers on the Ariane 5. Figure Vernier rocket engines. Rotating nozzles Rotating nozzles are slant cut and must be used at least in pairs, figure This slant cut creates an area of underexpansion on one side. This generates an unbalanced side load on the inner wall of the longer side of the nozzle. Rotation of the nozzle about the longitudinal axis of the launch vehicle moves this side load to any point to provide roll, pitch, and yaw control. The Russian Proton launch vehicle uses six liquid first-stage engines with rotating nozzles. Figure Rotating nozzle. Figure Secondary fluid injection. Secondary fluid injection Secondary fluid injection, or liquid thrust vector control, involves multiple injection ports in an annular manifold, which is installed around the periphery of the nozzle, figure A secondary fluid is injected through these ports into the exhaust stream to change the thrust vector. The only fluid-injection thrust vector control systems in use today use reactive liquids. The pressure imbalance created by the combustion of reactive fluids produces the side forces for vectoring the thrust.

55 2 55 The solid strap-on boosters on Titan systems, American Rocket (AMROC) launchers, and India s Polar Satellite Launch Vehicle (PSLV) are the only systems today that use this type of thrust vector control. On the Titan IV, nitrogen tetroxide propellant, stored in a large external tank, is injected into the solid booster exhaust stream through a 24-nozzle manifold. The PSLV and AMROC use strontium perchlorate and LOX, respectively, as reactive liquids Solid, liquid, and gas propulsion systems Rocket propulsion is vital in any successful space program. Without it, there would be no payloads in space. In this section, you will learn about the types of propulsion systems and the characteristics used in our space business. Solid-propellant rocket motors People have used solid-propellant rocket motors for thousands of years. The ancient Chinese used skyrockets for celebrations as well as for weapons. The rockets red glare, in our national anthem, reminds us that early American s used rockets during the War of Jet-assisted takeoff (JATO) units, used to decrease aircraft takeoff roll or as takeoff assist units for lifting heavy loads, are familiar to many Air Force personnel. In space lift, solid-propellant motors are used for additional thrust at liftoff. Major components The solid-propellant rocket, as shown in figure 2 45, is comparatively simple. The major components are the combustion chamber, the igniter, and the converging-diverging nozzle, and the case that holds the propellants. Because of its simplicity, the solid motor is inherently more reliable and cheaper to produce than the liquid rocket engine. Figure Solid-propellant rocket. Improvements As you know, the use of additives, new chemicals, and the improved design of high-volumetric loading propellant grains are all improving the thrust of solids. The other approach for increasing performance is to increase the mass ratio of the motor. In this area, scientists and engineers have expended much energy in designing cases that are lightweight and stronger. Research seems to point to the two following possible solutions: make lightweight but strong cases of metals, such as titanium or design filament-wound cases of fiberglass or nylon tape impregnated with epoxy-type glues. Reinforcements We make the filament-wound cases even lighter as we design reinforced propellant grains to assist in supporting the vehicle. We can form these reinforced grains by molding the propellant around

56 2 56 aluminum or other metal-additive wires. The engine consumes these reinforcing materials during combustion. Reinforced grain motors are usually regressive burning so that combustion-chamber pressure decreases near the end of burning to allow the use of very lightweight cases. Today, industry can make both solid and liquid motors in a variety of sizes from very small attitude control and docking motors up to millions of pounds of thrust. Liquid-propellant rocket engines A liquid propulsion system, as shown in figure 2 46, consists of propellant tanks, propellant-feed system, thrust chamber, and such controls as regulators, valves, and sequencing and sensing equipment. Figure Liquid-propellant engine. The propellants can be monopropellants, bipropellants, or tripropellants, and may be either storable or cryogenic fluids. Liquid and gas propellants used in the aerospace industry are normally maintained within military specifications (MIL-specs) to ensure quality. Monopropellants systems Monopropellants are the least complex of these systems. Here, there is only one propellant tank, a single-feed system (usually pressure-fed), and a comparatively simple injector (since the engine does not require a mixing fuel and oxidizer). Monopropellant rockets are in use today but do not develop high thrust. However, the simplicity of monopropellant engines makes them adaptable and frequently desirable for use in attitude control or small velocity corrections in deep space. The simplicity and reduced weight of monopropellant engines makes them convenient for smaller satellites. Bipropellant systems The liquid-bipropellant system in common use is more complex. Some bipropellant systems use pressure-fed propellant flow. Here, pressurizing the tanks with an inactive gas, such as nitrogen or helium, forces propellants from the tanks to the engine. Igniting either a solid-propellant grain or some of the vehicle s liquid propellants in a gas generator designed for this purpose can create a pressurizing gas. Gravity and pumps Pumps and gravity can feed bipropellants to the engine, as was done in some of the earlier booster engines. However, the most commonly used method today is a combination of pumps and

57 2 57 pressurizing tanks to provide positive pressure to the pumps feeding the engines. The bipropellant system is complex in comparison to solid-propellant systems because of the multiple pumps, the need to maintain the correct oxidizer-fuel mixture, the effect of injector design upon stable combustion, and the need for thrust chamber cooling. Centrifugal pumps Often the vehicle s engine uses a centrifugal pump driven by a gas turbine to pump the propellants through the injector into the combustion chamber. A separate gas generator supplies gases to drive the turbine, or the engine bleeds these gases from the combustion chamber. Development of reliable turbopumps presents many challenges in design, materials, testing, and operational use. In some instances, more than 50 percent of the design effort for an engine is devoted to the turbopump. Turbopumps Turbopumps must pump fuel and oxidizer simultaneously at different rates and be able to withstand high-thermal stresses induced by the +1,500 F turbine gases while pumping cryogenic fluids with temperatures as low as -423 F (liquid hydrogen). Tripropellant systems Tripropellants are fairly new engines and still in the development stage. Tripropellants are being developed because several chemical reactions can create a greater thrust and efficiency by using a third chemical. The problem is it adds one more space vehicle system to monitor and control. As technology improves, reliability and efficiency of these systems will be seen in the future. Critical designs Developers must ensure that the vehicle can maintain adequate seals at these temperature extremes, since the propellants would explode if they were to come in contact inside the pump. Pump design is critical because the pumps must develop high-propellant pressure. Higher combustion chamber pressure means higher ISP, and pump outlet pressure must be higher than chamber pressure if propellants are to flow into the chamber. Individual designs Since components and controls for liquid engines can be designed for individual control, they have the potential for throttling and multiple restarts. Thus, these engines are attractive for in-space maneuvering. Gases The feed system of a liquid-propellant rocket engine transfers the liquid propellants from the vehicle storage tanks to one or more thrust chambers. The most common type of gas-pressurization system is the gas-pressure-feed system. Gas-pressure-feed systems are widely used in the space lift business. Gas-pressure feed In a gas-pressure feed, high-pressure gas displaces the propellants that are fed into the tanks under a regulated pressure. The stored high-pressure gas furnishes the pressurization energy. The thrust of a pressurized-gas rocket propulsion system is determined by the magnitude of the propellant flow as controlled by the gas-pressure regulator setting. For low thrust and short duration, this feed system is generally lighter and superior to other more complicated ones. Early systems In early systems, nitrogen or even air (German V 2) was frequently used for logistics reasons. As it became more readily available, helium found increased usage because of its substantially lower molecular weight and thus reduced total pressurant weight. For these reasons, the helium systems are the most widely used system.

58 2 58 Rapid pulsing For spacecraft or satellite attitude control and maneuvering, the use of a pressurized-feed system, together with storable liquid propellants, permits rapid pulsing of several thrust chambers for spacecraft stabilization, station keeping, or rendezvous and position control. In the next lesson, you ll learn about propellants Solid-, liquid-, and gas-propellant systems Over the years, rocket engines have used a variety of fuels from liquids, for example: gasoline, alcohol and kerosene to solids using a rubberized base. The difficulty arises in finding the right balance between mission requirements and propellant-system efficiency. For example, the mission may require a large satellite to be placed in high orbit. To meet the mission requirements, engineers must determine which propellant system effectively and efficiently accomplishes the mission; considering cost, amount of propellant, and operational environment. Keep in mind added propellant means more weight. More weight means more thrust to break gravitational pull. In this section, you ll learn about the characteristics of solid, liquid, and gas propellants. Solid propellants Solid propellants will burn on their exposed surfaces to produce hot gases. Solids contain all the substances needed to sustain combustion. They consist either of fuel and oxidizer that does not react below some minimum temperature or of compounds that combine fuel and oxidizer qualities (nitrocellulose or nitroglycerin). Scientists mix these materials to produce a solid with the desired chemical and physical characteristics. We commonly divide solid propellants into the two classes: composite (or heterogeneous) and homogeneous. Composites Composites are heterogeneous mixtures of oxidizer and organic fuel binder. The fuel contains small particles of oxidizer dispersed throughout. We call this fuel a binder because the oxidizer has no mechanical strength. Usually a crystalline, finely ground oxidizer such as ammonium perchlorate, is dispersed in an organic fuel such as asphalt. The oxidizer is approximately 70 to 80 percent of the total propellant weight. There are a large number of propellants of this type. Homogeneous Homogeneous propellants have oxidizer and fuel in a single molecule. Scientists base most of these homogeneous propellants on a mixture of nitroglycerin and nitrocellulose and call them double-base propellants. This name distinguishes them from gunpowder that uses either one or the other of the components as a base. Nitroglycerin is too sensitive to shock and has too much energy to be used safely by itself in an engine. However, it forms a suitable propellant when combined with the less energetic, but more stable, nitrocellulose. Ideal propellants Ideal solid propellants possess the following characteristics. High release of chemical energy. Low molecular weight combustion products. High density. Readily manufactured from easily obtainable substances by simple processes. Safe and easy to handle. Insensitive to shock and temperature changes with no chemical or physical deterioration while in storage. Ability to ignite and burn uniformly over a wide range of operating temperatures.

59 2 59 Nonhygroscopic (nonabsorbent of moisture). Smokeless and flashless. It s improbable that any propellant ever has all of these characteristics. Propellants used today possess some of these characteristics at the expense of others, depending on the application and the desired performance. Finished propellants The finished propellant is a single mass called a grain or stick. A solid-propellant rocket has one or more grains that constitute a charge in the same chamber. Use of solid propellants was limited until the development of high-energy propellants and processing techniques for making large grains. Now we can make single grains in sizes up to 22 feet in diameter. Restricted or unrestricted In addition to the composite and homogeneous categories, we class solid propellants as restricted or unrestricted. Restricted Today, most large, solid-propellant rockets contain restricted burning charges. A restricted grain delivers smaller thrust for a longer time. Propellants are restricted when we use inhibitors to restrict burning on some surfaces of the propellant. Inhibitors are chemicals that do not burn or burn very slowly. An inhibitor applied to the wall of the combustion chamber reduces heat transfer to the wall. We call this use a liner or insulator. Unrestricted Charges without an inhibitor are unrestricted burning charges. These burn on all exposed surfaces simultaneously. The unrestricted grain delivers a large thrust for a short time. Chemical composition The operating pressure, thrust, and burning time of a solid-propellant rocket depend upon the following characteristics: Chemical composition of the propellant. Its initial grain temperature. The gas velocity next to the burning surface. The size, burning surface, and geometrical shape of a grain. A given propellant can be cast into different grain shapes with different burning characteristics. Figure Typical grain shapes. Thrust The thrust of a rocket is proportional to the product of the exhaust velocity and the propellant flow rate. Large thrust requires a large flow rate, a large burning surface, and a fast burning rate. Rate The speed of the flame passing through a solid propellant in a direction perpendicular to the burning surface determines the burning rate of a solid propellant. Burning rate depends on the initial grain temperature and the operating chamber pressure.