LAUNCHES AND LAUNCH VEHICLES. Dr. Marwah Ahmed

Transcription

1 LAUNCHES AND LAUNCH VEHICLES Dr. Marwah Ahmed

2 Outlines 2 Video (5:06 min) : Introduction Expendable Launch Vehicles (ELVs) Placing Satellite into GEO Orbit

3 Introduction 3

4 Introduction 4 A satellite cannot be placed into a stable orbit unless two parameters that are uniquely coupled together- the velocity vector and orbital height- are simultaneously correct. A Geo-satellite, must be in an orbit at a height of 35, km above the surface of the earth (42, km radius from the center of the earth) with an inclination of zero degrees, an ellipticity of zero, and a velocity of m/s tangential to the earth in the plane of the orbit, which is the earth s equatorial plane. The further out from the earth the plane of the orbit is, the greater the energy required from the launch vehicle to reach that orbit.

5 Introduction 5 In any earth satellite launch, the largest fraction of the energy expended by the rocket is used to accelerate the vehicle from rest until it is about 32 km above the earth. Most launch vehicles have multiple stages and, as each stage is completed, that portion of the launcher is expended until the final stage places the satellite into the desired trajectory. Hence the term: expendable launch vehicle (ELV). The space shuttle, called the Space Transportation System (STS) by NASA, is partially reusable. Hence the term: reusable launch vehicle (RLV).

6 Figure 2.15 (p. 43) Schematic of a Proton launch (after reference 5). Satellite Communications, 2/E by Timothy Pratt, Charles Bostian, & Jeremy Allnutt Copyright 2003 John Wiley & Sons. Inc. All rights reserved.

7 Expendable Launch Vehicles (ELVs) 7 The decision on which particular rocket to use in a given situation will depend on a variety of factors:

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9 Figure 2.16 (p. 45) Representative ELVs (after reference 5). CCAS, Cape Canaveral Air Station; VAFB, Vandenburg Air Force Base. Satellite Communications, 2/E by Timothy Pratt, Charles Bostian, & Jeremy Allnutt Copyright 2003 John Wiley & Sons. Inc. All rights reserved.

10 Placing Satellite into GEO Orbit 10 Some of the launch vehicles deliver the spacecraft directly to GEO orbit (called a direct insertion launch) while others inject the spacecraft into a GEO Transfer Orbit (GTO). There are three basic ways to achieve GEO Orbit: GEO Transfer Orbit and Apogee Kick Motor (AKM). GEO Transfer Orbit with slow Orbit Raising. Direct Insertion to GEO

11 GEO Transfer Orbit and AKM 11 The initial approach to launching geostationary satellites was to place the spacecraft, with the final rocket stage still attached, into low earth orbit. After a couple of orbits, during which the orbital elements are measured, the final stage is reignited and the spacecraft is launched into a geostationary transfer orbit. The GTO has a perigee that is the original LEO orbit altitude and an apogee that is the GEO altitude. The rocket motor fires at apogee, it is commonly referred to as the apogee kick motor (AKM). The AKM is used both to circularize the orbit at GEO and to remove any inclination error so that the final orbit of the satellite is very close to geostationary.

12 Figure 2.19 (p. 48) Illustration of the GTO/AKM approach to geostationary orbit (not to scale). The combined spacecraft and final rocket stage are placed into low earth orbit (LEO) around the earth. After careful orbit determination measurements, the final stage is ignited in LEO and the spacecraft inserted into a transfer orbit that lies between the LEO and the geostationary orbit altitude: the so-called geostationary transfer orbit or GTO. Again, after more careful orbit determination, the apogee kick motor (AKM) is fired on the satellite and the orbit is both circularized at geostationary altitude and the inclination reduced to close to zero. The satellite is then in GEO. Satellite Communications, 2/E by Timothy Pratt, Charles Bostian, & Jeremy Allnutt Copyright 2003 John Wiley & Sons. Inc. All rights reserved.

13 13 GEO Transfer Orbit with slow Orbit Raising An alternative to using a single high-thrust boost is to employ the spacecraft propulsion system to add velocity incrementally, through a number of successive burns. They are activated when the satellite is at apogee, so that perigee is raised by corresponding increments. The process takes from a matter of days to perhaps months, depending on the thrust available from the spacecraft propulsion system. Low-level thrusting from ion propulsion produces the longest period for orbit raising but yields greater efficiency

14 Figure 2.20 (p. 49) Illustration of slow orbit raising to geostationary orbit (not to scale). The combined spacecraft and final rocket stage are placed into low earth orbit (LEO) around the earth. As before (see Figure 2.19), the spacecraft is injected into GTO but, in this case, once the satellite is ejected from the final rocket stage, it deploys many of the elements that it will later use in GEO (solar panels, etc.) and stabilizes its attitude using thrusters and momentum wheels, rather than being spin-stabilized. The higher power thrusters are then used around the apogee to raise the perigee of the orbit until the orbit is circular at the GEO altitude. At the same time as the orbit is being raised, the thruster firings will be designed gradually to reduce the inclination to close to zero. Satellite Communications, 2/E by Timothy Pratt, Charles Bostian, & Jeremy Allnutt Copyright 2003 John Wiley & Sons. Inc. All rights reserved.

15 Direct Insertion to GEO 15 Place the satellite into GEO. The final stages of the rocket are used to place the satellite directly into GEO rather than the satellite using its own propulsion system to go from GTO to GEO.

16 16 Q and A