Satellite Engineering

Transcription

1 Satellite Engineering Universidad de Concepción November 2009 Gaëtan Kerschen Space Structures & Systems Lab University of Liège

2 Satellite Engineering Universidad de Concepción November 2009 Day 3: Satellite Propulsion

3 Launch Vehicles The rest? Mostly intercontinental ballistic missiles (ICBM) 3

4 Launch Vehicle: Order of Magnitudes 90%: propellant 9%: structure 1%: payload 4

5 Ascent Propulsion Ascent propulsion is probably the factor over which a spacecraft designer has the least control. Rarely is a mission so important that a specific engine or launch vehicle will be designed to fit its needs. Yet it sets severe limits on payload, mass, volume and configuration! 5

6 Satellite Propulsion. Why? Venus Express (ESA) 570 kilograms of propellant on board; the propellant mass is almost half of the overall spacecraft weight! 6

7 Why? Venus Express (ESA) Major spacecraft manoeuvres, like the injection into orbit around Venus, are performed by firing the main engine while minor manoeuvres are made using four pairs of thrusters located at the bottom corners of the spacecraft. 7

8 Propulsion for Orbit Transfer Apogee motor for orbit circularization (Eutelsat) 8

9 Propulsion for Orbit Control A nominal orbit is defined for the satellite. This is the orbit the satellite should maintain. The orbit is controlled by applying forces to the satellite. Forces are applied by ΔV actuators. Launch vehicle ~ 10 6 N, 10 minutes, ΔV~10 km/s Apogee/ perigee motors ~ 10 4 N, 1 minute, ΔV~2 km/s Station keeping ~ [ ] N, intermittent, ΔV~0.35 km/s (7 years) 9

10 Propulsion for Orbit Control 6 thrusters for orbit control (Meteosat) 10

11 Propulsion for Attitude Control A nominal attitude is defined for the satellite. This is the attitude that the satellite should ideally maintain. The satellite attitude is controlled by applying torques to the satellite. Torques are applied by attitude actuators. 11

12 Attitude Control vs. Orbit Control How many DOFs does a (rigid) satellite possess? Trajectory dynamics Orbit control Attitude dynamics Attitude control Linear momentum Angular momentum Motion of the center of mass Motion relative to the center of mass Focus in this lecture Next lecture 12

13 Orbit and Attitude Are Interdependent Examples: 1. In LEO, the attitude will affect the atmospheric drag which will affect the orbit 2. The orbit determines the spacecraft position which determines both the atmospheric density and the magnetic field strength, which will, in turn, affect the attitude But this dynamic coupling is often ignored, and the time history of the spacecraft position is assumed to be known and to be an input for ADCS 13

14 Outline 1. Basics of space propulsion 2. The different engines 3. Orbit transfer 4. Orbit control 5. When propulsion does not suffice 14

15 1. Thrust: Theoretical Definition The objective of a rocket engine is to produce thrust T: Thrust is the result of all internal and external forces due to pressure and viscous effects developed by the fluid on all components of the engine. T Pressure imbalance at the nozzle exit T = pn ds F F Σ ax viscous, int viscous, ext Not a helpful definition for a practical measure of thrust! 15

16 1. Thrust: Useful Equation T = mv + ( p p ) A ej e a e (use of momentum balance) Ambient pressure Nozzle exit pressure Thrust is greater in a vacuum than at sea level 16

17 1. Thrust: Equivalent Exhaust Velocity T = mv + ( p p ) A = mv ej e a e eq 17

18 1. Performance Index: Specific Impulse t Ttdt () T = 0 = t mdt 0 Total impulse of the mission (change in momentum) I v [ m/ s] sp eq m Conventional method of comparing propellants: the higher the specific impulse, the less propellant is needed to gain a given amount of momentum. 18

19 1. Performance Index: Specific Impulse I sp t Ttdt () = 0 eq t T v gm = g mdt g [] s Advantage of the definition: specific impulse is measured in seconds in any consistent system of units 19

20 1. Performance Index: Specific Impulse Theoretical specific impulse of some propellants Energetic oxidizer but very difficult to contain; no practical use so far Storable: liquids at room temperature LH2+LOX: cryogenics but best performing practical propellants (Ariane V, SSME) No oxidizer! 20

21 1. ΔV Requirement ΔV Budget Thrust is not an essential quantity per se. One of the main products of the mission design is a statement of the ΔV required for the mission (orbit transfer and orbit control): ΔV (GTO GEO) 1.5 km/s In some cases, it is as important as power and mass budgets. For instance, it can be a principal design driver and impose complex trajectories for deep space probes! 21

22 1. Tsiolkovsky s Rocket Equation (1903) T dv = m dt T = mv eq dv = v Useful to convert ΔV in terms of propellant needs! eq dm m ( ln ln ) v v = v m m f i eq f i Mission requirement m i m i Δ v= veq ln = Ispg0 ln m f m f Clear impact of I sp! Propellant needs 22

23 1. But Coquilhat ( , Belgian) Established the rocket equation in 1873! Trajectoires des fusées volantes dans le vide, Mémoires de la Société Royale des Sciences de Liège. Recent discovery. 23

24 1. Delta-V: Order of Magnitudes (1000,0.288) Δ m/m Δ v (m/s) Isp=300s 24

25 1. Digression: Single-Stage Rocket 100 Δ v= v ln = v ln 5 = 1.61v ej ej ej [ ] 5 7 km/s 80%: fuel 10%: dry mass 10%: payload V ej = [3-4] km/s What to conclude? 25

26 Outline 1. Basics of space propulsion 2. The different engines 3. Orbit transfer 4. Orbit control 5. When propulsion does not suffice 26

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28 2. Rocket Engines 28

29 2.1 Chemical Engines Generate thrust by accelerating a high-pressure gas to supersonic velocities in a converging-diverging nozzle. The high-pressure gas is generated by high-temperature combustion/decomposition of propellants. Solid, monopropellant and bipropellant systems. 29

30 2.1 Chemical Engines: Monopropellant Hot gases are obtained by decomposition of a single propellant and are expelled through a nozzle generating thrust. Hydrazine is often used and is injected into a catalyst bed: Exothermic reaction: N 2 H 4 H 2, N 2 and NH 3 (ammonia) 30

31 2.1 Chemical Engines: Monopropellant Stored easily (freezing point at 2ºC). Simplicity (no oxidizer). Medium performance. 31

32 2.1 Chemical Engines: Monopropellant Astrium CHT 1 N: Hydrazine Burn life: 50h Length: 17cm Attitude and orbit control of small satellites and deep space probes. Herschel, Globalstar Astrium CHT 400 N: Hydrazine Burn life: 30m Length: 32cm Ariane V attitude control system 32

33 2.1 Chemical Engines: Bipropellant An injector introduces the oxidizer and the fuel into the combustion chamber. Continuous and rapid combustion then occurs. 33

34 2.1 Chemical Engines: Bipropellant Cryogenics (fluorine, oxygen and hydrogen) have never been used in satellites (storage reasons). Nitrogen tetroxide (N 2 O 4 ) and monomethylhydrazine (MMH) is the dominant combination. 34

35 2.1 Chemical Engines: Bipropellant Complexity and cost. High-performance system. Wide range of thrust capability. Versatility (pulsing, restart, variable thrust). 35

36 2.1 Chemical Engines: Bipropellant Astrium S 10 N: MMH (Fuel) N2O4-MON1-MON3 (Oxidizers) Attitude and orbit control of large satellites and deep space probes Venus Express, Arabsat Astrium S 400 N: MMH (Fuel), N2O4-MON1-MON3 (Oxidizers) For apogee orbit injection of GEO satellites and for planetary orbit maneuvers of of deep space probes Venus Express, Artemis 36

37 2.1 Chemical Engines: Solid The oxidizer and fuel are stored in the combustion chamber as a mechanical mixture in solid form. When the propellants are ignited, they burn in place. 37

38 2.1 Chemical Engines: Solid Simplicity. Reasonable performance. No restart (single burn)! Useful for orbit insertion or apogee kick motor. 38

39 2.1 Chemical Engines: Solid ATK Star 27 (TE-M-616) 27 kn: Burn time: 34s Length: 1.3m Gross mass: 361 kg Apogee motor (GOES,GPS) 39

40 2.1 Comparison of Chemical Engines 40

41 2.2 Electric Engines Low-thrust propulsion. 41

42 2.2 Electric Engines: Ion Engines A cathode extracts electrons from the propellant which is ionized. The ions are accelerated by a static electric field. Propellant: Ar, Xe 42

43 2.2 Ion Engine: Deep Space 1 (1998) 43

44 2.2 Ion Engine: EADS Astrium Astrium RITA 150 mn: Xenon Beam voltage: 1200V Burn time: >20000h Gross mass: 154 kg Stationkeeping, orbit transfer, deep space trajectories RITA-10 (Artemis) 44

45 2.2 Chemical vs. Electric Engines Astrium CHT 1N: Astrium CHT 400N: 210s 220s Monopropellant Astrium S 10N: 291s Astrium S 400N: 318s. Bipropellant ATK STAR 27: 288s Solid Astrium RITA-150: s Electric [Cold gas: ~50s Liquid oxygen/liquid hydrogen 455s ] 45

46 2.2 Chemical vs. Electric Engines RITA, Astrium The Ion Propulsion System for the Future 46

47 Outline 1. Basics of space propulsion 2. The different engines 3. Orbit transfer 4. Orbit control 5. When propulsion does not suffice 47

48 3. First Motivation Without maneuvers, satellites could not go beyond the close vicinity of Earth. For instance, a GEO spacecraft is usually placed on a transfer orbit (LEO or GTO). 48

49 3. From GTO to GEO: One-Impulse Ariane V is able to place heavy GEO satellites in GTO: perigee: km and apogee: ~35786 km. GTO GEO 49

50 3. From GTO to GEO: One-Impulse For an orbit with a perigee at 320 km and an apogee at km, what is the velocity increment required to reach the geostationary orbit? a ra + rp = = 2 v p = v a = GTO 10.13km/s 1.61km/s 24430km GEO v circ = = 3.07km/s Answer: 1.46 km/s (apogee motor) 50

51 3. Delta-V Budget: GEO 51

52 3. Two-Impulse Transfer: Hohmann The transfer between two coplanar circular orbits requires at least two impulses Δv 1 and Δv 2. In 1925, Walter Hohmann conjectured that The minimum-fuel impulsive transfer orbit is the elliptic orbit that is tangent to both orbits at its apse line. The rigorous demonstration came some 40 years later! 52

53 3. Two-Impulse Transfer: Hohmann Δv 2 v circ = μ r v ellip 2 1 = μ r a r 2 r 1 2μr μ 2 Δ v1 = r ( ) 1 r1+ r2 r1 Δv 1 2μr μ 1 Δ v2 = + r ( ) 2 r1+ r2 r2 53

54 3. Tangential Burns or Not? The major drawback to the Hohmann transfer is the long flight time. Time of flight can be reduced at the expense of an acceptable increase in Δv. A possible solution is a one-tangent burn. It comprises one tangential burn and one nontangential burn. 54

55 3. Tangential Burns or Not? Vallado, Fundamental of Astrodynamics and Applications, Kluwer,

56 Hohmann for interplanetary transfer Dep. ΔV 1 Arr. Arr. ΔV 2 Dep. Dep. Arr. 56

57 3. Second Motivation: Plane Change A launch site location restricts the initial orbit inclination for a satellite. Which one is correct? For a direct launch 1. launch site latitude desired inclination. 2. launch site latitude desired inclination. 57

58 3. Hint ˆK Ĵ Î i ˆ 1 h 1 ( ). cos z cos r v K = = h r v 58

59 3. Launch Azimuth Inclination, degrees Lat 0 deg Lat 20 deg Lat 40 deg Lat 60 deg Launch azimuth, degrees 59

60 3. HST and Cape Canaveral Hubble orbit inclination is 28.5º What is the latitude of Kennedy Space Center (Cape Canaveral)? 60

61 3. Inclination Change Cranking maneuver: with a single Δv maneuver, one wants to change only the inclination of the orbit plane. Where should we apply this maneuver? 61

62 3. Inclination Change The two orbit planes intersect in the equatorial plane. We can therefore change the inclination at an equator crossing, by a simple rotation of the velocity vector. v r δ v v View down the line of intersection of the two orbital planes 62

63 3. Delta-V Computation ( ) Δ v= v v = v v uˆ + v uˆ v uˆ 2 1 r2 r1 r ( ) 2 Δ v = v v + v + v 2v v cosδ r2 r v = v, v = v r1 r2 1 2 δ Δ v= 2vsin 2 63

64 3. Inclination Changes Are Expensive 1.5 δ v (% of v) (60º, 100%) (24º, 41.6%) δ, degrees 64

65 3. Remark: Hubble Servicing Too costly! Instead, NASA has chosen to have another shuttle ready to lift off to retrieve the astronauts if needed. 65

66 Outline 1. Basics of space propulsion 2. The different engines 3. Orbit transfer 4. Orbit control 5. When propulsion does not suffice 66

67 4. First Motivation: Orbit Raising ISS reboost due to atmospheric drag (ISS, Shuttle, Progress, ATV). 67

68 4. Second Motivation: Stationkeeping A GEO satellite orbit changes over time due to perturbations: 1. Inclination modified by the lunisolar attraction: N/S. 2. The longitude is modified by the tesseral terms of the geopotential: E/W. 68

69 4. GEO Satellites: Longitude Drift C 2,2 corresponds to the equatorial ellipticity. 69

70 4. GEO Satellites: Stationkeeping Box A stationkeeping box is defined by a longitude and a maximum authorized distance for satellite excursions in longitude and latitude. For instance, TC2: -8º ± 0.07º E/W ± 0.05º N/S 70

71 4. GEO Satellites: Delta-V Budget 71

72 Outline 1. Basics of space propulsion 2. The different engines 3. Orbit transfer 4. Orbit control 5. When propulsion does not suffice 72

73 5. How to Go to Saturn? V V E J G A 73

74 5. Gravity Assist Also known as planetary flyby trajectory, slingshot maneuver and swingby trajectory. Useful in interplanetary missions to obtain a velocity change without expending propellant. This free velocity change is provided by the gravitational field of the flyby planet and can be used to lower the delta-v cost of a mission. 74

75 5. Basic Principle Resultant V out SOI Resultant V in Planet s sun relative velocity Δv = v v, out, in 75

76 5. Basic Principle Inertial frame Frame attached to the train Frame attached to the train Inertial frame A gravity assists looks like an elastic collision, although there is no physical contact with the planet. 76

77 5. Cassini: Swingby Effects V V E J S 77

78 Satellite Engineering Universidad de Concepción November 2009 Day 3: Satellite Propulsion 78