PH 508: Spacecraft design and operations

1. PH 508: Spacecraft design and operations Dr Mark Price (mcp2@star.kent.ac.uk) , Prof Mark Burchell (convener), Prof Richard Holdaway (CCLRC), Dr Vicky Fitzgerald. Spring 2011.

2. Contact details • Dr. Mark Price • Room 103C • E-mail: mcp2@star.kent.ac.uk • Lectures notes will eventually be available on Moodle, but can be downloaded now from: http://astro.kent.ac.uk/~mcp2/teaching/PH508

3. Contact hours (36 in total) • 30 hours of lectures • Low Earth Orbit (4 lectures and 1 workshop – MCP) • Spacecraft Systems (11 lectures and 1 workshop – MCP) • Project Management (5 lectures and 1 workshop – VF) • Orbital Mechanics (10 lectures and 1 workshop – RH) • 4 workshops (weeks TBD) • 2 class tests (weeks 18 & 24)

4. Course synopsis: aims • To provide a basic understanding of the major subsystems of a spacecraft system. • To provide basic frameworks for understanding of spacecraft trajectory and orbits, including interplanetary orbits, launch phase and attitude control. • To provide an awareness of the basic ideas of how space is a (multi-billion dollar!) business opportunity and some of the management tools required in business.

5. Syllabus: I • Low Earth Orbit (4 lectures) • The vacuum, radiation environment and thermal environment that a spacecraft encounters in Low Earth Orbit (LEO) and that environment’s effect on the spacecraft materials (electronics, superstructure etc.). • Spacecraft Systems (11 lectures) • An introduction to spacecraft and their environment. Covers Spacecraft structures and materials, thermal control, power systems, attitude control systems, the rocket equation and propulsion.

6. Syllabus: II • Project management (Dr. Vicky Fitzgerald) • Explains the evolving framework in which world-wide public and private sector space activities are conceived, funded and implemented. • Introduces the basics of business planning and management applicable to any project! • Orbital mechanics (Prof. Richard Holdaway) • Using celestial mechanics (Newton’s laws) to the real world application of satellite/spacecraft missions. • Basic equations of motion are outlined in order to give an understanding of the causes and effects of orbit perturbations. • Descriptions are given of different types of orbit and methods are outlined for the determination and prediction of satellite and planetary orbits • Assessment of mission analysis problems such as orbital choice, ground station usage, satellite station-keeping and orbital lifetime.

7. Learning outcomes • An understanding of the way in which space missions are configured both from the point-of-view of the constituent subsystems, mission profile (i.e., the project aims) including the influence of the space environment. • Appreciate the constraints and trade-offs which led to one mission configuration over another. • Appreciate space activities from a commercial viewpoint and be familiar with basic management tools for planning work (e.g., Gant charts, Pert charts etc.) • Make (valid) approximations and solve problems using a mathematical approach.

8. ESSENTIAL reading • Spacecraft Systems Engineering by Fortescue, Stark and Swinerd (3rd edition). [*NB: No joke, this is pretty much ESSENTIAL ] Library has 6 copies (Classmark: TL875, location: Level 2 West. Also has some copies in core text collection (1 week loan?). Library has many copies (>15) of the second edition (same classmark and location) Amazon price: £37.95 Please contact me (mcp2@star.kent.ac.uk) if you really can’t source/afford a copy!

9. Recommended reading • Orbital Motion by A. Roy. (3rd edition) Library classmark: QB 355, 6 copies. Amazon price: £53.19 • Space vehicle design by Griffin and French [Classmark: TL 875] • Space mission analysis and design by Wertz & Larson [Classmark TL 790] • Satellite Technology and its applications by Chetty [Classmark TL796] • Spacecraft Attitude determination and control by Wertz [Classmark TL3260] • Rocket and Spacecraft propulsion by Turner [Classmark: TL 872]

10. Introduction: Space mission architecture Basic elements of a space mission

11. Introduction: Spacecraft subsystems F&S, Fig. 1.3, Page 7

12. Spacecraft environment Crude overview: [Read: chapter 2, F&S] • Ground phase (vehicle construction) • Pre-launch phase (payload and rocket integration) • Launch phase • Space operations phase • Other (planetary, asteroid belt, cometary environments, de-orbital/end of life phase)

13. Ground phase: I Can be sub-divided further into: • Manufacture stage • Assembly stage • Test and checkout stage • Handling stage • Transportation stage • Storage (prior to rocket/payload integration)

14. Ground phase: II Manufacture and construction stage • Could be argued that this phase is the same as for any other industrial product. Incorrect! • Spacecraft manufacture is very expensive, and very few spacecraft are actually made. • Spacecraft (even the lowly comms satellite) are very complicated with many subassemblies and built-in redundant systems. • Operational constraints mean that each spacecraft (or rocket) is only used once (slight exception is the space shuttle) and cannot be ‘test driven’. It has to work, and it has to work first time! • [Q: Approximate cost of comms./science satellites?]

15. Ground phase: III Manufacture and construction stage (continued) • The end user environment (see later in course) imposes unusual constraints in terms of mass, volume, power, allowable materials, reliability, technology etc. over conventional manufacturing. • All this pushes up the cost, construction time and complexity of the end product (spacecraft or rocket). • Need to impose the highest standards of quality control to guarantee the manufactured end assembly.

16. Ground phase: IV Test and checkout stage • A time-consuming and therefore expensive process. Remember: it has to work, and it has to work first time! • Example: A chip fails during a PCB test. What do you do? • Answer: Replace ALL chips from the same manufacturer on all PCBs. • Example: A solar panel generates insufficient power for the end user requirement. What do you do? • Answer: Rebuild panel, delaying the mission by 6 months. • HST mirror!

17. Ground phase: V Handling stage • To be ‘space qualified’ all components and assemblies have to handled in a clean environment. • Human operatives have to wear paper coveralls, gloves, hairnets, facemasks. • All handling of sub-assemblies is documented meticulously (do not drop a spanner onto the spacecraft, or one of the flight detectors on the floor!)

18. Ground phase: VI Transportation stage: • Manufacture and checkout facility may be a long distance (>1000 km) from the launch site (end user requirement, depends on the required orbit) • Vibration and shocks are monitored during transport by gauges. • Transported in a sealed environment to prevent ingress of dust, moisture and temperature extremes. • Just moving a spacecraft is expensive!

19. Ground phase: VII Storage: • Missions get delayed, sometimes for years. Estimated cost of HST storage was $500M - $1Bn! • ‘Ground Phase’ duration can last years. Time to build a typical Spacecraft ~3 years, longer for scientific payloads. Recent attempts have been made to try and ‘production line’ the process particularly for telecommunication satellites due to commercial pressures. • ‘Ground Phase’ environment: ‘one-g’, standard temperature and pressure, in a clean room. [Aside: clean rooms are quantified by the number of particles per cubic foot (US). A class ‘1000’ being dirtier than a class ‘100’. Spacecraft and scientific payload assembly is normally done in a class ‘100’ or ‘1000’]

20. Prelaunch phase • Spacecraft is delivered to the launch site for integration with rocket. • The whole assembly is then moved to the launch-pad. • This period can last days, but delays can (and do) occur. • Environment: ‘one-g’ gravity, temperature and pressure that of the launch site, cool dry air can be flowed over the spacecraft to give a certain degree of environmental control.

21. Launch phase: I • Duration: typically ~1000 seconds, from ground to orbit. • Environment: Vibration/noise, acceleration shock (‘g-force’), mechanical shock (vibration), thermal changes (ground – space), pressure change (1 atmosphere – vacuum). • Vibration/noise caused by: • Burning of fuel • Operation of pumps, compressors, turbines etc. • Aerodynamic buffeting

22. Launch phase: II Vibration/noise environment • Two modes of transport to payload • Vibration through payload mounting structure • Acoustically through the atmosphere • At its worst (peak intensity) at launch (from reflections off ground, main firing of rocket) and during transonic flight through the atmosphere. Uneven air flow over the rocket causes buffeting/shearing forces.

23. PH 508: Spacecraft design and operations: lecture #2 Dr Mark Price (mcp2@star.kent.ac.uk) , Prof Mark Burchell (convener), Prof Richard Holdaway (CCLRC), Dr Vicky Fitzgerald. Spring 2011.

24. Launch phase: III - Noise Typical Ariane 4 & 5 noise spectrum

25. Launch phase: IV - noise • Definition of noise intensity, I • I has units of decibels, ‘dB’. • P is the overpressure (i.e., a ‘relative’ pressure) referenced to 20 millipascals (at 1kHz).

26. Launch phase: V - vibration • Vibration is given by spectral density, units of g2/ Hz (g = acceleration) Ariane 4 vibration Spectrum (F&S, Fig 2.2). Actual shape is rocket and payload dependent.

27. Launch phase: VI - vibration • Both vibration and noise can damage a payload. • Designers/engineers do ‘destruction’ tests on engineering/flight spare components to make sure the flight model Spacecraft will survive. • The fairing (the detachable cover over the Spacecraft) vibrates. Due to weight constraints, the fairing has to be lightweight and thus is prone to vibration.

28. Launch phase: VII - acceleration Acceleration (‘g force’ – normalised to Earth standard gravity) • Acceleration is not constant. Has several peaks and troughs during the launch phase. • These peaks and troughs happen as various stages ignite and drop-off and during jettisoning of the payload fairing. • Early ‘brute force’ rockets had high values of ‘g’. Early Mercury rockets peaked at ~6.7g. • Space shuttle has a ~3g sustained peak. A sustained thrust reduces the peak g experienced.

29. Launch phase: VIII - acceleration Note: starts at 1g (ground) ends at 0g (free-fall). Ariane 4 acceleration profile (F&S – Fig. 2.3)

30. Launch phase: IX - acceleration

31. Launch phase: X - acceleration • If –gz (ie., ‘up’) is > 5g blood supply to brain stops (heart is pumping against 5g). Leads to unconsciousness. • Can survive front-back motions at higher g as limitation is the squashing/tearing of soft tissue in the lungs. • During an uncontrolled descent, some Russian cosmonauts survived >18g! (Soyuz 18 + others?) • For very short time periods (<msec) accelerations can be very much higher (~hundreds of g) as fairings detach etc.

32. Space operations phase: I – thermal environment • Heat experienced by payload is not due to the radiative/conductive heat from the rocket motor. Payload is well insulated from this • The payload fairing is forcing its way through the atmosphere and gets hot (think inverse heat shield). • Atmospheric frictional heating occurs and this heat can be conducted to the payload or radiatively from the fairing’s inner surface. • Effect decreases with altitude due to decreasing atmospheric density.

33. Space operations phase: II – thermal environment • Total energy input, E, into payload can be calculated via: • Where dx is each interval of distance travelled • F is the drag force experienced travelling dx

34. Space operations phase: III – thermal environment • The drag force, F, is defined as: • CDis the drag coefficient - a function of atmospheric density, ρ. Typical values are between 0.5 – 2. Also a function of altitude • A is the cross-sectional area of the spacecraft in line of flight • v is the velocity.

35. Space operations phase: IV – thermal environment • Emery equation difficult to solve exactly as some terms are altitude dependent. • Main points: • Energy input is large • Dominated at low altitude by increasing velocity (acceleration through thick atmosphere). • Decreases at high altitude at ρ decreases. • But this is energy input. The temperature is a function of the heat capacity of the fairing and the way it dissipates heat and how that heats the payload.

36. Space operations phase: V – thermal environment • Fairing temperature: • Atmospheric frictional heating • Specific heat capacity • Radiative, conductive and convective heat loss • Payload temperature determined by: • Radiative and conductive thermal pathways from fairing. • Then direct radiative input after fairing is jettisoned from space environment. Jettison occurs at an altitude of ~100 km. • Heat input into Ariane V is typically 500 W m-2 with a peak of 1135 W m-2.

37. Space operations phase: VI – thermal environment

38. Space operations phase: VII – pressure environment Atmospheric pressure/density • Affects heating of fairing and thus payload • Affects the noise/vibration environment • Affects the final velocity achieved Thus is very important!

39. Space operations phase: VIII – pressure environment • As the atmospheric pressure drops, the pressure in the payload bay drops. The depends on the venting through the fairing. • Need good venting paths to avoid sudden pressure drops and large pressure differentials across the payload (‘pop!’). • For Ariane rockets pressure venting is ~10 mBar s-1. • For the Space Shuttle, the venting pressure rate is controllable.

40. Space operations phase: IX – pressure environment • How does the atmospheric pressure change with density?

41. Space operations phase: X – pressure environment • However, the absolute pressure varies not just with altitude, but with the Sun! Due to fluctuating energy input from the Sun, weather, time-of-day, solar cycle. COSPAR Int. Ref. Atmosphere (1972)

42. Space operations phase: XI – pressure environment Different species concentration as function of altitude (US Standard atmospheric Model).

43. End • Things to ‘take home’ • The design and implementation of a space mission is a complicated and expensive task. • Each separate phase has to implement the highest possible level of quality control. It has to work, and it has to work first time! • Many different things to consider when designing a mission: power requirements, weight, thermal control, mechanical robustness, system redundancy, etc.

44. The space environment: I [Will crop up again in PH608, and probably PH711] • Can be broadly categorised into: • Near Earth Environment • Deep Space • Other ‘local’ environment (planetary orbits, asteroid belt, cometary etc.). • As ‘Near Earth’ is local space we’ll start with the general case: deep space.

45. The space environment: II Deep Space: gravity • “Zero gravity” . Not true. You are always subject to gravity, but in a freefall state (or coasting) you appear to have ‘Zero gee’. • ‘Zero gee’ is now going out of fashion to be replaced with the more correct term: “micro-gravity (μg)”. • Spacecraft vibrations can shake the structure giving rise to a μg environment.

46. The space environment: III Deep space: μg • Advantages: • Small forces -> light structures can be employed • Cheap to launch (but objects still have mass and inertia!). • Disadvantages: • Low self-damping -> vibration prone (rigidity more important than strength, determines limiting mass). • Difficult to test material behaviour on the ground • Fluid flow problems in μg environment (‘bubbles’) • Need active pumping/circulation system (no gravity feed) • Humans? Sleeping, eating, respiration etc. all affected by μg environment.

47. The space environment: III Deep space: pressure (or lack of!) • At Geostationary Earth Orbit (GEO, altitude ~36,000 km) pressure ~10-15 Pa! • At altitudes >120 km don’t really use pressure units, but a ‘number density’ (number per cubic metre, m-3). The molecules of a gas are too separated to interact, so can treat as separate species.

48. The space environment: IV Different species concentration as function of altitude (US Standard atmospheric Model). [F&S, Pg. 22]

49. The space environment: IV • At an altitude of ~400 km the number density is approximately 1012 – 1014 atoms per m3 depending on the species and solar activity. Typical number density at low altitude inside the atmosphere ~1024 m-3. • The result of such a low pressure is outgassing. Solids give off materials contained within them when the ambient pressure ~ vapour pressure (10-11 – 10-4 Pa)

50. The space environment: V • Outgassing examples: • Metals → adsorbed/absorbed gases and water on surface. • Polymers →volatile components (normally organic) which are part of their matrix material. • Composite →absorbed water. • This process starts immediately the pressure drops and can last months until all trapped molecules are released.