Systems Considerations and Design Options for Microspacecraft Propulsion Systems

1. Systems Considerations and Design Options for Microspacecraft Propulsion Systems Andrew Ketsdever Air Force Research Laboratory Edwards AFB, CA Juergen Mueller Jet Propulsion Laboratory Pasadena, CA

2. OUTLINE • Introduction • Microspacecraft • Micropropulsion • Scaling Issues • Micronozzle Expansion (AIAA 99-2724) • Ion Formation • Combustion and Mixing • Heat Transfer • MEMS Devices • Systems Considerations • Conclusions

3. Introduction • Microspacecraft will require a propulsive capability to accomplish missions • Microspacecraft - AFRL Definition • Small Spacecraft 1000 - 100 kg • Microspacecraft 10 - 100 kg • Nanospacecraft 1 - 10 kg • Microspacecraft will be resource limited • Mass • Power • Maximum Voltage • Volume

4. Introduction • Micropropulsion Definition • Characteristic size • Maximum producible thrust • Any propulsion system applicable to 100 kg or less spacecraft • At least two sub-classifications • Small-scale thrusters • Scaled down versions of existing thrusters • Reduced power, mass, thrust level • MEMS thrusters • Require MEMS/novel fabrication techniques • Performance scaling issues

5. Introduction • A wide range of micropropulsion concepts will be required • High thrust, fast response • Low thrust, high specific impulse • Micropropulsion systems which have systems simplicity or benefits will be advantageous • Performance is always the driver; however, total systems studies must be performed • Tankage, power required (power supply mass), integration, propellant feed system, MEMS component performance (limitation?), …

6. Introduction • Micropropulsion systems of the future will have to perform as well as large-scale counterparts • Robust • Reliable • Efficient • Long lifetime • Micropropulsion systems today • Losses due to characteristic size • Spacecraft limitations on mass, power, volume • Lagging development of MEMS hardware

7. Scaling: Microscale Ion Formation • Containment of electrons • Transport of electrons to discharge chamber walls is major loss mechanism for ion micro-thrusters • Typically magnetic fields are used to contain electrons and increase ionization path length l = 1 / nosi Rg = me vo,perp / (q B) • Want Rg < discharge chamber radius 10 cm diameter => B = 0.1 Tesla 1 mm diameter => B = 10 Tesla (Yashko, et al., IEPC 97-072)

8. Scaling: Microscale Ion Formation • Grid acceleration and breakdown • Micro-ion thruster grids will have to hold off significant potential differences • Lower ionization => higher accelerating potential for high specific impulse • Voltage isolation with very small insulator thicknesses • Material dependencies • Two modes of breakdown

9. Scaling: Microscale Ion Formation • Micro-ion thruster modeling issues • Lower degrees of ionization => more influence of neutral flow behavior • Traditionally, PIC codes assume some uniformly varying neutral flowfield • Coupled approaches (DSMC/PIC) may be required • For very low ionization, a de-coupled approach to plasma and neutral flow may be useful • May be only data available for some systems • VALIDATION DATA REQUIRED

10. Scaling: Micro-Combustion • Advanced liquid and solid propellants are targeted at mission requirements involving • High thrust • Fast response • Scaling issues arise which may limit characteristic size • Mixing length required for bi-propellants • Residence time in combustion chamber • Combustion instabilities • Heat transfer

11. Scaling: Micro-Heat Transfer • Radiation • qrµ AT4µ L2 T4 • Can be a major loss mechanism at high temperatures • Conduction (1-D) • qc= k A (dT/dx) µ L ∆T • High thermal conductivity can be good and bad • Can remove heat from places which otherwise might reach Tmax • Can remove heat from propellant at walls causing inefficiencies

12. Scaling: Micro-Heat Transfer • Material thermal expansion also a major issue

13. Scaling: MEMS Propulsion Support Hardware • Example: MEMS valves • Legendary issue associated with MEMS valve leakage • MEMS valves with acceptable leak rates are currently being developed • Neglected issues associated with propellant flows (gas and liquid) through MEMS devices • Characteristic size of flow channels (rarefied flow even at high pressure) • Transient flow

14. Scaling: MEMS Propulsion Support Hardware • Small impulse bit maneuver • Microspacecraft slew maneuver - 1 µN-sec • Two possible scenarios for achieving small I-bit • Reduce thrust • Reduce valve actuation time • For 1 µN-sec impulse bit and 1 mN thrust, valve actuation time on the order of 1 msec required • Open questions regarding the flow uniformity over the valve actuation (affects prediction of I-bit) • Longer valve actuation may imply more uniformity but also implies very low thrust level

15. Systems: General • Microspacecraft will need to be highly integrated to effectively utilize limited resources • Full systems approach will be required for micropropulsion performance studies • Intrinsic performance (thrust stand performance) will be modified by systems considerations • Propellant storage tank mass, valve leakage, power, power supply mass, propellant feed system complexity • Simplified thrusters with lower intrinsic specific impulse may win out over complicated high Isp concepts • Dual (or more) use systems have added benefit

16. Systems: Micro-Ion Thrusters • Low ionization can be countered in ion-type thrusters with large accelerating potentials • Limitations on power available • Limitations on mass available • Applied magnetic fields do not scale favorably • Relatively large mass for permanent magnets • High power requirements for solenoids • Beneficial designs • No use of magnetic fields or accelerating grids • No use of valve or other flow components • Power requirements met with pulsed operation Optimization

17. Systems: Micro-Chemical Thrusters • Propellants store naturally as liquid or solid • Corrosive propellants add system complexity • Example: hydrazine compatibility with silicon • MMH/Nitrogen Tetroxide, chlorine triflouride, … • Cryogenically stored propellants probably not an option for most microspacecraft • Beneficial propellants • Easily handled and stored on-orbit • Non-corrosive • Green

18. Systems: Micro-Chemical Thrusters • Pressurant gases not desired unless they have a dual purpose (e.g. propellant for cold gas ACS) • MEMS (or other) propellant pumps not desired • All component materials will need to survive harsh environments from tanks to nozzles • Corrosive propellants / combustion products • High temperature • Monopropellants appear attractive but also have limitations (e.g. high temp. catalysts)

19. Conclusions • The future of MEMS-scale micropropulsion will depend on novel approaches to scaling and system limitations • Micropropulsion devices which have overall system benefits and simplicity are desired even if intrinsic Isp is lower • Microspacecraft system limitations must be addressed • Simply scaled down versions of existing thrusters may not work on the MEMS level

20. Conclusions • Impacts • Micromachining for materials other than silicon and derivatives • Improved thermal, electrical, mechanical properties • High resolution thrust stands capable of measuring micro-Newton thrust levels • Very low mass flow (fluid and gas) measurement techniques • High spatial resolution diagnostics • Improvements in other microspacecraft subsystems • Power - improved solar arrays, MEMS batteries, …