1、Systems Considerations and Design Options for Microspacecraft Propulsion Systems,Andrew Ketsdever Air Force Research Laboratory Edwards AFB, CAJuergen Mueller Jet Propulsion Laboratory Pasadena, CA,OUTLINE,Introduction Microspacecraft Micropropulsion Scaling Issues Micronozzle Expansion (AIAA 99-272
2、4) Ion Formation Combustion and Mixing Heat Transfer MEMS Devices Systems Considerations Conclusions,Introduction,Microspacecraft will require a propulsive capability to accomplish missions Microspacecraft - AFRL Definition Small Spacecraft 1000 - 100 kg Microspacecraft 10 - 100 kg Nanospacecraft 1
3、- 10 kg Microspacecraft will be resource limited Mass Power Maximum Voltage Volume,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 ve
4、rsions of existing thrusters Reduced power, mass, thrust level MEMS thrusters Require MEMS/novel fabrication techniques Performance scaling issues,Introduction,A wide range of micropropulsion concepts will be required High thrust, fast response Low thrust, high specific impulse Micropropulsion syste
5、ms 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?), ,Introduction,Micropropulsi
6、on 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,Scaling: Microscale Ion Formati
7、on,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 / no si Rg = me vo,perp / (q B) Want Rg B = 0.1 Tesla 1 mm diameter = B = 10
8、 Tesla (Yashko, et al., IEPC 97-072),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 insulat
9、or thicknesses Material dependencies Two modes of breakdown,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
10、) 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,Scaling: Micro-Combustion,Advanced liquid and solid propellants are targeted at mission requirements involving High thrust Fa
11、st response Scaling issues arise which may limit characteristic size Mixing length required for bi-propellants Residence time in combustion chamber Combustion instabilities Heat transfer,Scaling: Micro-Heat Transfer,Radiation qr AT4 L2 T4 Can be a major loss mechanism at high temperatures Conduction
12、 (1-D) qc = k A (dT/dx) 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,Scaling: Micro-Heat Transfer,Material thermal expansion also a major issue,Scaling: MEMS Propulsion S
13、upport 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 eve
14、n at high pressure) Transient flow,Scaling: MEMS Propulsion Support Hardware,Small impulse bit maneuver Microspacecraft slew maneuver - 1 N-sec Two possible scenarios for achieving small I-bit Reduce thrust Reduce valve actuation time For 1 N-sec impulse bit and 1 mN thrust, valve actuation time on
15、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,Systems: General,Microspacecraft will need to be highly integrated to effectively u
16、tilize 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
17、 Simplified thrusters with lower intrinsic specific impulse may win out over complicated high Isp concepts Dual (or more) use systems have added benefit,Systems: Micro-Ion Thrusters,Low ionization can be countered in ion-type thrusters with large accelerating potentials Limitations on power availabl
18、e 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 pul
19、sed operation,Optimization,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
20、for most microspacecraft Beneficial propellants Easily handled and stored on-orbit Non-corrosive Green,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
21、 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),Conclusions,The future of MEMS-scale micropropulsion will depend on novel approaches to
22、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,Conclusions,Impact
23、s 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, ,
