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
OUTLINE
• Introduction
– Microspacecraft
– Micropropulsion
• Scaling Issues
– Micronozzle Expansion (AIAA 99-2724)
– 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
– Microspacecraft
– Nanospacecraft
1000 - 100 kg
10 - 100 kg
1 - 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 versions 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 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?), …
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
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 / no si
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)
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
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
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
Scaling: Micro-Heat Transfer
• Radiation
– qr  AT4  L2 T4
– Can be a major loss mechanism at high temperatures
• Conduction (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
Silicon
Silicon Dioxide
Silicon Carbide
Silicon Aerogel
Silicon Nitride
Aluminum (6061-T6)
Teflon
Thermal
Max.
Conductivity, Temp.
k (W/mK)
(ÞC)
157
1350
1.38
1200
75-155
1400
0.017
1200
30.1
1300
104.7
660
0.35
340
• Material thermal expansion also a major issue
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
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
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
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
Optimization
• 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
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
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)
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
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, …