In-Space Propulsion Systems
Low Thrust
Micropropulsion
Michael M. Micci
The Pennsylvania State University
Presented at the
NASA Technology Roadmaps: Propulsion and Power Workshop
National Research Council
California Institute of Technology
Pasadena, CA
March 22, 2011
My background
Micropropulsion for Small Spacecraft, edited by M. M. Micci
and A. D. Ketsdever, Progress in Astronautics and Aeronautics,
Vol. 187, AIAA, 2000.
30 years on the faculty of The Pennsylvania State University.
Experimental experience with:
Solid and liquid propellant rockets
Microwave and RF plasma electrothermal thrusters
Miniature RF and microwave ionization ion thrusters
Year sabbatical in electrospray lab at University of London, QM.
Micropropulsion
2.2.1.4
By definition “Low Thrust”
But also:
• Low electrical power
• Low mass
• Low volume
• Precise thrust and impulse bits
• Low cost
Micropropulsion
Propulsion for microsats, nanosats, and CubeSats.
But they’re not just for small spacecraft anymore.
Useful anytime low thrust or impulse bits are required.
• Drag make-up (GRACE)
• Formation flying (LISA)
• Asteroid and planetoid orbits
• Close proximity (Inspector) missions
• Precise positioning missions (JWST occulter)
• Franklin and Edison microspacecraft missions
Micropropulsion
You can’t just scale down a current thruster and expect the same
performance.
Physical processes detrimentally affecting micropropulsion:
• Large surface to area ratios.
Higher heat losses than larger scale devices.
• Small flow passages.
High viscous losses, both in nozzles and flow passages.
Subject to flow blockage due to contamination and bubbles.
• Smaller volumes for charged particle containment.
Lower charged particle residence times.
Higher magnetic fields required for charged particle confinement.
• Small thrust and propellant mass flow levels.
Difficulty making accurate thrust and flow rate measurements.
Micropropulsion
Eight technologies listed in NASA Roadmap
Chemical
2.1.7.1
2.1.7.2
2.1.7.3
Solids
Cold Gas/Warm Gas
Hydrazine or H2O2 Monopropellant
Electric
2.2.1.4.1
2.2.1.4.2
2.2.1.4.3
2.2.1.4.4
2.2.1.4.5
Microresistojets
Microcavity Discharge
Micropulse Plasma
Miniature Ion/Hall
MEMS Electrospray
Solids
2.1.7.1
Advantages
High TRL level (>6).
Simplicity.
No need for liquid or gas storage and management.
Disadvantage
No controllability.
Comment
No discussion of digital microthrusters, which have
been investigated and would provide controllability
and scalability.
Cold Gas/Warm Gas
2.1.7.2
Advantage
High TRL level (>6) due to simplicity.
Disadvantages
Low performance (Isp) compared to other devices.
Leakage from small valves.
Need to contain high pressures if high performance
is desired.
Comment
Do we really want to invest more in this due to low
performance?
Hydrazine or H2O2 Monopropellant
2.1.7.3
Advantages
High chemical performance (Isp) and controllability.
Disadvantages
Lower TRL levels for smaller thrusters.
High heat losses for smaller thrusters.
Comment
No discussion of low toxicity (HAN-based)
monopropellants which would simplify handling while
improving performance.
Microresistojets
2.2.1.4.1
Advantages
High TRL level due to large scale heritage.
Simplicity.
Disadvantage
Low performance (Isp) compared to other devices.
Comments
Do we really want to invest in this due to low
performance?
No discussion of Free Molecular Micro-Resistojet
(FMMR) developed by AFRL and Air Force Academy.
Microcavity Discharge
2.2.1.4.2
Advantages
Higher performance than resistojets.
Scalable to very small dimensions (MEMS based)
as well as to large scale to obtain high thrust.
Disadvantages
Low TRL levels.
High heat losses and electrode erosion.
Comment
No discussion in Roadmap of RF or microwave
discharges, both of which are under development and
show the potential for increased performance and
longer lifetimes due to electrode-less operation.
Micropulse Plasma
2.2.1.4.3
Advantages
Easy to miniaturize.
Can use solid propellants.
Disadvantages
Pulsed operation effect on power system design
(capacitors and switches).
Low overall system efficiencies.
Electrode erosion due to pulsed operation.
Comment
No discussion of Micro Pulsed Plasma Thruster
developed to a high TRL level by AFRL.
Miniature Ion/Hall
2.2.1.4.4
Advantages
Potential for high performance (Isp and efficiency).
Uses inert propellants (xenon).
Disadvantages
Lower charged particle residence times.
Need to increase magnetic field strengths to maintain
charged particle confinement.
Need small electron sources (hollow cathodes).
Comment
No discussion of miniature (1 cm) low power (10 W)
ion thrusters using RF and microwave ionization
developed at Penn State and in Japan.
MEMS Electrospray
2.2.1.4.5
Advantages
High TRL level (7), scheduled for LISA Pathfinder.
High performance (Isp and efficiency).
Can take advantage of MEMS technology.
Scalable to high thrust.
Disadvantages
Propellant distribution to large numbers of emitters.
Flow blockage due to contaminants and bubbles.
Electrochemical degradation of emitters.
Comment
Shows great promise if above problems can be solved.
Micropropulsion
Other thoughts
• Micropropulsion is a relatively young technology but is
poised to make a large impact on NASA current, planned
and unforeseen missions; near the “tipping point”.
• Micropropulsion advances technology that has application
outside of aerospace, for example electrosprays and
miniature plasma sources in the biomedical and electronic
fabrication areas.
• Micropropulsion, through its small size, allows substantial
small business, academic and student participation.
Micropropulsion
Summary of Roadmap comments
• Many potential high-performing concepts will require
an investment to increase TRL levels but are worth it
and are near term.
• Too many non-NASA concepts are not discussed in
the Roadmap (“Not invented here?”).
• Too much emphasis on low performing technologies.