1. 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

2. 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.

3. 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

4. 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

5. 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.

6. Micropropulsion Eight technologies listed in NASA Roadmap Chemical 2.1.7.1Solids 2.1.7.2Cold Gas/Warm Gas 2.1.7.3Hydrazine or H 2 O 2 Monopropellant Electric 2.2.1.4.1Microresistojets 2.2.1.4.2Microcavity Discharge 2.2.1.4.3Micropulse Plasma 2.2.1.4.4Miniature Ion/Hall 2.2.1.4.5MEMS Electrospray

7. 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.

8. 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?

9. Hydrazine or H 2 O 2 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.