Electric Propulsion for Space Travel

Transcription

1 Electric Propulsion for Space Travel Sarah Cusson Ph.D. Candidate NASA Space Technology Research Fellow University of Michigan Plasmadynamics and Electric Propulsion Laboratory

2 Agenda History of electric propulsion and Hall thrusters Motivation for electric propulsion Basic physics of electric propulsion Hall thruster performance models Current research on Hall thrusters Focus on nested-channel Hall thrusters and magnetic shielding

3 History First electric propulsion system to fly was in 1964 Deep Space 1 was the first mission to use EP as primary propulsion (NSTAR ion engine) Boeing satellites now offer all electric systems (702SP using XIPS) 10x more efficient than conventional chemical system First launch was March 2, 2015 on a SpaceX Falcon 9

4 Many commercial and defense Earth satellites rely on Hall thrusters for orbit maintenance SMART-1 ESA mission with a HET Hayabusa FalconSat AEHF NASA selected the HERMeS (Hall Effect Rocket with Magnetic Shielding) for the ARM (Asteroid Retrieval Mission) Consists of kw Hall thrusters First time NASA has selected Hall thruster as primary propulsion Current Programs

5 AEHF The start of the Hall thruster boom Satellite was initially inserted into initial GTO 225 km x km Liquid Apogee Burn was supposed to raise perigee to km When they first tried to turn on the liquid engines, the engine failed to turn on. Tried again, failed again. The team determined there was a propellant line blockage and they couldn t fire without threat of explosion. Used hydrazine engines to raise perigee to 4900 km Hall thrusters raised the rest of the orbit (took 1 year instead of 3 months)

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7 Electric Propulsion is ideal for all NASA Robotic Missions Low-disturbance High V space missions High-precision

8 Motivation Δv = u e ln m rocket + m fuel m rocket Δv is defined solely by the mission. u e is the velocity of the propellant coming out of your rocket Typical values: Chemical Rocket: 2-5 km/s Electric Rocket: km/s u e = g 0 I sp where g 0 is gravity on Earth s surface and I sp is the propellant s specific impulse Additionally, electric propulsion is safer for ground handling and the spacecraft in case of propellant tank failure

9 EP can deliver up to 3000x the dry mass compared to chemical propulsion M final M initial e V U e Advanced Propulsion Isp 4000 s 1-out-of out-of-250,000 Conventional Propulsion Isp 400 s

10 Electric Propulsion Operating Ranges Hall thrusters satisfy a niche between ion engines (~5000 s) and chemical propulsion (~400 s) Plasma is relatively dense (10 12 cm -3 ) and energetic (~50 ev) good source for space physics experiments Victor, A.

11 High Payoff Trip Time [days] USAF has shown that high power electric propulsion is ideal for their mission LEO to GEO orbit transfer ΔV 5.8 km/s, 5000 kg wet mass 400 km circular starting orbit in LEO 20-kW EP 40-kW EP 60-kW EP Biprop kW 60-kW 20-kW Chemical Bipropellant Spacecraft Dry Mass [kg] Maximize Dry Mass High Payoff High Payoff 5000 Minimize Time Evolving Space Power Capabilities Next Generation High-Power EP MISSION ADVANTAGES Increased Maneuverability, High Payoff Rapid, Sustainable Repositioning Improved GEO Payload Delivery, Reduced Time Mission Enabling (Space Tug, Rescue) TECHNOLOGY DRIVERS 1.Processing Increased Space Power Levels, High Thrust Density ( kw) 2.High-Efficiency Operation Over Range of Moderate Isp ( sec) 3.Low-Mass Propulsion System for Advanced Space Power Generation Brown, et al., Development of High Power Electric Propulsion Technology for Near-term and Mid-term Space Power, JANNAF-1193, 57 th JANNAF Propulsion Meeting, USAF-SPONSORED RESEARCH EFFORTS Near-Term: Nested Channel Hall Thrusters (5-500 kw) Mid-Term: FRC Technology (50 kw to >1 MW)

12 Chemical Propulsion Vs Electric Propulsion V=50 km/s Chemical Propulsion MR (M f /M o ) = 1/150,000 Electric Propulsion MR = 1/5 ELECTRIC PROPULSION Low thrust (mn to N) Only work in vacuum Very efficient/lightweight Isp up to s Acceleration (mg to mg) Can use just about anything for propellant (e.g. noble gases like xenon) Fires for months to years CHEMICAL PROPULSION High thrust (N to kn) Work in space or on the ground Very inefficient/heavy Isp up to 450 s Acceleration 0.1 to 1 g Fires for minutes to hours

13 Basic Schematic of a Chemical Rocket Engine

14 Electric Propulsion Electric Propulsion uses heat, electromagnetic fields and electric fields to ionize gases and then accelerate them

15 Basics of Electric Propulsion Inject a neutral gas Ionize it Low temperature plasma cloud of electrons, ions and neutrals Temperatures on order 1-5 ev Electrons can get up to 20 ev Accelerate it using electric fields to produce thrust A + - C

16 Single Particle Motion Magnetic Field F = m a = q(e + v x B) ω c = qb m r L = v th ω c mt B ω c is the frequency r L is the Larmor radius distance from the magnetic field line that the particle gyrates about r L

17 Single Particle Motion with Electric Field Particle still gyrates the same as with magnetic field, but now also moves along the magnetic field line v ExB = E B

18 Gridded Ion Thruster Neutral Xe Injection Anode Body Magnetic Field Windings Cathode e - e - Xe + Xe + Beam Neutralizing Cathode Xe + Xe + e - Propellant Ion Beam V d V accel V x

19 Electrons trapped by the magnetic field Larmor radius on order of 0.3 cm magnetized Neutral gas ionized by electron bombardment (Hall current ExB drift) Ions aren t magnetized Larmor radius closer to 1 meter Ions are accelerated by electric field Cathode neutralizes ion beam Hall Thruster

20 Hall Thruster Cross Section

21 Increased Secondary Electron Emission Leads to Adverse Performance Effects Wall material shown to affect electron conductivity to the anode Increased electron currents to anode found for materials with higher SEE coefficients, resulting in efficiency drop electron back-current SEE likely the source of enhanced electron conductivity, but details of mechanism still under debate Increasing SEE * Figure from Barral, S., Makowski, K., Peradzynski, Z., Gascon, N., and Dudeck, M., "Wall material effects in stationary plasma thrusters. II. Near-wall and in-wall conductivity," Physics of Plasmas 10, 10, (2003). * Figure from Raitses, Y., Smirnov, A., Staack, D., and Fisch, N. J., "Measurements of secondary electron emission effects in the Hall thruster discharge," Physics of Plasmas 13, 1, (2006).

22 HALL THRUSTER PERFORMANCE MODEL Hofer-Reid Model Utilizes Plume Data to Understand Loss Mechanisms h a - Anode thrust efficiency (cathode & magnet ignored) h 2 T 2mP h h h h h a q v d b m a d ExB - h q NASA-173Mv2 IC, OC ITC, ETC 10 mg/s ITC ETC h q - Charge utilization efficiency h v - Voltage utilization efficiency h b - Current utilization efficiency h m - Mass utilization efficiency h d - Divergence Normalized di/dv RPA - h v Vmp = Vd - Vloss FWHM Xe + species fraction Xe 2+, Xe 3+ species fraction Xe2+ Xe Discharge voltage (V) 900 Plasma potential (Volts from ground) Emissive/Langmuir (RPA Correction) - h v Discharge voltage (V) NASA-173Mv2 10 mg/s IC, OC ITC ITC, ETC ETC Faraday h m, h b, h d Energy / Charge (Volts) 500

23 Current Utilizations dominates Performance Loss Mechanisms Results: Voltage utilization >90% Current utilization ~70-80% Divergence utilization >90% Charge utilization >95+% Mass utilization >90% Thruster efficiency (50-70%) Current Utilization is the primary performance Sink; i.e., large electron discharge current h b Illustrates the need to understand interior Hall thruster physics

24 H6 Hall Thruster H6 Hall Thruster World s Most Efficient Xenon Hall Thruster The H6 is a 6 kw Hall thruster designed in collaboration with AFRL and the University of Michigan The thruster was designed to serve as a high-performance test bed for fundamental studies of thruster physics and technology innovations High-performance is achieved through advanced magnetics, a centrally-mounted LaB6 cathode, and a high uniformity gas distributor Throttleable from 2-12 kw, s, mn At 6 kw, 300 V (unshielded): 0.41 N thrust, 1970 s total Isp, 65% total efficiency At 6 kw, 800 V (unshielded): 0.27 N thrust, 3170 s total Isp, 70% total efficiency Highest total efficiency of a xenon Hall thruster ever measured.

25 Plasmadynamics and Electric Propulsion Laboratory Founded in 1992 by Dr. Alec Gallimore One of the leading electric propulsion research centers Home to Large Vacuum Test Facility (LVTF) largest vacuum chamber in academia Current partners: Air Force Research Laboratory NASA (Glenn and JPL) Aerojet Rocketdyne Phase4

26 Facilities

27 Facilities NEOVac (Nanosatellite Engine Operation Vacuum Chamber) 3 ft x 5 ft Base Pressure ~1 x 10-7 Torr Pumping Speed of 1500 L/s on air Cathode Test Facility (CTF) 2 m x 0.6 m Base Pressure ~2 x 10-8 Torr Pumping Speed 1500 L/s on xenon Junior Test Facility Connected to LVTF (3 m x 1 m) Independently reach 5 x 10-5 Torr

28 Current Research Campaigns at PEPL High-speed Thruster Plasmadynamics Time-resolved Laser Induced Fluorescence Cube-sat Ambipolar Thruster (CAT) Nested-Channel Hall Thrusters Characterization of Background Effects Lifetime Studies

29 High-speed Thruster Plasmadynamics While thruster may seem steady state, there are actually large oscillations at high frequencies High-speed Dual Langmuir Probes High-speed Imaging Analysis Capture breathing and spoke mode characteristics Current work to develop other time-resolved diagnostics

30 Breathing Mode Oscillations Hall Thrusters tend to breathe at khz Slowed down 6000x

31 Laser Induced Fluorescence takes advantage of Doppler shifted wavelength seen by moving ions to measure velocity Great non-intrusive diagnostic Currently using time-averaged and time-resolved to see acceleration region of thrusters Future work includes: Cathode oscillation investigation TRLIF on a magnetically shielded Hall thruster Time Resolved Laser Induced Fluorescence

32 CubeSat Ambipolar Thruster CubeSat Ambipolar Thruster (CAT) uses an RF plasma source and a magnetic nozzle to provide thrust for maneuvering of CubeSats in orbit Currently, feed system testing and plasma diagnostics are occurring Aiming for launch within next couple of years

33 Background Environment Effects Thruster Operation is different in space and on the ground In experiments, performance has been affected depending on the background pressure Additionally, in space performance has been different than what was expected based on ground experiments Performance changes can t be explained by neutral ingestion alone Studies going on to determine what exactly is happening Electrical effects Need to be able to accurately predict performance in space

34 Nested Channel Hall Thrusters NASA wants to increase power levels of Hall thrusters go further faster University of Michigan, in conjunction with Air Force and NASA, developed nested-channel thrusters The X3, a 200-kW Hall thruster, is the biggest Hall thruster ever produced and can provide up to 15 N of thrust

35 Advantages of Nested Hall Thrusters Enhanced throttle-ability (Isp and power) Increased power throughout (kw to MW for nested cluster) Reduced mass/power and size (footprint)/power Improves packaging on launch vehicles for very high power Lower costs Comparison of 200-kW systems Diameter (m) NASA recognizes high power EP as enabling and critical technology for human exploration mission Air Force has identified nested-hall thrusters as enabling for time critical missions

36 200-kW Hall Thruster Compared to VASIMR VX-100 Thrust Power (Not including auxiliary systems) VASIMR: VX-200 Thruster 200-kW Hall Thruster 50-kW NASA Hall Thruster 200 kw 200 kw 50 kw Nominal 100 kw Demonstrated Specific Impulse sec sec sec. Thrust 5 N (25 mn/kw) 5 14 N (30-70 mn/kw) 3 N Propellant & Mass Flow Rate Argon 150 mg/sec. Krypton & Xenon 120 mg/sec.(kr) 200 mg/sec. (Xe) Krypton & Xenon 60 mg/sec.(kr) 100 mg/sec. (Xe) Thrust Efficiency 42% 63% (Kr) & 72% (Xe) 63% (Kr) & 72% (Xe) Thruster Specific Mass (Not including auxiliary systems) Major Dimension ~1.5 kg/kw <1 kg/kw ~1 kg/kw ~1.5 m width ~3 m length 0.6 m diameter 0.15 m length 0.5 m diameter 0.15 m length

37 Nested Channel Hall Thrusters

38 Magnetic Shielding

39 Magnetic Shielding Principles Currently, only single channel thrusters have shown magnetic shielding Examples: H6, MasMi, HERMeS, BPT H6 US H6 MS Performance has seen some, but little effect Thrust down 4% on H6MS Specific Impulse up 3% due to higher presence of doubly charged ions Divergence angle increase 5

40 The EP thrusters NASA would like to develop should be able to operate over a broad power and specific impulse range to allow for maximum mission flexibility. They should be capable of high thrust at a lower specific impulse of ~2000s but also be capable of propellant efficient operations at a higher specific impulse of ~5000s. To meet the needs of a variety of mission concepts, the engines should have an in-space lifetime goal of >50,000 hours and an operational (thrusting) goal of >10,000 hours. The engines should have an operational end-to-end total system efficiency goal of greater than 60% (the thruster efficiency alone will be higher). The High Power (50 kw to 300 kw per thruster) class EP engine Motivation for MS NHT

41 Nested Channel Hall Thrusters + = My Dissertation We are trying to combine nested-channel and magnetic shielding to create a long-life high-power Hall thruster that will enable the nextgeneration of space travel, including bringing humans to Mars

42 Questions?