2. Electric Propulsion Overview & Hall Thruster What is Electric Propulsion? “The acceleration of gases for propulsion by electrical heating and/or by electrical and magnetic body forces.” (Physics of Electric Propulsion, Robert G. Jahn, 1968) For Ve above 10,000 m/s, a propulsion system needs; Extremely high temperature of an insulated gas stream or Direct acceleration by applied body forces EP thrusters are very “fuel efficient” due to high exhaust velocity Ve. 2 How to accelerate? Electrothermal Electrostatic Electromagnetic Arcjet thruster (5-10km/s） MPD thruster (50-100km/s) Hall thruster Ion thruster (15-25km/s） (30-50km/s） 3 How much mass reduction expected? History of Himawari-5 (Metrological Sat.) 1995 Mar. Launched 2000 Mar. Life-time (5 years)Expired 2003 May Taken over by retired GOES-9 2005 Feb. Himawari-6 2006 Feb. Himawari-7 Hydrazine monopropellant thruster Momo ⇒ Ion Engine Propellant for NSSK 207kg(28%) ⇒ 21kg ‘Whether satellite Himawari-5’ Propellant for OTV 402kg(54%) ⇒ 40kg Total mass 747kg ⇒ 199kg 4 How Large ∆V mission is possible? Large ∆V = long-range missions (interplanetary flights) long-time missions (maintenance of satellite) Ex. Orbit to Mars ∆V = 14 km/s Typical ion thruster Ve = 30 km/s mi m fexp V V e 1.6 Cf. Typical chemical Ve = 3 km/s mi m fexp V V e 106 5 Electric Propulsion vs Chemical Propulsion Chemical Ve = 4.5 km/s Electric Ve = 30 km/s 6 Power Supply – The higher the Ve is, The Better? All EP thrusters must have electric power supply on-board the spacecraft. The weight of power supply scales monotonically with the power level , which is directly related to Ve. There exists an optimal Ve for a given ∆V and Total Mass vs. Ve (Isp). thrust level. 7 Transfer time Final net mass delivered to GEO as a function of LEO-to-GEO trip time for various propulsion options based on a 1550 kg Atlas IIAS-class payload with 10 kW power. (AIAA-95-2513) 8 Optimum exhaust velocity for OTV by Electric Propulsion (1) 2 mVprop e Thrust Efficiency th Ps: Available Power 2Ps Specific power of solar cell panels α=Ps/mpanel (W/kg) β= mpanel /Ps (kg/W) Typical β=0.05 kg/W Propellant consumption rate mmprop prop : Transfer Time 9 Optimum exhaust velocity for OTV by Electric Propulsion (2) 1 ΔV/V0=0.05 0.9 0.1 mmpay P prop 1 s 0.8 m m m 0.2 i i i 0.7 0.3 mprop Ps 0.6 11 ratio mmi prop 0.5 0.5 2 0.4 Payload V Ve 1 1 exp 1 0.3 0.7 Ve2 th 0.2 2 VV Ve 0.1 exp 1 exp VVe 2 th e 0 0.1 1 10 Ve/Vo VVe,opt 0 th Payload ratio and exhaust velocity V 10km/s for 30 days, 33km/s for 1 year. e,opt 10 History of Electric Propulsion Brief History of EP – It’s An Old Idea. In 1906, R. H. Goddard expressed informally many of the key physical concepts of EP. In 1911, Konstantin Tsiolkovskiy proposed similar concepts as Goddard. In 1929, Herman Oberth included a chapter on EP in his classic book “Man into Space.” In 1950s, Ernst Stuhlinger summarized his studies in his book “Ion Propulsion for Space Flight.” In 1960s, EP became a major component of space propulsion development. Ion thrusters in US Hall thrusters in the former Soviet Union 12 In the United states in ’90s PAS-5 Telstar Telstar (‘93-’95) 24 Arcjets Intelsat (GE Aerospace-Lockeed Martin) PanAmSat (‘90-’00) Intelsat(’93-’96) :40 Resistjets 60 Ion thrusters (Ford Aerospace-SS/Loral) (Hughes-Boeing) Aerojet 550W Arcjet Boeing NSTAR Ion engine 13 In Russia and Europe Russia SPT100 D-55 More than 100 Hall thrusters on Cosmos & Meteor series from ’70s. Europe PPS1350 SMART-1 Astrium and Alcatel are competing and cooperating for Hall thrusters in 2000s. 14 In Japan MELCO Ion thruster ISAS Ion Thruster ETS-series On Hayabsa explorer Ion thrusters No. Operation Satellite Type NASA NSTAR Ring Cusp 1 16kh DS1 XIPS13 52 55kh BS601HP Boeing Ring Cusp XIPS25 24 14kh BS702 UK10 Kaufman 2 0.7kh Artemis Astrium RIT10 RF 3 7.7kh Artemis, EURECA MELCO IES Kaufman 8 0.2kh ETS6, COMETS ISAS/NEC μ10 Microwave 4 40 kh HAYABUSA 15 Renewed interests in EP in recent years More on-board electric power In-space propulsion for manned asteroid/planet exploration All Electric satellites Courtesy NASA Boeing satellite 16 Summary of EP overview So, Do We Really Want an EP System? In general, large ∆V missions are appropriate for using EP systems. Factors in determining a suitable propulsion system: Ve, exhaust velocity EP Thrust CP Efficiency EP Power subsystem CP Propellant management subsystem ---- Mission trajectory and time CP/EP Interface with other subsystems CP Reliability (heritage, redundancy) CP Operation of spacecraft (orbit, trajectory) CP/EP Cost CP (advantage for) EP: electric propulsion CP: chemical propulsion 18 Operation of a GEO Satellite MBSAT: DBS communications satellite for Japan and Korea built by SS/L, 2004. The first US-built satellite with SPT-100. Orbit Control Maneuvering of chemical and Hybrid GEO satellites Propulsion NSSK EWSK NSSK Cycle EWSK Cycle System time/firing time/firing 20-N Bi-prop 1 per two 2 per two A few minutes A few seconds (x 2 units) weeks weeks SPT-100/Bi- 12 hrs. 2 per day A few seconds 2 per day prop by SPT-100 by SPT-100 by a Bi-prop by a Bi-prop 19 Operation of a GEO Satellite (2) • For the bi-prop system, 3 maneuvers in 2 weeks. • For the EP/Bi-prop hybrid system, 56 maneuvers in 2 weeks. • The hybrid system requires orbit control maneuvering every day due to its low thrust SPT-100. • One NSSK maneuver takes a few minutes for the bi-prop system and 12 hours for the hybrid system. • Orbit control ∆V: 95% for NSSK, 5% for EWSK • Large electric power (~ 2 kW) from both the solar cells and batteries is supplied for the hybrid system. Complex battery charging management, especially during the eclipse periods in spring and autumn. 20 Cost of EP systems (1) As a simple comparison, the number of orbital maneuvers for a GEO communications satellite by the hybrid system is 18 times that by the bi-prop system. An orbital maneuver is proceeded by a ranging, orbit determination, and control planning and followed by another ranging, orbit determination, and assessment. A rough estimate of cost increase in orbit control operations (with one operator + one orbital engineer) is ~ $8 million in 15 years (orbital lifetime). 21 Cost of EP systems (2) MBSAT carries the same number of bi-prop thrusters as the satellite that does not carry SPT-100s. MBSAT: 12 bi-prop thrusters + 4 SPT-100 thrusters Chemical propulsion satellite: 12 bi-prop thrusters ~ $20 million cost increase due to four SPT-100 thrusters plus gimbals, PPU, and PFS Trade-off between the bi-propellant mass and the mass of SPT-related hardware and xenon. The decrease in launch cost is minimal. Having an EP system, with less propellant mass and longer mission lifetime, would NOT REALLY save the total cost when considering actual satellite procurement and operations!! 22 EP CAN…. can have an advantage with their low thrust level. Extremely small attitude disturbance by their maneuvering. Suitable for satellites with flexible structures. Suitable for missions with fine maneuvering requirement such as co-location and formation flying. can appeal to missions of extremely high ∆V requirements. Impractical to carry the necessary propellant mass of a chemical rocket system Interplanetary missions Deep-space probes can continue to evolve. Higher reliability Lower cost Greater variety of the combinations of thrust, Isp, and power levels. 23 Hall Thruster Hall Thruster Principle: Overview Acceleration Electrostatic acceleration Propellant channel (Xe) Axial E and radial B Xe+ Cathode as electron emitter Hall current Design criteria: (Larmor radius) Anode E electron B Cathode (Hall parameter) (Ion mean free path) : Channel length 25 Hall Thruster Principle: Electric Field 100 ~ 1000 V DC E field Plasma is highly Plasma conductive → Electric field is shielded Anode r B field sideCathode z Channel wall Radial magnetic field to confine electron motion Φ B Electron gyroradius must r be much smaller than channel length Electron Larmor radius Anode Channel exit Cathode 26 Hall Thruster Principle: Hall Current E x B drift movement of E x B direction charged particles B electron motion E Electron-induced Hall current Electron Hall current For effective confinement of B electron motion, electron E Hall parameter must be much larger than 1 Collisions contribute to cross B-field transports (Hall parameter) = (Gyrofrequency) x (mean free time) Electron motion with collisions 27 Hall Thruster Principle: Closed Annular Cavity Hall current B E Operation of Hall thruster Linear Hall-effect ion source1 1https://www.ulvac.co.jp/ 28 Hall Thruster Principle: Ion Generation Energy loss: Energy gain: Electrons are very lightweight Ionization, Joule heating Wall loss, etc. Φ Te Electrons are heated to Ionization 20 eV = 232,000 K region Collisional ionization Anode Channel exit Cathode First ionization energy [eV]: + Argon: 15.8 Xe Krypton: 14.0 Xenon: 12.1 Electron 29 Hall Thruster Principle: Ion Acceleration Ions are smoothly accelerated by electric field Ion Little disturbance by magnetic field or collisions Anode r z Channel wall Ion Larmor radius Ion mean free path Exhaust velocity estimation Discharge voltage: Acceleration voltage: If 90% of is ionized, Ion mass (Xe): Elemental charge: 30 Hall Thruster Principle: Recap Acceleration Electrostatic acceleration Propellant channel (Xe) Axial E and radial B Xe+ Cathode as electron emitter Hall current Design criteria: (Larmor radius) Anode E electron B Cathode (Hall parameter) (Ion mean free path) 31 Magnetic Layer vs.