intro to electric propulsion

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EP – Electric Spacecraft Propulsion Concepts, Physics, Case Studies Dr. Dieter M. Zube Aerojet General, Redmond, WA May 2007 History / Definitions Types of EP Electrothermal Electrostatic Electromagnetic -Propulsion Outlook

History / Definitions: 

History / Definitions Propulsion requirements are defined as “Dv” - changes in velocity Propulsion potential of an ideal rocket, derived from impulse conversation law: Tsiolkovsky’s rocket equation: m0/m1: mass at the beginning of the maneuver divided by mass at the end of the maneuver function of spacecraft design = (mstructure + mpropellant) / mstructure weight of the structure > 6-7% of the fully loaded rocket (compare to nature’s example, the egg: 5-10%) practical limits of the velocity increment Dv:

Further exploring the Rocket Equation: : 

Further exploring the Rocket Equation: ce: average velocity of the thruster exhaust particles, thrust F divided by flow rate important figure of merit for propulsion system: specific impulse (aka “Isp”): Nice feature: the unit of the specific impulse is independent from the unit system! [specific impulse]metric, imperial = s

Further exploring the Rocket Equation: : 

Common question during the mission planning phase: How to increase Dv without increasing mpropellant? Increase ce! (not always maximize ce!) Chemical rockets: Use chemical energy stored in the propellants to increase thermal energy of the reaction products, convert thermal energy into kinetic energy in a de-Laval (converging – diverging contour) nozzle best we can hope for: hydrogen (atomic!), k=1.67, T = 3000 K: ce = 11,150 m/s, Isp = 1,130 s, in reality: < 550 s (fluorides…) Chemical rockets are “energy limited” Further exploring the Rocket Equation:

Further exploring the Rocket Equation: : 

Useful equation to assess the propulsion capability without a specific S/C in mind: total impulse = total propulsion potential of a system Dt = time duration of a single maneuver mtotal propellant: characteristic of the propulsion system, e.g. demonstrated throughput capability of a thruster Further exploring the Rocket Equation:

Further exploring the Rocket Equation: : 

Further definition for rocket engine types: monopropellant: single component, usually a liquid decomposed on a catalyst bi-propellant: two components - oxidizer and fuel - which react with each other solid propellant: explosive material burns inside the rocket under controlled conditions cold gas system: pressurized gas is released through a valve to provide thrust Further exploring the Rocket Equation:

Further exploring the Rocket Equation: : 

Solution: → add additional power to the propellant! use of electric power first envisioned by: (insert name of your favorite rocket pioneer) Tsiolkovsky, Oberth, Goddard, … first practical applications: late 1940s in the U.S.A.: E. Stuhlinger Further exploring the Rocket Equation: Power Source (solar electric, solar thermal, battery, nuclear thermal (RTG), nuclear electric) Power Converter / Conditioner / Processor (PCU / PPU) converts “bus power” into the voltages and currents the thruster “needs” Thruster converts electric power into propulsive power

What is the Power of a Rocket Engine?: 

What is the Power of a Rocket Engine? Power: definition of propulsive power of a rocket engine: Examples: 22 N = “5 lbf bi-prop thruster” Isp = 320 s: P = 35 kW! Space Shuttle Main Engine: 2 x 106N = 470,000 lbf, 420 s : P = 4.2 GW! electric power available on spacecraft: Sputnik: 5 W 1960s: < 100 W 1980s: < 1 kW today: 10 kW ISS 60 kW → for the foreseeable future, EP is no match to chemical thrusters for launch applications EP is power limited

Efficiency of an EP thruster: 

Efficiency of an EP thruster Efficiency: common definition of efficiency for EP: (may or may not include PCU conversion efficiency, hPCU > 90%) Acceleration of the spacecraft: → high Isp will cause low acceleration, long trip time! design trades: Isp (or type of EP and propellant mass) vs. trip time vs. system complexity

Types of EP: 

Types of EP History

EP Isp vs. thrust: 

EP Isp vs. thrust

Electrothermal Thrusters: 

Electrothermal Thrusters → “afterburner” for a chemical thruster: raises the temperature of the exhaust gases best propellant: low mass particles (→ hydrogen) Resistojet: Gas flows over heat exchangers, raises temperature to 1800 K most common application: “afterburner” to a monopropellant hydrazine thruster Isp = 300 s, thrust < 0.8 N= 0.18 lbf, 600 W advantages: simple concept, no PCU (“direct drive”), low power level disadvantages: challenging to build, limited life, low Isp

Heat exchanger concepts for resistojets: 

Heat exchanger concepts for resistojets

Arcjet Thrusters : 

Arcjet Thrusters takes the resistojet concept one step further: gas flows through an electric arc, is ionized (< 15%), expands through nozzle Isp = 600 s, thrust 0.23 N = 52 mlbf, 2 kW advantages: simple design, use of hydrazine, low maneuver duration (1 h once every week for NSSK) disadvantages: exotic materials, low efficiency (35%), low Isp; no “schedule and forget” maneuver execution, ground crew has to monitor maneuver

Electrostatic thrusters: 

Electrostatic thrusters Ion thrusters: ionized propellant is accelerated in an electric field velocity of an ion after accelerating in an electric field thrust = “current density” (current per exit area) j: , l = electrode gap power per exit area: → the higher the particle mass, the less power for a given thrust! propellant selection criteria: large m/q (“mass to charge ratio”), mercury, noble gases (xenon), cesium (“double charged ions”)

Methods of ionization : 

Methods of ionization How to add energy to the propellant atoms / molecules to cause ionization? electron bombardment (Hughes, now Boeing XIPS thruster) radio frequency (RIT Germany, Great Britain) field emission (esa) Microwave ion contact

ion thruster cross section (electron bombardment): 

ion thruster cross section (electron bombardment)

Ion thruster grid electron optics: 

Ion thruster grid electron optics heavy, fast, charged particles hit the grid: high erosion rates “intelligent” grid design (spacing, hole size, magnetic field, material) reduces erosion keyword for optic design: “deceleration grid”

Ion thruster essential: The neutralizer cathode: 

Ion thruster essential: The neutralizer cathode to neutralize ion plume: ejects low energy electrons into the plume also used as plasma contactors / neutralizers on ISS

The NSTAR and the NEXT engines: 

The NSTAR and the NEXT engines

Ion engine performance: 

Ion engine performance XIPS-25: 0.063 mN = 14 mlbf thrust, 2,800 s Isp, 1.4 kW used on (now) Boeing GEO S/C advantages: high Isp disadvantages: low thrust, long maneuver times; intensive operator support needed; long lifetime requirement (> 10,000 h = 416 d!); “grid short” The Deep-Space 1 mission

electromagnetic thrusters: 

electromagnetic thrusters Hall thrusters (Hall Effect thrusters HETs) “gridless ion thruster” combination of electrical and magnetic fields cause “azimuthal” Hall current jq, which causes the desired axial Lorentz force acceleration no easy set of equations described the HET mode of operations… need cathode to neutralize plume

HETs : 

HETs developed in the U.S. in the early 1960s, development ceased because of lack of efficiency, continued in the SU, renewed interest in the U.S. since the mid 1990s, several companies now offer and use HETs for on-orbit applications

HETs performance: 

HETs performance SPT-100: 83 mN = 19 mlbf, 1,600 s, 1.4 kW Aerojet: 300 mN, 2,200 s, 4.5 kW advantages: decent thrust at decent Isp disadvantages: long life time, high erosion rate, not yet completely understood physics make it difficult to plan for all eventualities, erosion of electrodes can lead to premature failure

Pulse Plasma Thrusters - PPTs : 

Pulse Plasma Thrusters - PPTs discharge between two electrodes ionizes propellant (e.g. Teflon!), Lorentz forces accelerate ions advantages: compact, self sustained, Teflon as propellant!, very predictable thrust pulse, very low thrust, low power level disadvantages low efficiency, low Isp, very low thrust, high energy capacitors for PCU < 300 s, < 30 W, impulse bits 100 mNs, >107 pulses

Magneto Plasmadynamic thrusters MPD : 

Magneto Plasmadynamic thrusters MPD use only Lorentz forces to accelerate propellants self field and applied field magnetic field designs low efficiencies, promise of higher efficiencies at higher power levels (> 1 MW, MPD “works” only at very high currents…), limited use in plasma wind tunnel applications, academic interest, pulsed applications to reduce power need limited flight tests to demonstrate the concept (Japan)

Micro-Propulsion : 

Micro-Propulsion use on small spacecraft (< 50 kg, in some cases < 5 kg) experimental missions (universities) science missions (ultra precise station keeping) e.g. Laser interferometry to search for gravity waves “free flying” S/C to inspect satellites and other S/C thrust level small (N), low propellant mass (few 100 g), adequate to the S/C size cold gas thrusters long history, low performance, but reliable (< 75 s Isp, mN thrust) PPTs (see above) exotic concepts: VAT vacuum arc thrusters: similar to PPT electromagnetic acceleration, self consuming electrode PPT propellant concepts “self consuming S/C” (remember the rocket equation and the mass ratio!)

Micro-Propulsion : 

Micro-Propulsion Somewhat more practical -propulsion concept currently under evaluation: FEEP (field emission electric propulsion) ion thrusters high electric field near “needle tip” style electrodes provide enough force to ionize propellant N thrust, Isp 800 – 5000 s (same ion optics concept as standard ion thrusters) developed in Europe in the 1970s, but never flown propellants: mercury, cesium (both highly toxic), indium

Conclusion, Outlook : 

Conclusion, Outlook EP concerns: with all this fantastic performance, why aren’t more EP thrusters flying? cost of development long lifetime, reliability concerns EMI perception (so far, unfounded!) plume contamination (ion thrusters, Hall thrusters) need for operator interaction “EP operation is cost intensive” EP’s future looks bright! use of arcjets, Hall thrusters, ion thrusters for GEO NSSK use of ion thrusters for interplanetary missions NASA’s space exploration initiative


References Stuhlinger, E.: “Ion Propulsion for Space Flight”, McGraw Hill, New York 1964 Jahn, R.G.: “Physics of Electric Propulsion”, McGraw Hill, New York, 1968 Sutton, G.P.: “Rocket Propulsion Elements”, Wiley, New York, 1992 Filliben, J.D.: “Electric Thruster Systems”, CPTR-97-65, Chemical Propulsion Information Agency, John Hopkins University, 1997 Auweter-Kurtz, M.: “Lichtbogenantriebe für Weltraumantriebe”, Teubner Verlag, Stuttgart, 1992 Aerojet capability and product brochures NASA websites for photos

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