AE6450 Rocket PropulsionElectric Propulsion-1Copyright © 2003-2006 by Jerry M. Seitzman. All rights reserved.Electric (Rocket) PropulsionEP OverviewAE6450 Rocket PropulsionElectric Propulsion-2Copyright © 2003-2006 by Jerry M. Seitzman. All rights reserved.Basics Rocket Propulsion ElementsPropellantEnergy SourceStorageFeed SystemStorageConversionAcceleratorsame in chemical rocketsAE6450 Rocket PropulsionElectric Propulsion-3Copyright © 2003-2006 by Jerry M. Seitzman. All rights reserved.Electric Propulsion System ElementsPropellantEnergy SourceStorageFeed SystemStorageConversionThrusterGas (Xe), liquid (N2H4), solid (teflon)chemical, nuclear, radiation (solar)Thermodynamic (nozzle), electric, electromagneticto electricity at proper V, DC/AC( freq), current, steady/pulsedAE6450 Rocket PropulsionElectric Propulsion-4Copyright © 2003-2006 by Jerry M. Seitzman. All rights reserved.Electric Propulsion - AcceleratorsElectrostatic,Static E field (alone) accelerates charged particles (ion engines, colloidal and Hall(?) thrusters)Lorentz,Magnetic and elec. fields accelerate ionized propellant(MPD, pulsed plasma thrusters)Pressure,Electrically heat propellant and use nozzle expansion(resistojets, arcjets)Accel. Force<10-4−10-6<10-4<10-32,000-10,000+1,000-10,000300-1,500Isp(s)ElectrostaticElectromagneticElectrothermaleFrBjvv×p∇WeightThrustAE6450 Rocket PropulsionElectric Propulsion-5Copyright © 2003-2006 by Jerry M. Seitzman. All rights reserved.PropulsionTechnologyOrbitInsertionOrbitMaintenanceandManeuveringAttitudeControlTypicalSteady StateIsp[sec]Thrust[N]Advantages DisadvantagesCOLD GAS√√60-250 0.1- 50- Simplicity- Safe- LowContamination- Low SpecificImpulseCHEMICAL√280-300√√140-240√√√305-460√√√313-322(a) Solid(b) LiquidMonopropellantBipropellantDual ModeHybrid√√250-3500.1 to12x106- High Thrust- Heritage- Moderateperformance- Combustioncomplications- Safety concernsNUCLEARTHERMAL√√750-6000 Up to12x106High SpecificImpulse- Unproven- Politicallyunattractive- Expensive- LowThrust/weightNON CHEMICAL√√300-1500√√1000-10,000Electro-Thermal ( Arcjets, Resistojet)Electro-Magnetic(Plasma)Electro-Static√√2000-100,0000.0001to20Very highspecific impulse- High systemmass- Low thrust levelLimited heritageComparison to Chem./Nucl. RocketsAfter Evans, Telesat Canada internal courseAE6450 Rocket PropulsionElectric Propulsion-6Copyright © 2003-2006 by Jerry M. Seitzman. All rights reserved.Potential Advantages: Ideal Rocket Equationfrom M. Walker• Higher ue⇒⇒⇒⇒ Lower propellant mass/Greater payload mass fraction– ideal case (neglects change in gravitational potential of propellant -“gravity penalty”)Typical velocity increment ∆Vrequirements (in km/s)euVinitialfinaleMM∆−=AE6450 Rocket PropulsionElectric Propulsion-7Copyright © 2003-2006 by Jerry M. Seitzman. All rights reserved.Gravity Losses• Can reduce advantage of EP if propellant is “carried”over large change in gravitational potential• Example: orbit escape– ∆Vlong duration spiral burn ≈ 2.3×∆Vimpulse burn– part of Ispincrease needed to meet higher ∆V• Not issue for station keeping (no change in gravitational potential) • Multiple (short) firings for large orbital changes can also require nearly same ∆V as Hohmann (impulsive) transfer– short thrust durations at same gravitational end-points as Hohmann transferAE6450 Rocket PropulsionElectric Propulsion-8Copyright © 2003-2006 by Jerry M. Seitzman. All rights reserved.Potential Commercial Advantages(spiral)from M. WalkerAE6450 Rocket PropulsionElectric Propulsion-9Copyright © 2003-2006 by Jerry M. Seitzman. All rights reserved.EP Power Requirements• Jet Power• Increase in Isp(or ue) entails increase in power– ∝ ue2for constant flowrate– ∝ uefor constant thrust• For comparison, chemical rocket with thrust of just 1000 lbf(4.5 kN) and Isp= 350s, Pj=7.7MW ⇒ Pj=66 MW for elec. rocket with Isp=3000s and same thrust• EP tends to be power-limited to low thrust (and low acceleration) levels221ejumP&=()eeuum&21=euτ21=()espgIτ21=AE6450 Rocket PropulsionElectric Propulsion-10Copyright © 2003-2006 by Jerry M. Seitzman. All rights reserved.Required Supply Power• Supply Power• Overall conversion efficiency has 3 main componentsTjsPPη=total efficiency of energy conversion thppSTηηηη=energy conversion - of raw energy source to electricitypower preparation and conditioning electronics (losses in electronics)thruster efficiency -only part of input elec. energy converted to KE• ~100% for photovoltaics –direct conversion of photons(does NOT include fractionof solar radiation that can beconverted to electricity by typical solar array, ~18-25%)• 10-40% for nuclear thermal,thermodynamic cycle limits = lots of waste heat• ~92% (electrostatic)• ~98% (steady arc jets)• 40-70%AE6450 Rocket PropulsionElectric Propulsion-11Copyright © 2003-2006 by Jerry M. Seitzman. All rights reserved.Mass Requirements• What limits available power to EP systems?– mass of power plant and associated systems• If power plant mass is significant fraction of propellant mass, then some advantages of higher specific impulse operation are lost– “payload” may consist mostly of power plant/electronics for propulsion systemAE6450 Rocket PropulsionElectric Propulsion-12Copyright © 2003-2006 by Jerry M. Seitzman. All rights reserved.Mass Requirements (con’t)• Power source masssssourcepowerPMassβ≅nearly linear relationshipspecific mass (e.g., kg/kW)Note: sometimes αused for specific mass, but also used by others for specific power (kW/kg), i.e., α= 1/β•βs~7-25 kg/kWelecfor solar arrays (depends on cell design, substrate)•βs~2-4 kg/kWthermalfor nuclear reactors (depends on shielding)– to reject waste heat, also require additional mass for radiatorsβR~0.1-0.4 kg/kWwaste heatAE6450 Rocket PropulsionElectric Propulsion-13Copyright © 2003-2006 by Jerry M. Seitzman. All rights reserved.Mass Requirements (con’t)• Power preparation and conditioning• Large variation with type of EP device (especially if need high voltage or power, and short pulse forming electronics/switches)–βpp~0.2 kg/kWelecfor typical arcjets–βpp~20 kg/kWelecfor PPT (pulsed plasma