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Plasma and Nuclear Propulsion
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Thrust and Specific Impulse
• Thrust is defined as the force generated by an engine or rocket
• For rockets Fthrust = ce*dm; dm = fuel mass flow rate
• Specific Impulse measures the efficiency of a rocket engine (not a physical quanFty).
• It is effecFvely equal to the thrust divided by the amount of fuel used per unit Fme.
• It is measured by a quanFty called Isp = ce/g
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Types of Electric Propulsion 1. Electrothermal – uses electricity to heat a neutral gas examples: arcjet
2. Electrosta/c – uses a staFc electric field to accelerate
a plasma. StaFc magneFc field are someFmes used to help confine the plasma, but they are not used for acceleraFon. examples: gridded ion thruster
3. Electromagne/c – uses electric and magneFc fields to accelerate a plasma. examples: hall thruster, pulsed plasma thruster
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Electrothermal: Arcjet How they work: 1. Neutral gas flows through the
propellant flow. 2. An electrical arc forms between the
anode and cathode. 3. A small amount of the neutral gas is
ionized to form the arc. 4. The remaining gas is heated as it passes
through the arc. Propellant: Hydrazine Ammonia
Exhaust speed: 4-‐10 km/s
Thrust range: 200-‐1000 mN*
Power required: 400 W – 3 kW
Efficiency: 30-‐50%
* 1 mN is about the weight of a sheet of paper. 4
ElectrostaFc: Gridded Ion Thruster
ElectrostaFc: Gridded Ion Thruster Vital Stats:
Propellant: Argon, Krypton, Xenon
Exhaust speed: 15-‐50 km/s
Thrust range: 0.01-‐200 mN*
Power required:
1-‐10 kW
Efficiency: 60-‐80%
Advantages: 1. High exhaust speed 2. High efficiency 3. Inert propellant
Disadvantages: 1. Complex power processing 2. Low thrust 3. Grid and cathode lifeFme
issues 4. High voltages 5. Thrust density is limited
Uses: 1. StaFon keeping 2. Orbital change
LEO to GEO 3. Primary propulsion
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* 1 mN is about the weight of a sheet of paper.
ElectrostaFc: Gridded Ion Thruster Gridded Ion Thrusters have been flown as the primary propulsion of several satellites:
Deep Space 1 (NASA; Braille, Borrelly) Dawn (NASA; Ceres & Vesta) Hayabusa (JAXA; sample from Itokawa)
Deep Space 1’s NSTAR Thruster: 1. Exhaust speed 35 km/s 2. Used 74 kg of Xenon fuel 3. Low thrust (92 mN) over a long Fme (678 days) 4. Δv due to thruster (4.3 km/s)
DAWN’s Ion Engine: 1. Exhaust speed 31 km/s 2. Low thrust (90 mN) over a long Fme (longer
than DS1) 3. Larger Δv than DS1
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ElectromagneFc: Pulsed Plasma Thruster (PPT)
How they work: 1. Arc ablates material off the Teflon
surface. a. Material is ionzied b. Current flows through the arc.
2. Current generates a magneFc field. 3. MagneFc field and current interact to
accelerate the plasma. Propellant: Solid Teflon
Exhaust speed: 6 -‐ 20 km/s
Thrust range: 0.05 -‐ 10 mN*
Power required: 5 -‐500 W
Efficiency: 10%
* 1 mN is about the weight of a sheet of paper. 8
ElectromagneFc: Pulsed Plasma Thruster (PPT)
Advantages: 1. Simple design 2. Low power 3. Solid fuel
a. No propellant tanks/plumbing b. No zero-‐g effects on propellant
Disadvantages: 1. Low thrust 2. Low efficiency 3. Toxic products
Uses (flown in space): StaFon keeping Precision poinFng
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ElectromagneFc: Hall Thruster How they work: 1. Cathode releases electrons which
ionize propellant. 2. Electrons from ionizaFon move in a
circular papern (create current). 3. Current interacts with radial magneFc
field to produce ion acceleraFon. 4. Cathode electrons neutralize the
beam.
Propellant: Xenon or Argon
Exhaust speed: 15 -‐ 20 km/s
Thrust range: 0.01 -‐ 2000 mN*
Power required: 1 W -‐ 200 kW
Efficiency: 30-‐50%
* 1 mN is about the weight of a sheet of paper. 10
ElectromagneFc: Hall Thruster Advantages: 1. High exhaust velocity 2. Simple power supply 3. Inert propellant 4. High efficiency 5. Desirable exhaust velocity
Disadvantages: 1. High beam divergence 2. LifeFme issues (erosion)
Uses (flown in space): StaFon keeping Orbital transfer (LEO to GEO) Primary Propulsion (SMART-‐1)
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Variable Specific Impulse Magnetoplasma Rocket (VASIMR)
How it works (VX-‐200): 1. Helicon ionizes neutral gas (30 kW). 2. Plasma flows along field lines and is compressed. 3. Ion Cyclotron Resonance HeaFng (ICRH) is used to
heat the ions (170 kW). 4. MagneFc nozzle converts temperature into directed
flow. 5. Plasma detaches from the magneFc field.
VASIMR
Advantages: 1. Variable exhaust speed 2. High exhaust speed 3. Variable thrust 4. High thruster 5. No grids or anode/cathode 6. Variety of fuels (H, Ar, Ne)
Disadvantages: 1. SuperconducFng magnets
required 2. PotenFal detachment issues 3. PotenFal energy conversion
issues 4. Requires nuclear reactor
EP Summary Types of EP: Electrothermal: resistojet, arcjet Electrosta/c: gridded ion thruster Electromagne/c: Hall thruster, PPT, MPD thruster, VASIMR
Advantages: High exhaust velocity High propellant efficiency High spacecraq speeds
Disadvantages: Power intensive Very low thrust (in space only) AcceleraFon takes Fme PotenFal lifeFme issues
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Nuclear Propulsion
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Radioisotope Thermal Generator (RTG):
RTGs have a good service history, but are still controversial.
How they work: • RadioacFve decay (oqen 238Pu) • Heat generated in decay • Thermocouples convert heat to
electricity
AddiFonal informaFon: • 10s-‐100s of Waps • 3-‐7% efficient • Well suited to deep space roboFc
missions • US has Flown 45 RTGs in 25 missions
• Voyager 1& 2 • Cassini (870 W -‐ shown leq) • Galileo (570 W) • Viking 1 & 2 • Pioneer • Ulysses
RadioacFve Heater Units: • 1 Wap of heat power • Used to keep spacecraq warm • US has flown more than 240
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Nuclear Propulsion Now we’re really gerng into the border of science ficFon.
However, real research is being done or has been done to seriously invesFgate several nuclear propulsion concepts.
Types of nuclear propulsion: 1. Nuclear pulse propulsion – uses nuclear explosions to
propel a spacecraq
2. Nuclear thermal propulsion – uses the heat of a nuclear reactor to heat a gas which is expelled for thrust
3. Nuclear electric propulsion – uses electrical power from a nuclear reactor to power an electric thruster
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Nuclear Pulse Propulsion Also called external pulsed plasma propulsion.
Uses nuclear explosions to generate thrust.
Programs: 1. Project Orion (1958 – 1963) 2. Project Daedalus (1973 – 1978) 3. Project Longshot (1987-‐1988)
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Project Orion Study by General Atomics led by Ted Taylor and Freeman Dyson
Goal: High thrust with high exhaust speeds
How it works: 1. Drop nuclear bomb out the back of the spacecraq 2. Nuclear bomb detonates about 60 m behind the spacecraq 3. Explosion hits a steel plate, which propels the spacecraq forward.
Note: shock absorber is required for human payload due to the high g involved.
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Project Orion Performance: EsFmated thrust > 1 mega-‐newton EsFmated exhaust velocity:
20 – 30,000 km/s EsFmated spacecraq speed:
0.03c – 0.1c (c = speed of light)
PotenFal Missions: Fast travel through solar system with massive payloads
Single stage to Mars Saturn’s moons Jupiter’s moons
Asteroid deflecFon Interstellar travel
PotenFal Problems: Plate ablaFon/damage Nuclear fallout on Earth High acceleraFon rate Crew shielding
Project Orion was terminated by the ParFal Test Ban Treaty of 1963.
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Orion
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Commercializing Human Space Flight
New Commercial Space
• NASA COTS/CRS – Orbital Sciences – SpaceX
• NASA CCDev Partners – Blue Origin – Boeing – Paragon – Sierra Nevada – United Launch Alliance
• Space Tourism – Bigelow Aerospace – Space Adventures – Virgin GalacFc – XCor
Falcon 1/1e: • 2 stages: LOX-‐Kerosene • 670 kg (1010 kg) to LEO • Achieved orbit: Sept., 28, 2008
• 2/5 successes • $10.9 M
Falcon 9: • 2 stages: LOX-‐Kerosene • 10,450 kg to LEO • 4,540 kg to GTO • Dragon Capability • Maiden Flight: June 4, 2010
Placed test payload in orbit • Cost: $45.8 – $55.1 M • Flight 2: Tuesday, Dec 7, 2010
– First Dragon test flight – First private company to return a capsule from orbit.
• Next launch with docking to ISS soon (5/19?)