Plasma and Nuclear Propulsion

1 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 quanty). • It is effecvely equal to the thrust divided by the amount of fuel used per unit me.

• It is measured by a quanty called Isp = ce/g

2 Types of Electric Propulsion 1. Electrothermal – uses electricity to heat a neutral gas examples: arcjet

2. Electrostac – uses a stac electric field to accelerate a plasma. Stac magnec field are somemes used to help confine the plasma, but they are not used for acceleraon. examples: gridded ion thruster

3. Electromagnec – uses electric and magnec fields to accelerate a plasma. examples: hall thruster, pulsed plasma thruster

3 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 Electrostac: Gridded Ion Thruster Electrostac: Gridded Ion Thruster Vital Stats: Propellant: Argon, Krypton, Xenon Exhaust speed: 15-50 km/s Thrust range: 0.01-200 mN* Power 1-10 kW required: Efficiency: 60-80%

* 1 mN is about the weight of a sheet of paper. Advantages: Disadvantages: Uses: 1. High exhaust speed 1. Complex power processing 1. Staon keeping 2. High efficiency 2. Low thrust 2. Orbital change 3. Inert propellant 3. Grid and cathode lifeme LEO to GEO issues 3. Primary propulsion 4. High voltages 5. Thrust density is limited 6 Electrostac: 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 me (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 me (longer than DS1) 3. Larger Δv than DS1

7 Electromagnec: 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 magnec field. 3. Magnec 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 Electromagnec: 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): Staon keeping Precision poinng 9 Electromagnec: Hall Thruster How they work: 1. Cathode releases electrons which ionize propellant. 2. Electrons from ionizaon move in a circular paern (create current). 3. Current interacts with radial magnec field to produce ion acceleraon. 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 Electromagnec: 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. Lifeme issues (erosion)

Uses (flown in space): Staon keeping Orbital transfer (LEO to GEO) Primary Propulsion (SMART-1) 11 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 Heang (ICRH) is used to heat the ions (170 kW). 4. Magnec nozzle converts temperature into directed flow. 5. Plasma detaches from the magnec 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. Superconducng magnets required 2. Potenal detachment issues 3. Potenal energy conversion issues 4. Requires nuclear reactor EP Summary Types of EP: Electrothermal: resistojet, arcjet Electrostac: gridded ion thruster Electromagnec: Hall thruster, PPT, MPD thruster, VASIMR

Advantages: High exhaust velocity High propellant efficiency High spacecra speeds

Disadvantages: Power intensive Very low thrust (in space only) Acceleraon takes me Potenal lifeme issues

14 Nuclear Propulsion

15 Radioisotope Thermal Generator (RTG):

How they work: Addional informaon: • Radioacve decay (oen 238Pu) • 10s-100s of Was • Heat generated in decay • 3-7% efficient • Thermocouples convert heat to • Well suited to deep space roboc electricity missions • US has Flown 45 RTGs in 25 missions • Voyager 1& 2 • Cassini (870 W - shown le) • Galileo (570 W) • Viking 1 & 2 • Pioneer • Ulysses

Radioacve Heater Units: • 1 Wa of heat power • Used to keep spacecra warm • US has flown more than 240

RTGs have a good service history, but are still controversial. 16 Nuclear Propulsion

Now we’re really geng into the border of science ficon. However, real research is being done or has been done to seriously invesgate several nuclear propulsion concepts.

Types of nuclear propulsion: 1. Nuclear pulse propulsion – uses nuclear explosions to propel a spacecra

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 17 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)

18 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 spacecra 2. Nuclear bomb detonates about 60 m behind the spacecra 3. Explosion hits a steel plate, which propels the spacecra forward.

Note: shock absorber is required for human payload due to the high g involved.

19 Project Orion Performance: Esmated thrust > 1 mega-newton Esmated exhaust velocity: 20 – 30,000 km/s Esmated spacecra speed: 0.03c – 0.1c (c = speed of light) Potenal Missions: Fast travel through solar system with massive payloads Single stage to Mars Saturn’s moons Jupiter’s moons Potenal Problems: Asteroid deflecon Plate ablaon/damage Interstellar travel Nuclear fallout on Earth High acceleraon rate Project Orion was terminated by the Crew shielding Paral Test Ban Treaty of 1963. 20 Orion

21 Commercializing Human Space Flight New Commercial Space

• NASA COTS/CRS • Space Tourism – Orbital Sciences – Bigelow Aerospace – SpaceX – Space Adventures • NASA CCDev Partners – Virgin Galacc – Blue Origin – XCor – Boeing – Paragon – Sierra Nevada – United Launch Alliance 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?)