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Chapter 17: Electric Propulsion

Three Types of Electric Propulsion (EP):

• Electrothermal

– Propellant is heated electrically, causing it to expand

thermodynamically and accelerated through nozzle

– Examples: resistojet, arcjet

• Electrostatic

– Propellant consists of charged particles, which are accelerated

using an electric field

–Example: ion, field emission, electron bombardment

• Electromagnetic

– Propellant is a highly ionized plasma, which is accelerated

using a magnetic field

–Example: Hall, pulsed plasma, magnetoplasmadynamic

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Comparison to Chemical Rockets

General Characteristics

of EP Thrusters:

• Low thrust (< 2N)

• High Isp

• Low SPC (1/Isp)

• Long burn time

Applications:

• Stationkeeping

• Interplanetary

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EP Required Power vs. Specific Impulse

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Typical Operating Parameters for Thrusters

with Flight Heritage (Goebel)

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Electrothermal: Resistojet

• A current is passed through a high resistance metal element

– P = IV and V = IR P = I2R

• Propellant is heated by convection/contact with heated metal parts

• Propellants: H2, O2, N2, CO2, NH3, CH4 , N2H4

– Highest Isp: H2

– Advantages of Hydrazine (N2H4)

· Decomposes to NH3 and H2 which preheats propellant

· Thrust = 0.01N to 0.4N, Isp = 200 sec to 350 sec

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DeTurris - Aero 540From D. Goebel

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Electrothermal: Arcjet

• Heats propellant using electrical arc instead of metal

components (Tarc = 10,000K to 20,000K)

• Issues:

– Arc instability

– Electrode deterioration

• Propellants: H2, O2, N2, CO2, NH3, CH4

– Hydrogen or Hydrazine (N2H4) Commonly Used

· Isp = 400 sec to 1000 sec

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Arcjet Schematic

Arc

Diffuse

anode

discharge

Inner flowOuter flow

Hydrogen

flow

Hydrogen

flow

Cathode

Anode

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Electrostatic Propulsion

• Types (same acceleration, different ionization schemes):

– Electron Bombardment Thrusters

· Positive ions are produced by bombarding propellant particle

with electrons

– Ion Thrusters

· Positive ions are produced by forcing propellant vapor

through an ionizer

– Field Emission or Colloid Thrusters (Electrospray)

· Liquid metal propellant droplets are negatively or positively

charged by passing them through an intense electric field in a

small tube (wicking)

Principal: Accelerate charged propellant particles with an electric field

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Ion Thrusters

• Idea first came up in early 1900’s, started to fly in 1960’s

– Propellant was originally mercury, which is toxic or

cesium which is corrosive and reactive

– Now ion thrusters use xenon which is high

performance, but expensive

– For testing, argon is low performance, but inexpensive

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DS1 XIPS

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Ion Thruster Schematic

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Description of Ion Thruster

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XIPS Operations

• First used on Deep Space One as a primary propulsion

system

• PAS-5 satellite, launched in 1997, may have been first

commercial XIPS

• Hughes satellite first to make use of XIPS on earth orbiting

satellites

• European Space Agency currently using XIPS for

SMART-1 moon mission

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Electrospray Thrusters - A type of thruster that accelerates

charged particles produced from electrified liquid surfaces

through an electrostatic field. Two examples are:

• Colloid (have already flown)

- Accelerates charged droplets or ions and use

solvents such as doped glycerol as propellant

• Field Emission Electric Propulsion

- Utilizes liquid metals as propellant

Ref: “MIT Open CourseWare Session 20: Electrospray Propulsion”

Electrostatic Alternatives to Ion Thrusters

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Electrospray Thruster Schematic

• Apply voltage on emitters to produce a taylor cone

• Extract ions

• Accelerate electrostatically

Ref: Ziemer, J. K., et al., “Colloid Micro-Newton Thrusters for the Space Technology 7 Mission,” IEEE Paper 978-1-4244-

3888- 4/10, 2010.

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Plasma Density Measurement

• Use Langmuir Probe to measure electron temperature

• Probe voltage swept across the plasma voltage range

• Probe currents then translated into electron density

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Ion Thruster Performance Optimization

• Uneven beam density causes localized hot spots, creating

thermal fatigue and premature failure of the accelerator grids

• Use changes to the geometry to even out the plasma

distribution at the thruster exit by:

– Adding a magnet ring

– Switching magnet orientations and/or location

– Changing propellant flow rate

– Moving cathode forward or backward

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Electrostatic Propulsion Performance

Electrical power input Acceleration of ions

• Electrical power is used to produce charged particles and to accelerate the ions

• Since positively charged particles are coming out of the thruster, Gauss’s Law says to balance the charge so the net total charge is zero. The added electrons do not contribute to thrust.

High velocity

beam of

charged particles

e i totalQ Q Q Eds

Q=electric charge

E = electric field

ds = surface

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Ion Thrust Equation

where I=current, M=mass, e=charge=1.6x10-19

from conservation of energy

V=voltage through which ions are accelerated

2beam

eVv v

M

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Ion Specific Impulse

cosexsp m

vI

g

for chemical rockets

for electric propulsion

mwhere is the mass utilization efficiency

or2 1

cossp m

eI V

M g

with correction

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• Xenon, I=4 amps, V=1500 V, Power=6kW, F=0.25N

• Low thrust, low thrust requirement

– Small forces at GEO required to remain stationary

• Xenon is used as propellant because its mass is 131.3

times that of a proton and produces much more thrust

• Thrust has 2 corrections:

– Thrust vector is off axis – the particles are not

coming out on axis, off by

– Double ionization – some particles are double charged. However this correction, , is less than 5% of the thrust,

so that F

cos

Xenon Example

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Magnet Materials

MaterialResidual Induction

(Gauss)

Maximum

Operating

Temperature (oF)

Comments

Ceramic 2300-3850 400Cheap kitchen

magnets

Samarium Cobalt 8700-10500 482-572Expensive Rare

Earth Magnets

Neodymium 10800-12300 176-212Relatively Cheap and

available

ALNICO (Al-Ni-Co) 7400-12800 975-1020

Expensive. Cast or

sintered into various

shapes easily

demagnetized

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Electromagnetic Propulsion

BIF

I

B

F

ElectrodesArc

• An arc forms between electrodes

– Arc = plasma of ablated electrode material and/or

propellant vapor

• F acts on the arc perpendicular to the current flowing

through the arc, I, and the magnetic field, B

• The arc is accelerated out of the thruster

Maxwell’s Equation:

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Hall Effect

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Hall Thruster

Used extensively in

Russia starting in 1971

Japan started use in

satellites in 1995

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Hall Thruster Beam Neutralization

• Electrons in beam both

ionize and neutralize

• Acceleration is both

electrostatic and

electromagnetic

Ref: Goebel and Katz

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Magnetoplasmadynamic (MPD) Thruster

• High current arc ionizes propellant

• Lorentz force accelerates charged particles

• High input power produces high thrust

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Electric Propulsion Performance

resistojet 300 0.5-1 N2H4

arcjet 500-600 0.9-2.2 N2H4

ion 2500-3600 0.4-4.3 Xenon

Hall 1500-2000 1.5-4.5 Xenon

PPT 850-1200 <0.2 Teflon

Thruster Isp (sec) Input Power (kW) Propellant

Ref: Goebel and Katz

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EP Power Supplies

• Batteries (low power, short life)

– Primary – consume active materials (convert chemical to electrical

energy)

– Secondary – reversible reaction (rechargeable)

• Fuel Cells (low power, short life)

– Direct conversion of chemical energy to electrical energy

• Solar Cells (low to medium power, long life)

– Photovoltaic energy conversion

• Nuclear reactors and thermoelectric isotope generators

(medium to high power, long life)

– Nuclear reactor driving mechanical system

– Nuclear heat is converted to electricity directly

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Sutton EP Performance Comparisons

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