chapter 17: electric propulsion · electrothermal: resistojet •a current is passed through a high...
TRANSCRIPT
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DeTurris - Aero 540
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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