electric propulsion for rockets - prof. seitzman...
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Electric Propulsion-1
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Electric Propulsion for
Rockets
Electric Propulsion-2
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved.
Electric Propulsion System Elements
Propellant Energy Source
Storage
Feed System
Storage
Conversion
Thruster
Gas (Xe), liquid (N2H4), solid (teflon)
chemical, nuclear, radiation (solar)
Thermodynamic (nozzle), electric, electromagnetic
to electricity at proper V, DC/AC( freq), current,
steady/pulsed
Power source separate
from propellant
Isp not limited by
chemical bond energy
2
Electric Propulsion-3
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved.
EP Power Requirements
• Jet Power
• Increase in Isp (or ue) entails increase in power
– ue2 for constant flowrate
– ue for 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)
2
21
ej umP
ee uum2
1 eu2
1 espgI2
1Minimum possible
power required
Electric Propulsion-4
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved.
Required Supply Power
• Supply Power
• Overall conversion efficiency has 3 main
components
T
j
s
PP
total efficiency of
energy conversion
thppST
energy conversion - of raw
energy source to electricity
power 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 fraction
of solar radiation that can be
converted 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%
3
Electric Propulsion-5
Copyright © 2003-2004, 2017 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 system
Electric Propulsion-6
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved.
Mass Requirements (con’t)
• Power source mass
sssourcepower PMass nearly linear
relationship
specific mass (e.g., kg/kW)
Note: sometimes used for specific mass, but also used by others for
specific power (kW/kg), i.e., = 1/
• s~7-25 kg/kWelec for solar arrays (depends on cell design,
substrate)
• s~2-4 kg/kWthermal for nuclear reactors (depends on shielding)
– to reject waste heat, also require additional mass for radiators
R~0.1-0.4 kg/kWwaste heat
4
Electric Propulsion-7
Copyright © 2003-2004, 2017 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)
– pp~0.2 kg/kWelec for typical arcjets
– pp~20 kg/kWelec for PPT (pulsed plasma
thrusters)
pppppp PMass
Electric Propulsion-8
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Electric Rocket Propulsion
A. Background
5
Electric Propulsion-9
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Electric Propulsion - Accelerators
Electrothermal Electromagnetic Electrostatic
Accel.
Force
Pressure,
Electrically heat
propellant and use
nozzle expansion
Lorentz,
Magnetic and elec.
fields accelerate
charged particles
Electrostatic,
Static electric field
alone accelerates
charged particles
Isp(s) 300-1,500 1,000-10,000 2,000-100,000+
<10-3 <10-4 <10-410-6
eF
mF
p
Weight
Thrust
Electric Propulsion-10
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Electrical Heating
• Current passing through
conductor heats it by
amount proportional to
its resistance
• Wire
• Gas (Plasma)
Discharge
– resistance due
to collisions
RJQ 2
Current
(A=C/s)Resistance
(Ohms)
J
Cathode Anode
e- i+J
VE V E
6
Electric Propulsion-11
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Forces on a Charged Particle
• To examine how to use electrical energy to
accelerate a propellant, consider acceleration
of a particle with mass m and charge q
pBuqEqdt
udm
Electrostatic
Force
Lorentz
ForceCollisional Force
(Momentum Transfer)
E
qe
F
+
Bq mF
+
u
u
B
mF
Elec. Field Mag. Field
(1)
Electric Propulsion-12
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Motion of Charged Particle in E&B Fields
• How does charged particle move in electric and magnetic fields
• Electric field only
– electron lighter, higher accel.
• Magnetic field only
– particle gyrates(centripedal accel.)
– radius of gyration
– frequency of gyration
– No work; B and F perpendicular
Em
q
dt
ud
Bum
q
dt
ud
E
+-
u
B
rg2qB
Bumr
g
m
qBg
7
Electric Propulsion-13
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
J
B
Induced Magnetic Fields
• In general, current flow induces a B field
– B field induced by
linear current
– B field induced
by coil
B
From Space Propulsion Analysis
and Design, Humble, Henry and
Larson, 1995
Electric Propulsion-14
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Motion of Charged Particle in E&B Fields
• Crossed E and B fields
– E field accelerates
particle upward
– B field causes
acceleration
perpendicular to u
– overall result is drift
velocity normal to E and B
E
B
du
heavy
light
2B
BEu
d
mrg mg 1
8
Electric Propulsion-15
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Plasmas
• Gas composed of equal “amount” of negatively
and positively charged particles
– electrically neutral
– negative particles usually e-
– positive particles are positive ions
– typically most of gas molecules remain
neutral
• Momentum equation for plasma
Bjpuut
u
Current Density
(A/m2) 321
,, uuuu
Electric Propulsion-16
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Induced E Fields – Hall Effect
• Electron acceleration
– lighter e- accelerate more quickly
and accommodate to flow field (most of j)
– collisional coupling (momentum) of electrons
and heavies (ions and neutrals) is weak
plasma
resistivity (m)
en
p
en
BjBujE
e
e
e
Hall Effect
accelerates heavy
particles in charge
neutral plasma
2envmecee
(2)
Induced
E field due
to plasma
motion
electron
pressure
termcollision freq.
electrons with heavies
9
Electric Propulsion-17
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Electric Rocket Propulsion
B. Examples
Electric Propulsion-18
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Electrothermal Rockets
• Resistojet (1W-10kW)– walls hotter than gas,
lower Tmax
– can combine withchemical heating, e.g.,hydrazine monopropellant
• Arcjet (1 kW- 10 MW)– high Tarc>Twall
(1-4104 K)– must mix with
rest of gas– very high To,
frozen flow losses in nozzleSpace Propulsion Analysis and Design,
Humble, Henry and Larson, 1995
Insulation
arc unstable
10
Electric Propulsion-19
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Resistojets
• Heating methods
– flow over coils of wire
– flow through hollow tubes
– flow over heated cylinder or plate
• Material limits temperature/performance
– conducting materials < 2700 Krhenium, refractory metals/alloys (tungsten, tantalum, molybdenum), platinum, cermets
– max Isp ~ 300 sec
• Power supply
– AC or DC
– Power TcmRVQp 2
Electric Propulsion-20
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Arcjets
• Gas heated by electrical discharge (arc)
– electrically conducting gas: plasma
• Joule heating
• Nozzle and electrode erosion
– high temperature, current arc
• Isp for H2 ~1200-1500 sec
TcmEjQp
enpjBujjee
2en
p
en
BjBujE
e
e
e
Hall Effect
normal to j
Resistive Heating
Work fromEM force
enpjBjujee
2Work from
pressure grad.
from (2)
no work
11
Electric Propulsion-21
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Arcjets for Space Missions• Application designs
– from station keeping of moderate-sized spacecraft (few kW, hydrazine)• 1st satellite station keeping application - December 1994 - Lockheed Martin Astro Space
Series 7000 Telstar 401 satellite (owned by AT&T)
– to piloted Mars mission (100 kW, hydrogen)
• Advantages
– high specific impulse, approximately 600 s (hydrazine), 800 s (ammonia, TRW/AFRL
ESEX project) to 2,000 s (hydrogen)• compared to that used hydrazine decomposition thrusters ( Isp ~220 s) and
resistojets (Isp ~300 s)
• Improves propellant efficiency and allows satellite to carry larger payload instead of the
additional propellant normally required for long duration missions
– similarity between hydrazine arcjets and hydrazine thrusters and resistojets already
integrated into spacecraft for station keeping
• Drawbacks
– excessive heating limited life and/or advanced materials
– higher frozen flow losses in nozzle (high T), equivalently low thrust efficiencies
(~35% of input power is converted to useful thrust)
– possible plume contamination
Electric Propulsion-22
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
1.8 kW Hydrazine Arcjet
Space Propulsion Analysis and Design,
Humble, Henry and Larson, 1995
• Telstar IV application– stationkeeping
• Hot gas from catalytically decomposed liquidN2H4
• Heated valveprevents N2H4
from freezing
• Isp~570 sec
• ~230 mN(~40mg/s)
12
Electric Propulsion-23
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Hydrazine Arcjet
1999 Test Firing of a Primex (now Aerojet)
MR-510 Hydrazine Arcjet
A shipset of Aerojet MR-510 Hydrazine
Arcjets and a Power Conditioning Unit
• Thrust 222-258 mN
• Input 4340 W (into Power Supply [PCU] for 2
Power arcjets); 2000 W input to each arcjet
from PCU)
• Isp > 570-600 s
. • 16 Lockheed Martin A2100
spacecraft (satellites) with MR-510
systems have been launched
• Each spacecraft has 4 thrusters and
1 PCU; 2 at a time used for N-S
station keeping orbit maneuvers
Electric Propulsion-24
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Arcjet with Applied Magnetic Field
• High current density in arctends to destroy electrodes
• Add soleniod B Field– adds azimuthal drift
velocity to arc– improves azimuthal
symmetry of gas temperature
– reduces local electrodeheating, reduces erosion
– needed for high powers(100 kW)
• Microwave discharge– alternate to arc heating
Space Propulsion Analysis and Design,
Humble, Henry and Larson, 1995
13
Electric Propulsion-25
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Electrostatic Thrusters• Ion engine example
• Near vacuum required
• Ion source
– usually electronbombardmentplasma
– also ion contact: propellant flowingthrough hot poroustungsten
– field emission: chargedspray droplets/particles
• Electrons added toneutralize exhaust
– thermionic emitters
Space Propulsion Analysis and Design,
Humble, Henry and Larson, 1995
L
A
Accelerator
Electric Propulsion-26
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Electrostatic Thruster Performance• Maximum exhaust velocity (Isp)
theoretically limited by voltage
difference across accelerator
• High thrust requires high mass
flowrates and therefore current
(and number, n) densities
• Ion current limited
by space-charge
– field from lots of ions
overcomes applied
E field
inletexite
VVm
qu 2
enquj
2
23
max
2
9
4
L
V
m
qj o
s
m
MW
V )volts(800,13
for singly ionized
meter
Farado
1210854.8 permittivity
of free space
Child-Langmuir
Law
accelerator electrode
spacing
22
23
8
maxm
)volts(1044.5
m
Amps
LMW
Vj
for singly ionized
14
Electric Propulsion-27
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Electrostatic Thruster Thrust
• Maximum thrust
• High requires high V and aspect ratio
– space chargeD/L ~1
– use many small ion beams toget more thrust
ee
uqmjAum
eu
q
mAj
maxmax V
m
q
q
mA
L
V
m
qo
22
9
42
23
22
max98 LVA
o
A=cross-sectional area flow
for circular cross-section
of diameter, D 22
max92 VLD
o
NewtonsVLD in1018.6 2212
DL
Electric Propulsion-28
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Electrostatic Thruster Thrust
• For fixed specific impulse
– best propellant for high thrust
• heavy molecules (particles)
• xenon (Xe) good choice (MW=131.3)
espuI
qmA
max
sp
accelo
Iq
mA
L
V
m
q2
23
max
2
9
4
15
Electric Propulsion-29
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Electrostatic Thruster Power
• Electrical energy converted to kinetic
energy with some efficiency
• Must also account for energy used to
create ions – ionization losses
– minimum loss given by ionization
potential for atom times current
VjAVIPower
2
2
1econv
umVI
qm
mIP
IIlossion
Electric Propulsion-30
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Electron Bombardment Xe+ Engine Example
• Operating conditions
– V=700 V, L=2.5 mm, 2200 holes (grids)
each with D=2.0 mm
– MW(Xe)=131.3, I(Xe)=12.08 eV
• Determine
– ,
– ue, Isp
– m
– minimum power required (including
ionization loss)
.
16
Electric Propulsion-31
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Solution
2212
max1018.6 VLD
gridN /1094.17005.2/21018.6 62212
mNgridNgridstotal
3.4/1094.12200 6
,max
smsmMWVue
3.131700800,13800,13
smue
860,31 sIsp
3248
sgume
41034.1
mmqumPPP Ielossionexhaust 2
min 2kg
kgMWm25
27
1019.21067.1
Cq 1910602.1
for singly ionized molec.
WW 18.19.67
WP 1.69min maximum effic. =67.9/69.1
= 98.3%
practically < 60-70%
flown
engines up to
~100-200mN
flown engines up to
few kW
Electric Propulsion-32
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Hall Thruster
• Hall effect most noticeable at low particle densities
• Stationary Plasma Thruster (SPT)
– developed in Russia
– 10’s kW
– axial current flowacross radial Bgenerates azimuthal electron flow
– induced (axial) Hall Eaccelerates ions
– also called “gridless” (neutral) ion thruster
– 1500-2000 s Isp with >50% efficiency using Xe and no oscillations like MPD Space Propulsion Analysis and Design,
Humble, Henry and Larson, 1995
17
Electric Propulsion-33
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Electromagnetic Propulsion Systems
• Use applied or induced magnetic fields to
produce acceleration of propellant
– high currents/
powers required
to produce
significant
induced fields
– high power available
only (normally) in
pulsed operationSpace Propulsion Analysis and Design,
Humble, Henry and Larson, 1995
Electric Propulsion-34
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Pulsed Plasma Thruster
• Propellant produced by vaporizing solid material with discharge
• B field induced by discharge also acts to accelerate vaporized propellant
• Advantage of simplicity
• Accelerationforce~ j2
(discharge current)
Space Propulsion Analysis and Design,
Humble, Henry and Larson, 1995
18
Electric Propulsion-35
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
Magnetoplasmadynamic (MPD) Thrusters
• Resemble arcjets
• Lower flow densities to attain higher exhaust velocity
• Diffuse discharge,low erosion
• Self field requires high J
• Applied B field
– allows higher Vat lower discharge currents
– increase accel.
– larger Hall effect
Self-Field MPD Thruster
Applied-Field MPD ThrusterSpace Propulsion Analysis and Design,
Humble, Henry and Larson, 1995
Electric Propulsion-36
Copyright © 2003-2004, 2017 by Jerry M. Seitzman. All rights reserved. AE4451 Propulsion
MPD Thrusters (con’t)
• Most efforts focused on applications with
exhaust velocities (Isp) greater than arc jets
• Typically require higher powers than currently
available on in-space vehicles
• Exhaust speed
– limited by erosion
and oscillations at high j
mjue
2
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