The advanced electric propulsion program is directed towards lowering the specific impulse and increasing the thrust per unit of ion thruster systems. In addition, electrothermal and electromagnetic propulsion technologies are being developed to attempt to fill the gap between the conventional ion thruster and chemical rocket systems. Most of these new concepts are exagenous and are represented by rail accelerators, ablative Teflon thrusters, MPD arcs, Free Radicals, etc. Endogenous systems such as metallic hydrogen offer great promise and are also being pursued

Finke, R. C.

English

NASA Technical Reports Server

ELECTRICPROPULSIONTECHNOLOGY
RobertC. Finke
NASALewisResearchCenter
Propulsion systems can be classified into two basic categories:
I. Endogenous; which use energy stored within the propellants to create thrust.
Solid rockets, liquid rockets, cold gas systems, etc. are all well known
examples of endogenous systems.
II. Exogenous; in which the energy is supplied to the propellant from an outside
power source. Al electric propulsion systems are exogenous although some like
electrically augmented hydrazine are a combination of the two.
WHATISELECTRICPROPULSION?
ELECTRICPROPULSIONISA PROCESSINWHICHELECTRICAL
ENERGYIS USEDTOACCELERATEA PROPELLANTTO HIGH
VELOCITYCREATINGTHRUST.
r.onditlml_
Ul_st_
Pro_el_
23
https://ntrs.nasa.gov/search.jsp?R=19800022946 2020-03-21T17:35:46+00:00Z
The most significant advantage of an exogenous system is thatif external
energy is available for accelerating a propellant, the resulting specific impulse
and total impulse can be greatly in excess of that that can be stored in an endogenous
device. Thus an ion thruster system with an Is_ of 3000 sec would require 2000 kg
of propellant as compared to 15,000 kg of propeYlant for a Centaur with equivalent
tota] impulse. The dry weights of the two systems are also similar, resulting in
a significant advantage for the ion thruster system.
Electric propulsion devices are inherently low thrust devices. A cluster of
ten 30-cm thruster systems provides a 0.3 pound thrust to the system for up to
15,000 hours of operation. The low level continuous thrusting characteristic of
Electric Propulsion allows very fragile large space structures to be transported by
these class of propulsion systems, assembled, from LEO to GE0.
In addition, since propellant is a very small fraction of overall system mass,
weight growth of the payload during the construction phase of the project can be
accommodated by thrusting for a longer period of time; increased mass then merely
requires longer trip times.
CHARACTERISTICS OF ELECTRIC PROPULSION
e HIGH SPECIFIC IMPULSE
LARGE TOTAL IMPULSE FOR LOW MASS
MINIMUM PROPELLANT REQUIREMENTS
e LOW THRUST
LOW "G" LOADING ON SPACECRAFT STRUCTURES
PRECISION POINTING CAPABILITY PROVIDED
e HIGH POWER REQUIRED
EXCELLENT MATCH WITH HIGH POWER PAYLOADS
e ORBIT TRANSFER TIME/PAYLOAD TRADE AVAILABLE
e COMPATIBLE WITH LONG TERM SPACE STORAGE/OPERATIONS
24¸
There are three generic classes of electric propulsion devices, all of which are capable of high Impulse.
The electrostatic devices, in particular are capable of a wide range of specific Impulses.
• ELECTROTHERMAL
In the electrothermalrocket electric power Is used to heat the propellantto a hlgh temperature. The heating
may be accomplishedby producingan electric dischargethrough the propellantgas (arcjet) or by flowing the
prope]lantgas over surfaces heated with electricity(resistojet).
The electrothermalrocket is similarIn some respects to the chemical rocket. Although there is no combustion,
the propellantgas is heated to high temperaturesand expanded through a nozzle to produce thrust. This rocket
can achieve propel]antexhaust velocitieshigher than those of chemical rockets because the energy added to the
gas moleculesmay be larger than the energy availablefrom combustion. Material failure at high temperature,
however, places a practicalupper limit on the amount of energy that can be added to the prope]lant. Other
factors, such as breakup, or dissocfation,of the propeITantgas molecules,which absorbs energy without raising
gastemperaturemuch, also limit the exhaust velocity.
e ELECTROMAGNETIC
The second general type of engine is Me electromagneticthruster,often called the plasma thruster. In this
thruster,the propellantgas Is ionized to form a plasma, _InlchIs then acceleratedrearward by electrlc and
magnetic fields.
In a plasma, the electronsand the ions are swirling about In a random manner much 11ke atoms In a gas. The
plasma can conduct electric current just as a copper wire can conduct current. It Is this conductivitythat
makes possible acceleratingthe plasma electrlcallyand magnetlcally. When an electric current Is made to pass
through a plasma In the presence of a magnetic field, a force is exerted on the plasma. Because of this force,
the plasma is acceleratedrearwa_. Thus, a plasma thruster Is quite similar to an electric motor with the
plasma replacingthe moving rotor.
e ELECTROSTATIC
The third type of electric rocket engine Is the electrostaticthruster. (Best known of thi_ type Is the Ion
thruster or ion engine.) As In Me plasma thruster,propellantatoms are ionized by removing an electron
from each atom. In the electrostaticthruster,hov_ever,the electrons are removed form the ionizationregion
at the same rate as Ions are accelerated rearward.
The most successfulelectrostatlcthruster presently availableis an electron-bon_)ardmentthruster conceived
and developedat the NASA-Lewls ResearchCenter. This thruster operates as follows. When heated, the pro-
pellant evaporatesand forms a vapor,which is fed into the thruster dischargecha_er. In the chant)er,
electrons are knocked out of many of t_hepropellantatoms to form ions. Thls ionizationIs accomplishedIn a
gentle electric dischargewherein electrons in the dischargehlt _lectrons in the atom and displace them from
the structureof the atom. The electrons and the Ions form a plasma In the lonlzatlonchaunber.The
electric field between the screen and the acceleratordraws Ions from the plasma. These Ions are then
acceleratedout through many small holes in the screen and acceleratorelectrode.
WHY- ELECTRICPROPULSION?
• CHEMICALENERGYISLIMITEDTOSPECIFICIMPULSESSO0SEC.
• ELECTRICPROPULSIONISCAPABLEOFA BROADRANGEOFSPECIFICIMPULSE,
• ELECTROTHERMAL • ELECTROMAGNETIC (I ELECTROSTATIC
.xSO-/200 SEC 200- 2000SEC 1500- 100,000SEC
25
ELECTROSTATIC
Applications of electric propulsion are many and varied. Electrostatic thrusters
with their capability for a broad range of specific impulse and ability to scale
and throttle over a wide thrust range, are suitable for primary propulsion appli-
cations for planetary and earth orbital missions and as auxiliary propulsion
devices for attitude control and stationkeeping of geostationary spacecraft.
Operation with a wide variety of propellants has been demonstrated from the heavy
metals such as mercury or cesium to gases such as argon, xenon, neon and nitrogen.
With an electrostatic thruster system, it is possible to tailor the thruster systems
very closely to the application.
ELECTROMAGNETIC
Electromagnetic thruster systems offer the promise of reduced complexity of power
systems and high thrust density. In general they are plasma devices and are thus
self-neutralizingeliminating the need for a neutralizer system.
One sub-class of electromagnetic thruster can accelerate solid project files. This
class represented by the rail gun and mass driver may make possible the direct
launch of payloads from earth to space, or the augmentation of booster capabilities
via an electric catapult device.
ELECTROTHERMAL
Electrothermal thrusters most resemble the classical chemical rocket. Many such
as electrically augmented catalytic hydrazine are techniques to increase the Iso
from chemical reaction by the addition of electric power. Others, such as the r
free radical propulsion concept represent a way to use electrical energy to dis-
sociate H2 and utilize the high temperatures of recombination to obtain high Isp
at high thrusts.
ELECTROSTATIC
BASELINEHG e e e
ADVANCEDHG e e e
INERTGAS e e
ELECTROMAGNETIC
M P D , ,
MASSDRIVER • e e
RAIL • • •
ELECTROTHERMAL
FREERADICAL e •
RESISTOJET • •
26
LSS PROPULSION REQUIREMENTS
Scenarios presently being considered for Large Space Structures (LSS) will require
technology advancements to enhance the capabilitiesof existing propulsion systems,
both for o_it raising and for on orbit applications. Almost all studies of LSS
have indicated that for balancing out solar pressure, configuration control and
maintaining required pointing accuracy will require propulsion systems with a
specific impulse well beyond that obtainable from chemical systems.
In addition, the cost of transporting heavy, high volume chemical propellant systems
from ear_ to orbit will become prohibititve as system requirements increase.
In an attempt to minimize mass to orbit, LSS will be designed to be relatively
fragile structurally. Large impulsive loads could literally destroy the LSS. In
this respect, electric propulsion systems are well matched to LSS since accelera-
tions produced by propose_ and existing electric propulsion systems suitable for
LSS are all less than lO"_ g's.
LSS PROPULSIONREQUIREMENTS
e TOTALIMPULSEANDMISSIONLIFEREQUIREMENTSWILLEXCEEDPRESENTCAPABILITIES
e LIFE/CYCLECOSTSA MAJORFACTOR
- MINIMIZETOTALSYSTEMASSREQUIREDINSPACE
- MINIMIZEPROPULSIONSYSTEMVOLUME/LENGTH/MASS/COST
- MAXIMIZEINHERITANCEANDUTILITYOF SYSTEMCONCEPTS(s)
e MANY LSS ORBITTRANSFERANDON-ORBITAPPLICATIONSREQUIRELOWACCELERATION
e PROPELLANTAVAILABILITYANDECOLOGICALCONCERNS,
27
PROPULSION CONCEPTS
The advanced chemical propulsion program is structured towards the development
of technology for high IsD, low thrus_ long life thruster systems suitable
for taking payloads from LEO to GEO orbit. The Advanced Electric Propulsion
program is directed towards lowering the specific impulse and increasing the
thrust per unit of ion thruster systems. In addition, electrothermal and
electromagnetic propulsion technologies are being developed to attempt to
fill the gap between the conventional ion thruster and chemical rocket systems.
Most of these new concepts are exagenous and are represented by rail accelerators,
ablative teflon thrusters, MPD arcs, Free Radicals, etc. Endogenous systems
such as metalic hydrogen offer great promise and are also being pursued.
PROPULSIONCONCEPTS
3000-__
EP
2000-
S
I000-JADV_
Specific600Impulse,
Sec. 400- FADV.___
200'
I00 a I I I '
O.I I.0 I0 I00 !000
THRUST,Lbs.
28
ELECTRIC PROPULSION TECHNOLOGY
506 - 55 - 22
FY
I '79 80 81 82 83 84
SUBPROGRAM
8-Cm HG
EXTENDED PERFORMANCE Heat Pipe 6 KW HG. Thruster Ex. Perf.
Magnetics Thruster Limits Life Verif.
V V V V
Simplified 20m Lb/KW
30-Cm PPU 30-Cm 15 KG/KW
Reqs Thruster Technology
V V V
Prelim Simplified Inert Inert
Scalable 6 KW Hg/Inert Thruster Thruster
ADVANCED CONCEPTS Inert Beam PPU Perf. Life
Thruster Supt_ly B.B. Doc Verif.
V V _ V V
RF 2-10 MLB Free Radical
Cathode Xenon Prelim
Eval Thruster Perf. Eval
V V V
Direct
Drive
Tech.
V
Point Environ.
Prelim. Reduced Design Interaction LSS
ANALYSES E.P. Sys. Aux. Prop. Electric Analyses Propulsion
Def" Def. E.O. Sat Complete Req. Def
V V V V V
• ELECTRIC PROPULSION - RESEARCH AND
ADVANCED CONCEPTS
J - JPL Js - JSC
L - LEWIS M - MSFC
_o__.0_.,-0s_ I I 1 L h 'SPACE SYSTEMS DIVISION FY 78 FY 79 FY 80 FY 81 FY 82 1 FY 83 [
Complete Thrust Complete
Stand and Preliminary Concept
o MPD ACCELERATOR Efficiency Measurement Evaluation
V _
lO-m Complete
Prelim. 2-m Performance Concept
o MASS DRIVER Design Demo Evaluation Evaluation
O
Complete Eval Complete Complete Theoretical &
MESC-Thruster Parametric Experimental
with Inert Study MPD Study of
o BASIC THRUSTER PHYSICS Gas _ Thruster Sputtering
Complete Large _,_7_C_ete Eval._iL_complete Feas. Complete
10N Source Textured Dental Study of Prel. Study Tex-
Development Implants Diamond Fi Ims tured
_ Surgical
o ION BEAM APPLICATIONS _7 _ _ x_7 Implants
Complete Eval. Complete Rectang.
Textured Large Indust. Ion Source
Crystal Align-
ment Surfaces f_7,,Ly/7
Complete Prelim.
Study of an Adv.
Propulsion Concept


Electric propulsion technology

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