The topics covered are presented in viewgraph form and include the following: engine lifetime assessment; lithium magnetoplasmadynamic (MPD) thruster development; and radiation-cooled, applied-field engine testing

Goodfellow, Keith

Pivirotto, Tom

Polk, Jay

English

NASA Technical Reports Server

N93-1 63 
THE MPD THRUSTER PROGRAM AT JPL
Keith Goodfellow, Tom Pivirotto, and Jay Polk
Jet Propulsion Laboratory
Pasadena, California
MPD THRUSTER ACTIVITIES AT JPL
Engine Lifetime Assessment
Methodology for Determining Life
- Electrode Modelling
- Experimental Program
Lithium MPD Thruster Development
Technology Review and Modelling
- Mission Analysis (APC Group)
- Technology Development
Radiation-cooled, Applied-field Engine Testing
- Anode Thermal Management
- Pumping Speed Improvements with a
Gasdynamic Diffuser
Dual-beam Thrust Measurements
61
https://ntrs.nasa.gov/search.jsp?R=19930009449 2020-03-17T07:55:52+00:00Z
DEFINING ENGINE LIFETIME
.q
.J
Required
Service Life
FaJlur¢/Succcss
Data
O0
O0
O0
Engin_rlng
Analysis
l..i_
Engine lifetime, requirements and
operating experience
• CURRENT STATUS
- Required service life is not well defined
- Critical failure modes have not been
identified
- No theoretical or experimental characteri-
zation of life distribution
• IMPORTANT OBSERVATIONS
Life distribution characterization by
system-level operating experience is not
feasible
Engine lifetime is inherently probabilistic
PROBABILISTIC FAILURE ASSESSMENT
ENGINEERING ANALYSIS
QUANTITATIVE
FAILURE MODEL
PROBABILISTIC
FAILURE MODELING
1
UNCERTAINTY OF
ENGINEERING ANALYSIS
PARAMETERS AND
• MODELS
\
ESTIMATED FAILURE
PROBABILITY
!OPSRAT'N___jGEXPER__.__,EN__CE
I PHYSICAL i
PARAMETER
l I INFORMATION T
I 7
I SUCCESStFAILUREi11 DATA
f-
STATISTICAL [ANALYSIS
1
FAILURE RISK
ACCEPTABLER_SK ABLER,SK
ACQUIRE AI_DITIONAL
INFORMATION
• REDUCE DRIVER UNCERTAINTY
• CHARACTERIZE ENVIRONMENT
• MEASURE/VERIFY LOADS
• CHARACTERIZE MATERIALS
• VALIDATE MODELS
REDUCE REQUIREMENTS IMPROVE DESIGN OR
AND/OR INCREASE PRODUCTION QUALITY
INSPECTION FREQUENCY • REDUCE SEVERITY
• REDUCE MANUFACTURING
VARIABIUTY
62
QUANTITATIVE CATHODE FAILURE
MODELLING
Row Model Input Parameters Experiments
Near-Cathode
Plasma Model Heat Flux Model _ Work FunctionModel
Thermal Model
Erosio! Model
Gas Tran)Ixm Model
CATHODE EROSION MODELLING
RATES
MECHANISMS
MI;L'rL_o
F_JEL"'I'ION
OF
DROPLETS
l DIFFUSION i
ADSORPTION IN
SOLID
CONVECTION / DIFFUSION THROUGH AMBIENT GAS I
I
II
Fd Fca Fc Fs
MELTING CHEMICAL EVAPORATION SPUTTERING
ATTACK
63
COMPARISON OF CALCULATED AND
MEASURED CATHODE EROSION RATES
a_
.__ 0.1
0.01
0.001
c_ ._dmut_l Um_,_ty Based on
Sc_ddvky A_d_
Diffur_on-IL, xdled Evaporation Rate
2_ _o 6obo _o
Time (s)
Cathode erosion measurements performed
with Stuttgart thruster NCT-1 at 2500 A,
1.0 g/s of argon, 71 kWe and 20 Ton" ambient
pressure
• Diffusion-limited evaporation of
tungsten is the dominant mechanism
Model underpredicts erosion rate by
a factor of 6, reflecting uncertainties
in transport rate through concentration
boundary layer
Calculated erosion rates are based on
measured temperatures--thermal model
required for fully predictive capability
CATHODE THERMAL MODELLING
HT8 - 1D thermal model with variable grid spacing and non-linear
thermal and electrical conductivity. Allows specification of radiation,
conduction, convection and arc attachmentboundary conditions on
ends and inner and outer radii.
AFEMS - Commercial 2D finite-element model with nonlinear
material properties. Very flexible solid modeUer for geometry
specification, but definition of boundary Conditions is more
cumbersome than in HT8.
• Fully 2D version of HT8 under development.
64
NEAR-CATHODE PLASMA MODELLING
J [Argon. Te=1.0 eV
3000 lionization Fraction=l 0 "_
[At=60 A/cm2K 2
2000 [Pre ss'ure= 10 Toff
Work Funcdon=3.0 eV 4.3 eVShealh Potential=8.0 V 3.5 eV
4.0 eV lo.ovj, _ s.ov __ .
/\ A "%/h 1
-_ / / \ / \ /_,,,,"X/" \\ 4.3ev
I \ \ _-.---._ 4.3evI \ \ \ "Wv
-|000 i , , i , ,J , i ,i
2000 2200 2400 2600 2800 3000 3200 3400 3(230
Surface Temperature (K)
Work Function=3.
_ 100.
I" // Ix_..on _.,,_,lo,,.-Io"/_ I'_,o,vc,."e
o.I / / IS__ath_t.mla-g.ov
0.01 / LPressure" !° T°rr
2000 2200 2400 2600 2800 30110 3200 3400 3600
Surface Temperature (K)
• The model describes the electrostatic
sheath, presheath and ionization zones
Current and heat fluxes are calculated
as functions of gas properties, therm-
ionic properties, surface temperature
and sheath potential
Terms normally neglected in high-
pressure noble gas arc models are
included to allow accurate modelling of
low-pressure alkali metal arcs
COMPARISON OF CALCULATED AND
MEASURED TEMPERATURE DISTRIBUTIONS
3400,
3200.
3000.
2800-
m
& 26o0-
E
_" 24oo.
2200-
20O0
0
I-- lvl'easured Temperature
•._ IJ-2000 A
".. _,_ I_1.0_sA_
--*Calculated Temperalure
.I:1991A Vs=7.6 V _'_.
Ar _=43 eV '_,_-.
p:7,6 Tort AR:60 A/cm'ZK z _"_,_,
_b 2b 3b ,_
Dislaace from Tip (nun)
Cathode model geometry and results
The model includes radiation, con-
duction out the base and heat input over
the first 20 mm from the near-plasma
model
The model reproduces the tip temperature
and shaft behavior for reasonable values
of the input parameters
Width of the attachment zone and the high
gradient in the middle are not predicted--
this may be due to 2-D effects, axially
varying gas properties, or convection
65
CATHODE WORK FUNCTION MODELLING
r_
io !.
t0 7 .
lo _.
105 .
104 _
t0_.
l0 2.
10 _ .
io°.
i0 i .
1ooo
[_ Tungslen ] 1
I- -" Pully-ihorialed Tungsten[ I
I- Th-W Model I 1
[ ........ Li Model I T K Operating I
r Time s I
if'i, Ton') "_ I
/../__ l_oo_-_-__ _ I
/. ,/_. (15.s) -"_- :'_---7
I"i_l\ <.741_ "-=.-"..... z_.-/ I
/ • t # _/ i/ .- / I
! fl _.oo7)-\.,," / I
iii ,"_------.- / I
1500 2000 2500 3(]00 3500
Cathode Surface Temperature (K)
Emission capability of tungsten metal
with Th and Li adsorbed on the surface.
• "Activator" may be electropositive
material in the cathode bulk or in the
propellant
• Two models were developed for cathode
additive transport and propellant-surface
interaction
• Th-W effect on work function is limited
by depletion of thorium additive
• Li supply from propellant is unlimited,
but surface coverage depends on gas
pressure and temperature
• There is considerable uncertainty in
model input parameters
CATHODE TEST FACILITY
_2j.: . '.L; II. •- :,: :_ _.).,.:iI:_"::• :::'_i,:_
II
66
CATHODE TEST FACILITY
• Demonstrate feasibility of new cathode concepts
• Measure cathode temperature distributions and erosion
rates to validate models
• Measure model input parameters
• Collect success/failure data in long endurance tests
ANODE MODELLING
Objective: Determine failure mechanisms, model life distribution
and develop methods for thermal management
Finite element model of existing anode design is complete
Subsequent tasks:
- Apply sheath analysis to anode region
Review existing data and theoretical treatments of magnetic
field effects in the anode region
Formulate proper boundary conditions for anode thermal
models
Develop an improved anode radiator design
67
LITHIUM MPD THRUSTER
TECHNOLOGY REVIEW
(Presented at the SEI Technologies Conference, Sept. 1991)
The review was motivated by Russian and US data from the 60's
and 70's indicating substantial performance and cathode lifetime
gains with alkali metal propellants
Scope
Critical review of existing data
- Analysis of the physical basis for performance and lifetime gains
Examination of systems and testing considerations
Conclusions
The available data are persuasive and provide a sound rationale for
renewed examination of alkali metal propellants, particularly lithium
Alkali metals offer a tremendous advantage in facility
pumping requirements
- The greatest risk is the potential for spacecraft contamination
LITHIUM MPD THRUSTER
TECHNOLOGY DEVELOPMENT AT JPL
Funded by NPO in FY92 to develop a lithium feed system
- Reservoir and vaporizer designed and under construction
Flow rate calibration system design complete, components
under construction
Test facility design nearly complete, construction to be completed
in FY93
6' x 15' double-walled stainless chamber with 27' long
extension to be used as a beam dump pumped by a 20"
diameter oil diffusion pump
Initial testing of 100 kWe-class radiation-cooled engine to begin
in FY93
68
LITHIUM MPD THRUSTER
TEST CHAMBER
LITHIUM MPD THRUSTER
TEST FACILITY
Supplies [._J [_.J [_ower_"]_ 1Tower I
Van with Control Equipment
69
RADIATION-COOLED, APPLIED-FIELD
ENGINE TESTING
Operation of radiation-cooled anode
up to a power level of 80 kWe was
demonstrated on ammonia with no further
anode degradation beyond initial melting
encountered in earlier testing with
argon propellant
The testing confirms the results of
simple thermal modelling which indicated
that the open-throated configuration
could tolerate higher heat loads
MPD ENGINE PLUME DIFFUSER STUDIES
+
v
d:
[....
80-
60-
40-
20-
0
0
Amlnmaa
• 646G o 646G
• 1171G
• iii • 116
a
13 a a
a
a a
llmql_ a ••Aml•
0 o-°
_ao a _ WixlLD.ilI_
Aa . • OG o OG
a• • 646.0 o 555G
• 10720 • Ill0G
_o ,b +b +b
Power_W)
I00
Tank pressures are generally higher with
ammonia compared to argon, but the diffuser
still has a strong effect on the backpressure
The gasdynamic function of the diffuser and
its effect on thruster operation are still not
well understood
PRELIMINARY THRUST MEASUREMENTS
80-
70-
60-
30-
20-
l0
/ [ o Archcad xl_st [
J ]o Magnet Thrusts[
Engine Powex OcWe)
The measurements were made with
ammonia propellant and an applied field
strength of 646 G
The magnet thrust appears to be approx-
imately constant, while the engine thrust
increases linearly with power
Similar trends are observed when plotted
versus j2 and JB z
70


The MPD thruster program at JPL

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