NASA Technical Reports Server (NTRS) 20040095949: Rarefied Aerothermodynamic Predictions for Mars Global Surveyor

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by  NASA Technical Reports Server (NTRS)

Rare(cid:12)ed Aerothermodynamic Predictions for
Mars Global Surveyor
1 1 2 2
R. G. Wilmoth , D. F. G. Rault , R. W. Shane , R. H. Tolson
1
NASA Langley Research Center, Hampton, VA, USA
2
George Washington University, Hampton, VA, USA
1 Introduction
MarsGlobal Surveyor(MGS) successfully completed its(cid:12)rst phaseof aero-
braking in early 1998 and is the (cid:12)rst planetary mission to use aerobrak-
ing as a primary means of customizing its orbit to achieve its mission
objectives [1]. The aerobraking requirements together with post-launch
anomalies [2] presented a unique challenge to provide accurate predictions
of the aerothermodynamic environment of the spacecraft in the rare(cid:12)ed
transitional(cid:13)ow regime. Direct Simulation Monte Carlo(DSMC) and free-
moleculartechniqueswereusedtoprovideheatingandaerodynamicpredic-
tionsandtoinvestigateavarietyofrare(cid:12)ed(cid:13)owphenomenaacrossthe(cid:13)ight
regime[3, 4,5]; MGSisthe (cid:12)rstmajorplanetarymissionin whichrare(cid:12)ed-
(cid:13)ow predictions have played such a critical role all the way through the
design, mission planning, and operational phases. This paper summarizes
thesestudieswithemphasisontransitional-(cid:13)owandgas-surfaceinteraction
phenomena.
2 Computational Approach
Analyses were carried out using the NASA LaRC 3D DSMC (LaRC-3D)
code[6],theDAC(DSMCAnalysisCode)code[7],andcompanion3Dfree-
molecularcodesdevelopedinrecentyearsforanalyzingcomplexgeometries.
The DSMC codes di(cid:11)er primarily in their computationalgrid methodology
and modeling of the surface geometry. However, both DSMC codes use
essentiallythesameDSMCproceduresandphysicalmodelsandarecapable
ofhandlingthecomplexgeometryoftheMGSspacecraft. TheDSMCcodes
are complemented by 3D free-molecular codes which use analytical free-
molecularanalysisandline-of-sightshadowingtechniquestomodelthe(cid:13)ow
aboutcomplexgeometries. AnalysisofMGSaerodynamicsandheatingwas
performed primarily using LaRC-3D in the design phase and in the early
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post-launch, pre-aerobraking phase, while DAC was used primarily in the
laterpost-launch,pre-aerobrakingphaseandin the earlyoperationalphase
of aerobraking.
Calculationswereperformedforavarietyofsolarpanelpositionsandspace-
craftattitudesandforatmosphericdensitiesrangingfromfree-molecularto
transitional (cid:13)ow (Knudsen numbers < 0.1). The composition of the Mars
atmosphere was assumed to be 95.37% CO2 and 4.63% N2 by mole. Colli-
sionsweremodeled with the VariableHard Spheremodel ofBird[8]usinga
reference temperature of 3000 K, a temperature exponent of 0.806638,and
(cid:0)10 (cid:0)10
reference diametersof 3:8969(cid:2)10 m and 3:3487(cid:2)10 mforCO2 and
N2,respectively. Gas-surfaceinteractionsweremodeledasacombinationof
di(cid:11)useandspecularscattering. Thee(cid:11)ectsofCO2evaporationwerestudied
and modeled as a simple steady-state outgassing process.
3 Spacecraft Geometry and Modeling Issues
The geometry of the MGS spacecraft in the aerobraking con(cid:12)guration is
shown in Fig. 1. The solar panels are swept back to reduce aerodynamic
heating andto provideaerodynamicstability. The pre-launchdesigncalled
for both panels to be swept 30 deg to give essentially a symmetric con-
(cid:12)guration. However, when the panels were deployed after launch, the -Y
panel failed to fully extend due to what was determined to be a jammed
hinge between the yoke and the inner solar panel [2]. This failure left the
-Y panel approximately 20 deg short of full deployment and resulted in a
con(cid:12)guration that would not have been su(cid:14)ciently sti(cid:11) when subjected to
aerodynamic moments about the X-axis. The problem was resolved by ro-
tating the -Y panel 180 deg about its principal axis and sweeping the yoke
51 deg aft tocompensate forthe 20 deg shortfallin paneldeployment. The
+Y panel of this revised aerobraking con(cid:12)guration was swept to approxi-
mately 33.8deg to providethe desired aerodynamictrim about the X-axis.
The revised con(cid:12)guration also provided greater sti(cid:11)ness to aerodynamic
momentsaboutthe X-axisalthoughsmallde(cid:13)ectionsofthe-Ypanelabout
the unlatched hinge were expected. These de(cid:13)ections were computed and
taken into accountwhen extractingatmosphericdensity from the on-board
accelerometer measurements [9].
4 Results
4.1 Aerodynamic Heating
One of the early design considerations was the aerodynamic heating that
wouldoccuronthesolarpanelsduringaerobraking. Althoughtheestimated
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heating levels were relatively low, the allowable temperatures on the solar
panel still limited the depth to which the spacecraft could penetrate the
Martianatmospherewithout exceedingthesetemperatures. The maximum
atmospheric densities expected at periapsis placed the spacecraft well into
transitional(cid:13)ow,andDSMCanalyseswereconductedtore(cid:12)nethepredicted
heating levels.
Figure 2 shows density contours around MGS at densities of 12 and 120
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kg=km where the higher value is approximately the maximum expected
density at periapsis. These predictions exhibit the typical di(cid:11)use shock
layers that occur in rare(cid:12)ed, transitional (cid:13)ows. The shock layer e(cid:11)ec-
tively\shields"thebodyfromreceivingthefullkineticenergyoffreestream
molecules,andlocalheatingratesaresigni(cid:12)cantlylowerthanfree-molecular
values as seen in Fig. 3. This (cid:12)gure shows heat transfer coe(cid:14)cients, CH
1 3
((cid:17)q_=(2(cid:26)1V1)), along a diagonal cut on the windward side of the inboard
solarpanel, and reductions up to 40% are seen near the center of the panel
at the highest density. Such predictions were important for \calibrating"
temperaturesensorsonthe panelsthat wereusedtoassessthe actualheat-
ing levels in (cid:13)ight.
4.2 Drag During Aerobraking
Aerobrakingdependsonaerodynamicdragtoslowthespacecraftbyasmall
velocity increment, (cid:1)V, on each pass. The total reduction in velocity re-
quired is determined by the (cid:12)nal orbit one wishes to achieve. The (cid:1)V
obtained on each aerobraking pass is directly proportional to the atmo-
spheric density and the drag coe(cid:14)cient of the spacecraft. To minimize the
time requiredto achieve(cid:12)nal orbit, it is desirableto penetrate asdeeply as
possibleintotheatmosphereandtomaximizethedragcoe(cid:14)cient. Figure4
shows predictions of the drag coe(cid:14)cient over the density range of interest
forMGS.Predictionsforthesymmetricsweeppre-launchcon(cid:12)gurationob-
tained with LaRC-3D and the revised sweep con(cid:12)guration obtained with
DAC are shown to demonstrate that drag variations for the two con(cid:12)gura-
tions are essentially the same. The drag coe(cid:14)cient is reduced up to 15%
at the highest density, and although this reduction may seem small, its
e(cid:11)ect on the total number aerobraking passes required can be signi(cid:12)cant.
Therefore,DSMCpredictionswereimportantintheearlydesigntoprovide
accurate predictions of the drag coe(cid:14)cient of MGS.
The impact of drag on aerobraking time is seen in Fig. 5 which shows a
history of MGS orbit parameters at periapsis over approximately the (cid:12)rst
eight months of operation. The original mission called for a target orbit
period of approximatelytwo hours to be achieved in about 120 days. (The
exact time required was subject to relatively large uncertainties in atmo-
sphericdensity.) However,excessivede(cid:13)ectionsofthepartially-deployed-Y
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solar panel during the (cid:12)rst few weeks caused a 30-day halt to aerobraking
while these de(cid:13)ections were analyzed. When aerobraking resumed, it was
decided to minimize the panel loads by aerobraking at lower densities and
to extend the totalaerobrakingphase toabout 18monthswhich includesa
six-month non-aerobraking period for obtaining science data.
4.3 Aerodynamic Solar Panel De(cid:13)ections
The partially-deployed -Y panel was not \latched" into a rigid position,
and de(cid:13)ections were expected during aerodynamic loading. These de(cid:13)ec-
tions were also expected to reduce the drag coe(cid:14)cient and alter the aero-
dynamic trim. Therefore, additional analyses were performed to provide
a more complete aerodynamic database for various panel de(cid:13)ections, and
two of the more important quantities, the drag coe(cid:14)cient, CD, and the
yawing moment about the X-axis, Cm;x, are shown in Fig. 6 for panel de-
(cid:13)ections of up to 20 deg. Drag reductions of 10-15% are predicted at the
maximum panel de(cid:13)ection, and trim angle, (cid:12), about the X-axis is shifted
by about one-half of the de(cid:13)ection angle. These predictions were used to
extract atmospheric densities from the onboard accelerometer data during
each aerobrakingpass, and aniterative procedurewasrequiredto compute
the panel de(cid:13)ection consistent with the axial force coe(cid:14)cient, Cz, corre-
sponding to that de(cid:13)ection. Figure 7 shows typical axial force coe(cid:14)cients
and computed panel de(cid:13)ections derived from (cid:13)ight data, and the de(cid:13)ec-
tions show relatively good agreement with independent measurements of
de(cid:13)ections made with an onboard sun sensor [9].
4.4 Gas-Surface Interaction E(cid:11)ects
4.4.1 Momentum Accommodation
Therevisedaerobrakingcon(cid:12)gurationplacedtheglass-coveredsolarcellsof
the -Y panel on the windward side, and di(cid:11)erences in momentum accom-
modation coe(cid:14)cients between the glass and the composite material on the
windward side of the +Y panel were expected. The main e(cid:11)ect of these
accommodationcoe(cid:14)cient di(cid:11)erences wasexpected to be a changein aero-
dynamictrimabouttheX-axis. Therefore,free-molecularcalculationswere
(cid:12)rst performed for accommodation coe(cid:14)cient di(cid:11)erences of up to 0.3 (ac-
tualdi(cid:11)erenceswereexpectedtobelessthan0.2),andthepredictedshiftin
yaw trim angle is shown in Fig. 8. Although the predicted shifts are small,
they were important in assessing panel de(cid:13)ections in the early passes, and
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a few DSMC calculations were made at a nominal density of 81 kg=km to
further re(cid:12)nethe predictions. At this transitional(cid:13)owdensity, the e(cid:11)ect of
accommodationdi(cid:11)erencesisdiminished considerablybecauseofmolecular
collisions near the surface that cause partially accommodated molecules to
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Marseille, France, July 26-31,1998
have multiple surface collisions.
4.4.2 CO2 Evaporation
Accelerometer (cid:13)ight measurements have shown perturbations in axial ac-
celeration during certain aerobraking passes that cannot be explained by
known atmospheric density perturbations or by spacecraft geometry. One
explanationconsideredisthatthe\cold"solarpanelsmightadsorbsu(cid:14)cient
CO2 which then rapidly evaporates during aerodynamic heating, and this
e(cid:11)ect has been qualitatively evaluated using a simple \outgassing" model.
EvaporationofCO2 wasmodeledonthewindwardsurfacesofthesolarpan-
els at variousarbitraryratesto determine the e(cid:11)ect on axialforce, and the
results are shown in Fig. 9. Evaporation (cid:13)ux rates of up to four times the
incident freestream(cid:13)uxweresimulatedat the relativelyrare(cid:12)edfreestream
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density of 12 kg=km . Figure 9 shows the net change in axial force and
the components caused by the \shielding" e(cid:11)ect produced by evaporating
molecules colliding with freestream molecules (a reduction in axial force or
drag) and the \thrust" exerted by the evaporating molecules as they leave
thesurface. Thesee(cid:11)ectscanbequalitativelyunderstoodbyexaminingthe
streamlines around the spacecraft shown in Fig. 10 which clearly show the
\shielding" e(cid:11)ect as well as the streamlines emanating from the surface.
The evaporation rates simulated in this brief study did not produce su(cid:14)-
cientchangesinaxialforcetoexplaintheobserved(cid:13)ightdata. Furthermore,
estimatesofthequantityofCO2 thatmightbeadsorbedsuggestthatevap-
oration rates higher than those simulated are unlikely, although the model
used to maketheseestimates is subject to largeuncertaintiesdue to uncer-
tainties in surface phase e(cid:11)ects and heat of evaporation.
5 Concluding Remarks
MarsGlobalSurveyorhasbeenhighlysuccessfulindemonstratingthataero-
brakingcanbeusedtotailoraplanetaryorbit,andrare(cid:12)ed(cid:13)owsimulations
have contributed signi(cid:12)cantly to the successful design, mission planning,
and (cid:13)ight operations. Engineering (cid:13)ight data obtained to date has pro-
vided added con(cid:12)dence in the aerothermodynamic predictions, and both
the engineering and science data are providing a wealth of information on
planetary aerobrakingand on the Mars environment.
References
[1] Dallas, S. Sam, Mars Global Surveyor Mission, Proceedings of IEEE
AerospaceConference,Vol.4,SnowmassatAspen,CO,Feb.1-8,1997,
pp. 173-189.
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21st International Symposium on Rare(cid:12)ed Gas Dynamics
[2] Lyons, Daniel T., Mars Global Surveyor: Aerobraking with a Broken
Wing, AAS Paper 97-618,Aug. 1997.
[3] Rault, D. F. G., RCS Plume E(cid:11)ect on Spacecraft Aerodynamics, 20th
Rare(cid:12)ed Gas Dynamics Symposium, Bejiing, China, August, 1996.
[4] Shane,R.W,Rault,D.F.G.,andTolson,R.H.,MarsGlobal Surveyor
Aerodynamics for Maneuvers in Martian Atmosphere,AIAAPaper97-
2509, June, 1997.
[5] Wilmoth, R.G.,Rault, D.F.G.,Cheatwood,F.M., Engelund,W. C.,
and Shane, R. W., Rare(cid:12)ed Aerothermodynamics Predictions of Mars
GlobalSurveyor,SubmittedtoJournalofSpacecraftandRockets,1998.
[6] Rault, D. F. G., Towards an E(cid:14)cient Three-Dimensional DSMC code
for Complex Geometry Problems, 18thRare(cid:12)edGasDynamicsSympo-
sium, Vancouver, British Columbia, July 1992.
[7] Wilmoth, R. G., LeBeau, G. J.; and Carlson, A. B., DSMC Grid
Methodologies for Computing Low-Density, Hypersonic Flows About
Reusable Launch Vehicles, AIAA Paper 96-1812,June 1996.
[8] Bird,G.A.,MolecularGasDynamicsandtheDirectSimulation ofGas
Flows, Clarendon Press, Oxford, 1994.
[9] Tolson,R.H.,Keating,G.M.,Cancro,G.J., Parker,J.S.,Noll,S.N.,
andWilkerson,B.L,Application ofAccelerometerDatatoMarsGlobal
Surveyor Aerobraking Operations, Submitted to Journal of Spacecraft
and Rockets, 1998.
Figure 1: MGS Spacecraft in AerobrakingCon(cid:12)guration.
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Marseille, France, July 26-31,1998
r ¥=12kg/km3 r ¥=120kg/km3
1.0
1.1
1.1 Free-Molecular
1.5
Max»30 Max»60 C 0.8 r¥=12kg/km3
H r¥=60kg/km3
0.6
2.1 r¥=120kg/km3
1.1 0.6
1.1 1.5 1.1 1.5 1.1 0.40.0 0.6 1.2 1.8 2.4
Distance,m
Figure 2: Nondimensionalized Figure 3: Transitional E(cid:11)ect on
Density Contours, (cid:26)=(cid:26)1. Solar Panel Heat Transfer.
2.4 101 KnHS,¥ 100 10-1 Altkitmude, Pheoruiords, kmV,/s
300 50 5.2
Period 40
2.0 200 Velocity 4.8
30
C
D 20
1.6 FPrreee-L-MauonlecchuMlaordLeiml(iLtaRC3D) 100 Altitude 4.4
Pre-LaunchCurveFit Target 10
RReevviisseeddAMeordoeblrCakuirnvgeMFoitdel(DAC) Period
0 0 4.0
1.2 0 60 120 180 240
10-1 100 101 102 ElapsedTime,days
r ,kg/km3
¥
Figure 4: Drag Predictions for
Figure 5: MGS Aerobraking His-
Various Geometry Models and
tory. (Values given at periapsis.)
DSMC Codes.
Kn
HS,¥
101 100 10-1 0.4
2.4
ZeroDeflection
10-degDeflection
20-degDeflection
0.2
2.0
CD,trim Cm,x
0.0
1.6
ZeroDeflection
10-degDeflection
20-degDeflection
(2nd-OrderCurveFits)
1.210-1 100 101 102 -0.2-20 -10 0 10 20
r ,kg/km3 b ,deg
¥
(a) Drag Coe(cid:14)cient. (b) Yawing Moment.
Figure 6: E(cid:11)ect of -Y Panel De(cid:13)ection.
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21st International Symposium on Rare(cid:12)ed Gas Dynamics
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2.1
6
eg 5
d
n, 4
Cz 2.0 deflectio 32
el Aero Calc
an 1 Sun Sensor
P
1.9 0
-1
-200 -100 0 100 200 -200 -100 0 100 200
Time from periapsis, sec
Time from periapsis, sec
(a) Axial Force Coe(cid:14)cient. (b) Measured and Calculated
Panel De(cid:13)ection.
Figure 7: Axial Force and De(cid:13)ections Extracted from Flight Data.
Note:ShiftisinDirectionof
PanelwithLowerA.C.
4 16
3 12 Eva"pTohrrautsint"goGfas
Shiftin Free-Molecular
bdterimg, 2 (rT¥=ra8n1siktgio/nkmal3) FIninocrrAceexa,ias%el 48 NetChange
1
0
0 ShieldingEffect
0 0.1 0.2 0.3 -4
DifferenceinAccommodationCoefficients 0 1 2 3 4
Between+Yand-YPanels EvaporationFlux/FreestreamFlux
Figure8: E(cid:11)ectofDi(cid:11)erentialAc-
Figure 9: E(cid:11)ect of CO2 Evapora-
commodation on Yaw Trim.
tion on Axial Force.
CO Evaporation
NoOutgassing Rat2e=4xIncident
FreestreamFlux
3
Figure 10: E(cid:11)ect of CO2 Evaporation on Streamlines. (cid:26)1 = 12 kg=km .
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