This presentation was part of the session : Short CoursesSixth International Planetary Probe Worksho

Laub, Bernard

Venkatapathy, Ethiraj

Wright, Michael J.

English

Scholarly Materials And Research @ Georgia Tech

Preliminary - For Discussion Purposes Only Page 16th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008Thermal Protection System (TPS) Design and theRelationship to Atmospheric Entry EnvironmentsByBernard LaubDr. Michael J. WrightDr. Ethiraj VenkatapathyNASA Ames Research CenterMoffett Field, CAPreliminary - For Discussion Purposes Only Page 26th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008Agenda• Why Ablative TPS?• What is ablative TPS?• Entry environments for planetary probes– Key Physical Challenges– Sample Entry Environments• TPS Selection– Failure modes– Heat flux, pressure, atmospheric composition– Heat load• TPS Testing• SummaryPreliminary - For Discussion Purposes Only Page 36th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008Why Ablative TPS?Preliminary - For Discussion Purposes Only Page 46th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008What is ablative TPS?Preliminary - For Discussion Purposes Only Page 56th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008How hot is hot?• All materials are (potentially)ablative materials– If exposed to typical entryheating, any material willget to temperatures where itwill either melt, vaporize,oxidize, sublime, etc.• For comparison, thetemperature of the sun is≈ 6000 K• The gas near the heatedTPS surface (behind theshock) is at much highertemperatureRadiation equilibriumPreliminary - For Discussion Purposes Only Page 66th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008Aerothermodynamics of Planetary EntryqcondqcqradqreradqmdotDesign Problem: Minimize conductioninto vehicle to minimize TPS mass/riskqcond = qc + qrad – qrerad – qmdotIncident AeroheatingMaterial ResponseSurface EnergyBalanceHot Shock Layer(up to 20000 K)Thermochemicalnonequilibrium,Ionization, Radiation“Cool” Surface(2–3000 K)Surface kinetics,AblationPlanetary AtmospheresMars&Venus: CO2/N2Titan: N2/CH4Giants: H2/HeEarth: N2/O2Boundary Layer(2–6000 K)Turbulence,Ablation productmixing, RadiationblockageVAfterbody FlowUnsteady non-continuum vorticalflowfieldPreliminary - For Discussion Purposes Only Page 76th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008• Reentry generates a lot of energy:• Fortunately, most of this energy does notreach the surface– >90% of total energy is dissipated via thebow shock heating the atmospheric gasesEnergy Loss over TimeEnergy (MJ)EndParachuteDeployAtmosphericInterface18(99.998%)0.9(99.94%)0.2(99.98%)1.28 x 105(88%)84(94%)105(92%)1.07 x 10614141260Galileo ProbeGenesisMERStardust CapsulePreliminary - For Discussion Purposes Only Page 86th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008Broad Range of Entry EnvironmentsNASA entry probes have successfully survived entry environmentsranging from the very mild (Mars Viking ~25 W/cm2 and 0.05 atm.)to the extreme (Galileo ~30,000W/cm2 and 7 atm.)101001000100001000000.01 0.1 1 10Mission EnvironmentsPeak Heat Flux (W/cm2)Stagnation pressure (atm)Mars Viking (2.8%)MER (12%)MPF (8.2%)Stardust (22%)Apollo (13.7%)Genesis (18%)Pioneer Venus (13%)Galileo (50%)Values in parentheses are TPS mass fractionPreliminary - For Discussion Purposes Only Page 96th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008Representative EnvironmentsVenus Earth Mars Jupiter Saturn Titan Neptuneq (kW/cm2) 2-7 0.6-2 0.05-0.5 30-60 2-5 0.05-0.25 2-10Q (kJ/cm2) 10-20 10-40 5-10 200-500 50-150 2-6 100-400p (atm) 10 0.25-0.5 0.25-0.5 5-10 0.5-5 0.25 0.5-5Direct EntryVenus Earth Mars Jupiter Saturn Titan Neptuneq (kW/cm2) 1-2 0.5-1 0.05-0.3 N/A 3-10 0.05-0.15 3-10Q (kJ/cm2) 40-80 20-50 10-30 N/A 200-500 5-12 500-2000p (atm) 0.3 0.25 0.25 N/A 0.5-1 0.1 0.5-1.5AerocapturePlanned missions will require TPS able to survive a broad rangeof entry conditionsPreliminary - For Discussion Purposes Only Page 106th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008How do ablative materials manage energy?• Surface reradiation is themost effective energyrejection mechanism (60-80%)– Carbon or materials thatform carbonaceous charsare desirable as they attainvery high surfacetemperatures and havehigh emissivity– Ablation, even in thepresence of exothermicoxidation, consumesenergy (20-40%)– Only a small fraction of theincident heating isconducted into the TPSmaterial (10-20%)Preliminary - For Discussion Purposes Only Page 116th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008Material Performance LimitsOptimal performance regime is balanced between ablative andinsulation efficiency. When material is used outside of optimalzone, inefficient performance leads to non-minimal mass fraction.1101001000100001000000.001 0.01 0.1 1 10 100Limitations of ablative TPS classesPeak Heat Flux (W/cm2)Stagnation pressure (atm)surface spallation threshold(inefficient performance)StardustApolloGenesisGalileoPioneer VenusMars VikingMERMPFLow density TPSMid-density TPSHigh-density TPSPreliminary - For Discussion Purposes Only Page 126th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008Example failure modesSpallation Loss of  liquid layerPreliminary - For Discussion Purposes Only Page 136th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008TPS Selection• Objective is minimum TPS mass with reliable performance– Reliable performance implies that material failure modes are wellunderstood and environmental conditions leading to failure will not beencountered (or approached) for the selected mission– Low density materials are (typically) better insulators than high densitymaterials– High density materials are (typically) better ablators thanlow density materials• Ablation is good - it absorbs energy– Too much ablation may not be good if it leads to shape changethat influences aerodynamics• TPS selection involves a balance between ablation and insulationperformance and manufacturability– Select the lowest density material that can handle* the range ofenvironmental conditions (heat flux, pressure, shear, atmosphere)– Material should provide effective insulation for imposed heat load– Procedures for material fabrication, installation, inspection, etc., should beestablished and, preferably, demonstrated*Material should have demonstrated reliability at extreme conditions of interestPreliminary - For Discussion Purposes Only Page 146th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008TPS TestingArc Plasma Facilities• Have been used for over 40 years to study TPS materialperformance– Two classes:• Low enthalpy, high pressure, high heat flux (high β vehicles)• High enthalpy, low pressure, low-moderate heat flux (low β vehicles;lifting entry, aeroassist, aerocapture, planetary entry, etc.)• Significant flexibility– Pressure: nozzle geometry, test article design, gas mass flow rate– Enthalpy: gas mass flow rate, electrical power– Gas composition: most facilities operate with air, but tests havebeen conducted with N2, CO2, H2/He, etc. gas streams• Amenable to sophisticated (non-intrusive) diagnostics– Surface visibility (film or video), surface pyrometry, PLIF,emission spectroscopy, etc.• Capability to simultaneously simulate conditions representativeof flight (e.g.,            )  is rare.– Requires strategic test planning– Typically, cannot simulate time-varying conditions (trajectories)! H , ˙ q, pPreliminary - For Discussion Purposes Only Page 156th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008TPS Testing (concluded)• Inability to simulate the actual flight environment in arc jetsresults in significant uncertainties in ground test to flighttraceability• Well-designed ground-test program should cover the range ofconditions anticipated in flight– Typically, ground tests cannot simulate some aspects of the flightenvironment• Turbulent flow• High shear• High pressure gradient• Combined convective/radiative heating• Mechanism-based modeling allows extrapolation with someconfidence– Identification of surface response mechanisms and development ofhigh fidelity model significantly reduces performance uncertaintiesin flight– Remaining uncertainties can only be addressed through flight testwith instrumented TPSPreliminary - For Discussion Purposes Only Page 166th International Planetary Probe Workshop, Atlanta, GeorgiaShort Course on Extreme Environments Technologies06/21-222008Summary & Conclusions• Atmospheric entry qualifies as an extreme environment• Selection of an ablative TPS for a given mission is governed by theseverity of the entry environment– High density materials minimize ablation but result in a heavy TPS– Low density materials minimize insulation thickness and result in a light TPS– Optimum material (among those available) is the lowest density materialthat does not produce excessive ablation while performance is far fromfailure thresholds• Arc plasma facilities produce the best simulation of the entryenvironment– Actual flight conditions (typically) cannot be simulated– Requires testing over broad range of conditions to understand performancemechanisms– Mechanism (physics- and chemistry-) based models enable extrapolationfrom ground test to flight• Ablative materials have been successfully used for thermal protectionfor 50 years and will continue to be used in the foreseeable future

Thermal Protection System (TPS) Design and the Relationship to Atmospheric Entry Environments

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Thermal Protection System (TPS) Design and the Relationship to Atmospheric Entry Environments

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