Preliminary Thermal-Mechanical Sizing of Metallic TPS: Process Development and Sensitivity Studies

17 Pages·2002·1.3 MB·English

by  Poteet, Carl C.

AIAA 2002-0505
Preliminary Thermal-Mechanical Sizing of
Metallic TPS: Process Development and
Sensitivity Studies
Carl C. Poteet
Hasan Abu-Khajeel
Su-Yuen Hsu
NASA Langley Research Center
Hampton, VA 23681-2199
40th Aerospace Sciences Meeting & Exhibit
14- 17 January 2002
Reno, Nevada
For permission to copy or to republish, contact the copyright owner on the first page.
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AIAA2002-0505
PRELIMINARY THERMAL-MECHANICAL SIZING OF METALLIC TPS:
PROCESS DEVELOPMENT AND SENSITIVITY STUDIES
Carl C. Poteet*
NASA Langley Research Center, Hampton, Virginia
and
Hasan Abu-Khajeel*, Su-Yuen Hsu*
Lockheed Martin Space Operations, Hampton, Virginia
Abstract Introduction
The purpose of this research was to perform Thermal Protection Systems (TPS) on Reusable
sensitivity studies and develop a process to perform Launch Vehicles (RLV) are required to be light weight
thermal and structural analysis and sizing of the latest while providing protection from heating during reentry
Metallic Thermal Protection System (TPS) developed at and insulation to cryogenic fuel tanks during ground
NASA LaRC. Metallic TPS is a key technology for hold. Recent design goals for RLV have called for
reducing the cost of reusable launch vehicles (RLV), "Commercial Aircraft Like" operations, which further
offering the combination of increased durability and increases the importance of the TPS. To meet these
competitive weights when compared to other systems. goals, TPS must not only be a good insulator capable of
Accurate sizing of metallic TPS requires combined withstanding cryogenic and reentry temperatures, but it
thermal and structural analysis. Initial sensitivity must be durable and easily maintained. To increase
studies were conducted using transient one-dimensional RLV operability, the TPS may be required to withstand
finite element thermal analysis to determine the exposure torain and hail.
influence of various TPS and analysis parameters on ARMOR TPS is sized herein to meet the insulation
TPS weight. The thermal analysis model was then used and structural requirements resulting from the
in combination with static deflection and failure mode groundhold cryogenic environment as well as the ascent
analysis of the sandwich panel outer surface of the TPS and reentry aerothermal heating environments. The
to obtain minimum weight TPS configurations at three analysis focuses on sizing of the fibrous insulation layer
vehicle stations on the windward centerline of a and sizing of the honeycomb sandwich onthe ARMOR
representative RLV. The coupled nature of the analysis TPS outer surface. Dimensions for other components
requires an iterative analysis process, which will be are based on work reported by Blosser, et al.3
described herein. Findings from the sensitivity analysis Insulation layers are sized by the aerothermal heating
are reported, along with TPS designs at the three RLV and cryogenic conditions experienced in the three
vehicle stations considered. environments, while the honeycomb sandwich panel on
the ARMOR TPS outer surface is sized considering
aerodynamic pressure, acoustic pressure, and thermal
gradients. Since the thermal performance is dependent
onthe structure and the structural response is dependent
on the temperatures in the TPS, the thermal and
structural analyses are coupled, requiring an iterative
Copyright © 2002 by the American Institute of analysis alternating between thermal and structural
Aeronautics and Astronautics, Inc. No copyright is analyses.
asserted in the United States under Title 17,U.S. Code. This paper represents one of several reporting onthe
The U.S. Government has a royalty-free license to development of Adaptable Robust Metallic Operable
exercise allrights under the copyright claimed herein Reusable (ARMOR) TPS at NASA Langley Research
for Governmental Purposes. All other rights are Center. 1-5 ARMOR TPS, as shown in Figure 1,
reserved bythe copyright owner. employs a light weight metallic structure to encapsulate
*Research Engineer, Metals and Thermal Structures high efficiency fibrous insulation and react
Branch, Structures and Materials Competency aerodynamic pressure to the vehicle structure. The goal
*Aeronautical Engineers, Lockheed Martin Space of ARMOR TPS development is to improve operational
Operations. features, increase adaptability (by allowing attachment
to different tank and structural configurations), and
American Institute of Aeronautics and Astronautics
reducethe weightfrompreviousmetallicTPS
intertank structure is not shown. The sandwich panel
design2s'3. Operabilitycan be increasedby
uses graphite epoxy facesheets with Korex ©honeycomb
modificatioonfTPSdesignparametesruschasouter
and TEEK 14cryogenic foam. The cryogenic foam is
faceshegeatug(etoimprovreesistantcoedamagfreom
used to limit heat flow into the tank during groundhold
hail,raina, ndorbitadlebrisa)ndstandodffistancfreom
and prevent air liquefaction in the gap between TPS and
thestructuroertank(whichgreatlyimproveosrbital
tank during the vehicle groundhold and ascent
debriismpacret sistan1c)e.
conditions. The TPS bottom comers are mechanically
ThefinalTPSdesignrseporterdepreseanntominal
attached to a TPS support system (TPSS), which is
desigwnhichw,hileexhibitinigmproveddurabilitaynd
bonded to the tank wall (Figure 3). TPSS is used to
operabilictyomparetodpreviouTsPSdesignhsa,snot
attach TPS to tank structure while accommodating
beensizedtomeeat specifidcurabilitoyroperability
differences in shape between the outer mold line of the
criterias,uchasacertainsizehail,impacetnergyo,r
vehicle and the tank, and to form acavity for purging of
spacdeebripsarticlesize.Thesceriteriaarediscussed
the system. Purging is performed with gaseous
byDorseey,tal2.
nitrogen during vehicle groundhold to reduce heat flow
into the cryogenic fuel tank and to neutralize any
ARMOR TPS Design potential tank leaks. An air purge after landing is
Figure 1 shows a fabricated ARMOR TPS panel assumed to be a standard operational procedure and is
along with a cutaway view showing details of the inner performed using blowers attached 30 minutes after
structure. The sandwich panel is exposed to ascent and vehicle touchdown to cool the tank and support
reentry heating as well as aerodynamic and acoustic structure.
pressure. One of the primary functions of the sandwich Figure 4 is a schematic of the TPSS used in this
panel is to re-radiate heat, dramatically reducing the study, which is composed of two graphite epoxy tabs.
amount of heat absorbed by the TPS. Panel to panel The lower tab is bonded to the tank wall and the upper
gaps are sealed by overhanging metal foil to prevent tab is then mechanically attached to the lower tab. A
ingress of hot gases during reentry. Flutter analysis of felt layer is bonded to the upper surface of the upper
these seals isreported by Chen, et al.4 Pressure loading tab. The 3" by 3" area on the upper tab surface is used
is reacted to the box beam picture frame through four to attach the comers of four adjacent TPS panels. If
thermally compliant supports. 3 The supports are necessary, the upper tab can be made out of a higher
arranged in a circular pattern and have low bending temperature material to reduce the amount of insulation
stiffness to allow nearly free in-plane thermal required.
expansion of the sandwich panel while resisting
translation and rotation. Bulged, compliant sides, made Analytical Method
of thin gauge metal foil, form the sides of the TPS panel
and block the radiative heat transfer path in the panel to Aerothermal Environment and Trajectory
panel gaps. The interior of the TPS panel is filled with Vehicle loads, aerothermal environment, and
SaffilTM high efficiency fibrous insulation. 6 A thin trajectory information was obtained from Dorsey, et.
gauge metal foil closes out the bottom of the TPS panel. al.,2for aRLV lifting body configuration designated 3c.
Several mesh covered vents are incorporated into the This data was used to determine outer surface heating
metal foil backing to allow the TPS internal pressure to and pressure gradient loads acting on the outer
be maintained at local atmospheric pressure. honeycomb sandwich panel. Three vehicle stations
Selection of materials for ARMOR TPS depends on were chosen along the windward centerline for
the maximum surface temperature experienced. For analysis: STA 240, STA 802, and STA 1200, where
regions of the vehicle where temperatures are under numeric values represent distance from the vehicle nose
1100 °F titanium alloys can be used. Regions in excess in inches. Figure 5 shows the location of the vehicle
of 1100 °F use Inconel 617 for the outer honeycomb stations relative to the cryogenic fuel tanks. Station 240
sandwich panel and compliant sides and Inconel 718 for is on the LOX tank near the nose of the vehicle.
the thermally compliant supports. Stations 802 and 1200 are on the LH2 tank, where STA
Design of thermal protection systems is dependent 802 is near the middle of the RLV and STA 1200 is
on the underlying structure. In this analysis, a single near the engines. Aerothermal heating rates2are shown
stage to orbit (SSTO) RLV is studied that uses foam- as a function of time for the three vehicle stations in
filled-honeycomb-sandwich semi-conformal LOX and Figure 6. As can be seen, the heating at STA 240 is
LH2 tanks analyzed in the study by Wang, et al.7 significantly higher than the heating at STA 802 and
Figure 2 shows the semi-conformal tanks, where the 1200.
LOX tank is forward and the LH2 tank is aft. The
2
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Aerothermhael atingwascalculateudsingthe
equation: k;
kg m
q = h(H_ - H e) 2 2-o_ 2y 1 )_
where h, the heat transfer coefficient, and H.... the
recovery enthalpy, are time dependent quantities where kg* is the temperature-dependent gas thermal
obtained from the aerothermal environment data. Hg is conductivity for air, c_ is the accommodation
the atmospheric gas enthalpy, and is calculated using coefficient, 7is the specific heat ratio for air, and Pr is
the empirical equation: the Prandtl number. Lo is the characteristic length of
the enclosure. In this work, the characteristic length
943.6 was assumed to be the core height. The mean free path,
Hg = 0.2345 *T + 9.786E - 6* T 2 + -- - 1.57
T )_,is given by:
where the units of Hg are Btu/lbm. TPS outer surface
2c= KBT
temperature is represented by T. Using the recovery
enthalpy boundary condition is more accurate than
applying a heat flux, since the influence of TPS surface where KB is the Boltzmann constant, dg the gas
temperature is included. collision diameter, and T and P the temperature and
pressure, respectively. Radiation inside honeycomb
Thermal Sensitivity Studies core was approximated using a rod element with an
Thermal sensitivity studies were conducted to equivalent conductivity calculated using the equations
determine the effect of key assumptions and parameters developed by Swann and Pittmanl°:
on TPS insulation requirements and weight. The areas
studied were: purging during groundhold, reentry
krad = 4(OTLg3L
purge initiation time, reentry initial temperature, and
TPSS temperature limit. A one-dimensional transient
heat transfer finite element model, including elements where Tavg is the average rod element nodal
temperature, cyis the Stefan-Boltzmann constant, L is
to model the effects of heat shorts, was created for use
the honeycomb core height, and (is given by:
in the sensitivity studies and for later use in the sizing
analysis. Studies by Blosser have shown that one
dimensional models reasonably predict temperatures in _"= 0.664(fl + 0.3) (-°69) _°1"63(fl+1)(0.89)
TPS systems. 8
In this equation, 8 is a uniform emissivity value inside
Thermal Finite Element Model
the honeycomb and _3is given by:
Figure 7 shows a diagram of the thermal finite
element model of the TPS/TPSS/Tank system. The
L
TPS/TPSS/Tank system is shown schematically in
d
Figure 3. In the model diagram, surfaces are depicted
by open circles, and were used to apply boundary
conditions and keep track of surface related quantities, where disthe honeycomb cell size.
such as coating emissivity and surface area. Nodes are The primary mode of heat transfer through the TPS
represented by filled circles, and rod heat transfer will be through the Saffil fibrous insulation layer, due
elements are represented by lines. to its large area. Saffil thermal conductivity is highly
The honeycomb sandwich on the TPS outer surface pressure dependent, so it was necessary to model both
was modeled using three rod heat transfer elements in temperature and pressure dependency of the insulation
parallel, along with increased thermal capacitance at the layer material properties.
end nodes to account for facesheet thermal mass. The In addition to heat transfer through the insulation,
three elements were used to model solid conduction the heat shorts resulting from the compliant sides and
through the core, gas conduction in the enclosed thermally compliant supports were also included, as
honeycomb, and radiative heat transfer between the well as a model of the box beam on the lower surface of
outer and inner facesheets and the core, respectively. the TPS that included four elements in parallel to
The gas thermal conductivity was determined using9: simulate solid conduction through the box beam sides,
3
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solid conduction through the mechanical fasteners, gas applied on the surface labeled "Outer Surface BC".
conduction, and radiation. Data for calculation of aerothermal heating on the TPS
The TPS panel rests on a Nomex felt pad at each outer surface was obtained from the RLV 3c ascent
corner and is mechanically fastened to the TPSS. The aerothermal data file.2 Radiation to space was modeled
TPSS was modeled with two solid conduction elements, assuming an emissivity of 0.86 and 0.8 for TPS with an
one representing the Nomex pad and the other Inconel 617 and titanium outer honeycomb sandwich
representing mechanical fasteners. A cavity is formed panel, respectively.
between the back of the TPS panel and the outer surface The reentry loadcase applied aerothermal heating
of the tank. Heat transfer across the cavity was and radiation to space boundary conditions on the
modeled with two elements in parallel, one modeling "Outer Surface BC" surface. All other surfaces were
gas conduction in an enclosure and the other modeling adiabatic. As with the ascent loadcase, emissivities of
radiation between infinite parallel plates. Finally, the 0.86 and 0.8 are assumed for TPS with Inconel 617 and
foam filled honeycomb sandwich tank structure was titanium outer honeycomb sandwich panels,
modeled with four elements in series, representing solid respectively. It takes approximately 43 minutes for the
conduction through the cryogenic foam and honeycomb RLV to touch down, however peak temperatures in the
core. The thermal capacitance of the end nodes was tank wall often occur after touch down. For this reason,
increased to account for facesheet thermal mass. it isnecessary to extend analysis to simulate the vehicle
sitting on the runway. At 43 minutes the boundary
Thermal Load Cases condition applied to "Outer Surface BC" is changed
The boundary conditions were varied to represent from aerothermal heating and radiation to a convection
the thermal conditions expected during the RLV flight boundary condition with air temperature set at 70 °F. It
cycle. Three transient thermal loadcases were defined: was assumed that an air purge is initiated in the cavity
groundhold, ascent, and reentry. region 30 minutes after touchdown in order to cool
Groundhold analysis assumed the cavity between down the TPS and tank, with 30 minutes being an
the back of the TPS panel and the outer surface of the estimate of a reasonable amount of time to hook up
sandwich tank to be purged with gaseous nitrogen. ground based blowers to the RLV. Since the purge is
This was simulated by applying convection boundary performed in the area of the TPS / tank system that will
conditions to the surfaces marked "Purge BCs" in be most sensitive to over-heating, i.e. the TPSS and the
Figure 7. Purge temperature was assumed to be -160 tank wall, it is assumed that the purge works very
°F. A heat transfer coefficient of 1.0E-3 Btu/s-ft-R was quickly to reduce temperatures. The analysis is
used to represent forced convection heat transfer, based therefore concluded at the initiation of purging.
on the work reported in Reference 11. Based on Sensitivity studies were performed to assess the
empirical calculations, this corresponds to a flow rate benefits of performing reentry purging more quickly
on the order of 3 ft/s. The purge boundary condition after touchdown, or even while the RLV was traveling
drives the node at which it is applied to within a few at subsonic speeds via an air scoop, and will be reported
degrees of the purge gas temperature, effectively acting inthe results section.
like a prescribed temperature boundary condition, so
that increasing flow rate beyond 3 ft/s will not Insulation Sizin_ Criteria
significantly influence the results. A prescribed Both the Saffil insulation thickness and the foam
temperature boundary condition is applied to the filled honeycomb core thickness were sized using
surface marked "Inner Surface BC" to model the effect iterative thermal analyses, increasing or decreasing
of cryogenic fuel, where temperatures of 423 and 300 layer thicknesses until an optimum solution was
°F were used for the LH2 and LOX tank, respectively. reached. Saffil insulation thickness was minimized
In addition, a convective boundary condition, with heat with constraints that temperature limits in the TPS,
transfer coefficient of 6.94E-4 Btu/s-ft-R is applied to TPSS, and tank were not exceeded during ascent or
the surface "Outer Surface BC", allowing convection to reentry loadcases, resulting in at least one critical node
ambient air at 70 °F. The heat transfer coefficient is with temperature equal to a temperature constraint
obtained from Reference 13 and represents typical (within a +/- 5 °F tolerance). The foam filled
launch pad conditions. honeycomb thickness was optimized such that heat flux
In the ascent loadcase the purge boundary condition into the cryogenic fuel was under 0.01 Btu/s-ft 2and air
was removed. The same cryogenic boundary condition liquefaction was prevented during groundhold and
on "Inner Surface BC" used in the groundhold loadcase ascent. The heat flux constraint is based on
was used in the ascent loadcase. Finally aerothermal conservative estimates of heat flux into the shuttle
heating and radiation to space boundary conditions are external fuel tank reported in Reference 11. A pressure
American Institute of Aeronautics and Astronautics
dependernetlationwasusedforairliquefactiotno
preliminary sizing of TPS. This allows calculation of
allowaccuratedeterminatioonf air liquefaction
AP using the formula:
....
conditiondsuringascent. Thiscanbecomean
importanctonsideratiwonhenpurgingis performed
duringgroundholdA.llfinalconstraiwntasplacedon aerodynamic = Plocal static -- Patmospheric
thefoamfilledhoneycomthbattheminimumthickness
equalthethicknesosf thetankasdeterminebdy where Plocalst3tic is the inward acting component of local
structursailzing7. aerodynamic pressure and P3tmospheric is the local
atmospheric pressure at the current vehicle altitude. A
positive value indicates inward acting pressure.
Loads Table Generation
To perform combined thermal and structural
Structural Model
analysis and sizing, a loads table was needed after each
A structural finite element model was used to
thermal analysis to allow determination of cases to be
calculate deflection of the outer honeycomb sandwich
structurally analyzed. A representative loads table is
shown in Table I for STA 1200. The loads table is used panel and consisted of: outer honeycomb sandwich
panel modeled with composite shell elements,
to collect thermal analysis, aerothermal environment,
thermally compliant supports modeled with bar
and trajectory data for several different structural load
elements, and box beam frame modeled with bar
cases. Load cases were defined for both ascent and
elements. Figure 8 shows the resulting finite element
reentry. Ascent cases were liftoff, maximum normal
model. Uniform pressure loading was applied on the
force, maximum pressure gradient, maximum thermal
sandwich outer surface, and temperatures were applied
gradient, and maximum axial acceleration. Reentry
over the entire model. Loads were obtained from the
cases were maximum thermal gradient, maximum
loads table at specific times of interest.
surface heat flux, and maximum pressure gradient.
Degraded material properties were used for Inconel
Data collected for each case includes temperatures,
617 foil facesheets to account for the effect of brazing
atmospheric pressure, static normal pressure acting on
in the fabrication process. In addition, temperature
the TPS surface, and vehicle accelerations. Acoustic
dependent properties were used for Inconel 617 and Ti
pressure is calculated based on dynamic pressure, as 1100.
described in Reference 2.
The model was constrained at points A, B, C, and D
Pressure gradient acting on the TPS outer
as shown in Figure 8. All points were allowed z
honeycomb sandwich panel was determined using the
rotational freedom. All other degrees of freedom at
equation: Point A were fixed. Points B and C both had z
rotational freedom, in addition point B had translational
APultimate,TPS+ = 1.4(Ap .... dynamic + 3Ap........... tic ) freedom in the x direction and point C had translation
freedom in the y direction. Point D was given
translational freedom in the x and y direction as well as
1.4(Ap dynami-c- 3Ap
APultimate,TPS_ = .... ........... tic )
z rotational freedom. All other degrees of freedom at
points B, C, and D were fixed. The boundary
where APultimate,TPS+ and APultim3te,TP S_ represent the conditions represent mechanical attachment to
maximum inward and outward pressure expected from expansion slots, as described in Reference 3. Figure 9
the combination of aerodynamic pressure and three shows a representative deflection (plot of z
standard deviations of acoustic pressure, acting in either displacement) resulting from an inward acting
the positive or negative direction. A factor of safety of (positive) pressure.
1.4is applied tothe loads.
The ARMOR TPS design forms an aerodynamic Iterative Sizin_ Method
shell that carries aerodynamic pressure on the TPS Thermal-mechanical sizing of the TPS panel was
outer surface. The inside of the TPS panel is vented to performed following the logic shown in Figure 10. The
local atmospheric pressure. In reality there may be process consists of the following steps: Making an
variations between TPS internal pressure and local initial guess of design parameters, performing iterative
atmospheric pressure due to a pressure lag effect, transient thermal analyses to size insulation layers,
however data on this effect was not available, and the creating a loads table from thermal analysis results and
assumption that TPS internal pressure equals local vehicle data, static deflection analysis of the outer
atmospheric pressure was deemed adequate for honeycomb sandwich, and finally local failure analysis
of the outer honeycomb sandwich. There are two
5
American Institute of Aeronautics and Astronautics
primarysizingloopsintheprocessa: ninnerloop faceshetehticknes(0s.006t"o0.016i"nincremenotfs
occursin thestructuraslizingof thehoneycomb0.001")andhoneycomdbepth(0.25"to 1.00"in
sandwictohdetermintheeminimumweighdtesigtnhat incremenotsf 0.05")wasconsideredT.he0.006"
satisfiebsothdeflectioanndlocalfailurecriteriat,he facesheethticknesrsepresenmtsinimumfacesheet
outerloopisusedforconvergenocfethestructural gaugeforthemateriaclsonsidereadn,disbasedon
parametvearluegsuesseindthethermaalnalysiwsith manufacturcinognsiderationItsm. aybenecessatory
thoseobtainethdrougshtructuraanlalysisT.hisprocess increastheeminimumgaugienthefuturetoaccounfotr
hastoberepeatefdoreachvehiclestationanalyzed.criteriasuchasgrounhdail,flyingthrougrhain,etc.a,s
Threewindwarcdenterlinveehiclestation(sSTAw) ere describeidn Referenc2e. Also,four different
selectefdoranalysisS: TA240,STA802andSTA honeycomspbecificatio(rnibsbonthicknesxscellsize)
1200w,hertehenumericvaallueisthestationlocation wereconsidered0:.002"xl/8"0,.002"x3/160".,002"
ininchesfromthenoseoftheRLV. STA240was xl/4",and0.002"x3/8"A. routinewascreatetdo
selectebdecausiteisnearthenoseandexperiencesautomaticaallnyalyzaenddetermintheeweighotfall
significanetntryheatingS,TA802isroughlhyalfthe honeycomdbesignsa,totalof 704designsin, the
vehiclleengthfromthenoseandisrepresentaotivftehe designspacdeefinebdythesandwicphanevlariables.
vehiclewindwardacreagaerea,andSTA1200is Fromthis,a tableof candidatdeesignss,ortedby
locatendeartheenginews,herehighacoustilcoading increasinwgeight,wascreatedfor eachmaterial
occurdsuringascent. (Incone6l17andTi1100).
Severasltructuraclomponenwtseresizedin a Theloweswteighhtoneycomsabndwicphanetlhat
companiostnudy3t:hermallcyompliansut pportbso,x passeddeflectiocnriteriawasthenanalyzetodcheck
beama,ndTPSsupposrttructureI.t waspossiblteo forlocalizefdailures.Locahloneycomstbresfsailure
sizethesecomponenintdsependenfrtolymthepresent criteriawerecheckeudsingHypersiz®e1ra2ndincluded:
activity.Inadditionth,etankstructurweasoptimized in-planteensilefailureandin-planseheafrailureofthe
intheworkbyWange,t.al.7 facesheeitnst,racelluladrimplingofthefacesheets,
InordetrostartthesizingprocessshowinnFigure transverssheeafrailureofthecorea,ndcorecrushing.
10,an initialguessof insulationandstructural Atthispointt,hefirstdecisiobnoxinFigure10has
parametvearlueissmadbeaseodnpreviouesxperiencebeenreachedI.f thecandidahteoneycomsabndwich
analyzingTPS. Thermaalnalysisandsizingof passethselocalizefdailurecriteriaa,nalyspisroceedtos
insulatiothnickneisssthenperformeudsingthethermal theseconddecisiobnox.Otherwisteh,enextheavier
finiteelememntodeal,spreviousdlyescribed. honeycomsabndwicthhatpassethdedeflectiocnriteria
Nexta,loadstableisgenerateTdh.eloadstableis isselecteadndcheckefodrlocalizefdailureT. hisinner
usedto collectthermaalnalysisa,erothermaaln,d loopisrepeateudntiladesiginsfoundthatpasses.
trajectordyatafor severadlifferensttructuralolad Theseconddecisionbox,shownin Figure10,
cases.Criticalloadcasewseredeterminefrdomthe compartehsestructurpaalrameteurssedinthethermal
loadstableforstructuraanl alysiasndsizing.Linear analysitsothestructurpaal rametedresterminefrdom
staticdeflectioannalysoisfthehoneycompabnedlueto thestructuraalnalysisI.f theparametearrsewithin
thermaalndpressurgeradientwsasperformewdith toleranceth,eanalysisfor thisvehiclestationis
NASTRAND.eflectiolinmitswereimposetodprevent complete.Otherwiseit,is necessatroyreturnto
boundalrayyetrransitioanthighMachnumbearsndto thermaalnalyswisiththeupdatesdtructurpaalrameters.
prevenptermanecnotmpactioonffibrousinsulation. Tolerancweasdefineads+/-0.002fo"rfaceshegeatuge
ExcessidveeflectioonftheTPSoutesrurfacceanresult and+/-0.05"forhoneycomdbepth. In all cases
in anearlytransitioonf flowtypefromlaminatro honeycomgbaugeandhoneycomcbell sizewas
turbulentF. orthisreasona,tvelocitiegsreatetrhan matchedexactlybetweenthermalandstructural
Mach5.0,a deflectiolnimitbasedonTableII is analysis.
imposeodnthehoneycomsabndwicphane2l.InTable
II, L isthediagonalelngthoftheTPSpanel.The Results
seconddeflectiolinmit,imposetdoprevenptermanent
insulatiocnompactioisni,neffecftorallloadcasaens,d
Thermal Sensitivity Study
requirethsattheTPSoutersandwicphanedleflection
Thermal sensitivity studies were performed to
notexcee1d0%ofthetotaTl PSpanethl ickness.
examine the effect of:
Twodifferenmt aterialwsereconsiderefodrthe
1. Groundhold purging on system weight
outehroneycomsabndwichIn:cone6l17andTi 1100.
2. Earlier reentry purging
Materiaslelectiownasbaseodnmaximumtemperature
3. Assumed initial reentry temperature
reacheadndtimeatthattemperatureA. rangeof
American Institute of Aeronautics and Astronautics
4. IncreasinTgPSStemperatulimreit
driven by the 300 °F temperature limit of the TPSS and
Structurpaal rametewrsereheldconstanthtroughout
tank structure. By increasing the initial temperature
thisstudyw, ithemphasoisndeterminitnhgeinfluence
from 70 to 250 °F, the allowed change in temperature
ofthestatedchangeosnSaffilandcryogenifcoam
of the TPSS and tank is reduced from 230 °F to 50 °F,
filledhoneycomlabyewr eight.
which means that the heat capacity of the TPSS and
Weighdtsirectlryelatetdoinsulatiolanyetrhickness
tank that can be used to store the absorbed energy
(Saffil,cryogenfiocaminsulationc,ompliansut pports
during reentry isreduced by a factor of 4.6.
andsidesa,ndcryogenifcueltankhoneycomcbore)
The TPSS and cryogenic fuel tank facesheets were
werecalculatefodrtypicaTl PSdesignasndcompared
assumed to be made of graphite epoxy with a maximum
totheoveralwl eightoftheTPS,TPSSa,ndtank
temperature limit of 300 °F. In all analyses, the TPSS
sandwicphanel.Ascanbeseenin Figure11,the temperature limit constraint was active in the sizing of
weightdirectlyrelatedtoinsulatiolnayerthickness
Saffil insulation. It was anticipated that using a
amounttosbetwee3n4%and39%oftheoveraTllPS
/ material with a higher temperature limit for the TPSS
tank system weight, depending on which vehicle station would significantly reduce TPS weight. For this final
is examined. sensitivity study, the temperature limit of the TPSS was
Figure 12 compares insulation weight of systems increased to 350 °F, which resulted in the tank structure
that were purged during groundhold with unpurged temperature limit becoming the active constraint during
systems. At STA 240 and STA 1200 purging had no Saffil insulation sizing. Since the tank structure
effect on system weight. This is due to the constraint temperature limit constraint was already close to being
that cryogenic foam insulation thickness be greater than active, only a small weight reduction was seen at STA
or equal to the core thickness determined by structural 240 and STA 1200, with a larger 12% weight reduction
analysis of the tank. At STA 802 purging resulted inan at STA 802. From these results, it appears that to
increase in required cryogenic foam thickness to significantly reduce TPS weight, both the TPSS and
prevent violation of the constraint limiting heat flux tank temperature limits need to be increased.
into the tank. Insulation requirements can be reduced at
this vehicle station by reducing the purge temperature. Thermal-Mechanical TPS Sizin_
Most analyses assumed that a reentry purge was TPS panel sizing required iteration between thermal
performed 30 minutes after vehicle touchdown, which analysis and structural analysis. The results reported
would be accomplished by attaching external blowers are for the final, converged solution.
to the RLV to vent the cavity between TPS and tank. In Table III shows the results of thermal sizing of the
Figure 13, the effect of purging at an earlier time is insulation layers at three vehicle stations. Maximum
examined. Times of 4365 and 3465 seconds TPS surface temperatures ranged from 1514 °F at STA
correspond to 30 and 15 minutes after touchdown, 240 to 1140 °F at STA 1200. Inconel 617 TPS was
respectively. Purging at 2360 seconds represents used for STA 240 since the maximum surface
purging while the RLV is still in the air at subsonic temperature is well above the temperature limit of Ti
velocity via an air scoop. As can be seen there is only a 1100. Two cases were examined at STA 802. Case
small benefit to purging earlier. However, if purging number 2 used Inconel 617 TPS and case number 3
could be initiated at supersonic velocities it is likely used Ti 1100 TPS. This was done to assess the
that there would be a significant weight savings. potential benefits of running Ti 1100 past the material
In reentry vehicle insulation sizing the initial temperature limit. At STA 1200 Ti 1100 TPS was
temperatures of the TPS and tank are uncertain used, since the maximum surface temperature is close
parameters. These parameters are determined by the to the temperature limit of Ti 1100. Reentry insulation
specific operation of the vehicle prior to reentry, and ranged from 1.89" to 3.08", with all cases being sized
are not known at a preliminary design stage. Figure 14 by the TPSS temperature limit. Cryogenic insulation
shows that there is significant sensitivity of TPS thickness ranged from 0.62" to 0.82"; with sizing
insulation related weights to assumed initial driven by either allowable heat flux into the fuel tank
temperature. This information may be useful from an during groundhold or minimum structural thickness of
operations standpoint, since it indicates that measures the fuel tank sandwich panel.
taken on orbit to reduce vehicle temperature prior to Table IV shows results from structural sizing of the
reentry will significantly reduce TPS weight. It is TPS panel honeycomb sandwich panel. In all cases,
interesting to note that there is a large jump in honeycomb core was selected with 0.25" cell size and
insulation related weight between assumed initial 0.002" ribbon gauge. Honeycomb thickness ranged
reentry temperatures of 70 and 250 °F. This results from 0.30" to 0.75", depending on vehicle station and
from the fact that reentry insulation sizing is being material used. Facesheet thickness was normally sized
American Institute of Aeronautics and Astronautics
bytheminimummateriaglaugeconstrainotf0.006", obtainefrdomReferenc1e5,andpropertiefosrInconel
butwas0.008a"tSTA1200. 617andIncone7l18wereobtainefrdomReferen1c6e.
The"SizingInformations''ectionof TableIV MateriaplropertiefsortheSaffilfibrousinsulation
reporttsheloadcastehsatproducethdeworsltoading layewr ereobtainefrdomReferen6c.eWeightfsorthe
conditionfosrsizingofthehoneycompabnedl ueto "boxbeam","complianstupport"a,nd"compliant
deflectioanndlocaflailuremodes".CriticaDleflection sidesc"omponernetpsresennotminawleights.
Loadcasied"entifietsheloadcastheatresulteidnthe ComponeanntdtotaTl PSweightfsoreachcaseare
maximuhmoneycompabnedleflection"C. riticaLlocal summariziendTableVI. Thereisa13%decreasine
FailureLoadcasree"porttsheloadcastheatproducedTPSpanewl eighbtetweeCnase1sand2,duetothe
thelowesmt arginosfsafetuysedinsizingforlocalized decreasiendsulatiorenquiremeanttSsTA802.Case3
failuremodes.Finallyt,he"CriticalLocalFailure useTsi 1100insteaodfIncone6l17forthehoneycomb
Mode"liststhelocalfailuremodewiththelowest sandwichas well as "complianstupport"and
marginofsafety. "complianstides"componenrtes,sultingin a 19%
ExamininthgeresultsforSTA240inTableIV,it weightreduction. Thereducedweightof the
canbeseenthatboththecriticadl eflectioanndlocal honeycomsabndwicchontributteos58%ofthisweight
failureloadcasweasthemaximumpressurgeradient reductionw,iththereducewdeighotfthe"compliant
caseduringascenwt,herethepressugreradienatcting support"and "compliantsides" components
ontheouterTPSsurfacies1.76psia.Thisloadcasecontributintog38%oftheweighrteduction.It is
occur6s0seconadfstelriftoff. obvioutshattitaniumispreferabaletSTA802if the
Case2sand3wereperformeadtSTA802,with outersurfacceanbemadetowithstantdhe1225°F
Incone6l17honeycomsabndwicphaneulsedinCas2e reentrtyemperaturTeh.eseveraecoustpicressuarend
andTi1100honeycomsabndwicphaneulsedinCas3e. thermaglradienrtessultinfgromincreasinhgoneycomb
Inbothcasesth,ecriticadl eflectioanndlocalfailure thicknesrsesultin a 49%increasien honeycomb
loadcasweasduetoenginaecoustpicressudreuringthe sandwicwheighattSTA1200(Cas4e). Changeins
initialsecondosfliftoff.Pressugreradienotntheouter othecromponewnetightasreminimaalndoveraTllPS
surfacweas1.3psiap,rimarildyuetoenginaecoustics.weighistincreasbeyd18%.
A thickerhoneycomsbandwicwhasrequiredwhen
titaniumwasused.Howevear,swillbeseent,he Conclusions
titaniumsandwicwhasstillsignificanltilgyhtetrhanthe
Thermal sensitivity studies were performed to
Incone6l17sandwich.
determine the influence of analysis and design
Vehiclestation1200requiredthethickesotuter
parameters on insulation sizing. It was found that
honeycomcobre0,.75".Structurdaelsigantthisstation
groundhold purging at 160 °F will increase cryogenic
isinterestiningthatinitialsizingiterationwseredriven
insulation requirements at some vehicle stations. If
byliftoffacousticrse,sultinignincreasehdoneycomb
possible the purge temperature should be lowered. In
corethicknessH.owevear,sthehoneycomcobrewas
addition, performing reentry purging was not effective
madethickerthermagl radienttshroughthe core
in reducing insulation requirements for the initiation
becammeoresignificantotpanedl eflectiounn,tilthe
times examined. It is possible that purging at
reentrmyaximumthermaglradienlotadcabseecamthee
supersonic velocity would be beneficial. Initial
structuraslizingdriver. Thisloadcasoeccurs38
temperature of the TPS and tank on reentry has a large
minuteisntoreentrys,hortlybeforetouchdownT.he
effect on insulation sizing, with initial temperatures
thermagl radienits 356F (resultingin inward
greater than 70 °F resulting in significant insulation
concavepdanelshapew) ithminimalaerodynamic
weight penalties. Finally, it was seen that in general
pressureT.hecriticalloadcasfoerlocalizefdailure
increasing the TPSS temperature limit did not
sizingwasstilltheliftoffconditiodnu,etothe4.26psia
significantly reduce TPS weight when tank structure is
pressuregradientp,redominantdlyueto engine
a epoxy composite.
acoustics.
A sizing process was created for metallic TPS
TableV showsa representatiTvPeSweights
panels using a coupled thermal and structural analysis
calculationat STA 1200. Thetableincludes
approach. The process included insulation sizing using
assumptionfosr materialpropertiesand design
the thermal model generated for the sensitivity studies,
parameteorsnthelefthandsideanda weights
deflection analysis of the outer honeycomb sandwich
breakdowbyncomponeonnttherighthandside.Itis
panel using a linear static finite element model, and
assumethdatthehoneycomcobreandskinarejoined
local failure analysis of the honeycomb panel using
usingaLiquidInterfacDeispersio(nLID)technique
Hypersizer. Sizing was performed at vehicle stations
withpropertieasslisted.PropertifeosrTi 1100were
240, 802 and 1200 along the windward centerline of a
American Institute of Aeronautics and Astronautics
liftingbodyRLV. Inall casesth, eSaffilfibrous
6Blair, W.; Meaney, J. E.; and Rosenthal, H. A.:
insulationlayerwassizedduringreentryby the
Re-Design of Titanium Multi-Wall Thermal Protection
temperatulirmeitoftheTPSS.Thecryogenic-foam-
System Test Panels. NASA CR-172247, 1984.
filledhoneycomcboretankwallwassizedeithebry 7Wang, J. T.; et. al.: Cryogenic Tank Structure
structurlaoladingu,singresultsfromReferenc7e,or
Sizing with Structural Optimization Method. Presented
theconstrainotnheatflux intothetankduring at 42ndAIAA /ASME / ASCE /AHS / ASC Structures,
groundholdA.tSTA240,neatrhenoseoftheRLV,
Structural Dynamics, and Materials Conference, April
theTPSoutehroneycomsabndwicphanewl assizedby
16 19, 2001/Seattle, WA. Available as AIAA-2001-
maximumaerodynampircessurdeuringascent.At 1599.
STA802thepanelwassizedby acoustilcoading SBlosser, M. L.," Advanced Metallic Thermal
duringliftoff.Finallya,tSTA1200thepanewlassized
Protection Systems for Reusable Launch Vehicles,"
byacombinatioofnacoustliocadindguringliftoffand
Ph.D. Dissertation, University of Virginia, May, 2000.
thermaglradienintduceddeflectiodnuringreentryI.n
9Kennard, E. H.: Kinetic Theory of Gases; McGraw
all casest,he criticallocalfailuremodeof the
Hill, New York, 1938.
honeycomsbandwicwhasintraceldlimplingof the
_°Swann, R. T., and Pittman, C. M., "Analysis of
facesheets.
Effective Thermal Conductivites of Honeycomb-Core
TPSpanewl eighdtecreasferodmSTA240toSTA
and Corrugated-Core Sandwich Panels," NASA TN D-
802duetodecreasiendsulatiornequirementIsf.Ti
714, April, 1961.
1100canbeusedattemperaturoefs1225°F,a
HPoteet, C. C., "Groundhold and Reentry Purge
significanwteighstavingcsanberealizeidntheregion
Parameter Trade Study," Lockheed Martin, Langley
ofSTA802.FinallyT,PSpanewl eighitncreaseast
Program Office, Hampton, VA, LMES SDSR 99-01,
STA1200dueto acousticpressuraendthermal 1999.
gradientsresultingfrom increasedhoneycomb
12Hypersizer Analytical Methods & Verification
thickness.
Examples, 3rd Edition, Collier Research Corporation,
Unpublished, 1999, available at collier-research.com.
References _3Gruszczynski, M. J., Thorp, V. L., Heim, W. J.,
1Poteet, C. C.; Blosser, M. L.: Improving Metallic and Swanson, N. J.: Design, Development, and Test of
Thermal Protection System Hypervelocity Impact the Atlas Liquid Hydrogen Propellant Tank Foam
Resistance Through Design of Experiments Approach. Insulation System. Proceedings from AIAA 26th
To be presented at the 40thAerospace Sciences Meeting Thermophysics Conference. AIAA Paper 91-1438,
and Exhibit, January 14 17, 2002/ Reno NV. 1991.
Available as AIAA-2002-0912. _4Weiser,E. S.; et. al.: Polyimide foams for
2Dorsey, J.T.; Chen, R.; and Poteet, C. C.: Metallic aerospace vehicles. High Perform. Polym. 12 (2000) 1-
Thermal Protection System Technology Development: 12. Printed inthe UK.
Concepts, Requirements and Assessment Overview. To _SHutt, A. J.; Parris, W. M.: "TIMETAL-1100
be presented at the 40 th Aerospace Sciences Meeting Sheet Properties", Titanium '92 Science and
and Exhibit, January 14 17, 2002/ Reno NV. Technology, The Minerals, Metals and Materials
Available as AIAA-2002-0502. Society, 1993.
3Blosser, M. L.; Chen, R.; Schmidt, I.; Dorsey, J.T.; 16Brown, W. F.; Mindlin, H.; and Ho, C. Y.:
Poteet, C. C.; Daryabeigi, K.; and Bird, K.: Advanced Aerospace Structural Metals Handbook, CINDA/USAF
Metallic Thermal Protection System Development. To CRDA Handbooks Operation, Purdue Univ., 1994.
be presented at the 40 th Aerospace Sciences Meeting
and Exhibit, January 14 17, 2002/ Reno NV.
Available as AIAA-2002-0504.
4Chen, R.; Blosser, M. L.: Metallic TPS Panel
Flutter Study. To be presented at the 40thAerospace
Sciences Meeting and Exhibit, January 14 17, 2002/
Reno NV. Available as AIAA-2002-0501.
5Blosser, M. L.: Investigation of Fundamental
Modeling and Thermal Performance Issues for a
Metallic TPS Design. To be presented at the 40th
Aerospace Sciences Meeting and Exhibit, January 14-
17,2002/Reno NV. Available as AIAA-2002-0503.
9
American Institute of Aeronautics and Astronautics