Thermal Protection Systems - NASA

1 The Space Shuttle design presented many Thermal insulationchallenges. The system not only had to perform well, it had to integratewith other subsystems. The Orbiter s surfaces were exposed toexceedingly high temperatures and needed reusable, lightweight,low-cost Thermal Protection . The vehicle also required low vulnerabilityto orbital debris and minimal Thermal conductivity. NASA decided tobond the Orbiter s Thermal Protection directly to its aluminum skin,which presented an additional challenge. The External Tank required insulation to maintain the cryogenic fuels,liquid hydrogen, and liquid oxygen as well as to provide additionalstructural integrity through launch and after release from the challenge and solutions that NASA discovered through tests andflight experience represent innovations that will carry into the nextgeneration of space InnovationsThermalProtectionSystemsIntro ductionGail ChaplineOrbiter Thermal Protection SystemAlvaro RodriguezCooper SnappGeminesse DorseyMichael FowlerBen GreeneWilliam SchneiderCarl ScottExternal Tank Thermal Protection SystemMyron Pessin Jim ButlerJ.

2 Scott SparksSolid Rocket Motor Joint An Innovative SolutionPaul Bauer Bruce SteinetzIce Detection Prevents Catastrophic ProblemsCharles StevensonAerogel-based Insulation System Charles StevensonOrbiter ThermalProtection SystemThroughout the design and developmentof the Space Shuttle Orbiter ThermalProtection System, NASA overcamemany technical challenges to attain areusable system that could withstand thehigh-temperature environments ofre-entry into Earth s von Karman, the dean ofAmerican aerodynamicists, wrote in1956, Re-entry is perhaps one of themost difficult problems one canimagine. It is certainly a problem thatconstitutes a challenge to the best brainsworking in these domains of modernaerophysics. He was referring toprotecting the intercontinental ballisticmissile nose cones.

3 Fifteen years later,the shuttle offered considerably greaterdifficulties. It was vastly larger. Itsthermal Protection had to be reusable,and this Thermal shield demanded bothlight weight and low cost. Therequirement for a fully reusable systemmeant that new Thermal protectionmaterials would have to be developed,as the technology from the previousMercury, Gemini, and Apollo flightswere only single-mission capable. Engineers embraced this challenge bydeveloping rigid silica/alumina fibrousmaterials that could meet the majorityof heating environments on windwardsurfaces of the Orbiter. On the nose cap and wing leading edge, however,the heating was even more extreme. In response, a coated carbon-carboncomposite material was developed toEngineering Innovations183 While the re-entry surface heating of theOrbiter was predominantly convective,sufficient energy in the shock layerdissociated air molecules and provided thepotential for additional heating.

4 As the airmolecules broke apart and collided with thesurface of the vehicle, they recombined in an exothermic reaction. Since the surfaceacted as a catalyst, it was important that theinterfacing material/coating have a lowpropensity to augment the reaction. Atomicrecombination influenced NASA s selectionof glass-type materials, which have lowcatalycity and allowed the surface of theOrbiter to reject a majority of the chemicalenergy. Engineers performed precise arc jet measurements to quantify this effect over a range of surface temperatures forboth oxygen and nitrogen resulted in improved confidence in theThermal Protection Protection System Could Take the HeatOrbiter remained protected during catalytic heating. FlowNitrogen and oxygen molecules are dissociated in the shock may recombineand form moleculeson the vehicle molecules in the shock layer separate into O+ and O- of atoms onthe surface of the vehicleadds heat of dissociation tothe Thermal Protection LayerBoundary Layerogen and NitrFlowogen and on the vehicle form moleculesAtoms may rthe shock layere dissociated in aroxygen molecules on the vehicle form moleculesecombineoms may r e.

5 Layer dissociated in oxygen molecules and O+Oseparate into in the shock layer Oxygen molecules the shock layer Oxygen molecules the Thermal Pradds heat of dissociation tothe surface of the vehicleRecombination of atoms on otection System. r adds heat of dissociation tothe surface of the vehicleRecombination of atoms onform the contours of these structuralcomponents. NASA made anexhaustive effort to ensure thesematerials would operate over a largespectrum of environments duringlaunch, ascent, on-orbit operations,re-entry, and re-entry, the Orbiter s externalsurface reached extreme temperatures up to 1,648 C (3,000 F). The ThermalProtection System was designed toprovide a smooth, aerodynamic surfacewhile protecting the underlying metalstructure from excessive loads endured by the systemincluded launch acoustics, aerodynamicloading and associated structuraldeflections, and on-orbit temperaturevariations as well as naturalenvironments such as salt fog, wind, and rain.

6 In addition, the ThermalProtection System had to resistpyrotechnic shock loads as the Orbiterseparated from the External Tank (ET). The Thermal Protection Systemconsisted of various materials appliedexternally to the outer structural skin of the Orbiter to passively maintain theskin within acceptable temperatures,primarily during the re-entry phase of the mission. During this phase, theThermal Protection System materialsprotected the Orbiter s outer skin fromexceeding temperatures of 176 C(350 F). In addition, they were reusablefor 100 missions with refurbishment andmaintenance. These materials performedin temperatures that ranged from -156 C (-250 F) in the cold soak of spaceto re-entry temperatures that reachednearly 1,648 C (3,000 F). The ThermalProtection System also withstood the forces induced by deflections of the Orbiter airframe as it responded to various external the vehicle surface, a boundary layer developed and was designed to be laminar smooth, nonturbulentfluid flow.

7 However, small gaps anddiscontinuities on the vehicle surfacecould cause the flow to transition fromlaminar to turbulent, thus increasing the overall heating. Therefore, tightfabrication and assembly toleranceswere required of the Thermal ProtectionSystem to prevent a transition toturbulent flow early in the flight whenheating was at its for the ThermalProtection System extended beyond the nominal trajectories. For abortscenarios, the Systems had to continue toperform in drastically differentenvironments. These scenarios included:Return-to-Launch Site; Abort OnceAround; Transatlantic Abort Landing;and others. Many of these abortscenarios increased heat load to thevehicle and pushed the capabilities ofthe materials to their Protection System MaterialsSeveral types of Thermal ProtectionSystem materials were used on theOrbiter.

8 These materials included tiles,advanced flexible reusable surfaceinsulation, reinforced carbon-carbon,and flexible reusable surface insulation. All of these materials usedhigh-emissivity coatings to ensure the maximum rejection of incomingconvective heat through radiative heat184 Engineering InnovationsOrbiter Tile Placement System Configuration High-temperature Reusable Surface Insulation TileLow-temperature Reusable Surface Insulation TileReinforced Carbon-Carbon CoatingFlexible Reusable Surface Insulation BlanketAdvanced Flexible Reusable Surface Insulation Blankettransfer. Selection was based on thetemperature on the vehicle. In areas in which temperatures fell belowapproximately 1,260 C (2,300 F),NASA used rigid silica tiles or fibrousinsulation.

9 At temperatures above that point, the agency used reinforcedcarbon-carbon. TilesThe background to the shuttle s tiles lay in work dating to the early 1960s at Lockheed Missiles & SpaceCompany. A Lockheed patentdisclosure provided the first descriptionof a reusable insulation made ofceramic fibers for use as a re-entryvehicle heat shield. In other phasedshuttle Thermal Protection Systemdevelopment efforts, ablatives and hotstructures were the early , tight cost constraints and astrong desire to build the Orbiter withan aluminum airframe pointed towardthe innovative, lightweight, andreusable insulation material that couldbe bonded directly to the airframe used two categories of ThermalProtection System tiles on theOrbiter low- and high-temperaturereusable surface insulation.

10 Surfacecoating constituted the primarydifference between these two reusable surfaceinsulation tiles used a black borosilicateglass coating that had an emittancevalue greater than and covered areasof the vehicle in which temperaturesreached up to 1,260 C (2,300 F).Low-temperature reusable surfaceinsulation tiles contained a white coating with the proper opticalproperties needed to maintain theappropriate on-orbit temperatures forvehicle Thermal control purposes. The low-temperature reusable surfaceinsulation tiles covered areas of thevehicle in which temperatures reachedup to 649 C (1,200 F).The Orbiter used several different types of tiles, depending on thermalrequirements. Over the years of the program, the tile composition changed with NASA s improvedunderstanding of Thermal majority of these tiles,manufactured by Lockheed Missiles & Space Company, were LI-900 (bulk density of 144 kg/m3[9 pounds/ft3]) and LI-2200 (bulkdensity of 352 kg/m3[22 pounds/ft3]).