!"#" FactsNational Aeronautics andSpace Administration

John F. Kennedy Space CenterKennedy Space Center, Florida 32899 FS-2004-07-012-KSC

Solid Rocket Boosters andpost-launch processingIt’s a familiar sight during a

Shuttle launch: When the twinSolid Rocket Boosters (SRBs)have expended all their fuel, theyjettison away from the orbiter withhelp from the Booster SeparationMotors, about 26.3 nautical milesabove the Earth’s surface. Theydon’t fall immediately becausetheir momentum keeps themtraveling upward for about 70seconds, to approximately 38.6nautical miles. Then they begintheir tumble back toward theocean.

The twin sets of boostersprovide 80 percent of the SpaceShuttle launch thrust. Each SRB ismade up of four “loaded” solidpropellant segments called SolidRocket Motors or SRMs. TheSRBs operate parallel with theSpace Shuttle main engines for thefirst two minutes of flight, provid-ing additional thrust needed toescape the gravitational pull of theEarth.

The spent SRBs are recov-ered, cleaned, disassembled,refurbished and reused after eachlaunch. The recovery and clean-

ing process, however, is not likedriving your car through a carwash. Much of it is dangerous,from diving in the open ocean tohandling hazardous materials.

To slow the boosters as theybegin their descent to the ocean,the nose cap separates at analtitude of 2.5 nautical miles andreleases a pilot parachute that, inturn, deploys a drogue chute.This action provides initial decel-eration and proper orientation forthe SRBs.

At 6,000 feet, the frustum isseparated when the ordnancesystem fires the separation ring,located between the frustum andforward skirt structure, containinga linear-shaped charge. Theseparation pulls out the three mainparachutes, providing a “soft”landing in the ocean for the SRBs.

Slowing from a speed of 230miles per hour, they impact at aspeed of 51 miles per hour. Theentire descent takes about 5minutes. Because the parachutesprovide for a nozzle-first impact,air is trapped in the empty (burnedout) motor casing, causing the

booster to float with the forward end approximately30 feet out of the water.

The boosters (with aft skirts still attached), frus-tums, and parachutes are recovered by two SRBretrieval ships: the Liberty Star and Freedom Star.The nose cap and the aft portion of the exit cone arethe only sections left behind. If the nose cap isneeded for a special reason, such as post-flightinspection, it will be included in the retrieval process ifit can be located.

The main parachutes are reeled on board the shipfirst, followed by the drogue parachutes, which arestill attached to the frustum. Once the frustum isabout 100 feet from the ship, the parachute lines arereeled in until the frustum can be lifted from the waterby a 10-ton crane on board the ship.

Recovery of the SRB is the last part of boosterretrieval.

Divers plug the nozzle of the SRB with an en-hanced diver-operated plug, or EDOP. A 2,000-foot-long airline attached to the EDOP is plugged intoan air compressor located onboard the ship. Air ispumped into the booster and forces all the water out,causing the SRB to topple over into a horizontalposition. When the flight hardware is safed, the shipsreturn to Hangar AF at Cape Canaveral Air ForceStation (CCAFS), Fla., where the disassembly of theSolid Rocket Boosters will begin.

While the SRBs are en route, a 200-ton, StraddleLift crane at the Hangar is moved into position overthe slip, and its lifting slings are dropped into thewater. Upon arrival, each ship noses up to a wharf soits SRB can be in line with the hoisting slip. The leadrope is wrapped around an electric capstan that helps

to draw the SRB into the slip over the slings. Inaddition, various technicians surround the SRB andhelp tow it gently into the slip.

In turn, the crane raises each SRB above thewater and all remaining saltwater contamination iswashed off. While an SRB is being removed fromthe slip, its frustum and parachutes are offloaded fromthe ships and placed on transportation platforms.

The parachutes will be moved to the ParachuteRefurbishment Facility for cleaning and reuse on afuture launch. The frustum, however, is moved insidethe Hangar AF high bay, where it will undergoassessment and disassembly.

When each SRB is ready, it will be moved to asafing area and lowered onto a rail dolly. Techniciansthen begin the initial safing process. Afterward, theSRB is driven through the wash bay for a cleaningand rinsing, then moved back to the safing area.Ordnance is removed from the forward skirt and thethrust vector control system is depressurized.Workers at CCAFS help maneuver the SRB into the

hoisting sling.2

SRB SEGMENTS

Assessment teams move in to determine if thebooster received any damage during flight or post-flight activities. Once the SRBs have received a cleanbill of health, they are moved into the east and westwash bays. In these bays, they receive the ultimatepressure cleaning – known as “hydrolasing” – thatremoves the foam, or Thermal Protection System(TPS), from around the tunnel covers, Solid RocketMotors and aft skirt attach points.

Hyrdolasing requires two workers: one to operatethe rotating head gun and the other to act as a safetymonitor. The gun operates at 15,000 pounds persquare inch, producing an extremely loud sound asthe water impacts the SRB. Hearing protection isrequired to work in and around the area.

Following hydrolasing operations, the SRB ismoved inside the Hangar so that the aft skirts can beremoved. Due to the hydrazine still inside the thrustvector control systems, the Hangar is cleared of allnon-essential personnel. Once an “all clear” has beenestablished, the aft skirt operations can begin. Aseparation ring is placed on the SRB to ready the aftskirt for demating. Technicians operate hydrauliccontrol panels that are connected by lines to theseparation rings. As hydraulic fluid flows through thelines, the aft skirt is forced away from the aft segment.

Next, the crane trolley backs up, clearing the aftskirt away from the SRB. The aft skirt is then rotatedand lowered onto blocks, lifted and placed on atransportation dolly, and moved inside the ThrustVector Control Deservicing Facility for hydrazineremoval and additional disassembly operations.

Inside, technicians prepare the aft skirts fordeservicing. The auxiliary power units are removed,followed by the thrust vector control system and,finally, the aft booster separation motors. The aft skirtis then transported to the Robotic Hydrolase Facilityfor removal of the TPS.

The SRBs are rinsed one more time and thenmoved inside the Hangar, where they remain for therest of the disassembly operations. The nozzle, whichis the component that helps steer the Shuttle during itsfirst two minutes of flight, is the next item to beremoved from the SRBs.

Nozzle removal is basically the same as aft skirtremoval. Both procedures require the use of a craneand hydraulic panels. The nozzle is pulled straight outand moved over to a transportation platform andplaced in the horizontal position. Once again, assess-ment teams move in to verify damage, if any. Thenozzles are placed inside a shipping container andsent to ATK (Alliant Techsystems) in Utah for finalprocessing.

The spent case segments are also sent back toUtah for build-up, but they must first be separatedfrom each other.

The forward skirt is separated first, followed bythe rest of the segments.

The pins attaching the segments to each other areremoved at the start. Separation rings are thenlowered into place. Each ring has three joints thathelp mold the ring around the segment. The air motoris turned on and the drive mechanism on the front ring

A spent SRB is lifted in a hoisting slip in the Hangar AFarea at CCAFS. It will be washed down and moved to theHangar.

After retrieval post-launch, an SRB is processed throughthe washing station at Hangar AF.

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rotates, causing the four jack screws to turn and pressagainst the pads on the second ring. This forces thefront segment to move slightly away from the backsegment. Technicians help finish the job by pushingthe rail car forward.

Because of the impact when the boosters fall intothe ocean, all internal segment debris ends up in theforward segments. This type of debris is rinsed outinto special carts and taken away. Assessment teamsmake one last check before the segments are placedon flatbed trucks.

The individual segments are transferred to arailhead located on the launch side at Kennedy SpaceCenter. Upon arrival, a segment is moved underneatha 60-ton crane that lifts it off the flatbed. A trainbacks up and the segment is lowered onto a rail car.Yellow transportation covers are placed over eachsegment and secured. The covered segments aremoved to J and J Railroad, where they will behooked up to yet another train for the long trip backto Utah.

Meanwhile, work at the Hangar continues. Theremaining light hardware – the frustum, forward skirtand aft skirt – must be refurbished. All three piecesgo through the same procedures following disassem-bly: hydrolasing to remove the TPS and mediablasting to remove paint coatings, as well as theapplication of alodine, primer, paint and sealants.

Let’s follow the frustum through the sequence.The frustum was offloaded from the retrieval ship

and moved inside the Hangar AF High Bay fordisassembly. Various parts, such as the flotationcurtains and foam blocks, now are removed. Someparts, like the barometric switch tubes, are sent to“Small Parts,” an area inside the Hangar.

Robotic hydrolasing is used to remove TPS fromthe exterior surface of the frustum. The frustum isplaced on a Mobile Access Refurbishment Stand, orMARS, and moved inside the Robotic HydrolaseFacility. The robot moves into position and theMARS begins to rotate on a turntable. After onecomplete rotation, the robot moves up slightly,removing TPS materials from the next portion of thefrustum. The robot continues in this fashion until itreaches the top of the hardware.

Putting the larger pieces of hardware through thisprocess removes the majority of the TPS, topcoat,

(Far left) Workers at Hangar AF,CCAFS, examine an aft sectionof an empty SRB.

(Left) The segment is in theWest Wash area at HangarAF, CCAFS, on a rail car dollyprior to SRB Aft Skirtdemating. Residual aft skirtfoam can be seen.

(Below) At Hangar AF, CCAFS,a technician cleans an areaaround the field joint of one ofthe SRBs recovered after alaunch.

(Below, far left) The railroadengine transports SRBsegments (in yellowcontainers) to Utah. Thesegments will be prepared fora future launch and returned toKSC.

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primer and black sealant.The stripped-down frustum is now transported to

a base support contractor, where it is blasted with aplastic media to remove remnants of the protectivefinish. The plastic bead blast strips the frustum downto bare metal, preparing it for the next stage ofprocessing: a water break test and alodine applica-tion.

The frustum undergoes extensive inspection,nondestructive evaluation, and repair proceduresbefore the application of protective finishes.

To help the frustum maintain a protective coatingof alodine, the entire surface must be clean. Thewater break test determines the success of the beadblasting. As water is sprayed over the frustum,technicians watch for trouble spots. The water mustflow down in sheets. If it beads up, then the frustumis not clean in certain areas and requires furthercleaning by hand.

After the frustum has passed the water break test,alodine is applied. The brownish liquid is used as apreventive measure for corrosion control and ishandled as a hazardous material. The technician fillsa large container with the alodine and moves aroundthe frustum while spraying. As the alodine hits the

structure surface, it “burns,” or leaves an etchingbehind. A second technician follows, rinsing off anyexcessive residue, which flows down into a specialdrain. The leftover alodine will be picked up by ahazardous materials group for disposal.

Using a system much like a shop vacuum, theexcessive water is removed from the frustum beforeit is moved next door to a paint and primer booth.

Technicians mix the primer and paint and applyboth to the larger pieces, both inside and out, usingspray guns attached to a roll-around canister. Theprimer is applied the first day, with the topcoatusually done the following day.

After the paint has cured, the frustum moves to alarge work area across from the paint booth, wheresealant is applied. Sealant is mixed with a catalyst tocreate a moisture-resistant material for certain areasof the vehicle where water intrusion is not desired.

Later, the frustum, aft skirt and forward skirt willbe moved to the Assembly and RefurbishmentFacility for final assembly build-up and testing.

All of these post-flight procedures are repeatedafter each mission.

At CCAFS, thefrustum of aforward skirt

assembly from aspent SRB is

removed from theretrieval ship. It

will be movedinside Hangar AF

High Bay fordisassembly.

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SpeedMission Elapsed Time Height (miles/kilometersEvent After Liftoff (feet/meters) per hour)

Booster separation from Shuttle 124 seconds 156,000 feet(47,549 meters)

Apogee (maximum height boosterreaches after separation) 196 seconds 238,000 feet

(72,542 meters)

Nose cap separation/pilot chute deploy 349 seconds 16,000 feet 360 mph(4,877 meters) (579 kph)

Drogue chute deploy 350 seconds 15,530 feet(4,734 meters)

Frustum separation/main chute deploy 371 seconds 6,450 feet 250 mph(1,966 meters) (402 kph)

Booster impact and main chuteseparation 414 seconds 50 mph

(81 kph)

Frustum/drogue chute impact 459 seconds 40 mph(64 kph)

A Solid Rocket Booster Retrieval Ship has arrived on scene after splashdown of an SRB segmentfollowing a Space Shuttle launch. The segment floats vertically until divers can insert an EnhancedDiver-Operated Plug (EDOP) into the nozzle of the booster. The EDOP pumps air into the emptycasing until all water is expelled and it falls horizontally and can be towed back to port.

FS-2004-07-012-KSC

Captions, front page:

(Top) The Solid Rocket Boosters on Space Shuttle Discovery spew a column of flame as itraces toward space on mission STS-105 to the International Space Station. The twin sets ofboosters provide 80 percent of the Space Shuttle launch thrust. Approximately 2 minutes afterlaunch, the boosters will be jettisoned into the Atlantic Ocean and recovered for future use.

(Middle) The SRB Retrieval Ship Liberty Star maneuvers a spent booster toward the HangarAF wharf at CCAFS so its SRB can be in line with the hoisting slip. A 200-ton, Straddle Liftcrane lifts the booster above the water and all remaining saltwater contamination is washedoff.

(Bottom) The individual segments are transferred to a railhead located on the launch side atKennedy Space Center. A train backs up and the segment is lowered onto a rail car. Yellowtransportation covers are placed over each segment and secured. The covered segmentsare moved to J and J Railroad, where they are hooked up to yet another train for the long tripback to Utah.

National Aeronautics and Space Administration

John F. Kennedy Space CenterKennedy Space Center, Florida 32899

NASA FactsNational Aeronautics andSpace Administration

Marshall Space Flight CenterHuntsville, Alabama 35812

FS-2004-08-97-MSFC August 2004

External Tank Return to Flight Focus Area

Thermal Protection System

NASA’s Space Shuttle Program hasinitiated an aggressive program tominimize any debris that could beproduced by the Space Shuttle’s elements:the Orbiter, the External Tank, the SolidRocket Boosters and the Main Engines.The Shuttle’s External Tank Project Officehas completed a top-to-bottom assess-ment of the tank’s Thermal ProtectionSystem and has examined all areas wherethe tank’s foam insulation, a component ofthe Thermal Protection System, is proneto loss.

Because the tank is not retrievable,engineers must rely on testing, computeranalysis, and video and photographicimaging to determine if there is apossibility of debris created during launchand ascent. These tests and analyseshelp determine the potential for foam lossand the possible ways to improve theoverall safety of the External Tank.

ExternalTank

ThermalProtection

System

The project office is also investigatingnew techniques that will allow the foam tobe inspected for internal defects withoutdamaging it. The initial focus is on man-ually sprayed closeout, or final, foamapplications. Although they are not yet fullydeveloped, non-destructive evaluationtechniques like backscatter radiographyand terahertz imaging could offer anadditional level of verification for the foam.

Backscatter radiography involvesinspection of a part by detecting theX-rays that are scattered back from thepart when it is illuminated with an X-raysource. It was originally developed formilitary use at the University of Florida inGainesville, Fla.

Terahertz imaging is a relatively newtechnology based on the terahertz (THz)range of the electromagnetic spectrum.A defect will cause the wave to reflectback to the receiver.

The main advantage of a terahertz imageris that it does not emit any radiation,capturing pictures of the natural terahertzrays emitted by almost all objects.Occupying a portion of the spectrumbetween infrared and microwaves, from1011 to 1013 Hertz, terahertz waves canpass easily through some solid materials,like walls and clothes, and can also befocused as light to create images ofobjects. The terahertz imaging is beingdeveloped in conjunction with NASA’sLangley Research Center inLangley, Va.

In addition to developing new non-destructive evaluation (NDE) techniques,the Project Office has created a stringentprocess control system to ensure that allnewly-applied foam meets NASAspecifications.

The new process controls include theinstitution of high fidelity mockups, videorecording of processes, simplification ofapplication design, acquisition of all pro-cess parameter data and the institutionof detailed spray instructions.

FOAM FACTSThe Space Shuttle's External Tank iscovered with spray-on foam insulationthat serves to insulate the tank beforeand during launch. The foam is one oftwo components in the External Tank'sThermal Protection System, or TPS.

There are two basic Thermal ProtectionSystems on the External Tank: One islow-density, closed-cell foam. The otherThermal Protection System component isa denser composite material called abla-tor, made of silicone resins and cork. Anablator is a material that dissipates heatby eroding.

The closed-cell foam used on the tankacreage is a Spray-On-Foam-Insulationoften referred to by its acronym as SOFI(pronounced so -FEE). The compositematerial is Super Lightweight Ablator,known as SLA (pronounced slaw).

The External Tank uses ablators on areasthat are subjected to extreme heat, suchas the aft dome near the engine exhaustand on protuberances that are exposedto aerodynamic heating, such as thecable trays.

The closed-cell foam used on the tankwas developed to keep the propellantsthat fuel the Shuttle's three Main Enginesat optimum temperature. It keeps theShuttle's liquid hydrogen fuel at minus423 degrees Fahrenheit and the liquidoxygen tank at minus 297 degreesFahrenheit -- even as the tank sits under

the hot Florida sun -- while preventing abuildup of ice on the outside of the tank.

The foam insulation must also be durableenough to endure a 180-day stay at thelaunch pad, withstand temperatures up to115 degrees Fahrenheit, humidity as highas 100 percent, and resist sand, salt, fog,rain, solar radiation and even fungus.Then, during launch, the foam musttolerate temperatures as high as1,200 degrees Fahrenheit generatedby aerodynamic friction and rocket exhaust.Finally, when the External Tank returns toEarth and begins reentry into theatmosphere -- about 30 minutes afterlaunch -- the foam helps hold the tanktogether even as temperatures and tankpressurization inside work to break up thetank, allowing it to safely disintegrate overa remote ocean location.

Though the foam insulation on the majorityof the tank is only 1-inch thick, it adds4,823 pounds to the tank's weight.Insulation on the liquid hydrogen tank issomewhat thicker -- between 1.5 to 2inches thick. Though the foam's densityvaries with the type, an average density isabout 2.4 pounds per cubic foot. Theaverage NCFI – NCFI is an acronym forNorth Carolina Foam Industries-- densityis 2.27 pounds per cubic foot.

The tank's foam is polyurethane-type foamcomposed of five primary ingredients:polymeric isocyanate, a flame retardant, asurfactant, a blowing agent, and a catalyst.A surfactant controls the surface tension ofa liquid and thus cell formation. Theblowing agent – originally CFC 11--creates the foam's cellular structure bymaking millions of tiny bubbles or foamcells.

Application of the foam, whetherautomated by computer or hand-sprayed,

is designed to meet NASA's requirementsfor finish, thickness, roughness, density,strength and adhesion. As in mostassembly production situations, the foamis applied in specially designed, environ-mentally controlled spray cells andsprayed in several phases, often over aperiod of several weeks. Prior to spraying,the foam's raw material and mechanicalproperties are tested to assure it meetsNASA specifications. Multiple visualinspections of all foam surfaces are alsoperformed after the spraying is complete.

Most of the foam is applied at LockheedMartin's Michoud Assembly Facility in NewOrleans when the tank is manufactured,including most of the "closeout" areas, orfinal areas applied. These closeouts aredone either by hand pouring or manuallyspraying. Some additional close-outs arecompleted once the tank reaches KennedySpace Center in Cape Canaveral, Fla.

There are four specially engineeredclosed-cell foams used on the tank. Thelarger sections of the tank are covered inNCFI 24-124, which accounts for 77percent of the total foam used on the tank.

NCFI 24-57, which has a slightly differentformulation than NCFI 24-124, is used onthe aft dome, or bottom, of the liquidhydrogen tank. PDL 1034, hand-pouredfoam used for filling odd-shaped cavities,and BX 250/265 foam is used on the tank's"closeout" areas. During the early days ofthe External Tank's development, PDLwas an acronym for Product DevelopmentLaboratory, the first supplier of that foam.

NCFI 24-124 and NCFI 24-57 aremechanically sprayed foams; BX 250/265is manually-applied, orhand-sprayed.

Environmental Protection AgencyIn 1987, the United States and 45 othernations adopted the "Montreal Protocol onSubstances that Deplete the Ozone Layer."Under the Protocol, class I ozone depletingcompounds, such as Chlorofluorocarbon11 known as CFC 11 -- the Freon-basedblowing agent used in the production ofthe External Tank's foam -- was to bephased out of production by the end of1995. Production of these compoundsafter 1995 is allowed only by "EssentialUse Exemption" and must have MontrealProtocol approval.

After extensive testing the External TankProject proposed hydro chlorofluoro-carbon HCFC 141b as the CFC 11replacement. HCFC 141b is a blowingagent more environmental regulationcompliant. At the same time, theEnvironmental Protection Agency allowedthe External Tank program to continue useof stockpiled supplies of CFC 11untilHCFC 141b was certified for use on theSpace Shuttle and phased in.

However, in 1999, the EPA proposed toexpand its regulations by implementing aban on nonessential products that release

class I ozone-depleting substances undersection 610 of the Clean Air Act. Under theproposed rule, sale and distribution of BX250, used to insulate part of the ExternalTank, would have been banned because itcontains CFC 11. NASA asked the EPA torevise the proposed rule to provide anexemption for BX 250 and other foamcontaining CFC 11 used in applicationsassociated with space vehicles.

The EPA allowed the exemption but limitedit to the Thermal Protection System of theShuttle's External Tank and only allowedthe use of CFC 11 as a blowing agentwhen no other chlorofluorocarbons areused in the foam product.

The "new" foam containing HCFC 141bwas first used on the liquid hydrogen tankaft dome of ET-82 and flew on STS-79 in1996. The foam was implemented on thetank's acreage, or its larger portions,beginning with ET-88, which flew on STS-86 in 1997. In December 2001, BX-265,which contains HCFC 141b, first flew as areplacement of BX-250. However BX250continued to be flown as BX-265 wasimplemented step wise through themanufacturing process.

NASA FactsNational Aeronautics andSpace Administration

Marshall Space Flight CenterHuntsville, Alabama 35812

FS-2004-08-95-MSFC August 2004

External Tank Return to Flight Focus Area

Forward Bipod Fitting

When the Space Shuttle returns toflight, the External Tank will have aredesigned forward bipod fitting – adesign that meets the recommendationof the Columbia Accident InvestigationBoard to minimize potential debris byeliminating the large insulating foambipod ramps. The new designeliminates these ramps in favor ofelectric heaters.

The insulating foam ramps were inplace to prevent ice buildup – anotherpotential debris source -- on the tank’sbipod fittings. Each external tank hastwo bipod fittings that connect the tankto the Orbiter through the Shuttle's twoforward attachment struts.

HistoryThe External Tank Project Office begandeveloping bipod redesign conceptsafter insulating foam from the left bipodramp area came off during the October2002 launch of Space Shuttle Atlantison the STS-112 mission. During thelaunch of Columbia on its STS-107mission in January 2003, a similar lossprompted NASA's Office of Space Flightto mandate a redesign of the bipodramp before the Shuttle fleet couldreturn to flight.

The bipod ramps were wedge-shaped foam structures, approximately30 inches long, 14 inches wide and12 inches tall. The ramps were appliedby hand spraying BX250/265 foam overthe bipod fittings during the final stagesof the tank's preparation. The final rampshapes were created by hand carvingthe foam to required dimensions.Bipod Location

Bipodramp, asflown

Dissection of existing bipod ramps ontanks in inventory conducted during theSTS-107 investigation indicated thathand-spraying over the complexgeometry of the fittings was prone toproduce internal voids and defects.These internal voids and defects havebeen shown to contribute to foam lossduring ascent.

Design ChangesThe bipod redesign will allow thefittings to fly mainly exposed --minusthe insulating foam ramps. The fittingsthemselves are the same basic designas before. However, to prevent iceformation while the shuttle sits on thelaunch pad loaded with extremely coldcryogenic liquid hydrogen fuel, theredesign adds four rod heaters placedbelow each fitting in a new copperplate. The copper plate with heaters issandwiched between the fitting and anexisting phenolic thermal isolating pad.This thermal isolator helps to reduceheat loss from the copper plate into theextremely cold liquid hydrogen tank.

The heaters are cartridge-type heaterswith a wire coil inserted into a tubefilled with magnesium oxide. They are0.25 inches in diameter and 5 inchesin length. Each heater can produce up

to 300 watts of power when operatedat 120 volts AC. The heaters will onlyfunction pre-launch, and will bepowered and monitored throughconnections in the Ground UmbilicalCarrier Plate, which separates whenthe shuttle is launched. The controlof the heaters will be through ground-based Programmable LogicControllers that will vary the heaterpower based on temperature sensorsco-located with the heaters at thecopper plates. Additional temperaturesensors on the bipod fittings willmonitor the fitting temperatures toensure they stay well above freezing.To minimize the potential for a launchscrub, the heaters and temperaturesensors have built-in redundancy topermit successful operation even in thepresence of certain hardware failures.

Other Fitting ModificationsAlthough the original bipod fittingswere covered with insulating foamramps, the bipod spindles, whichconnected the fittings to the struts,remained exposed. These spindleswere required to rotate to accom-modate the shrinkage of the tank thatoccurs when it becomes extremelycold. These spindles each containeda heater element, and it will no longerbe required.

Elimination of the spindle heatersmeant a smaller end cover could beused on the fitting. Since the fittingswill now fly exposed to the aerodynamicheating environment, the end coverswill get much hotter during flight and towithstand higher temperatures will nowbe made from Inconel 718. The fittingsthemselves are made from Titaniumand are already capable ofwithstanding these highertemperatures.

Bipod Redesign

The new design also requiresadditional cabling to operate theheating system. It includes eightcircuits – four for each bipod – thatrun from the External Tank GroundUmbilical Carrier Plate to the heatersunder the bipod fitting.

The new design is an alternativederived from three original redesignoptions proposed by the project officeto the Space Shuttle Program Require-ments Change Board on May 9, 2003,and later presented by the ColumbiaAccident Investigation Board tothe public.

TestingTesting is an important factor in anyredesign or modification because itvalidates the integrity of the design.Though testing cannot duplicate actualflight, it can significantly reduce riskbecause it allows for carefulobservation and precise control overthe test article. The new bipod fittingdesign has undergone wind tunneltests, structural tests, and thermaltests, during both its design andimplementation phases, to certify it isready for flight. These tests ensure thenew design does not affect the currentExternal Tank loads and stresses.

Structural testing performed at NASA'sMichoud Assembly Facility in NewOrleans demonstrated the loadcapability – its ability to withstandexternal forces acting on the structure --of the redesigned fitting. The structuraltesting included both the effects ofcryogenic -- subzero -- temperatures atthe fitting mounting area and the hightemperature effects of aerodynamicheating of the fitting itself.

Thermal testing performed at Eglin AirForce Base, Fla., demonstrated thecapability of the heater system toprevent ice or frost formation on thelaunch pad. These thermal testsencompassed all tanking and de-tanking scenarios, and theenvironmental chamber at Eglinpermitted all possible environmentalconditions (extreme combinations oftemperatures, humidity, and winds) tobe examined.

Wind tunnel testing performed at ArnoldEngineering Development Center atArnold Air Force Base, Tenn., demon-strated the design's aerodynamiccapabilities and its ability to resistaerodynamic loads and high temper-atures generated during ascent.

Although most of the foam that coveredthe bipod area has been eliminatedwith the redesign, the base area muststill be covered with hand-sprayedfoam. To ensure that this foam is freefrom internal defects that could causefoam loss during flight, the foamapplication techniques for this areahave been refined and thoroughlytested through a process Verificationand Validation program.

ImplementationThe bipod redesign will be retrofittedon the eight existing tanks andimplemented on all new tanks.

Lockheed Martin Space Systemswill do the work at NASA's MichoudAssembly Facility in New Orleans.Delivery of the first retrofitted tank tothe Kennedy Space Center in CapeCanaveral, Fla., is expected inOctober 2004.

NASA Facts National Aeronautics and Space Administration Marshall Space Flight Center Huntsville, Alabama 35812

FS-2003-09-112-MSFC September 2003

Improvements to the Space Shuttle’s External Tank As NASA prepares to return the Space Shuttle to flight, the Shuttle’s External Tank Project Office is assessing the design of the tank’s Thermal Protection System, or TPS, and examining any areas where the tank’s foam insulation, a component of the tank’s Thermal Protection System, has the potential to be lost. The focus of this effort is to minimize any debris the tank could potentially generate, and is one of the recommendations made by the Columbia Accident Investigation Board following the loss of the Space Shuttle Columbia and her crew. The External Tank is the "fuel tank" for the Shuttle’s Orbiter; it contains the propellants used by the Shuttle’s Main Engines. It is also the only component of the Space Shuttle that is not reused. Approximately 8.5 minutes into a Shuttle flight the tank – its propellant used -- is jettisoned and disintegrates over a remote part of the ocean. Because the tank is not retrievable, NASA’s engineers are unable to confirm by inspection how the tank’s insulation performed during ascent. Instead, they must rely on testing, computer analysis and video and photographic imaging to determine if there is a possibility of foam shedding during launch. The External Tank Project Office is re-evaluating the tank’s thermal protection and the potential for debris, through testing and analysis of some of its components, including areas that could be prone to foam loss such the protuberance airload ramps and the liquid hydrogen tank/intertank flange closeout area and an area that is prone to ice, the liquid oxygen feedline bellows. These tests and analyses are expected to determine the potential for foam loss in these areas and possible ways to improve the overall safety of the External Tank for the Space Shuttle system.

The project office is also reviewing how the thermal protection system is applied to the tank and examining new techniques that will allow the foam to be tested without damaging it. Liquid oxygen feedline bellows The liquid oxygen feedline bellows is part of the liquid oxygen feedline assembly that extends externally along the right side of the liquid hydrogen tank, up to and within the intertank, which joins the liquid hydrogen and oxygen tanks, and then to the aft dome, or bottom, of the liquid oxygen tank. The line is approximately 70 feet long and about 17 inches in diameter. The three liquid oxygen feedline bellows are joints that allow the feedline to move, or flex, when the tank is assembled and during installation of the feedline. The joints also allow the lines to adjust as the liquid hydrogen tank is filled and permit the line to adapt to the forces generated at liftoff. The feedline is insulated with foam. However, because the bellows must allow for movement, they are not insulated. The current configuration of the bellows allows ice to form during prelaunch tanking of the External Tank when moisture in the outside air contacts the cold surface of the uninsulated bellows; because the bellows are not insulated, they are subjected to the minus 423 degree temperature of the liquid hydrogen tank. Though there have been no reported losses of foam insulation from this area of the tank, photographs taken prior to launch indicate that ice does form. If ice in this area dislodged during liftoff, it could potentially damage the Shuttle system. Therefore, the project office is redesigning the bellows to eliminate the potential for ice debris and will retrofit the new design to the External Tank. Design concepts under consideration are a flexible heated bellows boot, or protective covering; use of

heated gaseous nitrogen (GN2) or gaseous helium (GHe) to purge the bellows prior to launch; and incorporating along with the use of a coating material that sheds water. These concepts are intended to either prevent ice formation on the bellows or to contain the ice during ascent.

Protuberance Airload Ramps, or PAL Ramps The External Tank’s protuberance airload ramps, known as PAL ramps, are foam ramps designed to reduce adverse aerodynamic loading, or forces, on the tank’s cable trays and its pressurization lines during launch. One of those ramps is near the top of the liquid oxygen tank, close to the nose cone; the other is below the intertank, near the top of the liquid hydrogen tank.

Presently, the three redesign options being evaluated are eliminating the ramps, reducing the size of the ramps to “mini” ramps, or building a “fence” to protect the cable trays and lines. The project office will continue to evaluate the redesign candidates and, after completing a comprehensive testing and analysis program on the options, select one for implementation. Liquid Hydrogen Tank/ Intertank Flange The External Tank’s intertank is the structural connection that joins the liquid hydrogen tank and the liquid oxygen tank. Flanges, which are joints that function like a tab or a seam on a shirt, are affixed at the top and bottom of the intertank so the two tanks can be attached to the intertank. The liquid hydrogen tank flange is at the bottom of the intertank. Once the two tanks are joined to the intertank, the flange is insulated with foam. There is a history of foam loss from this area during liftoff -- foam divots, or pieces, from this area have typically been less than 0.100 pound and considered a maintenance issue for the Orbiter. Therefore, the External Tank Project Office is conducting tests to determine why foam insulation in this area sheds. One modification being considered is to continually purge the intertank flange crevice with heated gaseous nitrogen or helium to prevent the formation of liquid nitrogen in the joint area of the LH2 tank to intertank. Although not proven, this is considered a possible contributor to foam loss. Another modification under consideration is to enhance or

improve the foam application procedures on the flange area.

TPS (Foam) Verification and Nondestructive Inspection of Foam In response to the Columbia Accident Investigation findings, the External Tank Project Office is also reassessing how the foam is both manually applied and hand-sprayed, on the tank. Using flight history, the project office has created a list of areas where such applications could later generate debris and is establishing requirements for additional quality tests for these applications. Included with this assessment is a review and update of the process controls, such as requiring at least two employees attend all critical hand-spraying applications to ensure proper processing and security.

NASA is also pursuing the development of nondestructive inspection techniques on the tank’s thermal protection system. The initial focus is on manually sprayed closeout, or final, foam applications. The project office is surveying state-of-the-art technologies and evaluating their capabilities. Initially, test articles with known defects -- such as voids in the foam -- were used to determine the detection limits of the various methods. Two technologies that show promise are terahertz imaging and backscatter radiography.

The main advantage of a terahertz imager is that it does not emit any radiation, capturing pictures of the natural terahertz rays emitted by almost all objects. Occupying a portion of the spectrum between infrared and microwaves, from 10¹¹ to 10¹³ Hertz, terahertz waves can pass easily through some solid materials, like walls and clothes, and can also be focused as light to create images of objects. The terahertz imaging is being developed in conjunction with NASA’s Langley Research Center in Langley, Va. Backscatter radiography was originally developed for military use at the University of Florida in Gainesville, Fla. The backscatter technique uses X-ray photons that bounce off the electrons of materials. The External Tank Project Office is conducting more comprehensive testing on these technologies.