Cryogenics 48 (2008) 223–231

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Cryogenics

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Aerogel insulation applications for liquid hydrogen launch vehicle tanks

J.E. Fesmire *, J.P. SassCryogenics Test Laboratory, NASA Kennedy Space Center, Mail Code KT-E, Kennedy Space Center, FL 32899, USA

a r t i c l e i n f o

Article history:Received 13 July 2007Received in revised form 4 January 2008Accepted 6 March 2008

Keywords:A. AdsorbentsB. HydrogenB. Liquid heliumC. Thermal conductivityD. Temperature sensors

0011-2275/$ - see front matter Published by Elsevierdoi:10.1016/j.cryogenics.2008.03.014

* Corresponding author. Tel.: +1 321 867 7557; faxE-mail address: james.e.fesmire@nasa.gov (J.E. Fes

a b s t r a c t

Solutions to thermal insulation problems using aerogel beads were demonstrated for space launch vehi-cles using a model of the space shuttle external tank’s liquid hydrogen (LH2) intertank. Test results usingliquid helium show that with aerogel, the nitrogen mass inside the intertank is greatly reduced and freeliquid nitrogen is eliminated. Physisorption within the aerogel was also investigated, showing that thesorption ratio (liquid nitrogen to aerogel beads) is about 62%. The insulating effectiveness of the aerogelshows that cryopumping is driven by thermal communication between warm and cold surfaces. Thistechnology can solve heat transfer problems and augment existing thermal protection systems on launchvehicles.

Published by Elsevier Ltd.

1. Introduction

Space launch vehicles experience a recurring problem with theuninsulated areas on the cryogenic propellant tanks and feedlinesthat allow air to condense or ice to form. These areas, includingflange joints, bracket supports, expansion bellows, and other cavi-ties, are uninsulated by design. Conventional thermal insulationmaterials, if applied to these critical system components, couldcause damage to the launch vehicle during flight because the vehi-cle’s mechanical articulations have been restricted or its cavitieshave been imperfectly sealed. New aerogel-based thermal insula-tion systems have been developed for use in critical locations onlaunch vehicles. Aerogel materials and a liquid oxygen (LO2) feed-line bellows have been previously described [1]. Recent work hasfocused on ambient-pressure insulation systems for liquid hydro-gen tank dome applications. This research was motivated by theneed to solve a longstanding problem on the liquid hydrogen(LH2) tank dome area of the space shuttle external tank.

Postflight photographs have shown a concentration of foam deb-ris around the intertank-to-LH2-tank flange joint of the externaltank (ET). In the cavity formed between the intertank wall, flange,and the LH2 tank dome (termed the ‘‘crevice” as shown in Fig. 1),there exists a periphery of bare metal at 20 K. The nitrogen purgegas within the intertank volume condenses on this cold surface. Li-quid nitrogen (LN2) accumulates in this crevice in the hours beforelaunch when propellant loading and conditioning are underway. Asmall amount of solid nitrogen (SN2) also forms on the colder side of

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the crevice. The LN2 can then seep through the flange joint to theunderside of the foam and into voids in the foam. Upon launch,the LN2 can vaporize due to the ullage pressurization (hot gas),aerodynamic heating, and reduction in ambient pressure. As thenitrogen expands from liquid to gas, the voids or discontinuitieswithin the foam can pressurize, creating foam debris that can dam-age the vehicle during flight. In addition, the lift-off mass penalty issignificant, up to 230 kg for both the liquid and solid nitrogen.

A thermal insulation system using bulk-fill aerogel material hasbeen successfully developed and demonstrated in a 1/10th scalemodel of the LH2 intertank flange area. This aerogel insulation sys-tem and method of application were developed over several yearsof materials research and demonstration testing at the CryogenicsTest Laboratory at NASA’s Kennedy Space Center.

2. Experimental investigations

Experimental investigations included performance testing ofcryogenic thermal conductivity, mass transport (cryopumping),and physical adsorption. Various types and forms of aerogel mate-rials were studied. Flexible blanket and bulk-fill aerogel materialsare now commercially available on a large scale for low-tempera-ture applications [2,3].

Aerogel beads or granules in bulk-fill form could provide thethermal insulation for the LH2 intertank flange area. The small sur-face area of the LH2 tank dome that is not covered by spray-onfoam insulation (SOFI) material would be insulated with this mate-rial, which has very low thermal conductivity. The aerogel is anopen-cell material with an average pore size of 20 nanometersand a surface area of approximately 800 m2/g. The average size

Fig. 1. The LH2 intertank flange area of the space shuttle external tank is shown on the side of the tank that attaches to the orbiter (left). A cutaway view of the intertankflange area shows the crevice (right).

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of the spherical beads is 1 mm. With a particle density of 140 kg/m3 and a bulk density of about 80 kg/m3, an interstitial space ofroughly 60% is typical. The aerogel beads are lightweight, easy tohandle and convey, and present no safety hazard. The bead parti-cles pack well, are capable of a high level of elastic compression,and are generally reusable. The material is commercially availablefrom Cabot Corporation under the trademark Nanogel [4].

2.1. Thermal conductivity

Thermal characterization of the aerogel materials in variousconfigurations was done using insulation test cryostats [2,3,5].The temperature profile for a 25-mm thickness of aerogel beadsfor an ambient pressure test is given in Fig. 2. The 2.5-mm layer po-sition gives a temperature difference of approximately 57 K. Basedon this information, an inference is made that the thickness re-quired to develop an isotherm in the range of 63 K (SN2)–77 K(LN2) from a cold boundary of 20 K (LH2) is less than 2.5 mm be-cause of the increased effectiveness of thermal insulation at lowerboundary temperatures [6].

Fig. 2. Experimentally measured temperature distribution through a 25-mm thickness onitrogen pressure.

2.2. Mass transport (cryopumping)

Cryopumping experiments for a cold boundary temperature of77 K were performed using both a closed-chamber apparatus (cryo-stat) and an open-chamber apparatus (cold column). Results showthat stabilization occurs in approximately 2 h after cooldown andisotherms form in approximately 3 h, as previously reported [1].Cryopumping effects between the boundary temperatures of300 K and 20 K were observed in the demonstration testing usingliquid helium (LHe) and were found to follow a similar pattern ofstabilization in the 2–3 h after cooldown. Thermal equilibrium oc-curred in approximately 8–10 h, as evidenced by on-off oscillationin the periodic flow rate of nitrogen purge gas supplied to theapparatus.

2.3. Physical adsorption (physisorption)

In general, aerogel materials have a very high capacity for sorp-tion of nitrogen molecules. All of the nitrogen mass that is addedduring the material cooldown process is sequestered inside the

f aerogel beads with boundary temperatures of 77 K and 293 K at one atmosphere

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nanoporous structure of the aerogel. The nitrogen is adsorbed tothe surfaces in monolayers of molecules. Differences in surfaceenergies do not permit condensation and formation of liquid mol-ecules within the pores [7].

The capacity of bulk-fill aerogel for physical adsorption, orphysisorption, of LN2 was measured using various methods. Inthe mass uptake method (cup test) shown in Fig. 3, the aerogel

Fig. 3. The nitrogen sorption capacity of the aerogel material is measured by themass uptake method using liquid nitrogen (cup test). Individual nitrogen moleculesare sequestered within the nanoporous internal structure.

Fig. 4. Cold finger experiment with no aerogel insulation inside tube

material is placed in a bottom-perforated cup, fully saturated usingLN2, and then placed on a precision scale to measure the weightgained. Nitrogen is taken up inside the nano-sized pores of theaerogel as the material surfaces are cooled. This process is knownas physisorption, meaning that layers of nitrogen molecules arestacked inside the porous structure. Results show that the sorptionratio (nitrogen to aerogel) is about 62% by volume. The sorption ra-tio on a mass basis is about seven to one. For the aerogel materialin the ET intertank case, the available sorption capacity for nitro-gen is many times more than the minimum necessary for the 8–10 h propellant loading timeline. This feature means that any extraaerogel insulation material in the intertank crevice would simplyprovide additional insulating benefit without intoducing any ad-verse effect.

2.4. Free liquid nitrogen experiment (cold finger apparatus)

The capability of the aerogel to insulate a cold metal surface be-low 20 K and eliminate formation of free LN2 was tested using anovel liquid nitrogen production method. This cold finger appara-tus uses gravity to visually determine if any collection and move-ment of nitrogen in the liquid state occurs when the cold surfaceis insulated with aerogel material. The ET intertank crevice is alsosimulated in that it does not positively contain LN2 that may col-lect. The cold finger apparatus has slits in the bottom analogousto the shim gaps in the intertank flange joint. The apparatus con-sists of a stainless steel cooling coil inside a clear plastic tube withslits cut just above a bottom foam plug. The tube is filled with aero-gel beads (or left empty) and purged with gaseous nitrogen. Thecooling coil is supplied from a vacuum-jacketed transfer hose con-nected to an LHe dewar, as shown in Fig. 4.

Three experimental configurations were made using the coldfinger apparatus. The first configuration, with no aerogel insula-tion, is shown in Fig. 5. In this case the free LN2 began flowing fromthe tip 6 minutes into the test and flowed continuously for thewhole ½-h test. The rate of nitrogen liquefaction was approxi-mately 0.42 ml per second as measured by weighing the LN2 that

(configuration 1) determined the rate of nitrogen liquefaction.

Fig. 5. Cold finger experiment with partially filled aerogel insulation (configuration 2) showed thermal stability even with oversaturated aerogel.

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was caught in an insulated cup after it flowed through the bottomdrain slits. At that rate, LN2 could fill the internal volume of theapparatus in about 2½ h. Configuration 2, in which the cylinderis partially filled with aerogel insulation, is shown in Fig. 6. Evenin this atypical case, none of the LN2 produced in the exposed coldcoil region above the aerogel was able to move to the bottom of theapparatus during the 1½-h test. The full aerogel insulation case,configuration 3, is shown in Fig. 7. No LN2 flowed from the appara-tus during the entire 4-h test. At the end of this test, the entire con-tents of the tube were spilled into a pan, and the aerogel materialwas found to be completely loose and friable with no condensednitrogen observed.

Fig. 6. Cold finger experiment with full aerogel insulation inside tube (configuration 3).

3. LH2 intertank demonstration test unit

The demonstration test unit depicted in Fig. 7 was designed todetermine the operational limits of the new aerogel insulation sys-tem. The test unit, at approximately 1/10th scale, is constructedfrom thin-wall stainless steel, with a G-10 feedthrough for theLHe supply and an aluminum lid. The crevice profile is approxi-mately full scale (with a 12.7-mm bottom surface and a 50-mm ex-posed cold metal tank surface).

Temperature sensors were located from the bottom of the jointcrevice to above the aerogel beads, as well as on the tank, intertankbarrel section, and foam surfaces, as shown in Fig. 8. Pressure

No liquid nitrogen was produced within the tube even after 4 h of cooling with LHe.

Fig. 7. Isometric cutaway view shows the main features of the demonstration testunit of the external tank’s LH2 intertank.

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sensors on the base of the crevice and on the lid measured anypressure difference from top to bottom. The intertank barrel sec-

Fig. 8. Section view of 1/10th scale demonstration test unit shows where temperature sright.

Fig. 9. Demonstration testing using an LHe tanker

tion is designed for full vacuum to allow cryopumping tests for aclosed-mass system. SOFI was applied to the sides of the unitand the dome of the tank in the thicknesses specified for flight con-figuration. Additional thicknesses of SOFI were applied to the cryo-gen feedthrough and the bottom of the unit to provide a stablethermal profile for the tank dome and intertank flange.

The test setup included an LHe supply system from a 42,000-ltanker, a GN2 supply system, temperature sensors, pressure trans-ducers, a GN2 flow control and metering system, and a full dataacquisition system (see Fig. 9). Viewports on the lid (two, eachwith a 50-mm diameter) allowed visual examination of the crevicearea. Cold helium gas was distributed through a spray header onthe underside of the tank dome to maintain a cold boundarytemperature (CBT) of 20 ± 5 K. Two electric tape heaters were af-fixed to the lid to maintain a warm boundary temperature (WBT)of 300 ± 2 K.

4. Test results

The test results show that the aerogel insulation material elim-inates production and accumulation of LN2 in the flange area of theintertank. Using this material also avoids any gas entrapment that

ensors are located. Closeup of the full-scale intertank crevice area is shown on the

(left). 1/10th scale demonstration unit (right).

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could cause collateral problems during ascent to orbit. Tests wereperformed in two series and are summarized as follows.

Part A of the test series included two tests performed with abare crevice and six tests with aerogel insulation installed. Forthese tests, the nitrogen gas was sealed in the intertank. The LHetanker provided a continuous liquid helium supply for long tests.The WBT was controlled by using lid heaters and a shield, or hat,to minimize the thermal influence of the cold helium boiloff va-pors. Visual observations were made at regular intervals by lookingthrough the two viewports on the lid of the test unit.

Observations from Part A of the testing showed that the aerogelinsulation dramatically reduced nitrogen cryopumping and gener-ally increased temperatures throughout the intertank. The appear-ance of the aerogel material remained unchanged throughout alltesting. In the tests with the bare (uninsulated) crevice, solid nitro-gen did not readily form due to the heat input from the intertankwall and the exothermic process of freezing. The LN2 in the barecrevice was observed to boil at the outer edge near the intertankwall, as was expected.

Fig. 10 compares the total mass flow of the nitrogen purge gasdue to cryopumping for tests run with and without aerogel insula-tion. Measurements are taken for the entire system, including thecrevice region (both with or without aerogel), the center feed-through, and the dome of the tank. Fig. 11 presents photographsof tests in which insulation is not used, which show LN2 accumu-

Fig. 10. Totalized GN2 flow for the Part A (closed system) test series shows the effect on tcooldown and stabilization phases.

Fig. 11. Photographs looking directly down at the crevice: tan area with black depth indicindicate the depth; wires are thermocouple leads. A test without the aerogel insulation,images. A test with aerogel insulation, showing an unchanged appearance, is shown on

lations, and photographs of tests in which the aerogel insulationis used, which show that the aerogel material is unchanged.Fig. 12 summarizes the steady-state temperatures and shows theeffect of insulating the cold metal surface of the intertank cavity.

Following Part A of the testing, which had a quantified GN2 flow,Part B of the test series was performed to obtain better visual andphysical evidence of the condition of the material in the crevice re-gion. The GN2 purge flow was maintained as before to keep a drynitrogen environment at ambient pressure inside the intertank,but the flow rate was not measured. Viewports were routinelyopened during the tests and used for access to the crevice area totake measurements, obtain samples, and probe the aerogel mate-rial. The following items were examined: LN2 level versus time,location of solid N2 formation, friability of the aerogel material,and whether LN2 was present in the aerogel material. Photos andvideo were taken at regular intervals during all tests.

Observations made in the Part B tests revealed that only a min-imal depth of aerogel insulation is required to prevent LN2 forma-tion and that the aerogel material remains completely dry andfriable (no free LN2). With just enough aerogel insulation to coverthe bare metal portion of the tank dome, LN2 was prevented fromaccumulating. Also, solid nitrogen forms more readily with smalleramounts of LN2 due to the heat capacity of the liquid. The aerogelinsulation system is tolerant of an imperfect installation as shownby tests in which a portion of the crevice was left bare. A test using

he crevice from having aerogel insulation versus the effect without insulation during

ator lines is the dome of the tank; red and green items are 14-mm diameter corks toshowing LN2 accumulation inside the crevice region, is shown in the left and centerthe right.

Fig. 12. Comparison of tests with and without aerogel, showing the effectiveness of the insulation system in keeping the warm side (intertank wall) thermally isolated fromthe cold side (tank dome). Temperature ranges at multiple locations are given for Tests 6 and 10 (with aerogel insulation) and Tests 7 and 8 (without aerogel insulation).

Fig. 13. Test of insulation system shows minimal aerogel spot at beginning (left) and end of test (right). A small amount of aerogel near bare metal portion of tank dome issaturated with nitrogen at the end of the test.

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an insulation system with a minimal aerogel spot on one side andbare metal spot on the other side is shown in Figs. 13 and 14. Thecrevice temperature is maintained at approximately 20 K duringall tests. Fig. 15, from another test of an insulation system with abare metal portion, shows that liquid nitrogen produced in theexposed crevice progresses into the adjacent aerogel material byphysisorption.

5. Analysis and discussion

A summary of the thermal effect of aerogel beads on the inter-tank is given in Fig. 16. The aerogel keeps the warm side warm and

Fig. 14. Test with aerogel insulation shows bare metal spot at beginning (top) andend of test (bottom). Liquid nitrogen is produced in the exposed crevice and is thenadsorbed by the adjacent aerogel material.

Fig. 15. Another test with exposed tank dome at 20 K. The aerogel material darkensover time as it is saturated with nitrogen by physisorption.

the cold side cold. A minimal depth of aerogel was found to be en-ough to prevent nitrogen from liquefying. In fact, the primaryrequirement is to simply cover the exposed bare metal portion ofthe tank dome. In practical use of the new insulation system onthe flight tank, the aerogel will be filled to a level that is convenientto verify.

Beyond the obvious advantages in eliminating LN2 in the creviceregion on the External Tank, there are ancillary advantages and dis-advantages to using the proposed aerogel insulation system.Advantages are as follows:

Fig. 16. Effect of the aerogel insulation system on temperature distribution withinthe intertank. The average steady-state temperature at each location was calculatedfor tests with aerogel (Tests 6, 9, and 10) and tests without aerogel (Tests 7 and 8).The reported temperature differences (tests with aerogel minus tests without ae-rogel) show that insulating the crevice resulted in warmer temperatures through-out the intertank.

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� Aerogel is high thermal performance material.s Has the lowest thermal conductivity of bulk-fill materials at

ambient nitrogen pressure at �14 mW/m-K (293 K and 77 Kboundary temperatures).

s Insulates all bare metal surfaces of the cavity.

� Material is cavity-filling, free-flowing, and hydrophobic.

s Withstands thermal cycling due to its excellent compressionstrength.

s Lightweight at � 80 kg/m3.

� High physisorption capacity of nitrogen.

s About 62% by volume (LN2:aerogel).s During cooldown, some nitrogen molecules are adsorbed

within the pores of the aerogel.s After cooldown, cryopumping processes are essentially

stopped.

� Total nitrogen purge uptake is dramatically reduced.

s Requires approximately 66% less compared to uninsulatedcase.

� Free LN2 in intertank flange and crevice region is eliminated.s For all phases of loading and launch operations.s Under both transient and steady-state conditions.

The only disadvantages identified are associated with installa-tion. The present aerogel insulation system would need to be in-stalled in the vertical position. In the demonstration unit, theaerogel stayed in place even with a forceful impingement of purgegas directed straight downward from the lid. In the flight tank theaerogel would be protected further by: (1) the presence of a stiff-ening ring immediately above the crevice area, (2) the favorablelocation of purge entrances to the large intertank volume, and (3)the very steep angle and depth of the crevice. An enhanced systemusing a breathable cap material to hold the aerogel in place couldbe developed if it were necessary to install the aerogel in a hori-zontal position.

Important findings also resulted from baseline demonstrationtests of the uninsulated intertank. Results showed a delicatethermodynamic balance that determined whether the nitrogenremained in the liquid state or solidified in the crevice. In onelong-duration test without aerogel, solid nitrogen was observed

to grow across the part of the crevice and subsequently melt beforethe end of the test. On other occasions, a probe was used to feel forthe bottom of the LN2-filled crevice. This technique indicated thatsolid nitrogen formed to a thickness of about 19 mm at the bottomof the crevice near the tank wall and tapered down toward theflange gap. The overall amount of LN2 present appeared to affectthe likelihood of solidification, with frozen nitrogen occurring morereadily in smaller amounts of liquid. Understanding this solidifica-tion process is important in determining the severity of the problemof liquid nitrogen seeping through the intertank flange.

A key advantage of the aerogel system is that it will remaincompletely loose and friable for the entire loading and launch pro-cess. Conventional attempts to solve the problem of LN2 in theintertank have relied on creating a perfect seal to isolate the nitro-gen from the cold surfaces. These approaches were not successfuldue to the difficulty of the intertank remaining sealed when ex-posed to a sudden thermal and pressure gradient during operation.A failed seal is potentially worse than no seal at all because the LN2

has a long time in which to accumulate and only a short time toescape through the failure point as an evaporated gas during vehi-cle ascent. Because the aerogel insulation system is not intended toseal out nitrogen, it avoids the problem of nitrogen entrapmentduring ascent.

6. Conclusion

A new thermal insulation system using a bulk-fill aerogel mate-rial was developed to keep liquid nitrogen from accumulatingwithin the intertank of the Space Shuttle. The proposed solutionis to insulate the exposed cold surface of similar cavities on theLH2 tank dome with aerogel beads. Typically, these cold surfaceareas are difficult or impossible to insulate with conventionalmaterials such as polyurethane foam. The aerogel insulation sys-tem eliminates all free liquid nitrogen within the crevice and thusremoves a cause of foam debris being created in the intertankflange area. And unlike conventional insulation materials, aerogelbeads do not restrict mechanical movements and do not rely onthe cavity being perfectly sealed. As an added benefit, the Shuttle’sflight weight can be reduced by up to 230 kg. The installation pro-cess is straightforward, and no adverse effects have been identified.

The operability of the aerogel-based thermal insulation systemhas been demonstrated in testing down to 20 K. The experimentaldesign, research, development, and demonstration testing wereperformed by the Cryogenics Test Laboratory at NASA’s KennedySpace Center. Sixteen tests were conducted using liquid heliumto provide the necessary cold boundary temperature for LH2 simu-lation. Laboratory testing of the liquid nitrogen sorption character-istics and other physical properties of the aerogel material was alsoperformed.

The new technology can improve thermal performance of fu-ture space launch vehicles. Using aerogel bulk-fill materials andother aerogel-based insulation systems will provide new optionsfor the design and safe operation of cryogenic systems.

Acknowledgements

The authors are glad to acknowledge the expert assistance ofmany engineers and scientists dedicated to the Space Shuttle Pro-gram. We especially appreciate the insight and assistance of Char-lie Stevenson, Trent Smith, Stephen Huff, and Wes Johnson for thedemonstration testing. This work was supported by the NASA Engi-neering and Safety Center (NESC) through the leadership of MikeKirsch and Andreas Dibbern. We thank Adam Dokos and the NASAPrototype Laboratory team for constructing the test unit.

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