NASA Technical Memorandum 101386 Technology Issues Associated With Fueling the National Aerospace Plane With Slush Hydrogen {I_ASA-TM- I0 13 _:0) II'CkNOLOG¥ I_,18ES AI..'.SOCIA'II_D W£'Ill I;"_EII_G _Ifl_ _at_I2CtiAI A_CS_AC_ _LA_K _I_H SLUSH l_l£_CG_h {HASh) S l: CSCL 21H 63/20 Ndg-1G123 Ned P. Hannum Lewis Research Center Cleveland, Ohio Prepared for the 7th Joint Intersociety Cryogenic Conference Symposium cosponsored by the ASME, AIChE, and IIR Houston, Texas, January 22-26, 1989 https://ntrs.nasa.gov/search.jsp?R=19890000752 2020-04-07T01:03:24+00:00Z
Transcript
NASA Technical Memorandum 101386
Technology Issues Associated WithFueling the National AerospacePlane With Slush Hydrogen
TECHNOLOGY ISSUES ASSOCIATED NITH FUELING THE NATIONAL AEROSPACE PLANE HITH SLUSH HYDROGEN
Ned P. HannumNational Aeronautics and Space Administration
Lewis Research CenterCleveland, Ohio 44135
ABSTRACT
The National Aerospace Plane is a horizontal takeoff and landing, single stage-to-orbit vehicle usinghydrogen as the fuel. The first flights are plannedfor the mid 1990's. The success of this importantnational program requires advancements in virtuallyevery discipline associated with both airbreathing andspace flight. The high heating value, cooling capac-ity, and combustion properties make hydrogen the fuelof choice, but the low density results in a large vehi-cle. Both the fuel cooling capacity and density areincreased with the use of slush hydrogen and result insignificant reductions in size of the vehicle. Anational program to advance this technology and to findengineering solutions to the many design issues is nowunderway. The program uses the expertise of the cryo-genics production and services industry, the instrumen-tation industry, universities, and government. Thisprogram will be discussed to highlight the ma_or issuesand display the progress to date.
INTRODUCTION
There are several compelling reasons to build andtest the National Aerospace Plane (NASP) X-30 researchaircraft. A horlzontal take off and landing spacelaunch vehlcle would have greatly reduced launch opera-tlons cost. A slngle stage-to-orbit is the most eco-nomic alternative for full reusabillty by mlnlmlzlngthe navigational and propulsion systems for return tothe launch site systems and by eliminatlng the stageintegration process. And, hypersonic cruise has manyappllcatlons in civil transportation and militaryactivity. The X-30 is, therefore, a test bed fordemonstratlng the advanced technologies that arerequlred for the next generation of flight.
The key enabling technology for hypersonic cruiseand single stage-to-orbit are air breathing propulsionsystems for the entire flight regime of zero to Mach 25.But, even with these propulsion systems, the vehlclewould be too large and too heavy using today's materi-als and fuels. Hydrogen is the fuel of choice, butits low density produces a large vehicle. The NASP
program is addressing these enabling technology areaswith strong programs in propulsion, materials, and inthe use of slush hydrogen.
The NASP X-30 requires a high energy propellantand active cooling. Hydrogen is the fuel of choicebecause of its high energy content and because of itscooling capability. Slush hydrogen is 16 percent moredense and, due to the addition of the heat of fusion,has 1B percent more cooling capability than liquidhydrogen. This Increased cooling capability is espe-cially important for the NASP because during some por-tions of the flight the cooling requirements exceed thepropulsion requlrement for hydrogen. The net effect onthe NASP of using slush rather than liquid hydrogen isto reduce the gross lift off weight of the vehicle byup to 30 percent. Most costing algorithms relate costto weight and, therefore, the use of slush hydrogen onthe NASP represents an important new technology. Slushhydrogen has been investigated by several researchersincluding the National Bureau of Standards. The advan-tages of using slush hydrogen in space vehicles wasrecognized and was considered for the Space Shuttle inthe early 1970's, Because the technology did not existbeyond the laboratory level it was not selected. Thecurrent NASP program is committed to advancing slushhydrogen technologies and demonstrating them on theX-30.
The current slush hydrogen program Includes, tovarying degrees, vehicle slush system design, flow com-ponent modeling, large scale production, ground storageand vehicle servicing, instrumentation, and safety cri-teria. Models are being developed/modlfied and pre-dicted results will be compared with experimentalresults. The objective is to advance the fundamentalunderstanding and to generate design information.
NASP OPERATION/ISSUES
Production
Methods. Slush hydrogen has been produced in labo-ratories by several researchers. The National Bureauof Standards has reported observations about slush
hydrogenproducedin smallglassdewarsusingtwodif-ferentproductionmethods.Otherresearchershavepro-posed,andin somecasesdemonstrated,othermethods.Themethodscanbecategorizedasevaporationandrefrigeration. Theevaporationmethodmostoften usedis referredto as thefreeze-thawmethod.In thismethod the tank pressure is lowered to the triple pointand solid hydrogen forms on the surface. The pressureis allowed to increase, heat leak through the wallsmelts the solid near the wa11, and the solid sinks intothe liquid. AS this process is repeated the solidfraction is increased up to about 30 percent before thesolid cannot be fully covered by the liquid. If theslush is maintained, the configuration (not the size)of the solid crystals changes with time. This processis called aging. With time (aging) the solids packmore densely and more freeze-thaw cycles can increasethe solid fraction to nearly 50 percent before thesolid cannot be covered by the liquid. The freeze-thaw method is well understood but has not been devel-oped at large scale. The disadvantages of thefreeze-thaw method is that it has been a batch processto date. Also, 16 to 20 percent of the liquid hydrogenis evaporated in the process and is either lost or
requires additional equipment to reliquefy. Anotherevaporation method For making slush is to spray liquidinto a near vacuum (less than the triple point pres-sure) to form solid crystals. This method requiresfluidizing with liquid to make slush and has not beencharacterized. Both of these evaporation methods havethe additional disadvantage of requiring subatmosphericequipment which introduces the possibility of leakingoxygen into the system and creating an explosivemixture.
Slush hydrogen has also been produced by refriger-ation methods. The most tested method is to flow liq-uid through a tube whose wall is cooled with eithercold gas or liquid helium. The solid crystals thatform on the surface are scraped off by an auger. Theadvantages of the auger method are that it is a contin-uous process (without aging) and that the equipment canbe operated above atmospheric pressure. The disadvan-tages are that the method is not well characterized
and, more importantly, requires an expensive refriger-ant. A magnetic refrigerator has also been proposedbut has not been characterized except at small scale.
The production of slush hydrogen Is seen as anengineering problem that Is manageable, The mostenergy efficient method has not been determined norhave trade studies been completed comparlng the capi-tal cost to the operating costs. Because of transferand storage conslderations to be discussed later, itseems apparent the slush should be produced at thevehicle loading site, Therefore, the logistics of sup-plying the site will be the same as with liquid hydro-gen. Certainly the production of slush hydrogen willbe more expensive than just liquid and the capitalequlpment will be rather expensive, but the payoff forthe NASP and for many other space vehicles makes thisan important technology for our nation to develop.
Issues. The issues associated with the productionof slush hydrogen are primarily englneerlng problems.
There are some very slgnifIcant technologles that arerequired. In subatmospheric systems where the contami-
natlon of the hydrogen with oxygen is a possibility,methods for detecting the presence of oxygen and exper-iments to determlne acceptable amounts of oxygen mustbe developed. The aging process that is required to
get the solid fraction up above 50 percent adds a sig-nificant tlme to the production which increases the
storage requirements and/or lengthens the turn around
time between flights. Methods to speed this aging
process must be found. These and other issues areshown in Table Ii
Ground Operations
Requirements, The requirements for the groundoperations segment to support the NASP are function-ally the same as for any research aircraft, but withadded emphasis on developing the technology for rapidturnaround. The use of a cryogenic propellant .in aresearch airplane is also new, but there is consider-able experience wlth rockets. The quantity/distancesafety requirements are assumed to be the same, butmore study is required, The use of slush hydrogen doesadd several new problems not treated in the use of liq-uid hydrogen in space vehicles. The low latent heat offusion means that low heat leak into the system willcause the solid hydrogen to melt. The result is thatthe storage and transfer systems must be designed tohave much lower heat leak than is required for liquidhydrogen systems. Starting with equal amounts of liq-
uid and 50 percent slush hydrogen in similar systemsthe amount of heat which would completely melt all ofthe solid in the slush would only vaporize 6.5 percentof the liquid hydrogen. The slush systems require sim-ilar insulation and handling technology as liquidhelium systems. Because low heat leaks into the slushwill cause changes in the density of the fluid, theslush systems are susceptible to acoustic instabili-ties, These instabilities may occur in the main flowlines and also in the instrumentation lines.
In the process of loading liquid hydrogen into aspacecraft it is necessary to top off the liquid asheat addition causes boiloff and the venting of gase-ous hydrogen. The process of loading and holding aslush hydrogen tank is different because heat additionmelts solid, lowers the density and causes liquid to bevented. The process of loading and holding a tank with50 percent solid will require considerably more turn-over of product to upgrade the slush in the tank.
Studies must be conducted but it seems apparent that
the degraded slush must be recirculated and reproc-essed. The thermal dynamics must be modeled and the
equipment to accomplish this upgrading process must bedeveloped.
To keep the weight of the NASP as low as possible,the fllght insulation system will not be as low In
heat leak as the storage system. Therefore, the longground holds that are llkely to be required for such a
complicated research aircraft will be a problem, espe-clally after roll out from the servicing facility. Aportable upgrade system may be required to solve thisproblem.
Even though many precautions are taken, debris ispresent in the liquid hydrogen. In loading space vehi-cles, the liquid hydrogen is filtered both as it isintroduced into the vehicle and again just before theengines. With slush hydrogen conventional filteringmethods cannot be used. Several alternatives have been
considered. The entire storage and transfer systemcould be malntalned at extreme levels of cleanlinessand the fuel filtered as a liquid before it is madeinto slush. Another method would be to design theengine pumps such that they could tolerate largerdebris. Since the slush will be melted in the pumps,filters could then be placed downstream of the pumpsand ahead of the propulsion equipment.
The method of transferring slush hydrogen fromthe storage tanks to the vehlcle must be studied fur-ther but initial studies indicate that pumped transferis preferred because there will be less heat additionto the fluid from the pumping than there will be to thestorage tank from a warm pressurant. The National
Bureauof Standardshaspumpedslushhydrogenwith noapparentproblemsbutmuchmoreinformationis needed.Anothertransfer issueis maintaininga critical veloc-ity to carry thesolidsalongwithminimalfrictionheatingof the fluid. Thiscritical velocity hasbeendeterminedfor smalllines but scalinganddesignrulesmuststill bedetermined.Pressuredropacrossorif-icesandventurishasbeenobservedto bevirtually thesameaswith triple point liquid.
Issues.Themajorissuesin storageandgroundoperationsare the lackof designcriteria andthe lackof modelsfor thefluid dynamicandthermaldynamicprocesses.Thefiltering andthefluid transfermeth-odswill also requireconsiderabletechnologydevelop-ment. Otherissuesareshownin TableII.
Flight Operations
Requirements. Because slush hydrogen is in equi-librium with vapor-solid-liquid only at the triplepoint of ]3.8 K and 52.8 mm Hg (1.02 psia) the pressur-ization of the flight tank to provide net positive suc-tion.pressure for pumps or to cause outflow presents adifficult problem. If gaseous hydrogen is provided aspressurant, cooling at the ullage-slush interface wouldcause the pressurant to be condensed and result in adrop in the tank pressure. This process falls toproduce the desired pressure or worse, can cause thetank to collapse structurally if it is not designed toaccommodate such loads. If temperature stratificationcould be maintained such that the slush-ullage inter-face is slightly above the triple point temperature,ullage condensation could be prevented. Vehiclemotion, and the perceived need to continually stir theslush, complicate this approach. An alternative is topressurize with a noncondensable like helium. Theproblem with this approach is the added weight of thehelium systems and the inert gas itself. Much morework must be done to determine the best way to pressur-ize slush hydrogen tanks. This is especially true forvehicle tanks where there is such a premium on low
weight and low volume.In the NASP mission profile there are times when
the cooling requirements for hydrogen exceed the pro-pulsion requirements. If, during these times, theextra hydrogen is routed through the propulsion system,the performance of the system is degraded even thoughadditional thrust is produced. An alternative is toroute the extra hydrogen back to the vehicle slushtank. Although the addition of this thermal energy tothe vehicle tank would degrade the cooling capabilityof the fluid, it would be adequate at other times inthe mission profile when the cooling requirements wereless demanding. This process is called recirculation.The technology issues associated with introducingwarmed fluid into the slush tank, with net outflowwhile maintaining pressure will require considerableexperimental work and some sophistlcated models.
Vehicles are usually tanked such that the propel-lant mass at lift-off is known to an accuracy ofI/2 percent. To achieve this kind of accuracy withthe slush hydrogen, two phase quantity gauges will berequired. Several techniques have been demonstrated inthe laboratory but selecting the best type to be usedin the NASP vehicle tank, which wlll have many internalobstructions, will be a significant challenge. Density
gauging methods that have been demonstrated in the lab-oratory use radiation attenuation and changes in thedielectric constant as indications of the density.There has been significant progress but demonstrationsin large tanks with internal obstructions are required.
Two-phase mass flow meters will be required, at leaston the research flights where redundant engine perform-ance measurements will be desirable. The most promis-
ing types are Coriolis effect meters and constant powerthermal meters. Other desirable flight instrumentationwould be liquid level sensors and fiber optic means formaking direct observations into the tank.
The hydrogen must be pumped to high pressure to beused as a coolant and as the propellant. The energyadded to the fluid in the pumping process will melt thesolids and, therefore, the hydrogen downstream of thepump will be at a lower temperature than if liquidhydrogen would have been introduced at the pump inlet.The National Bureau of Standards has pumped slushhydrogen with no observable damage to the pump but thehead rise was small and there are only limited data.More work is required to fully establish this technol-
ogy. An alternative is to screen the outflow from thevehicle tank such that only triple point liquid entersthe pumps. In this scenario the solids would be meltedby the addition of heat to the tank through the insula-tion or from the pressurant.
Issues. Pressurization is the most significant
single issue. Trades must be made for risk, weight andvolume using both hydrogen and helium or combinationsof the two as pressurant. Recirculation is the nextmost significant issue. Models for the thermal dynam-ics, stratification, tank motion and heat transfer mustbe developed for slush hydrogen and then these modelsmust be verified with experiments. Several new instru-ments are also required including density gauges todetermine the quantity of slush in the tank, flow ratemeters for performance measurements, liquid level sen-sors and fiber optic methods of observing the slush inthe tank. The technology for pumping slush hydrogenwithout damage to the pump and with predictable knowl-edge of the thermal condition of the fluid must bedemonstrated. The flight operations issues are shownin Table III.
SLUSH HYDROGEN RESEARCH PROGRAMS
In addition to the contract specific work that themajor airframe and propulsion contractors are doing toadvance the technologies for the National AerospacePlane, there is a large government-sponsored program tomature those technologies which are of a more genericnature. This group of tasks is called the TechnologyMaturation Program (TMP). The development of slushhydrogen technology is one of the major TMP tasks. TheNational Aeronautics and Space Administration (NASA)Lewis Research Center has the lead role for the slushhydrogen TMP task. Several organizations are currentlycontributing to this effort. The National Bureau ofStandards in Boulder, Colorado is working on instrumen-tation and physical properties. There is a contractwith the McDonnell Douglas Corporation and their sub-contractors, Wyle Laboratories, Martin Marietta DenverAerospace and Air Products and Chemicals, Inc. to dolarge scale experimental work in production, pressuri-zation, transfer, and modellng. NASA Lewis is alsodoing large scale experiments in productlon, storage,pressurization, transfer, slosh and instrumentation allwith particular emphasis on varying many parameters andverifying models. The University of Michigan is work-ing on the gellation of both hydrogen and slush hydro-gen and the University of California Los AlamosNational Laboratory is leading the safety tasks.
The tasks being worked by this slush hydrogen teamare discussed below.
Vehicle Slush System DesignThe NASP alrframe contractors are currently inves-
tigating concepts for the slush hydrogen systems andwill be making selections and beginning preliminarydesigns during 1989. There Is a dire need for designcriteria and for refilled models. Figure I shows thevehicle system deslgn data and activities that areplanned. In 1989 the physical properties of slush andgels will be defined experimentally and there will be athorough assessment of instrumentation. Pressurizationwill be demonstrated with both hydrogen and helium,critical flow velocities determined and several systemapplications simulated primarily using existing hard-ware. In 1990-91 the emphasis will be on scalechanges, broader ranges of critical parameters and thesimulation of critical vehicle operations. These datawill be used to verify models. The 1992-93 time framewill be focused on the demonstration of proceduresusing large scale hardware of the contractors designand/or concept.
Flow Component ModelinqFlow component modeling activities and planned
outputs are shown in Fig. 2. Many of the necessarymodels are in place and have been used successfully fortriple point and for subcooled hydrogen. Some modifi-cations are being made to properly model slush. Theseinclude condensation, adding the option of consideringthree phases and stratification in the ullage. Thermaldynamic and heat transfer models are being developed/modified for both tanks and lines. Slush models arealso being modified. These models will be verifiedusing the data from the various laboratory scale andlarge scale experiments.
Large Scale ProductionThe large scale production activities are shown in
Fig. 3. Because of the funding constraints in the pro-gram. the production activities are limited. Two dif-Ferent size augers are being tested. The major testfacilities are being supported by freeze-thaw produc-tion systems but the emphasis is not on studying thisproduction method. Safety issues are being studied andthere is a contract effort to conduct trade studies onproduction methods.
Ground Storage and Vehicle ServicinqThe ground storage and vehicle servicing technolo-
gies are being addressed by the slush hydrogen team asshown in Fig. 4. Other more design specific issues are
TABLE I. - SLUSH HYDROGEN PRODUCTION ISSUES
• Selection of slush production methodEnergy efficiencyCapital investment
• Safety
• Accelerating the aging process
• Oxygen contamination limits/detection
being addressed by the individual airframe contractors.Tanking, topping and upgrading tests and the deflnltlonof procedures and the correlation of models will be theprimary focus. Trade studies of various ground opera-tions scenarios as functions of cost, turnaround time
and risk will be conducted. In 1990-91 ground hold andinsulation schemes will be tested and the results willbe correlated with the models.
Instrumentation
The instrumentation tasks planned are shown inFig. 5. A survey of available instruments and of vari-ous laboratory methods to measure density, and flowrate with s]ush hydrogen was'completed in 1988. Sev-eral different density meters will be tested in 1989-90to determine the operating characteristics and applica-bility to the obstructed interior of the NASP vehicletank. Line density meters, flow meters and liquidlevel sensors are also being tested and evaluated.
Calibrations of these instruments wi]] be completed inthe 1989-91 period.
Safety Criteria
The safety criteria for slush hydrogen will bebased on the extensive set of criteria that exists forliquid hydrogen. The planned activities are shown inFig. 6. Studies will be conducted to determine whatdifferent procedures might be required with slush. Itis anticipated that some experimental work will also berequired to establish additional criteria. One ofthese may be to determine the explosive limits of fro-zen oxygen in s]ush hydrogen.
CONCLUDING REMARKS
The challenge of flying an airplane using liquidhydrogen as the fuel, including take off and landingoperations, has not been done. The National AerospacePlane will use slush hydrogen. The challenge is greatbut the benefit to both the NASP and to other spacevehicles makes this both an interesting and necessarytechnology for our nation to acquire. No real road-blocks have been identified to date by the nationalteam working on the problem. There are many engineer-ing problems but they all seem to have solutions thatdo not require major technical breakthroughs. Ifthese engineering problems can be solved in the nextfew years slush hydrogen will be the fuel of choicefor the National Aerospace Plane and for many otherapplications into the next century.
TABLE II. - SLUSH HYDROGENGROUND
OPERATIONS ISSUES
• Ground hold• Nith flight weight insulation• Need for umbilicals
• Transfer method
• Upgrading
• Acoustic instability
• Critical velocity
• Design criteria/models
• Filtering
• Safety procedures
TABLE III. - SLUSH HYDROGEN FLIGHT
OPERATIONS ISSUES
• Pressurization methods/models
• Rec]rculation
• Thermal dynamic/heat transfer models
• Instrumentation
• Quantity (density) gauge
• Flow meter
liquid level sensor
• Pumps
• Screens
PROGRAM MILESTONES
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Figure 3. Programs in Large Scale Production
PROGRAM MILESTONES
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