Patent Publication Number: US-2005136239-A1

Title: Multifunctional cryo-insulation apparatus and methods

Description:
PRIORITY CLAIM  
      This application claims the benefit of U.S. Provisional Patent Application No. 60/498,939 (Attorney Docket No. BING-1-1010), filed Aug. 29, 2003. 
    
    
     GOVERNMENT LICENSE RIGHTS  
      This invention was made with Government support under U.S. Government contract, Space Launch Initiative, Contract No. NAS8-01099, awarded by the National Aeronautics and Space Administration. The U.S. Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION  
      This invention relates generally to foam insulation and more specifically, to foam insulation for cryo-materials tanks.  
     BACKGROUND OF THE INVENTION  
      Typically, cryogenic propellant tanks, as in the Space Shuttle Orbiter external tank, are insulated with a light-weight (2-3 lb per cubic foot density) polymeric foam. This material, often polyurethane foam, however, is relatively weak structurally, and generally cannot endure temperatures higher than 250° F. to 300° F. Typical heat shield insulation for re-entry vehicle structures includes open celled ceramic tiles or blanket materials. These materials typically cannot be used at cryogenic temperatures because the breathable internal structure of the tiles or blankets permits air to liquify within the material, a process known as cryopumping.  
      Where cryogenic propellant tanks are utilized in spacecraft that leave and reenter the atmosphere, these tanks experience a very large range of temperatures near the outer surface of the tank. At the interface with the propellant, such as liquid hydrogen, the structure must endure temperatures as low as −423° F., while during re-entry the outer surface exposed to the atmosphere endures temperatures as high as 2500 degrees F. Cryogenic tanks and other equipment in other applications, from aircraft to hydrogen powered automobiles, may also be exposed to a wide range of temperatures.  
      Foams that can operate at higher temperatures than polyurethane foam have been tested for use as cryo-insulation on spacecraft propellant tanks. Higher operating temperature foams by way of example, but not limitation, include Rohacell foam manufactured by Rohm, and polyimide foams, including polyimide foams manufactured by Unitika, Ltd. Polyimide and Rohacell foams tolerate higher temperatures than polyurethane foams, some up to 500° F., but typically are more open-celled than polyurethane foams. Thus, when these foams are placed adjacent to cryogenic propellant tanks, the air in the open cells liquifies, and cryopumping occurs, often damaging the foam. Thus, polyurethane foams typically provide a better form of insulation than polyimide or Rohacell foams immediately adjacent to cryogenic propellant tanks because the polyurethane foams do not experience nearly as much air liquification within the foam.  
      The relative structural strength, however, of practically all insulating foams is somewhat limited. By way of example, this strength is typically not sufficient to permit direct bonding of a thermal protective system such as insulating ceramic tiles or blankets directly to the foam.  
      Alternatives to foam cryo-insulations for cryogenic tanks that will be part of a re-entry vehicle include vacuum structures. These involve a multiple wall tank with a vacuum maintained between the layers, with a re-entry thermal protective system installed to the outside layer. Alternately, structural supports may be utilized to mechanically hold the thermal protective system some distance from the outer wall of the cryogenic propellant tanks. Multiple wall tanks and structural supports for the thermal protective system typically involve greater weight than foam insulation for the same insulation values. This greater weight increases launch vehicle weight, and thus reduces the launch vehicle payload capacity. In other vehicles and cryogenic tank applications, such alternatives to foam cryo-insulations add weight and structural complexity.  
      Therefore, there is an unmet need to develop a cryo-insulation that can operate in a wide range of temperatures and still provide sufficient structural strength for the desired application, including, by way of example, keeping a thermal protection system, such as the ceramic tile or blanket materials, attached to its surface.  
     SUMMARY OF THE INVENTION  
      The present invention is directed to apparatus and methods for multi-layer foam structures. In one embodiment, a method includes filling a first portion of a receptacle with a removable filler. A second portion of the receptacle is filled with a first foam forming a first foam layer. The removable filler is removed, and at least part of the first portion of the receptacle is filled with a second foam, forming a second foam layer. In accordance with other aspects of the invention, the first foam may include a polyimide foam, and the second foam may include a polyurethane foam. Other aspects of the invention include a skin attached to the receptacle and the first foam, and the use of a hexagonal honeycomb matrix as a receptacle for the first foam and the second foam. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
       FIG. 1A  is an isometric drawing of a honeycomb hexagonal matrix utilized in an exemplary embodiment of the present invention.  
       FIG. 1B  is a cross-section of the honeycomb hexagonal matrix of  FIG. 1A .  
       FIG. 2  is a cross-section of the honeycomb hexagonal matrix of  FIG. 1A  coated with an adhesion promoter.  
       FIG. 3A  is a cross-section of the honeycomb hexagonal matrix of  FIG. 1A  partially filled with removable filler.  
       FIG. 3B  is a cross-section of the honeycomb hexagonal matrix of  FIG. 1A  partially filled with removable filler and polyimide foam precursor.  
       FIG. 3C  is a cross-section of the honeycomb hexagonal matrix of  FIG. 1A  with a skin, ready for heat curing.  
       FIG. 4A  is a cross-section of the honeycomb hexagonal matrix of  FIG. 1A  during installation of polyurethane foam.  
       FIG. 4B  is a cross-section of an integral multi-layer foam composite structure in accordance with an embodiment of the present invention.  
       FIG. 5  is a flowchart of a method of forming a multi-layer foam structure in accordance with an embodiment of the present invention.  
       FIG. 6  is a cross section of a multi-layer foam structure installed on a space-vehicle in accordance with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention relates to apparatus and methods for multi-layer foam structures. Many specific details of certain embodiments of the invention are set forth in the following description and in  FIGS. 1-6  to provide a thorough understanding of such embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments, or that the present invention may be practiced without several of the details described in the following description.  
       FIG. 1A  is an isometric view of a hexagonal celled honeycomb structure  10  utilized as a structural foundation or matrix for an exemplary multi-layer foam structure in accordance with an embodiment of the present invention. The honeycomb  10  includes a plurality of cells  12  defined by a matrix  14 . In this embodiment, the honeycomb  10  structure is a continuous sheet array of a plurality of hexagonal cells  12  of a reinforced polymeric material. The plurality of cells  12  are then filled, at least in part, with multiple layers of foam, as described below. By way of example, and not limitation, suitable honeycombs include Texas Almet, Inc., HRH 10 Aramid Fiber Reinforced Phenolic Resin, with a ⅜″ cell size, and a density of approximately 2 pounds per cubic foot. Other suitable honeycombs include HEXCEL HRP phenolic/fiberglass honeycomb manufactured of heat resistant phenolic resin, HEXCEL HRH 327 including glass fiber and polyimide resin, Kevlar honeycombs, Korex honeycombs, and metallic honeycombs.  
       FIG. 1B  is a side cross-sectional view of the honeycomb  10  of  FIG. 1A . The honeycomb  10  shown in side cross-sectional view includes cells  12 , and a matrix  14 . The honeycomb matrix  14  contributes structural strength to the integrally formed multi-layer foam structure.  
      As shown in  FIG. 2 , in an exemplary embodiment of the present invention, the honeycomb sheet  10  may be dipped in a adhesive promoter or primer  15  to promote adhesion of one or more of the foams being installed within the honeycomb  10 . By way of example, and not limitation, when polyimide foams are being utilized to partially fill the honeycomb, the adhesive promoter or primer  15  may be a polyimide adhesion promoter. In a particular embodiment, a suitable adhesive promoter  15  includes an RP50 polyimide adhesive solution reduced in methanol and NMP to 9% solids, supplied by Dr. Ruth Pater of the NASA Langley Research Center.  
      In one aspect, the primer  15  may be applied to the honeycomb  10  by first washing the honeycomb  10  in de-ionized water, and then dipping the sheet of the honeycomb  10  into a room temperature polyimide resin bath (not shown), air drying for 15-30 minutes, and then oven drying the honeycomb  10  with the adhesive promoter  15  in a circulating air oven (not shown) for one hour±10 minutes at 250° F. The honeycomb  10  is then cooled. This process substantially evaporates the solvent from the adhesive promoter  15 , thus “B-staging” the resin, but preferably does not cure the polyimide. The curing of the polyimide adhesion promoter  15  may occur during the expansion and cure of a polyimide foam later installed in the honeycomb  10 , as described more fully below. The polyimide adhesion promoter  15  provides adhesion between the polyimide foam (See  FIG. 3B ) and honeycomb  10 . This enhances the durability of the insulation and eliminates gaps through which air could otherwise travel and liquify below the surface of the insulation. In  FIG. 2 , the primer  15  (enlarged) is shown partially coating the honeycomb  10 . In this exemplary embodiment, the honeycomb  10  is dipped in a clean flat bottom tray large enough to accommodate the desired honeycomb panel size. The tray (not shown) may be filled with primer  15  to a depth equal to the desired thickness of the polyimide foam in the final composite foam structure (such as ½ the thickness of the honeycomb as shown in  FIG. 2 ). It will be appreciated that other primers may be used for other foam types, and that at least some foams suitably adhere to the honeycomb  10  without a primer  15 , depending on the foam and the desired application.  
      As shown in  FIG. 3A , the honeycomb  10  with the coating of adhesive promoter  15  is then partially filled with a removable filler  20 . In a particular embodiment, the removable filler  20  may be a silicon carbide sand. Silicon carbide sand is suitable as a removable filler due to its high thermal conductivity. This provides good heat distribution during cure of the polyimide foam layer, as described below. Depending upon the heat and other characteristics desired for the removable filler  20 , other sands or fillers may be utilized, including, for example, aluminum oxide sand. By way of example and not limitation, a suitable silicon carbide sand is Silicon Carbide 120-450 mesh, 220 grit, McMaster Carr Part No.3441k84.  
      In one embodiment, the removable filler  20  may be placed within the cells  12  of the honeycomb  10  by covering a mold or base  5  with a layer of silicon carbide sand. The silicon carbide sand filler  20  may be leveled to the desired thickness of a second foam  40  (See  FIG. 4B ) which is to be installed in the finished insulation panel. It will be appreciated that the portion of the honeycomb  10  or foam receptacle filled with the a second foam  40  (See  FIG. 4B ), and thus filled with filler  20  at this stage, may be any portion of the honeycomb  10 , from a very small portion to a very large portion, including from 1% to 99% of the honeycomb  10 , depending on the desired features of the final multi-layer foam structure.  
      By way of example, and not limitation, the second foam  40  (See  FIG. 4B ) may be a polyurethane foam, as described more fully below. The coated honeycomb  10  is placed onto the filler  20 , pressed into the filler  20 , covered with a caul plate (not shown), and tapped with a rubber mallet until it is seated in the mold with the filler  20  partially filling the cells  12 . This leaves a lower portion of the cells  12  filled with filler  20 . The balance of the cells  12  above the filler  20  may then be filled with a foam precursor  25 . In an example embodiment, the portion of the honeycomb  10  without promoter  15  is the portion placed into the filler  20 , because for some promoters  15 , filler  20  can interfere with binding of the promoter  15  to later added foam.  
      In  FIG. 3B , the honeycomb  10 , coated with primer  15 , is shown with its cells  12  filled with the filler  20  (lower portion of cells  12 ) and a foam precursor  25  (upper portion of cells  12 ). In a particular embodiment, the precursor  25  may suitably be a polyimide foam precursor. Polyimide foams typically endure higher thermal operating temperatures than polyurethane foams. In the present invention, the polyimide layer of foam precursor  25  is typically the foam layer furthest from the cryogenic propellant tank when the multi-layer foam insulating structure is applied to a cryogenic propellant tank. In one particular aspect, the polyimide foam precursor  25  may be polyimide friable balloons manufactured by Unitika, Ltd., as described in part in U.S. Pat. Nos. 6,133,330, and 6,180,746. Such polyimide foams suitably include, for example, TEEK-H Polyimide Friable Balloons manufactured by Unitika Ltd, of Kyoto Japan.  
      As further shown in  FIG. 3B , friable balloons of the foam precursor  25  have been leveled to the upper surface of the honeycomb  10 . The polyimide foam precursor  25  after cure offers a service temperature of 500 degrees F., which is 100 degrees higher than Rohacell foam, and 250 degrees higher than alternate polyurethane foams. Having a higher operating temperature foam installed immediately below the thermal protective system on a space vehicle may advantageously permit the thermal protective system to be thinner, and thus lighter. Put differently, higher operating temperatures at the boundary between a thermal protective system and the cryo-insulation resulting from a thinner thermal protection system are accommodated by the higher operating temperature of the polyimide foam. It will be appreciated that in other applications, such as those in vehicles, aircraft, or fixed equipment that incorporate cryogenic tanks or other equipment involving cooling, heating, or wide temperature differentials, the use of differing temperature accommodating foam composites, with a structural support, may be desirable.  
      It will also be appreciated that a first foam layer may also be installed in the honeycomb  10  by means other than filling with a precursor  25  and curing, such as spraying and machining away any excess foam. It will also be appreciated that a mold release may be desirable between the base  5  or other mold, and the precursor  25  filled honeycomb  10 . By way of example, suitable mold releases for polyimide foam precursors include Frekote 33.  
      In an exemplary embodiment of the present invention, a skin  30  may be added to the honeycomb  10  and the polyimide foam layer  25 , as shown in  FIG. 3C . By way of example, but not limitation, the skin  30  may be placed over the upper surface of the honeycomb sheet  10  and the precursor  25  after filling the cells  12  of the honeycomb  10  with the precursor  25 . In one embodiment, the skin  30  bonds to the edges of the matrix  14  of the honeycomb  10 , as well as to the polyimide foam formed during heating when the polyimide precursor  25  is cured. The skin  30  may be formed of any suitable material, including, for example, a bismaleimide (BMI) film adhesive such as a 2550G film adhesive commercially-available from Cytec. The BMI film may be heat cured and bonded to the cell walls  14  of the honeycomb  10 . When the assemblage of the skin  30 , honeycomb  10 , and the precursor  25  is heat cured, the BMI film adhesive skin  30  suitably bonds to the matrix  14  of the honeycomb  10  before the precursor  25  foams and expands. As the BMI film  30  is heat cured and bonded to the honeycomb  10 , it softens. Under pressure, the film  30  may form fillets at the junctures with the edges of the matrix  14  of the honeycomb  10 . These fillets (not shown) advantageously increase the strength of the bond of the skin  30  to the matrix  14  of the honeycomb  10 . It should be noted that such fillets typically do not form when a skin is bonded to a honeycomb that has already been filled with expanded foam. In that case, expanded foam already fills the honeycomb to the upper edges of the matrix  14 , leaving no space for filleting of the skin  30  against the matrix  14 . Traditionally it has been difficult to bond to foam-filled honeycomb because the previously installed foam prevents filleting thereby reducing the bond strength between the skin  30  and the honeycomb  10 .  
      Filleting helps to optimize the strength of the bond of the skin  30  to the honeycomb  10  matrix  14 . In the method of the present invention, because the first foam at this stage is a polyimide foam precursor  25  in the form of friable balloons that have not expanded at the time the adhesive skin  30  bonds to the matrix  14 , the precursor  25  does not impede the filleting of the adhesive skin  30 .  
      The skin  30  suitably may be held in position in the assemblage  32  by any suitable method during heat bonding of the skin  30 , and the heat curing of the precursor  25 . By way of example, but not limitation, as shown in  FIG. 3C , the assemblage  5  is inserted into an autoclave  3  for heat curing of the skin  30  and heat curing of the polyimide foam precursor  25 . The assemblage may be held together during cure by a suitable securing mechanism or clamp, including, for example, a vacuum bag  35 . In operation, when air  7  is evacuated from the vacuum bag  35 , the vacuum bag  35  collapses and holds the honeycomb  10  (in this example half filled with filler  20  and half filled with precursor  25  and covered with the covering skin  30 ) firmly on to the base  5 . It will be appreciated that a caul plate or other suitable weight or securing mechanism may be utilized to hold the assemblage  32  in position during transport and/or curing.  
      During cure, the autoclave may be maintained under  45  psi of autoclave pressure and the vacuum bag  35  may be vented to the atmosphere, with the result that pressure is maintained on the honeycomb  10 , precursor  25  and skin  30  during heat curing. By way of example and not limitation, at least one thermocouple  9  is typically installed in the polyimide foam  25  to monitor the temperature of the foam  25  during the cure process.  
      The assemblage  32  on the base  5  may then run through a multi-step heat cure process in the autoclave  3 . In one embodiment, a suitable heat curing process includes heating the assemblage  32  at a rate of 4 to 6° F. per minute to a temperature of about 375°±10° F. The assemblage  32  may then dwell at the elevated temperature for approximately 60 minutes±5 minutes. The assemblage  32  may then be heated at a rate of 4 to 6° F. per minute to roughly 482°±5° F. The assemblage  32  may again dwell at this elevated temperature for approximately 120 minutes±5 minutes. The assemblage  32  may then be heated again at rate of 1±0.9° F. to a temperature of about 550°±5° F. The assemblage  32  may again dwell at this elevated temperature for roughly 60 minutes±5 minutes. The assemblage  32  suitably may then be cooled at a rate of 5±3° F. per minute to below approximately 250° F. prior to removing from the mold or base  5 .  
      Control temperature may be suitably based on the average temperature of two thermocouples  9  in the precursor  25  at opposite edges of the honeycomb panel  10 , each located ¼ to ½ inch from the panel edge. In a particular aspect, the maximum difference between the autoclave air temperature and the assemblage  32  temperature may be limited to 375° F., and the maximum air temperature during cure may be prevented from exceeding about 575° F.  
      The above-described heat cure process may result in the skin  30  bonding to the matrix  14  of the honeycomb  10 . The polyimide foam precursor  25  then expands and cures, bonding to the matrix  14  with the assistance of the primer  15  previously applied to the honeycomb  10 . As the polyimide foam precursor  25  cures, it may also bond to the BMI film skin  30  as these two materials adhere to each other on curing.  
      With continued reference to  FIG. 3C , after heat curing, the assemblage  32  is removed from the autoclave  3 , and the vacuum bag  35  is removed. The honeycomb  10  with the now cured polyimide foam and attached skin  30  is removed from the filler  20  and inverted. As shown in  FIG. 4A , the resulting assembly  32  includes the honeycomb  10  with its cells  12  partially filled with cured polyimide foam  26 . The BMI skin  30  is attached to the polyimide foam  26  and the honeycomb  10 . It will be appreciated that removing the assemblage  32  from the sand and inverting it leaves a portion of the plurality of honeycomb cells  12  empty and open to be filled with a layer of second foam  40 .  
      In a preferred embodiment, the second foam  40  is a polyurethane foam layer. Specifically, with the assemblage  32  inverted, placing the skin  30  down, a remaining portion of the plurality of cells  12  of the honeycomb  10  may be filled with sprayed second foam  40 , sprayed from a spray polyurethane gun  41 . By way of example but not limitation, the second foam  40  may be sprayed on to the honeycomb  10  until an upper portion of the cells  12  of the honeycomb  10  are completely filled. In an exemplary embodiment, as the second foam  40  cures, it suitably self-adheres to the honeycomb  10  and the cured polyimide foam  26  without further steps or materials. The second foam  40  may expand and overfill the honeycomb  10 . The second foam  40  suitably may include polyurethane foam by Polymer Development Laboratories, Inc., product numbers 1034-2.5 and 1034-141. Other foams that may be utilized for the second foam  40 , include, by way of example, but not limitation, polyisocyanurate foam  
      Any second foam  40  overfill (not shown) may be machined off to the upper edge of the honeycomb  10  using a mill, a drill press fitted with a diamond grinder, or any other suitable removal process.  
      In a preferred aspect, the second foam  40  is a polyurethane foam sufficiently closed-celled to minimize or eliminate cryo-pumping when the second foam  40  is installed against a cryogenic propellant tank. Thus, in a particular embodiment of the present invention, the resulting exemplary structure  34  is a two-layer foam-filled honeycomb-matrix-core cryogenic insulation consisting of polyimide and polyurethane foams in an aramid/phenolic honeycomb.  
      It should be noted that the skin (or BMI adhesive film)  30  may suitably include a removable tear ply (not shown) on its outside surface, i.e. the side away from the honeycomb  10  and the polyimide foam  26 . Thus, when the multi-layer foam structure  34  is completed, and ready to be installed, the tear ply over the BMI film may be torn off. In this exemplary embodiment, the removal of the tear ply layer exposes a fresh surface for adhering a thermal protective system, or other structure or attachment to the skin  30 . By way of example, but not limitation, the tear ply may suitably be a sheet that may be laid against the BMI film skin  30  prior to cure, such as a teflon coated fiberglass release ply 200PFP-1 manufactured by Richmond Aircraft Products.  
      As further shown in  FIG. 4B , an exemplary integral multi-layer foam composite structure  34  of the present invention thus includes a skin  30  bonded to a honeycomb  10 , with the cells  12  of the honeycomb  10  half filled with cured polyimide foam  26 , and half filled with polyurethane second foam  40 . The skin  30  is suitably installed on the side of the honeycomb  12  adjacent to the cured polyimide foam  26 , in this exemplary embodiment.  
      The resulting structure  34  may be bonded to a cryogenic propellant tank. The structure  34  is bonded to the tank (not shown) with the side of the honeycomb  10  filled with polyurethane second foam  40  bonded towards the tank. This insulates the tank with the desired more closed-cell polyurethane second foam  40  closest to the tank, and higher temperature tolerant, but more open-celled polyimide foam  26  spaced away from the tank where the polyimide foam  26  is not subject to cryo-pumping. A significant portion of the internal structural strength of the multi-layer foam assemblage  34  is provided by the honeycomb  10 , permitting the assemblage to carry loads such as aerodynamic loads, or to be attached to other equipment. A thermal protective system, such as ceramic tiles or a ceramic blanket may be attached to the structure  34  by adhering it to the skin  30 , or if no skin is desired, to the honeycomb  10 . The thermal protective system is thus suitably secured against aerodynamic loads, because the skin  30  is cured and bonded to the polyimide foam  26  and the cell walls  14  of the honeycomb  10 , which provides sufficient structural strength to bear the loads. It will be appreciated that in alternate embodiments, a skin  30  may be applied to none, one or both sides of the honeycomb  10 .  
      An exemplary method of manufacturing the multi-layer foam structure of the present invention is outlined in a flow chart in  FIG. 5 . The method includes priming or resin coating a honeycomb at a block  110 , and drying the honeycomb at a block  120 . The honeycomb is partially embedded in a removable filler, in this example a sand embed, at a block  130 . Filling of an upper section of the honeycomb with polyimide friable balloons occurs at a block  140 . A BMI adhesive layer or skin may be applied, as desired, at a block  150 . The resulting resin coated honeycomb assemblage, partially embedded in the removable filler, with its upper section filled with polyimide friable balloons and covered with adhesive skin, is covered and vacuum bagged at blocks  160  and  170  respectively. The assemblage is, bonded, heat cured and the polyimide foam is expanded at a block  180 . The assemblage is unpacked and inverted at a block  190 , including removing the removable filler. At a block  200 , the inverted assemblage is sprayed with polyurethane foam. At a block  210 , the excess polyurethane is removed (e.g. by machining or other suitable process). As desired, if a tear ply was previously applied to the adhesive skin, that tear ply may be peeled off at a block  220 , preparing a fresh surface of the final multi-layer foam structure to adhere to another layer or other equipment, such as a thermal protective system for a spacecraft.  
      It will be appreciated that the structure and method of the present invention may be utilized with a variety of foam materials and different support structures and materials. For example, in alternate embodiments, the hexagonal cells  12  of the honeycomb panel  10  may be replaced with square, rectangular, circular, or any other suitably shaped cells. It will also be appreciated that the structure and the method of the present invention is not limited in applicability to cryogenic propellant tanks of space vehicles, but may be utilized in any application where a combination of insulating and structural features are desired. The structure and method of the present invention thus provide a means for a strong system that can combine the desirable features of at least two different types of foam insulation into an integral, easy to install package.  
       FIG. 6  is a cross-section of a section of a exemplary aerospace vehicle  200  incorporating an exemplary multi-layer foam structure  250  in accordance with an embodiment of the present invention. The aerospace vehicle has a tank wall  230  holding cryogenic propellant  235 . The tank wall  230  is internally reinforced by stringers  232 . The tank is connected to a lower stage through an intertank  220 , which is vented with an intertank purge  222 . In this example, the intertank  220  is not insulated with cryo-insulation. The outer surface of the tank wall  230  is covered with an exemplary multi-layer foam structure  250  in accordance with an embodiment of the present invention. The foam structure  250  is bonded to the tank wall  230  with adhesive (not shown). Attached over the foam structure  250 , on the side away from the tank wall  230 , is a thermal protection system  210 , in this example ceramic tiles. The thermal protection system  210  may be bonded direct to the foam structure  250  with adhesive (not shown), as shown in this example embodiment, or alternately, may be bonded to a skin (not shown) incorporated in the foam structure  250 , in the manner described above. The foam structure  250  may thus provide a lightweight cryogenic tank insulation, with increased heat tolerance towards the thermal protection system  210 , and resistance to cryo-pumping next to the tank wall  230 .  
      It may be appreciated that the aerospace vehicle  200  may be any model or type of vehicle that includes a tank for carrying cryogenic materials, including, for example, a planetary probe, a satellite or other type of spacecraft, a conventional or hypersonic aircraft, or a reusable orbital vehicle. In further embodiments, the vehicle  200  may be any type of land, sea, or undersea vehicle that is capable of transporting cryogenic materials, including automobiles, trains, ships, submarines, or any other suitable vehicle type.  
      While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.