Abstract:
A method of forming a heat shield that involves thermally stabilizing a plurality of phenolic microspheres; mixing the thermally stabilized phenolic microspheres with a phenolic resin to form a phenolic ablative material; compressing the phenolic ablative material into a honeycomb core; and allowing the phenolic ablative material to cure.

Description:
FIELD 
       [0001]    The present disclosure relates to heat shields, and more particularly to a heat shield constructed in part from a lightweight ablative material that is well suited for use on spacecraft and other aerospace vehicles. 
       BACKGROUND 
       [0002]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0003]    Spacecraft require safe, lightweight, affordable heat shields for protecting the spacecraft and its occupants during re-entry of the spacecraft into the Earth&#39;s atmosphere, or entry into a planet&#39;s atmosphere. Traditionally, the weight of the heat shield has been an important factor. The higher the weight of the heat shield panels used the lower the payload that the spacecraft will be able to carry. 
         [0004]    Previously manufactured heat shields have typically been made from mixtures of silicone resins with fillers, from epoxy-novolac resins with fillers, from phenolic resins with fillers, from carbon-carbon composites with backside insulation, from quartz-phenolic composites with backside insulation, or from Phenolic Impregnated Carbon Ablator (PICA) material. However, existing solutions can often add significant weight to a spacecraft. 
         [0005]    The cost of manufacturing previously developed heat shields for a spacecraft has also been an important concern for designers. Traditionally, the high cost of manufacturing heatshields for spacecraft, using complex processes, has contributed significantly to the overall cost of manufacture for a spacecraft. 
       SUMMARY 
       [0006]    In one aspect the present disclosure is related to a method of forming a heat shield. The method may comprise: thermally stabilizing a plurality of phenolic microspheres; mixing the thermally stabilized microspheres with a phenolic resin to form a phenolic ablative material; compressing the phenolic ablative material into a honeycomb core; and curing the phenolic ablative material under controlled heat and pressure. 
         [0007]    In another aspect the present disclosure relates to a method of forming a heat shield that may comprise: forming a phenolic ablative material; filling a rigid tool with the phenolic ablative material such that the material assumes a desired shape in accordance with a shape of an interior area of the tool; placing the tool in a vacuum while cooling the tool and the phenolic ablative material to freeze and dimensionally stabilize the phenolic ablative material to form a frozen phenolic ablative preform; removing the frozen phenolic ablative preform from the tool; and thawing and compressing the frozen phenolic ablative preform into a plurality of cells of a vented honeycomb core to form the heat shield. 
         [0008]    In another aspect the present disclosure relates to a heat shield that may comprise: a honeycomb core having a plurality of cells; a phenolic ablative material compressed into said cells; and the phenolic ablative material including a plurality of thermally stabilized phenolic microspheres and a phenolic resin. 
         [0009]    Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0011]      FIG. 1  is a side view of one exemplary spacecraft making use of a heatshield formed in accordance with the teachings of the present disclosure; 
           [0012]      FIG. 2  is a perspective view of one section of the heatshield shown in  FIG. 1 ; 
           [0013]      FIG. 3  is a cross section of the heatshield in accordance with section line  3 - 3  in  FIG. 2 ; 
           [0014]      FIG. 4  shows the carrier panel side of a honeycomb panel that has had venting slots cut into the cell walls; 
           [0015]      FIG. 5  is a flowchart of exemplary operations that may be performed to make the ablative material that is used in constructing the heatshield of  FIG. 1 ; 
           [0016]      FIG. 6  is an illustration of a mold tool being filled with the ablative material; 
           [0017]      FIG. 7  shows a rubber caul sheet being placed over the BPA mix and a vacuum bag being secured over the filled mold tool of  FIG. 5 ; 
           [0018]      FIG. 8  shows the ablative material being debulked prior to being frozen; 
           [0019]      FIG. 9  shows the vacuum bagging material being removed from the mold tool and the frozen preform; 
           [0020]      FIGS. 10A and 10B  show perspective views of the resulting ablative preform ready to be placed into the freezer for storage or to be pressed into a honeycomb core; 
           [0021]      FIG. 11  is a flowchart illustrating exemplary operations in forming the heatshield of the present disclosure; 
           [0022]      FIG. 12  is a partial side cross sectional view of the ablative preform positioned over the honeycomb core, and with the preform/honeycomb core assembly positioned within a mold tool that is enclosed within a vacuum bag ready for placing into the autoclave; 
           [0023]      FIG. 13  is an exemplary graph of the pressure and heat profiles used during green state curing of the assembly shown in  FIG. 12 ; 
           [0024]      FIG. 14  is an exemplary graph of the pressure and heat profiles used during post curing of the assembly shown in  FIG. 12 ; 
           [0025]      FIG. 15  is a perspective view of an exemplary closeout component that may be secured to a perimeter edge of the heatshield to close it; and 
           [0026]      FIG. 16  is a simplified side view showing how a monolithic heatshield may be formed using a plurality of sections of the heat shield described in the present disclosure. 
           [0027]      FIGS. 17A-17E  illustrate a sequence of operations showing the overall approach for a monolithic heatshield that is formed by first attaching the honeycomb core to a spacecraft structure and then processing the assembly; 
           [0028]      FIGS. 18A-18F  illustrate a sequence of operations for an alternate approach for constructing a monolithic heatshield that is formed by filling the honeycomb core on a tool that matches the heatshield structure, then processing, machining and attaching the ablative panel to the spacecraft in one piece; 
           [0029]      FIG. 19  illustrates another embodiment of the heat shield in which two preforms are used that have different constituencies of microballoons, to thus provide two layers of ablative material having different densities; and 
           [0030]      FIG. 20  shows the heat shield of  FIG. 19  after the two preforms are compressed into the honeycomb core. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
         [0032]    Referring to  FIG. 1 , an exemplary spacecraft  10  is shown incorporating a heatshield  12  in accordance with one embodiment of the present disclosure. The heat shield protects the spacecraft  10  and its occupants from the heat generated during reentry into the Earth&#39;s atmosphere, or during planetary entry. While the heatshield  12  is shown on a manned spacecraft, it will be appreciated that the heatshield  12  is well suited for use on a wide variety of other manned and unmanned space vehicles that are expected to encounter high temperatures on their exterior surfaces during travel through the Earth&#39;s, or a planetary atmosphere. The heatshield  12  is also potentially usable on other forms of vehicles, and possibly even on fixed (i.e., non-mobile) structures. The heat shield may find use on virtually any form of mobile airborne platform or ground based vehicle, or possibly even on marine vehicles. 
         [0033]    Referring to  FIGS. 2 and 3 , the heatshield is shown in greater detail. The heatshield  12  includes a core  14 , which in this example is a honeycomb core. For convenience the core  14  will be referred to throughout the following discussion as the “honeycomb core  14 ”. The honeycomb core has a plurality of intersecting wall portions  15  that form a plurality of cells  15   a.  An ablative material  16  is press fit into the cells  15   a  of the honeycomb core  14 . In  FIG. 3 , the honeycomb core  14  may be secured via an adhesive layer  18  to a carrier structure  20 . 
         [0034]    The honeycomb core  14  may be formed from a fabric of well known fiberglass, for example Style  120  (E-glass), which is impregnated with a phenolic resin. The honeycomb core  14  may also be formed from a Leno weave fiberglass or carbon fiber fabric having an open weave construction. This enables the ablative material  12 , when compressed into the cells  15   a  of the honeycomb core  14 , to fill the cells  15   a  and become an integral portion of the wall structure of the honeycomb core  14 . Prior to filling the cells  15   a  of the honeycomb core  14  with the ablative material  12 , the honeycomb core  14  may be cleaned with a radio frequency (RF) generated plasma field so that its surfaces are thoroughly conditioned for the remaining manufacturing operations to which the honeycomb core  14  will be subjected. The plasma field cleaning treatment is a process that is commercially available. One such company performing this process is 4 th  State, Inc., of Belmont, Calif. 
         [0035]    Referring to  FIG. 4 , following cleaning, and prior to filling the cells  15   a  with the ablative material  12 , the walls  15  of the honeycomb core  14  are partially slotted, preferably using a diamond edged cutting tool, on the side of the honeycomb core  14  that will be bonded to the heat shield carrier structure  20 . In  FIG. 4  these slots are identified by reference numeral  21  and shown in detail on a piece of the honeycomb core  14 . The slots  21  provide escape paths for ambient air that might otherwise create back pressure in the cells  15   a  during the subsequent honeycomb core  14  filling process, and for water and gases that evolve during a subsequently performed autoclave curing process. The air, water and gases are drawn off by a vacuum that is applied to a vacuum bag enclosing a preform that forms the ablative material  16 , the honeycomb core  14 , the carrier structure  20  and the tool. This process will be described in greater detail in the following paragraphs. 
         [0036]    The carrier structure  20  may be formed as a multilayer structure from one or more metal sheets, or possibly even as a honeycomb structure having metal, for example titanium, face sheets. For convenience the carrier structure  20  has been drawn as a single metal layer in  FIG. 3 . The adhesive layer  18  may be formed by any suitable adhesive, but in one example HT-424 adhesive, which is an epoxy-phenolic structural film adhesive commercially available from Cytec Industries, Inc. of West Paterson, N.J., is used as the adhesive. 
         [0037]    The ablative material  16  is uniquely formulated to form a lightweight, medium density, syntactic foam ablator compound. The ablative material  16  may comprise a mixture of phenolic resin, carbon fibers, silica (SiO 2  or Manville “Q”) fibers, phenolic microspheres and silica microspheres. The silica fibers and carbon fibers are used to provide structural reinforcement to the ablative material  16  and to enhance the thermal and ablative performance. The silica microspheres and phenolic microspheres are used as density reducing fillers that also enhance the thermal and ablative performance of the ablative material  16 . The phenolic resin, in one example, may be Plenco 11956 phenolic resin. The silica fibers may have a diameter of about 1.5 um. The carbon fibers may be milled fibers having a length of about 150 um and a diameter of about 7.4 um. One specific carbon fiber that is suitable for use is Asbury Graphite Mills AGM-94 milled carbon fibers. The silica microspheres may have a diameter of between about 20-250 um; and the phenolic microspheres may have a most common diameter of between about 20-100 um. In one implementation Phenocet BJO-0930 phenolic microspheres are used. It will be appreciated, however, that all of these dimensions may be varied to suit the needs of a particular application. 
         [0038]    The use of Plenco 11956 resin is particularly advantageous because it is a single component, water based resole phenolic resin that does not require adding flammable solvents, toxic curing agents, or reactive diluents to the basic phenolic resin, as with epoxy-novolac or some other types of phenolic resins. Because it is a liquid at room temperature it does not have to be heated to be blended with fillers. Because pure phenolic resin is a better ablator than typical curing agents or reactive diluents, the absence of such curing agents and reactive diluents from the phenolic resin helps to provide the ablator material  16  with superior thermodynamic response characteristics. The lack of a curing agent also allows the freshly mixed ablator material  16  to have a longer room temperature working life, since rapid cure does not initiate until it is heated to above 150° F. The fresh resin has a relatively long storage life at 0° F. of typically about four months and the room temperature working life for the ablator material  16  is five days, unlike epoxy-novolac, or some other phenolic resin based ablators. These characteristics of Plenco 11956 resin, along with the use of frozen preforms, give the ablative material  16  the working time needed to apply it to large monolithic structures that can be cured in one piece. Some other phenolic resins or epoxy-novolac material systems either have short working lives that limit the area of the heat shield that can be processed at one time, or they require that the ablative material be hand injected into individual honeycomb cells using heated caulking guns. 
         [0039]    Referring to flowchart  50  of  FIG. 5 , in forming the ablative material  16 , it is preferred, but not absolutely essential, that the phenolic microspheres are dried out using a heated inert atmosphere before they are used to make the ablative material  16 , as indicated at operation  52 . This serves to remove any water and volatiles that may remain in the microspheres from the manufacturing process used to make them, and it stabilizes the state of cure of the phenolic resin that comprises the microspheres. The silica microspheres may also be dried by the same process to remove adsorbed water. It is also preferred that the various constituent materials used to form the ablative material  16  are added in a specific order to avoid clumping, achieve even wetting of the fibers and the microspheres by the phenolic resin, and to obtain uniform blending of all ingredients. The mixing may be done in a commercial bread mixer that imparts high shear forces to the blend but does not chop or mill the fibers and microspheres. To this end, at operation  54  a quantity of phenolic resin is provided, which as explained above is preferably Plenco 11956 phenolic resin. At operation  56  the silica fibers are added to the resin and mixed to achieve uniform dispersion and wetting by the resin. At operation  58  the carbon fibers are then added to the phenolic resin/silica fiber blend and the mixing is continued. At operation  60  the silica microspheres are added to the blend and the mixing is continued. At operation  62  the phenolic microspheres are added to the blend. At operation  64  the mixing is continued until the final uniform wetting and consistency are achieved. The sequence of adding ingredients and mixing is carried out over a time span of typically between about 23 minutes-30 minutes. In laboratory testing the ablative material  16  had a density (virgin) of between about 0.417 g/cm 3  to 0.497 g/cm 3  (26-31 Ibm/ft 3 ); a thermal conductivity (virgin) at room temperature of 0.107 W/m-° K (0.62 Btu/hr-ft 2 ); an ablation onset temperature, in Nitrogen, of 396° C. (744° F.); a tensile strength through its thickness of 4.08 MPa (592 lb/in 2 ) and an effective heat of ablation of 69.9×10 3  KJ/Kg (at a plasma arcjet heat flux of 420 W/cm 2 ). At this point the ablative material  16  is ready to be manufactured into a preform. 
         [0040]    Referring to  FIGS. 6-10 , one method for forming a preform comprised of the ablative material  16  will be described. In  FIG. 6  the ablative material  16  is used to fill a mold  70 . The mold will have dimensions of length, width and thickness that correspond to the desired dimensions for the preform. In  FIG. 7  mold  70  is vacuum bagged with suitable bagging materials  72  and the ablative material  16  is debulked, (i.e., compressed to consolidate the granules of ablative material and remove voids by means of a partial vacuum drawn on the vacuum bag). In  FIG. 8  the mold  70  with the ablative material  16  is frozen at approximately −10° F. for about 4-6 hours. In  FIG. 9  the bagging material  72  is removed from the mold. The frozen ablative material preform  74  is shown in  FIGS. 10A and 10B . The preforms can be used immediately for filling honeycomb core  14 , or they can be freezer stored up to 2 months for later use. For a large heatshield  12 , that requires a plurality of preforms, the necessary quantity of preforms are made in advance and freezer stored until the time of final assembly. 
         [0041]    Referring now to the flowchart  100  of  FIG. 11  and the drawing of  FIG. 12 , a description of using the preform  74  to form the heat shield  12  will be described. At operation  102  the plasma cleaned honeycomb core  14  is slotted on the carrier structure side (as shown in  FIG. 4 ), using a diamond edged cutting tool, to thus form the slots  21 . At operation  104  the carrier structure  20  is bonded to the honeycomb core  14 . At operation  106  the honeycomb core  14  with the carrier structure  20  bonded thereto is placed in a mold tool  75  sized approximately to the dimension of the honeycomb core and its carrier structure. At operation  108  the frozen preform  74  is placed over an upper surface of the honeycomb core  14 , that being the surface opposite to that which the carrier structure  20  is secured to. The entire assembly is covered with vacuum bagging materials  77  as indicated at operation  110 . The assembly ready to be autoclave cured is shown in simplified form in  FIG. 12 . The core slots  21  that provide venting during filling and cure are shown in  FIG. 12  at the intersection of the honeycomb core  14  and the carrier structure  20 . 
         [0042]    Referring further to  FIG. 11 , the assembly of  FIG. 12  is then autoclaved cured to the “green state”, i.e. partially cured, as indicated at operation  112 . Exemplary temperatures and pressures that may be used during the autoclave cure cycle are illustrated in the graph shown in  FIG. 13 . During the autoclave curing cycle the preform  74  is thawed and squeezed into the cells  15   a  of the honeycomb core  14  under pressure until the cells  15   a  are completely filled with the material of the preform  74 , to thus form the heat shield  12 . Rather than by autoclave processing, the preform  74  could also be compressed into the cells  15   a  of the honeycomb core  14  by a mechanical press, as indicated by dashed lines  79 . 
         [0043]    Once the green state cure operation is complete, the heat shield  12  is removed from the mold tool  75  (or the mold tool disassembled), as indicated at operation  114 . At operation  116  excess material from the preform  74  that extends above the honeycomb core  14  may be removed by machining or other means, and the edges, or periphery, of the heatshield may be beveled to reduce the effect of shrinkage stresses during the subsequent postcure operation. At operation  118  the heatshield  12  is returned to the autoclave for postcure to the final cure state of the ablative material  16 . Exemplary temperatures and pressures that may be used during the autoclave postcure cycle are illustrated in the graph shown in  FIG. 14 . 
         [0044]    At operation  120  non-destructive examination of the heat shield  12  by x-ray and ultrasonic methods may be performed to: 1) verify the integrity of the adhesive bonds between the cured ablative material  16 , the honeycomb core  14 , and the carrier structure  20 ; 2) verify that the honeycomb cells are all filled completely, top to bottom, (i.e. the cell fill is free of voids); and 3) verify that there are no internal cracks in the cured ablative material  16 . At operation  122  the final outer mold line (OML) contour of the heat shield may be machined, if needed, to provide a particular, desired contour. At operation  124 , edge closeouts  125 , one of which is shown in  FIG. 15 , that have been manufactured in the same manner as the heat shield  12  described above, may be secured such as by an adhesive to the carrier structure  20  to close off the exposed peripheral edges of the heatshield  12 . The edge closeouts  125  may also be adhesively bonded to the edges of the main portion of the heat shield  12 , or they may be bonded only to the carrier structure  20  and the gaps between the closeouts  125  and the main portion of the heat shield  12  subsequently filled with a room temperature curing silicone elastomer. Also at operation  124  the edge closeouts  125  of the heat shield  12  are all non-destructively inspected. 
         [0045]    Referring to  FIG. 16 , when making a monolithic non-planar heat shield for the spacecraft  10  shown in  FIG. 1 , a slotted honeycomb cone  150 , similar or identical in construction to honeycomb core  14 , may be formed with the desired moldline needed to enable attachment of a finished heat shield to the outer surface of the spacecraft. In this regard a plurality of ablative preforms  74  may be cut to desired shapes and layed onto the honeycomb core  150 , which has been secured with an adhesive layer  18  to the carrier structure  20 , and then the entire assembly vacuum bagged and cured in an autoclave as a single piece assembly. Each of the preforms  74  may have chamfered edges  74   a  to help interlock with adjacently placed preforms. An alternative for making a monolithic, non-planar heat shield as shown in  FIG. 16  is to temporarily secure the slotted honeycomb core  150  to a male tool that matches the outer mold line contour of the spacecraft structure, then layup, vacuum bag, press into the core, and autoclave cure the ablative material  16  of the ablative preform  74  in the manner described above to produce a monolithic ablator. The monolithic ablator is then removed from the tool, the inner mold line contour is verified or machined, and the single piece is secured by an adhesive to the spacecraft structure outer mold line. 
         [0046]    In one variation of producing the ablative material  16 , the material  16  may be forced through a mesh screen, for example a 100 mesh screen, (meaning a stainless steel wire screen with 100 openings per inch that are about 0.005 inches on a side), to form a pelletized ablative material. The pelletized ablative material may then be distributed over the cells of the honeycomb core  14  to completely fill the cells prior to vacuum bagging of the honeycomb core. 
         [0047]    The methodology of the present disclosure thus provides a means for filling large areas of a honeycomb core structure at one time rather than filling each cell individually, or by filling tile size pieces of honeycomb core by machine or hand pressing material into both sides of the core, followed by curing and then machining to a finished shape for installation. This approach makes possible at least three major options for heat shield assembly. The first option is highly advantageous and involves pre-bonding the unfilled honeycomb core to the exterior of a spacecraft using an existing adhesive that has been certified for manned spaceflight. Thus, the present disclosure eliminates the need for the development and certification of a new attachment design for attaching the heatshield to a spacecraft using gore segments or tiles. The second advantageous option that the methodology discussed herein makes possible is that when a particular spacecraft design does not allow for processing the heatshield on the spacecraft, a single piece monolithic ablator assembly may be made on the side and then secured to the spacecraft by an adhesive in one operation. The third option, which has advantages for some spacecraft as well as for hypersonic aircraft, ground vehicles and stationary applications, is to make large preformed cured billets that are subsequently machined into panels, gores or large and small tiles. Options  1  and  2  are illustrated and compared in  FIGS. 17A-17E  and  18 A- 18 F, respectively. In  FIG. 17A  the honeycomb core  14  is first bonded to the carrier structure  20   a  that will ultimately form a portion of a spacecraft. In  FIG. 17B  the assembled honeycomb core  14  and carrier structure  20  are then cured in an autoclave  160 . The honeycomb core  14  may then be filled with the ablative material  16  and then cured in the autoclave  160  (FIG.  17 C). The outer mold line (OML) of the resulting cured assembly of  FIG. 17C  may then be machined to the desired shape and/or contour, as indicated in  FIG. 17D . The resulting product is shown in  FIG. 17E . In  FIGS. 18A-18F , option two described above is illustrated. The ablative material  16  is first compressed into the honeycomb core  14 . In  FIG. 18B , the assembly shown in  FIG. 18A  is then autoclave cured in the autoclave  160  to form assembly  170 . In  FIG. 18C  a machine tool  180  is used to machine the inner mold line (IML) of the cured assembly  170  to form machined assembly  185 , which is shown in  FIG. 18D . In  FIG. 18D  the machined assembly  185  is then bonded to the carrier structure  20  to form assembly  190 . In  FIG. 18E  the outer mold line (OML) of the assembly  190  is machined with a machine tool  200 . The finished product  205  is shown in  FIG. 18F . 
         [0048]    Referring to  FIGS. 19 and 20 , a heat shield  200  is shown in accordance with another embodiment of the present disclosure. The heat shield  200  involves using two or more frozen preforms  202  and  204 , which are each similar in construction to preform  74 , but where the two preforms have differing amounts (and possibly different types) of microspheres so that the two preforms  202 , 204  have differing densities. In this manner an ablative material may be produced that has a controlled density gradient through its overall thickness. This is beneficial because it is desirable to have a higher density material at the outer layer, where ablation occurs, but a lower density material at the inner layer in order to minimize thermal conductivity and overall weight. The challenge of attaching the individual preforms  202  and  204  to each other in a manner such that the bond therebetween can survive high temperatures is overcome by incorporating the density gradient in a single type of ablative material, and co-curing the two preforms  202 , 204  so that there is essentially no joint or seam between the two preforms in the finished product. The finished product is shown in  FIG. 20 . 
         [0049]    The heat shield  12  manufactured as described above is lighter than existing heat shields made from pre-existing approaches because of the greater mass efficiency of the ablator composition. The heat shield  12  also uses safe, non-toxic materials. The heat shield  12  allows two options for a monolithic heatshield design to be constructed that can be made in accordance with less complex manufacturing procedures traditionally employed in the manufacture of such heat shields. These benefits also help to reduce the cost of the heat shield  12  as well as the time needed to manufacture it. In particular, curing the ablative material  16  after it has been attached to the honeycomb/carrier structure avoids the need to form or machine a cured ablative material to match the contour of a heatshield carrier structure, which must take into account machining errors and any variations in each specific carrier structure piece. 
         [0050]    While various embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the present disclosure. The examples illustrate the various embodiments and are not intended to limit the present disclosure. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.