Abstract:
A method of forming ablative insulation coatings on aerospace structures and articles produced thereby. The method includes forming a mold having a cavity configured to complementarily receive at least a portion of the structure and directly molding the ablative coating to a surface of the structure by introducing a prepared ablative mixture, such as an epoxy resin, into the mold. The process may desirably include curing at atmospheric pressures to reduce the expense and equipment required in carrying out the process. The method of applying the ablative coating further includes features such as varying the thickness of the ablative coating from one portion of the component to another. Control of the thickness may be effected through use of spacers embedded into the ablative coating an formed of the same or similar material previously cured prior to placement in the mold.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to coatings of insulative and ablative materials. More particularly, the present invention relates to ablative coatings on exposed components of aerospace structures and a method of forming such coatings.  
           [0003]    2. State of the Art  
           [0004]    Ablative coatings are thermally insulating coatings typically utilized as heat shields for exposed surfaces of aerospace equipment such as rockets, missiles, space shuttles and similar vehicles. The ablative coating serves to protect the structure from high thermal energy experienced due to high velocity conditions, for example during launch or re-entry into the earth&#39;s atmosphere. The ablation of such a coating is a known phenomenon by which energy incident upon an ablating material is dissipated through vaporization of the material rather than conversion of the energy into heat. Thus, during exposure to the heat energy, the material of the ablative coating is eroded away through vaporization, dissipating the incident heat energy by converting the solid material into vaporous matter.  
           [0005]    Formation of ablative coatings of aerospace structures is conventionally accomplished through one or more processes known in the art. In one process the ablative coating is formed by molding the ablative material into a sheet and subsequently applying the sheet to a surface of the aerospace structure with an adhesive. Such a process is very labor intensive and time consuming. Molded sheets of ablative material, while exhibiting a certain amount of flexibility, are difficult to shape and conform to the complex shapes found in aerospace structures. Thus, a considerable amount of time is spent cutting, shaping and otherwise manipulating the sheet material in order to cover a desired surface. Additionally, much of the ablative material may be wasted in the trimming and shaping the sheet material.  
           [0006]    In some applications adhering the sheet material to the surface requires the structure to be placed in a vacuum bag or a similar process in order to accomplish proper adherence of the sheet material to the surface of the structure. This practice helps to minimize and hopefully eliminate voids between the sheet of ablative material and the surface of the aerospace structure. Additionally, once a surface is covered with the sheet material, substantial work is required to achieve a desired surface finish which may entail machining, grinding or sanding. Such post application surface work is also required with regard to joints formed at adjacent edges of abutting sheets of ablative material.  
           [0007]    Hand troweling is another conventional technique which has been utilized to apply ablative material to the surface of an aerospace structure. Hand troweling includes applying the ablative material to the surface of the aerospace structure by a hand trowel and allowing the ablative material to cure in place. This technique allows for the ablative material to be placed directly on the structure and also provides a means of coating complex shapes. However, hand troweling requires a high degree of skill, is very time consuming and labor intensive. Additionally, substantial rework is still required to produce the desired surface finish and shape. Also, it is difficult to monitor and control the thickness of the applied coating, particularly if the coating is placed over complex surfaces and the thickness is to be varied from one area of the aerospace structure to another.  
           [0008]    Yet another conventional technique of applying ablative coatings is to spray the coating directly onto the structure. Similar to the technique of hand troweling, spraying the ablative coating allows the material to cure in place. Also, spraying lends itself to coating aerospace structures having complex geometries. However, spraying typically requires the use of an ablative composition having a relatively low viscosity in order to pass the ablative material through the spraying equipment. Due to the nature of spraying, including the lower viscosity material, only a nominal thickness of the material may be applied to the aerospace component at one time. In order to achieve a sprayed on coating with any substantial thickness, multiple coatings must be applied. This becomes an extremely time consuming process, wherein a coating is applied and then cured to a specified level prior to application of a subsequent coating.  
           [0009]    Additionally, spraying does not provide adequate thickness control. Spraying is particularly deficient in applying ablative coatings which require a varied thickness over the surface of the aerospace component. Thickness control, in part, becomes a process of reworking the coating by hand after it is cured. Indeed, in some cases, surface work may be required between spray coatings adding to the time and labor required to achieve a satisfactory coating.  
           [0010]    The technique of spraying also results in incidental overspray. Overspray results in material waste and also requires the use and maintenance of special facilities, such as a spray booth, again increasing the cost of applying the ablative coating. Furthermore, ablative coatings often include a fibrous or particulate material in the ablative composition which poses additional problems for spraying. Fibers and particulates can clog the spraying equipment, requiring excessive cleaning and undue maintenance.  
           [0011]    As an example of the time and labor involved with spraying an ablative coating onto structures having complex shapes, FIGS.  1 A- 1 C show an aerospace component at various stages during spray application of an ablative coating. Referring to FIG. 1A, a ring strap  10  utilized in conjunction with an aeroskirt structure of a rocket is shown prior to application of an ablative coating. The ring strap  10  includes a number of nut plates  12  which protrude from an arcuate shaped plate  14 . Such a configuration makes it difficult to provide an ablative coating of consistent thickness. FIG. 1B shows the ring strap  10  with an ablative coating  16  applied by multiple sprayed coatings of ablative material. The ablative coating  16  is not conformal to the original geometry of the ring strap  10  and generally results in peaks  18  and valleys  20  formed in the surface of the ablative coating  16 . However, design requirements often specify a more conformal coating with a surface finish which is improved over that of a sprayed coating. Thus, as seen in FIG. 1C, substantial post-application rework is required to produce a substantially conformal coating  16 ′. Such rework may involve trimming, grinding, machining, sanding and the like. A substantial amount of time is involved in first, building up the ablative coating  16  (FIG. 1B), and second, re-working the ablative material to produce a conformal coating  16 ′ (FIG. 1C) with a satisfactory surface finish. Additionally, it is evident that a great deal of material is wasted in coating a part such as the snap ring  10  in such a manner.  
           [0012]    Furthermore, with each of the conventional techniques discussed above, variations exist in the resultant coatings from one component to another like component. For example, tight tolerances are difficult to maintain consistently regarding thickness, surface finish, density and other material characteristics. The ability to consistently apply coatings with repeatable and predictable results has been somewhat elusive with such techniques. Unacceptable variations are due in part to the manner in which the material is applied, and in part to the fact that operators of different skill levels may be applying the coating.  
           [0013]    In view of the shortcomings in the art, it would be advantageous to provide a method of applying ablative insulating coatings to structures in a reduced amount of time and with less wasted material. Additionally, it would be advantageous to provide a method which provides repeatable and predictable results.  
           [0014]    In providing repeatable results, the method would desirably minimize the opportunity for variance by reducing the number of variables affected by operators forming the ablative coating. Additionally, it would be desirable to provide a more standardized process such that variance would not be realized in the application of the material by one operator when compared to application by another operator.  
           [0015]    Such a method would also desirably result in improved surface finish and thickness control, thus minimizing the amount of touch up work required.  
         BRIEF SUMMARY OF THE INVENTION  
         [0016]    In accordance with one aspect of the invention a method of forming an ablative coating on a structure is provided. The method includes forming a mold with a cavity configured to cooperatively receive the structure or portion thereof which is to be coated. The structure or portion thereof is then placed in the mold cavity and a mixture of ablative material is introduced into the mold cavity, such as by pouring or by injection. The ablative material is then cured and substantially simultaneously adhered to a surface of the structure.  
           [0017]    Curing of the ablative material may be conducted under pressure, but more desirably occurs at atmospheric conditions to reduce the need for expensive equipment which might otherwise be required to carry out the process. The curing stage may also be effected in multiple stages, such as a first stage at a first temperature followed by a second curing stage at a second, elevated temperature.  
           [0018]    In accordance with another aspect of the present invention, another method is provided for forming an ablative coating on a structure. The method includes forming a mold with a cavity so as to cooperatively receive the structure or portion thereof which is to be coated. The structure or portion is then placed in the mold cavity. A mixture of ablative insulation material is prepared by combining and mixing a salt filled epoxy resin base with a fiber filled polyamide hardener and a silicone resin modifier. The ablative mixture is introduced into the mold cavity. The ablative insulation material is then cured and substantially simultaneously adhered to a surface of the structure.  
           [0019]    The ablative insulation material may be mixed under pressure so as to keep the material from becoming entrained with air. The pressure assists in collapsing any air bubbles formed during mixing.  
           [0020]    The material may also be heated prior to introduction into the mold to reduce viscosity and improve flowability of the material.  
           [0021]    Additional acts may also be incorporated into the method, as set forth herein, to enhance or alter the process in a desired manner.  
           [0022]    In accordance with another aspect of the invention an aeroskirt component is provided. The aeroskirt comprises a first structure, such as a snap ring, a door cover, a stiffening ring, or any other structure associated with an aeroskirt, and an ablative insulation coating which is molded directly to a surface of the structure.  
           [0023]    The aeroskirt component may further include at least one preformed spacer embedded in the ablative insulation coating. More desirably, the component may include at least two spacers embedded in the ablative coating with one spacer representing a first thickness and the second spacer representing a second thickness such that the ablative coating has a varied thickness over the surface of the component. The spacers may be advantageously formed of the same or similar material as the ablative coating. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0024]    The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:  
         [0025]    FIGS.  1 A- 1 C show an aerospace component with an ablative coating applied by a spraying process;  
         [0026]    [0026]FIG. 2 shows an aerospace structure requiring an ablative coating;  
         [0027]    [0027]FIG. 3A shows a mold for application of an ablative coating according to one aspect of the invention;  
         [0028]    [0028]FIG. 3B shows a component having an ablative coating applied with the mold of FIG. 2A;  
         [0029]    [0029]FIGS. 4A and 4B show a partial sectional view of another component having an ablative coating applied according to an aspect of the present invention;  
         [0030]    [0030]FIGS. 5A and 5B show plan and elevational views respectively of a component having an ablative coating applied in accordance with another aspect of the invention; and  
         [0031]    [0031]FIGS. 6A and 6B show various views of a mold according to another embodiment used in forming an ablative coating according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]    Referring now to FIG. 2, an aeroskirt  100  for assembly about a rocket casing is shown. As can be seen the aeroskirt  100  has numerous components and a complex geometry making application of an ablative coating difficult. For example, a snap ring  102  is located at the aft end  104  of the aeroskirt  100  which is to be individually coated. Similarly, an access door cover  106  is to be coated as an individual component. Additionally, the shell  108  of the aeroskirt  100  is to receive an ablative coating which varies in thickness across its surface. Each of these components poses different challenges in applying the ablative coating as shall be seen and discussed below.  
         [0033]    Various compositions are available for use as ablative coatings and the present invention may be practiced with such various available compositions. However, the present invention will be described in an exemplary manner with regard to the application of an ablative coating known as Sparesyl which is a proprietary coating prepared by Dexter-Hysol of Bay Point, Calif., for Boeing Corporation. Sparesyl is an epoxy ablative insulation system formed as a mixture of three different components. The first component is a salt filled epoxy resin which serves as a base. The second component is a fiber filled polyamide hardener. The third component is a silicone modifier. The mixture is combined in proportions of 100 parts (by weight) epoxy resin, to 86 parts polyamide hardener and 35 parts silicone modifier.  
         [0034]    Sparesyl may be mixed by either combining the proper proportions in a commercial mixer or by combining the components and mixing them by hand. Proper mixing is important since an improperly mixed composition may result in the coated parts being stripped and having to be re-coated.  
         [0035]    One particularly effective means of mixing the ablative composition is to use a commercial mixer such as a Hydra-Cat which is available from Graco, Inc., of Minneapolis, Minn. The Hydra-Cat acts as a proportioner assisting in the measurement and mixing of the three different components according to their proper ratios. Additionally, it mixes the composition under pressure and collapses air bubbles which may otherwise be formed during mixing. As shall be discussed in more detail below, it is important to produce a cured coating which is substantially free of air bubbles. Keeping the air from the final coating may be accomplished in different ways. However, by preparing the mixture in a machine such as the Hydra-Cat air bubbles are effectively removed prior to application of the composition to the aeroskirt  100  or other component. In addition to mixing the composition and removing air bubbles therefrom, the Hydra-Cat may be used to pump the material to its point of application.  
         [0036]    It is noted that other commercial mixers may be effective in mixing the ablative composition. However, use of other mixers may require the use of a separate pump and pressurizer in order to degas the mixture and to pump the mixture to its point of application.  
         [0037]    As stated above, an alternative method of preparing the Sparesyl composition is to mix it by hand. Hand mixing does provide the advantage of more precise measuring of the three individual components. However, hand mixing also tends to introduce more air into the resultant mixture. To reduce the amount of air incorporated into the mixture, the material may be allowed to stand for 40 to 60 minutes prior to application in order to allow the entrained air and volatile components to escape.  
         [0038]    Regardless of whether the composition is prepared through hand mixing or use of a commercial mixer, the epoxy resin is thoroughly mixed for a minimum of 20 to 30 minutes prior to combination with the hardener and modifier to ensure adequate suspension of solids which may have settled to the bottom of its container during storage. Similarly, the polyamide hardener should be thoroughly mixed for at least 20 to 30 minutes prior to adding it to the other two components to ensure proper suspension of solids.  
         [0039]    Referring to FIG. 3A the cross section of a mold  110  is shown with a snap ring  102  disposed therein for application of the ablative material to particular areas of the snap ring  102 . A base portion  112  of the mold  110  is shaped and configured to complementarily receive the snap ring  102  therein. A cover  114  is placed on top of the snap ring  102  to hold it into place during application of the ablative material. A space or cavity  116  is formed about the particular surface or surfaces of the snap ring which are to be coated with ablative material. This space or cavity  116  may be formed through design of the mold  110 , or may be formed by use of spacers  118  which serve to suspend the snap ring  102  above the base portion  112  of the mold  110  and act as a thickness indicator or gauge for the resulting ablative coating. The spacers  118  may be formed as previously cured ablative material, such as Sparesyl, and preferably the same type of ablative material which is to be introduced into the mold  110  to coat the snap ring  102 .  
         [0040]    The mixed ablative material may be introduced into the mold  110  through ports  120  and flowed into the space or cavity  116  formed between the base portion  112  of the mold  110  and the snap ring  102 . While the ports  120  are shown to be located on the sides of the mold  110 , it should be noted that other mold designs may also be used. For example, a port could be provided for each bolt cover  122  from the bottom of, or alternatively from the face of the base portion  112 . Alternatively, the mold  110  could be designed as an open mold having a partially open face which would allow the ablative material to be poured or injected into the mold.  
         [0041]    Prior to placing the snap ring  102  in the mold  110 , the mold is conditioned and prepared. One method of preparing the mold includes cleaning the interior surface of the mold  110  with isopropyl alcohol and then allowing it to dry for a period of time. After the mold  110  is dry, the interior surface is coated with a release agent such as, for example, Dexter Frekote® Release  700 , commercially available from Dexter Adhesive &amp; Coating Systems of Seabrook, N.H. After the release coat is applied to the interior surface of the mold  110  it is baked to drive the release coat into the pores of the mold  110  (this process is often referred to as seasoning the mold). As an example, after application of release coat, the mold  110  may be baked at a temperature of approximately 200° F. for approximately 6 hours to season the mold  110 . After the mold  110  has been seasoned a user coat of release agent may be applied and allowed to air dry for an adequate time, such as 30 minutes.  
         [0042]    After preparation of the mold  110 , the snap ring  102  is placed into the mold  110  and an adequate amount of ablative material is introduced into the mold  110 . The ablative material is then cured in place to form an ablative insulation coating  124  adhered to the snap ring. The ablative material may be cured under pressure, but is desirably cured at atmospheric pressure to reduce the cost of molds and molding equipment used in applying the ablative insulation coating  124 . One method of curing the ablative insulation coating  124  is to cure it at atmospheric pressure at a temperature of approximately 70° F. to 95° F. for approximately 8 hours. This initial curing period is then followed by a second curing stage at an elevated temperature, such as 110° F. for approximately another 8 hours.  
         [0043]    It is noted that if Sparesyl is being used as the ablative material, the material may be heated to slightly reduce the viscosity of the material and increase flowability during its introduction into the mold  110 . The material may also be heated during various stages of curing. However, it is desirable that the Sparesyl material not be subjected to a temperature exceeding about 120° F. prior to substantial cure of the material. Excessive heat may keep the material from curing properly and render the resultant coating ineffective as an ablative insulation.  
         [0044]    After curing, the mold  110  may be stripped from the snap ring  102  and the ablative coating  124  may be trimmed or buffed if needed to render a specified surface finish. Additionally, if so needed, the ablative insulation coating  124  may be “touched up” by applying a mixture of the ablative material to areas which may exhibit vugs such as blemishes or pock marks in locations where the ablative coating  124  failed to form properly in the mold  110 , or perhaps where the material stuck to the surface of the mold  110  during stripping of the snap ring  102 . However, it is noted that such touch up work is minimal relative to that which is required with other application processes. It is also noted that the surface finish produced by the presently described process of the invention is typically satisfactory without additional sanding or buffing being required.  
         [0045]    The method of applying the ablative insulation coating  124  as disclosed above allows for greater repeatability in producing multiple snap rings  112  (or other components as discussed below) with reduced variability in the resultant surface finish and thickness. Additionally, such a method reduces the subjective input from an operator since the operator doesn&#39;t have to determine whether the proper amount of material has been applied or if the proper thickness has been achieved. These variables are consistently controlled by the use of mold to apply the ablative insulation coating  124 .  
         [0046]    Referring now to FIGS. 4A and 4B, a partial sectional view of the shell  108  of the aeroskirt  100  is shown with an ablative insulation coating  124 ′ applied thereto according to the present invention. It is noted that the ablative insulation coating  124 ′ on the shell  108  is formed in a similar manner as described above with respect to snap ring  102 . However, the application of the ablative insulation coating  124 ′ to the shell  108  presents some additional issues which are also addressed by the present inventive method.  
         [0047]    The ablative insulation coating  124 ′ formed on the shell  108  varies in thickness from one section to another. For example, the exterior surface  126  of the ablative insulation coating  124 ′ is of a first thickness. The bottom surface  128  below the stiffening ring  130  is of a second thickness. Additionally, a first transition  132  of varying thickness is made from the exterior surface  126  to the bottom surface  128  of the ablative insulation coating  124 ′. Another transition  134  has varied thickness from the bottom surface  128  to the interior surface  136  of the ablative insulation coating  124 ′ which may exhibit a thickness different from those associated with the exterior surface  126  and the bottom surface  128 . The variation in thickness of the ablative insulation coating  124 ′ is easily controlled in the present application process by properly designing the associated mold (not shown) and by providing spacers  118 ′,  118 ″ and  118 ′″ of varied thickness where required. As noted above, such spacers  118 ′,  118 ″ and  118 ′″ are desirably formed of the same material as the ablative insulation coating  124 ′ and thus do not effect the performance of the resulting structure.  
         [0048]    As can be best seen in FIG. 4A, spacer  118 ′ may exhibit a different thickness than either spacer  118 ″ or  118 ′″. Utilizing spacers of differing thickness allows for increased versatility in molding the ablative insulation coating  124 ′ where complex shapes and varied thicknesses are required on a single component. Additionally, utilizing spacers  118 ′,  118 ″ and  118 ′″ which are formed of ablative insulation material allows for greater material compatibility and homogeneity. Such compatibility is particularly desirable with respect to material characteristics such as, for example, the coefficient of thermal expansion. By keeping such material characteristics substantially homogeneous throughout the ablative insulation coating  124 ′, the coating will perform its desired function with greater consistency and predictability.  
         [0049]    Referring to FIGS. 5A and 5B a door cover  106  which is used to cover an access port in the aeroskirt  100  is shown. The door cover  106  has been formed in general accordance with techniques described above. However, the door cover  106  has an additional feature incorporated into the applied ablative insulation coating  124 ′″. The door cover is removable from the shell  108  of the aeroskirt  100  to allow access to the rocket to which the aeroskirt  100  is assembled. In order to accommodate the placement of fasteners used to attach the door cover  106  to the aeroskirt  100 , stay-out zones  138  (FIG. 5B) are formed in the ablative insulation coating  124 ′″ during the molding process. The stay out zones devoid of ablative material may be formed by placing inserts  140  (FIG. 5A) at proper locations in the mold prior to introducing the ablative material into the mold. The inserts may be formed as buttons or bosses from a material such as Teflon® which may be easily removed from the cured ablative insulation coating  124 ′″ after stripping the mold from the door cover  106 .  
         [0050]    As can be seen in FIG. 5B the stay out zones  138  may be sized and configured to keep the ablative material out of specific areas such as fastener holes  142 . The stay out zones  138  may also be oversized to accommodate and provide clearance for the use of necessary tools, such as a wrench or socket, which may be required for inserting and attaching fasteners therethrough.  
         [0051]    Referring to FIGS. 6A and 6B, a mold  150  is shown which includes another technique of the present invention useful in forming the ablative insulation coating  124 ′″ on the door cover  106 . Particularly, FIG. 6A depicts one half  150 A of the mold showing various features formed therein, while FIG. 6B shows a cross section of the mold  150  including the door cover  106  of FIG. 5 being formed therein.  
         [0052]    The mold includes a first half  150 A and a second half  150 B with a cavity  152  formed in the first half  150 A for receipt of the door cover  106 . The two halves  152 A and  152 B may be held together by placing them in a fixture (not shown), by providing fasteners (not shown) through the two halves, or by other means known by those of skill in the art. An injection port  154  allows for introduction of ablative material into the cavity  152  from below. A relief portion  156  is also formed at the upper end of the mold  150  in the first half  150 A above the cavity  152 .  
         [0053]    It is noted that the injection port  154  may be placed at a different location within the mold  150  if so desired, or alternatively, an injection port need not be provided as the ablative material may be introduced from above through the opening above the relief portion  156 .  
         [0054]    During formation of the ablative insulation coating  124 ′″ excess material is filled into the relief area  156  and allowed to cure along with the ablative insulation coating  124 ′″. During curing of the ablative insulation coating  124 ′″ any air entrained in the uncured ablative material will rise into the relief portion  156  in the form of air bubbles. The air bubbles thus migrate away from the ablative insulation coating  124 ′″ into the material located in the relief portion  156 . The relief portion may then be trimmed off the door cover  106  in a post-cure operation. This technique is particularly useful when the material introduced into the mold  150  has been mixed by hand or by some method which introduced air into the ablative material. Of course, a similar relief portion  156  may also be incorporated when applying ablative material to other aeroskirt components. However, providing a relief in a mold is not necessary when the ablative material is mixed in a manner which degasses or removes the air from the mixture as described above herein.  
         [0055]    While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.