Abstract:
A method of forming a thermal protection system for high speed aircraft is described. The method includes mechanically working an uncured ablator material into a first surface of a felt layer such that the ablator material penetrates a distance into a thickness of the felt layer thereby forming an region that has a mixture of felt and ablator material, adding additional uncured ablator material to the worked ablator material, and curing the combined ablator material.

Description:
BACKGROUND 
     The field of the invention relates generally to ablative materials, and more specifically to a hybrid ablative thermal protection system. 
     Ablative materials have been used in a variety of applications to protect and insulate structures that are subjected to extreme thermal conditions. For example, many aerospace vehicles that traverse, exit, and enter the atmosphere of the Earth travel at high velocities, and as a result, their exterior aerosurfaces, and to some degree their substructure, experience high aerothermal loads. Aerothermal loads have been managed using a variety of techniques including insulation, radiant cooling, active cooling, conduction and convective cooling, and by phase change or ablative materials. Generally, ablative materials are applied to the affected aerosurfaces to absorb the extreme heat in order to insulate the vehicle from the thermal environment. 
     The thermal management technique of ablation has been widely used for a variety of applications since the early 1930s. Ablative materials were used in early rocket systems for nose cap protection and have also been used as re-entry heat shields on the Gemini and Apollo space vehicles, and further on many modern rocket nozzles. Many of these materials, although suitable for use in the aforementioned applications, have handling and longevity issues that preclude application on a system that is subjected to frequent handling and that may be stored for several years prior to use. 
     Known ablative materials comprise a variety of constituent components, each at certain percentages by weight or volume, to achieve the desired level of thermal protection and other physical properties. Generally, ablator compositions are a composite material comprising a resin matrix with a variety of filler materials to reduce the overall density or provide other physical properties. 
     Certain ablative compositions have included a variety of other constituent elements such as metal fillers, colloidal clay fillers, boron and oxygen compounds, polyurethane resins, a mixture of both epoxy and polysulfide resins, and many others too numerous to detail herein. The known art compositions, however, include numerous fillers to achieve a desired set of properties such as thermal, mechanical, and others. As a result, such compositions may be costly and difficult to fabricate with a relatively large number and variety of fillers. In addition, many known art ablator compositions demonstrate relatively low thermal and abrasion resistance performance under high heat flux and pressure loads, for example, as observed in Mach 6 to 8 vehicles. 
     Ablators with high shear, high heat flux capabilities generally have high densities. Lightweight thermal protection systems cannot withstand high shear, high heat flux environments. Accordingly, there remains a need in the art for an ablator composition that is reduced in weight from known ablative solutions that also improves thermal performance. Such an ablator would be low in density yet high in abrasion resistance and durability before, during, and after high thermal loads. The preferred ablator would also be relatively low cost and simple to fabricate. 
     BRIEF DESCRIPTION 
     In one aspect, a method of forming a thermal protection system for high speed aircraft is provided. The method includes mechanically working an uncured ablator material into a first surface of a felt layer such that the ablator material penetrates a distance into a thickness of the felt layer thereby forming an region that has a mixture of felt and ablator material, adding additional uncured ablator material to the worked ablator material, and curing the combined ablator material. 
     In another aspect, a thermal protection system for high speed aircraft is provided. The thermal protection system includes a felt layer comprising a first side, a second side, and a thickness separating the sides, the first side operable for attachment to a structure of the aircraft using an adhesive, and a layer of ablator material, a portion of the ablator material mechanically worked into the thickness of the felt layer from the second side prior to the curing of the ablator material. 
     The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an aerospace vehicle capable of traveling at high speed (greater than Mach 1) through the atmosphere of the Earth. 
         FIG. 2  is a cross-sectional view of an ablator composition formed on a structure of the aerospace vehicle of  FIG. 1 . 
         FIG. 3  is an illustration of the components of the ablator composition of  FIG. 2 . 
         FIG. 4  is a flowchart describing a process for assembling a thermal protection system that contains the ablator composition shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     The disclosed embodiments are directed to application of an ablator material, for example, Boeing lightweight ablator (BLA), to a needled felt of Nomex fibers, creating a dual component ablative thermal protection system. Nomex is a registered trademark of E. I. du Pont de Nemours and Company. The described configuration reduces structure temperature and thermal protection system weight compared to BLA only applications. 
     Referring to the drawings, the ablator composition of the present invention is applied to the exterior surfaces of an aerospace vehicle as illustrated and generally indicated by reference numeral  10  in  FIG. 1 . The aerospace vehicle  10  is shown flying through the atmosphere of the Earth, where high acceleration and velocities create extremely elevated thermal loads across the exterior surface, or aerosurface, of the vehicle  10 . Accordingly, the ablator composition  20 , shown in  FIG. 2 , provides thermal protection for the vehicle  10  during these extreme thermal conditions. 
     Referring specifically to  FIG. 2 , the ablator composition  20  is shown applied to a portion of the outer mold line (OML)  24  of the aerospace vehicle  10 . In addition to protecting the OML  24 , the ablator composition  20  further provides protection to substructure  30  adjacent the OML  24 . Accordingly, additional structure and/or systems within close proximity of the OML  24  are protected from the extreme thermal environment by the ablator composition  20 . 
     The application of the ablator composition  20  to an aerospace vehicle  10  as described herein should not be construed as limiting, rather this application is merely illustrative of one structure and one operating environment in which the described embodiments have utility. The ablator composition  20  described herein can further be employed with a wide variety of structures and systems that must withstand high thermal loads for an extended duration. 
     Now referring to  FIG. 3 , specific layers of the ablator composition  20  are shown and referenced to a structure  50 , for example, the outer moldline  24  of vehicle  10 . The layers include an adhesive layer  60  that is applied directly to the structure  50 , a felt layer  62  that is applied to the adhesive  50 , and a Boeing lightweight ablator (BLA) layer  64 . In one embodiment, the felt layer  62  is felt reusable surface insulation (FRSI) that is heat treated. In one specific embodiment, the felt layer  62  of FRSI is about 0.160 inches in thickness. 
     In one embodiment, a process of fabricating the ablator composition  20  has been modified to improve the adherence between the felt layer  62  and the BLA layer  64 , specifically to get an additional amount of the BLA layer  64  absorbed into the felt layer  62 , thereby increasing the mechanical lock between the two layers. 
       FIG. 4  is a flowchart  100  illustrating the process of fabricating the ablator composition. Specifically, the adhesive layer  60  is applied  102  to the structure  50 , for example, an aluminum panel. The felt layer  62  is then laid  104  onto the adhesive layer  60 . In one embodiment, pressure is applied to the felt layer  62  to improve adhesion between the adhesive layer  60  and the felt layer  62 . The adhesive layer  60 -felt layer  62  interface is cured  106  at room temperature. After curing, a first layer of uncured BLA  64  is applied  108  to the felt layer  62 . The uncured BLA  64  is then worked  110  into the felt layer  62  as further described below. After the working  110  step, a second layer of uncured BLA is applied  112  to the felt layer  62 -BLA  64  combination. As only a portion of the first BLA layer is worked in the felt layer  62 , and since the second layer of BLA  64  is applied  112  before the first layer of BLA  64  is cured, these are shown as a single layer of BLA in  FIG. 3 . After application  112  of the second layer of BLA  64 , the composition  20  is cured as further detailed in the following paragraphs. Application of the BLA, in one embodiment, is accomplished using a spraying process. 
     Working  110  the BLA-felt interface, in one embodiment, includes moving the nomex fibers of the felt layer  62  in several directions and causing rotation of these felt fibers so the BLA is, or can be, worked into the felt. In one specific application, the felt layer  62  is needled, for example using a spreading tool, so that the nomex fibers present a more porous surface to the BLA. During the application of the BLA to the felt layer  62 , a needle or other needle like device may be utilized as a depth gauge to check thickness. Specifically, by stabbing the needle through the uncured BLA and felt layer  62  until it bottoms out on the substrate, a thickness of the combination may be determined. In an alternative embodiment, the felt layer  62  is needled prior to application of the BLA to the felt layer  62 . In this alternative embodiment, the manufacturer of the felt layer  62  moves the nomex fibers of the felt layer  62  in several directions as described above. Subsequently, during application of the BLA, the needle is utilized as a depth gauge to check thickness as described above. 
     In one specific application, the BLA is worked into the felt layer  62  to a depth of about 0.03 to about 0.04 inches. In one embodiment, the BLA is thinned, for example using a solvent, prior to the mechanical working of the ablator material (BLA). 
     The curing of the BLA layer  64  includes curing at an ambient temperature for up to about 24 hours followed by an oven curing step. In one particular application, the oven curing process includes raising the temperature, from ambient to about 180 degrees Fahrenheit, at a rate of about 10 degrees per hour. The ablator composition is maintained at the increased curing temperature for up to 48 hours. The increased curing temperature causes a high temperature bond line to be formed between the felt and the BLA. This bond line is sometimes referred to as a mechanical lock between the BLA and the felt fibers. At least one result of the above described processes is that after the BLA has hardened, gets charred through use (producing silica), there is still a mechanical lock between the felt and the BLA. 
     In the above described process, the felt of the felt layer  62  traps solvent of the BLA. The trapping of solvent by the felt layer  62  is an undesired by product of the described processes. This trapped solvent tends to cause problems in curing of the felt layer/BLA interface predominantly by not allowing the BLA to fully cure. The development of the above described curing method has been added to the fabrication of the ablative composition to produce a successful bond between the BLA and the felt layer  62 . If the BLA is not fully cured, the thermal protection system will have underperforming mechanical properties. Therefore, the solvent has to be released from the thermal protection system. However, if the solvent is driven off too quickly it can cause a separation in the BLA/felt layer bondline due to the volatiles expanding faster than can be permeated through the BLA, which results in blisters within the uncured BLA. The specific slow cure process described in the preceding paragraph, with the slowly escalating temperature allows the solvent to be slowly flashed out of the BLA, without causing delaminations 
     The thickness of the felt layer  62  and the thickness of the BLA  64  are selected based on the particular application. In one embodiment, the BLA is a silicone ablator, filled with a commercial filler, and is described in U.S. Pat. No. 6,627,697, the disclosure of which is incorporated by reference. The described structure of ablator material worked into felt and subsequent curing provides an ablator that exhibits high shear and high heat flux capabilities, and the use of the felt layer helps reduce weight of the resulting dual component ablative thermal protection system as compared to BLA only ablator systems. 
     Referring back to  FIG. 3 , the combination of the adhesive layer  60 , the felt layer  62 , and the BLA layer  64  are generally formed on a structure  50 , as described herein. In practice, such a structure is generally a panel of an air vehicle. Upon curing of the ablator material, the panel may be attached to a vehicle frame. Before or after attachment, the cured ablative material may be milled to a desired thickness, for example, as required by the application. 
     The described embodiments illustrate the combination of the high shear and high heat flux capabilities of BLA with the low aerial weight and low thermal conductivity characteristics of a needled felt layer, for example, of nomex fibers. The manufacturing method creates a mechanical bond between the BLA and the felt that can withstand a higher use temperature than secondarily bonded BLA. As such, the embodiments combine a low thermal conductivity and low weight thermal protection system component with a high heat flux and high shear capable ablator. The improvement over the prior art occurs because the addition of the needled felt lowers the weight of the thermal protection system while maintaining the high heat flux and high shear capabilities. 
     Summarizing the described embodiments, the needled felt layer, for example, felt reusable surface insulation (FRSI) is bonded to the carrier structure with appropriate adhesive. In one specific application, RTV 560 is the preferred adhesive. BLA, thinned with OS-10 or other thinning fluid to aid in application, is manually pressed and worked into the felt surface fibers until a continuous layer of BLA is present in the felt. Before the BLA in the felt fibers is allowed to flash or cure, a second layer of BLA is then spray applied to the felt to achieve the desired overall thermal protection system thickness. The material is cured via room temperature and/or a heat cycle. The BLA surface is then machined to a desired outer mold line profile. 
     The addition of the needled felt of nomex fibers lowers an overall thermal conductivity of the described thermal protection system, which reduces backface temperature capabilities of the thermal protection system. Further, the felt is a lower weight material than BLA and its addition lowers the weight of the system when compared to BLA only thermal protection systems. The application technique, consisting of pressing the BLA into the felt surface to create a layer of BLA imbedded in the felt, provides a mechanical bond between the BLA and felt that when cured that can withstand a higher temperature than an adhesive bond between BLA and a structure. Simply, the BLA may get more brittle over time and the felt provides a type of strain relief function for the BLA. More specifically, the BLA will get more brittle during the dynamic heating conditions associated with flight. For example, the BLA becomes less elastic and more ceramic like when temperatures start to exceed 800 degrees Fahrenheit. The BLA characteristics do not change over time at room temperatures or at temperatures less than 500 degrees Fahrenheit. 
     The described embodiments provide a high heat flux, low density ablator capable of operating in a high shear environment. The use of a dual component thermal protection system uses the optimal performance characteristics of two different materials (felt and an ablator material such as BLA) to provide a dual component material with the best possible performance. The combination saves weight and decreases outer mold line thickness while maintaining a lower structure temperature, allowing hypersonic vehicles to fly faster and further since weight requirements are easier to meet and overall drag requirements are reduced. 
     The above described embodiments are especially suited for use upon a surface of a high speed air vehicle that is commonly referred to as an acreage, which is roughly defined as a top of such a vehicle. However, applications on other vehicles and vehicle locations are contemplated. 
     This written description uses examples to disclose various embodiments, which include the best mode, to enable any person skilled in the art to practice those embodiments, including making and using any devices or systems and performing any incorporated methods. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.