Abstract:
During the application of heat the intumescent coating is transformed into a swollen char material, which acts as a thermal barrier to eliminate or minimize incoming heat flux. It also acts as a mass transfer barrier, inhibiting oxygen from reaching the thermally insulative ablative material. During the intumescence process, the swollen material will also back fill into interstices within the ablative material and char to enhance their strength. The intumescent coating also acts as a moisture barrier to protect the thermally insulative ablative material from ambient elements such as moisture.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to ablator compositions and more particularly to an ablator composition, which utilizes an intumescent coating. 
     2. Description of the Related Art 
     Launch vehicle configurations often employ solid rocket boosters (SRB&#39;s) to augment the thrust of the main engine. Because of the proximity of the SRB plumes to the base region of the main engine, the convective and radiative heating augmentations to the base region of the main engine due to these SRBs are substantial. A layer of low temperature ablative (LTA) is needed to protect the structure in this region. The thermal, mechanical and chemical performances of the ablatives in the hostile environment produced by the rocket exhaust are of importance in the design of the thermal protection system. The requirements imposed on the insulation are as follows: 
     1. Ability to withstand the aerodynamic loads and aeroheating encountered during flight; 
     2. Protection and maintenance of the substructure below a critical temperature; 
     3. Light weight, low cost and ease to manufacture and to install; 
     4. Ability to withstand thermal shock due to launch plume heating; 
     5. Ability to withstand the mechanical and acoustic vibration environment; and 
     6. Chemical and mechanical compatibility with adhesive and substructure throughout the entire flight. 
     When an external heat flux is applied to the base of the main engine, the LTA material protecting this region may decompose in-depth and recede at its surface. The modes of surface recession may include combinations of phase change processes such as melting, sublimation, as well as, exothermic or endothermic chemical reactions such as oxidation and combustion. Similarly, in-depth decomposition, such as pyrolysis, may involve outgassing, phase change and chemical reactions. The ablation performance of these LTA are often characterized by q* or 
     37 heat of ablation” defined as 
     
       
           q *=qdot/mdot, 
       
     
     where qdot is the net heat flux=q hw −q rad ; 
     mdot is the rate of mass loss; 
     q hw =convective hot wall flux; and 
     q rad =net radiative heat flux. 
     The primary mechanisms for the LTA to counter the applied heat flux are high heat of ablation and low thermal conduction. Thus, the ideal properties of LTA include low density and thermal conductivity, ease of manufacturing and installation, and the ability to withstand flight conditions. 
     Another desirable property of an ideal LTA is to form strong char during the ablation process. If the strength of the char adhering to the surface is sufficient to keep it from being swept away by aerodynamic shear forces and acoustic vibrations, the performance of the insulation can be improved because of: 
     a) Increased thermal protection since less material is removed; 
     b) Increased thermal protection since the char in general is porous, lightweight and has low thermal conductivity; and 
     c) Increased radiant heat loss from surface since the char in general has higher emissivity and can withstand high temperature. The higher surface temperature also reduces convective heat gain. 
     Cork, with over 200 million cells per cubic inch, is often chosen as the LTA thermal protection system (TPS) because of the structure and mechanics of these cells. It is used as insulation material for launch vehicles because of its low density yet resilient mechanical properties; minimal cost; its ability to absorb vibration and withstand acoustic noise; and, its chemically stability. This natural product is cleaned, ground, mixed with various resins such as phenolic and formed into complex shapes. Common cork based TPS materials include cork epoxy, cork phenolic and cork silicone. 
     The combustion of cork and phenolic resin to form weakened char is the single most important failure mode of the cork phenolic heatshield materials. When the material is exposed to high heat flux and oxygen from ambient atmosphere, the cork-based ablatives quickly char and begin burning. Once ignited, the ablatives will continue to burn even after the external heat source is turned off. As the cork phenolic TPS ablates, the surface of the TPS will form char with cracks, the size of which increases with time. Eventually the remaining material will break and erode away due to the mechanical load or aerodynamic shear. 
     A typical launch vehicle may sit on the launch pad for days prior to flight, and often the TPS can absorb a significant amount of moisture if left unprotected. Existing families of launch vehicles often employ a coating of paint to seal the TPS. The launch vehicles may also have an additional layer of electrically conductive paint to ground electrical charges in the atmosphere. 
     The layer of LTA needed to protect the structure from excessive convective and radiative heating can add substantial weight, cost, technical risk and performance penalties to the launch vehicle&#39;s manufacturer and integration team. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is therefore a principal object of the present invention to provide an improved ablative composition, which can be applied to a substrate to protect the substrate from external heat flux. 
     It is another object to coat an ablative material such that during exposure to heat, the coating will swell to provide a thermal barrier, inhibit ambient air from contacting the ablative material, and provide a back fill into interstices within the ablative material and char to enhance their strength. 
     Another object is to provide an ablative composition with a moisture barrier. 
     Still another object is to provide an ablative composition with a layer of electrically conductive coating to ground electrical charges in the atmosphere. 
     Yet another object is to provide an ablative composition with a coating to reflect incoming radiant heat flux. 
     These and other objects are achieved by the present invention, which in its broadest aspects comprises a thermally insulative ablative material and an intumescent coating covering the thermally insulative ablative material. During the application of heat the intumescent coating is transformed into a swollen char material, which acts as a thermal barrier to eliminate or minimize incoming heat flux. It also acts as a mass transfer barrier, inhibiting oxygen from reaching the thermally insulative ablative material. During the intumescence process, the swollen material will also back fill into interstices within the ablative material and char to enhance their strength. The intumescent coating also acts as a moisture barrier to protect the thermally insulative ablative material from ambient elements such as moisture. The intumescent coating also acts as the electrically conductive paint to ground electrical charges in the atmosphere. The intumescent coating preferably contains particulate to reflect incoming radiant heat flux. 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a first embodiment of the ablator composition of the present invention. 
     FIG. 2 is a cross-sectional view of a second embodiment of the present invention having an electrically conductive coating and substrate secured thereto. 
     FIG. 3 is a schematic illustration showing application of the ablator composition to a base of a launch vehicle. 
     FIG. 4 is a graph of heat flux vs. time, illustrating the heat flux history of the base of a launch vehicle. 
     FIG. 5 is a graph of heat flux vs. time, illustrating the heat flux history of a cruise missile. 
     The same elements or parts throughout the figures are designated by the same reference characters. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings and the characters of reference marked thereon, FIG. 1 illustrates a first embodiment of the ablator composition of the present invention, designated generally as  10 . Ablator composition  10  includes a thermally insulative ablative material  12  and an intumescent coating  14  covering the thermally insulative ablative material  12 . 
     The thermally insulative ablative material  12  may be a cork phenolic material, which is particularly advantageous for launch vehicle applications. Other possible materials may include, but are not limited to, cork epoxies, cork silicones, silicones, carbon-carbons, carbon-phenolics, graphites, silicon phenolics and ceramics. 
     The thermally insulative ablative material  12  has a thermal conductivity in a range of 0.1 to 10 BTU-in/hr/ft 2 /ft, preferably in a range of 0.1 to 0.5 BTU-in/hr/ft 2 /ft. 
     The intumescent coating  14  will transform to a swollen char material upon heating. It has intumescent swelling in a range of 50% to 2000% of the original thickness of the coating. Preferably, the intumescent swelling is in a range of 200% to 1000%. 
     The intumescent coating  14  may comprise a number of commercially available paints. These paints often have ammonium polyphosphate as a swelling agent, which provides the necessary intumescence. Such paints are marketed by a number of commercial vendors, that include, for example Albi Manufacturing, American Vamag Company, Inc., Barnard Products, Inc., Carboline Company, Fiber Materials, Inc., Fire Research Laboratories, Flame Control Coatings, Inc., Flame Stop, Inc., Flamort Chemical Company, Gilman Paint, M.A. Bruder &amp; Sons, Materials Sciences and Technologies, Inc., NoFire, Inc., PPG Industries, Inc., Preservative Paint Company, Technical Coatings Inc., Textron Specialty Materials, Thermal Science, Inc., and Vimasco Corporation. 
     The intumescent coating  14  may be applied by spraying, rolling, trowelling, brushing or other conventional coating application methods. 
     The intumescent coating  14  has a thickness of 1 mil to 100 mils, preferably 5 mils to 20 mils. The paint should be of sufficient thickness to be an effective moisture barrier for a particular application. In an aerospace application, a launch vehicle may sit on the launch pad for several days. The thermally insulative ablative material  12  must therefore be coated with a moisture barrier. The ability of the intumescent coating  14  to inhibit moisture penetration in such an application can be enhanced by adding acrylic, latex, or epoxy to the intumescent coating  14 . 
     The temperature at which intumescence begins is known as the intumescence on-start temperature. The intumescent coating  14  has an intumescence on-start temperature in a range of 300° F. to 1600° F., preferably in a range of between 500° F. to 1000° F. 
     Since the external heat source often has a high percentage of radiative heat flux, the intumescent coating  14  should be capable of reflecting light in the range of 0.4 to 10 microns, preferably 1 to 4 microns. This can be achieved by adding metallic particulate of proper diameter. 
     A launch vehicle often has an electrically conductive coating to ground any electrical charge in the atmosphere. The quantity of metallic particulate in the intumescent coating  14  should be sufficient to provide the needed electrical conductance. Suitable additives include, for example: metallic particulate such as silver, copper, tungsten, and other heavy refractory metals, non-metallic particulate such as graphite, and microspheres coated with combination of such materials. These microspheres are often hollow to reduce weight, and are commonly referred to as micro-balloons. 
     During intumescence, the intumescent coating  14  will swell and act as a thermal barrier to eliminate or minimize the external heat flux. This coating  14  also acts as a mass transfer barrier to inhibit oxygen from reaching the surface of the thermally insulative ablative material  12 . The swollen intumescent coating  14  also back fills surface imperfections of the thermally insulative ablative material  12  and its char to strengthen such materials. 
     Referring now to FIG. 2, a second embodiment of the present invention is illustrated, designated generally as  16 , in which a substrate  18  supports the ablator composition  10 . The substrate  18  may be a structural material; for example, aluminum, titanium, or composite. The thermally insulative ablative material  12  may be secured to the substrate  18  with an adhesive  20  or fasteners (not shown). The thickness of the thermally insulative ablative material  12  should be sufficient to keep the substrate  18  or adhesive  20  below a critical temperature. 
     FIG. 2 also illustrates how an electrically conductive coating  22  may be applied over the intumescent coating  14  to provide the electrical grounding, as discussed above. 
     Referring now to FIG. 3, application of the ablator composition  10  to the base structure of a base  24  of a launch vehicle  26 , which may have one or more main engines  28 , is illustrated. (While FIG. 3 does not show any solid rocket boosters, it is understood that the present inventive concepts are applicable to situations where the launch vehicle may have none, or one or more strap-on solid rocket boosters.) Each of the solid rocket boosters may have its own motor. The main engines and the solid rocket boosters, when ignited, emit plumes of gas that may impact the launch pad  30  and recirculate. These plumes may heat the base  24 , convectively, or radiatively. As a launch vehicle ascends, the recirculating effect diminishes and the heating level drops. At higher altitudes these plumes expand and may interact with one another. This increases the heating of the base  24 . 
     Referring now to FIG. 4, a heat flux history of the base of a launch vehicle is shown. The heat flux history can be divided into a launch pad clearing phase  32  where the heat flux is high, followed by a low altitude ascent phase  34  where the heat flux is low. This is followed by a high altitude ascent  36  where the heat flux is high. With existing ablative designs, the high heat flux during the launch pad clearing phase may ignite the thermally insulative ablative material  12 , which continues to burn during the low altitude ascent phase  34  even though the heating level drops. The present invention utilizes the intumescent coating  14 , which is sized so that the temperature of the thermally insulative ablative material  12  remains below the ablation temperature of the thermally insulative ablative material  12  during the launch pad clearing phase  32 . Also, the intumescent coating  14 , acts as a mass transfer barrier, inhibiting oxygen from reaching the thermally insulative ablative material  12 . As noted above, the intumescent coating  14  also provides back filling. As a result, a thinner layer of thermally insulative ablative material  12  is required. It is understood that, while the present invention has been described with respect to its application to the base of a launch vehicle, its application is not limited to such a region. The present invention is particularly usefull in any region of the vehicle that senses a heating pattern similar to FIG.  4 . 
     Referring now to FIG. 5, a heat flux history of the outer surface of a cruise missile, is shown. In this example, the heat flux history can be characterized as an ascent phase  38  followed by a cruise phase  40 , followed by a reentry phase  42 . The intumescent coating  14  is sized so that the temperature of the thermally insulative ablative material  12  remains below the ablation temperature of the thermally insulative ablative material  12  during the ascent phase  38 . 
     Although the application of the ablator composition  10  of the present invention has been described with particularity with respect to its use on a launch vehicle it is understood that it may be used for other expendable aerospace applications. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.