The present invention relates generally to low temperature ablative compositions and more particularly to compositions comprising quantities of intumescent materials in addition to methods for mixing the compositions and forming ablative structures.
Ablative materials have been used in a number of applications to protect and insulate objects that are subjected to extreme thermal conditions. More specifically, extreme thermal conditions in aerospace vehicles have been managed using a variety of techniques including insulation and radiant cooling, active cooling, conduction and convective cooling, and by phase change or ablative materials. Generally, ablative materials are applied to the affected aerosurfaces and/or substructure to absorb the radiant and convective heat and to insulate the vehicle from the extreme thermal environment.
Aerospace launch vehicles having solid rocket boosters generate high convective and radiant heat near the base region of main engines. To prevent damage from the high heat, structure near the engines is typically protected with a layer of low temperature ablative (LTA) material. The LTA material generally insulates the structure by absorbing the heat through an ablation process, wherein the LTA material forms a char and thereafter burns for a period of time. During exposure to extreme heating and subsequent ablation, the LTA material may decompose and recede across its surface. The recession is generally due to phase change processes such as melting, sublimation, or chemical reactions including oxidation and combustion. Similarly, the decomposition is due to processes such as pyrolysis, phase changes, or chemical reactions.
The performance of LTA materials is often characterized by xe2x80x9cq*xe2x80x9d or xe2x80x9cheat of ablation,xe2x80x9d which is defined as:
q*=qdot/mdot;
where:
qdot=qhwxe2x88x92qrad; (net heat flux)
qhw=convective hot wall flux;
qrad=net radiative heat flux; and
mdot=rate of mass loss.
In order to adequately protect structure and systems from extreme thermal conditions, LTA materials must have a high heat of ablation in addition to low thermal conduction. Furthermore, LTA materials in aerospace applications typically have a low density in order to minimize weight, and are further able to withstand a variety of flight loads, such as aerodynamic shear forces, in addition to extreme heating.
When LTA materials are exposed to high heat flux and oxygen from the atmosphere, the LTA materials quickly char and begin burning. Once ignited, the LTA materials may continue to burn even after the heat source subsides. Accordingly, effective LTA materials typically form a strong char during the ablation process, which is sufficient to prevent separation of at least a portion of the LTA material from the structure due to aerodynamic forces, thermal shock, and vibrations.
Generally, the char provides increased thermal protection because less LTA material is removed during the ablation process. The char is also porous, lightweight, and has low thermal conductivity to further improve thermal protection. Additionally, radiant heat loss is increased since the char has higher emissivity and can withstand higher temperatures, and the higher temperatures further reduce convective heat gain.
Unfortunately, a critical failure mode of LTA materials is the formation of a weakened char. As the material forms a char and burns during the ablation process, cracks may form in the surface of the LTA materials. The cracks typically increase in size over time and eventually cause the LTA material to fracture and erode away due to aerodynamic forces. Therefore, effective LTA materials must be capable of forming a strong char.
LTA materials are also susceptible to moisture absorption due to their porosity and lightweight. Moisture absorption increases the weight of the LTA material and further contributes to weakened char during the ablation process. Accordingly, a thin layer of sealant or paint, such as Corlar(copyright), is applied over the top of the LTA materials, as a coating, to reduce moisture absorption. Unfortunately, the application of a sealant or paint increases the weight of the ablative composition, and further increases manufacturing cycle time and overall costs.
In addition to LTA materials, intumescent materials have also been used in high heat applications. Intumescent materials, generally defined as materials that swell when heated, have been used extensively as thermal barriers in the chemical and oil industries for fire protection. Unfortunately, intumescent materials have a high density and have been undesirable for use in weight sensitive applications such as in aerospace vehicles. Furthermore, the char that is produced by intumescent materials after being subjected to flames cannot withstand high aerodynamic shear forces.
Accordingly, there remains a need in the art for a lightweight ablative composition and methods of forming ablative structures that reduces the amount of ablation, strengthens the char, and protects the LTA against moisture absorption while improving manufacturability and reducing overall costs.
In one preferred form, the present invention provides an ablative composition that comprises a quantity of fire retarding intumescent material disposed within a low temperature ablative (LTA) material. To form an ablative structure, a quantity of LTA material is first applied to a substrate, such as an aerospace vehicle structure. An intumescent material is then mixed with a further quantity of LTA material, and the mixture is applied to the substrate, over the top of the first quantity of LTA material. Accordingly, intumescent material is disposed within the LTA material at the outer surface of the ablative structure, or ablative composition, to provide the requisite amount of thermal protection.
Preferably, the intumescent material is mixed with the LTA material during a spray forming process, wherein the LTA material is first deposited onto a substrate in streams during multiple passes of several spray heads to form an ablative structure. The intumescent material is then added to the LTA material during the final pass of the spray heads at the outer surface of the ablative composition.
Alternately, other known methods may be employed to apply the ablative composition to the substrate, such as manual troweling or pre-forming followed by a secondary bonding operation to the substrate. In addition, the ablative composition is cured onto the substrate, preferably at room temperatures, for a period of time that depends on the materials used and the amount of thermal protection required.
In another preferred form, the intumescent material is added to the LTA in increasing amounts towards the outer surface of the ablative composition, thereby forming a gradient of intumescent material. An increased amount of intumescent material is added to the LTA in each successive layer as layers of material are applied to the substrate using, for example, spray forming or manual troweling methods. As a result, the amount of intumescent material gradually increases towards the outer surface of the ablative composition for the required amount of thermal protection.
Under extreme thermal conditions, the intumescent material causes the LTA material to swell, and as a result, the LTA expands outward to block radiant heat and further expands inward to back-fill minor cracks. The swelling further prevents external heating and ambient oxygen from reaching the structure beneath the ablative composition. Advantageously, a stronger char is formed and the structure is adequately protected from the high heat.
Preferably, the LTA material is cork-based and further comprises epoxy. Additionally, the intumescent material is preferably ammonium polyphosphate (APP). The APP is added in a percentage between approximately 10% and 50% to the LTA, and the thickness of each layer applied to the substrate is between approximately 0.05 inches and 0.75 inches. Furthermore, the LTA and intumescent materials are cured at room temperature for approximately 10 minutes to approximately 24 hours, and preferably between approximately 1 and 4 hours. The amount of LTA and intumescent materials, and further the total thickness of the ablative composition, depends on the desired amount of thermal protection as described in greater detail below.
By adding the intumescent material to the LTA during the forming process, the added step of applying a coating of sealant or paint to prevent moisture absorption is eliminated. The ablative composition of the present invention further reduces the amount of ablation, strengthens the char on the surface of the ablative structure, and protects the LTA against moisture absorption.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.