New Resin Systems for Thermal Protection Materials

The invention provides a family of low density, flexible ablators comprising of a flexible fibrous substrate and a polymer resin. The flexible ablators can withstand a wide range of heating rates (40-540 Watts/cm2) with the upper limit of survivable heat flux being comparable to the survivable heat flux for rigid ablators, such as PICA and Avcoat. The amount and composition of polymer resin can be readily tailored to specific mission requirements. The material can be manufactured via a monolithic approach using versatile manufacturing methods to produce large area heat shields, which provides a material with fewer seams or gaps. The goals of the work are primarily twofold: (i) to develop flexible, ablative Thermal Protection System (TPS) material on a large, blunt shape body which provides aerodynamic drag during hypervelocity atmospheric entry or re-entry, without perishing from heating by the bow shock wave that envelopes the body; and (ii) to provide a relatively inexpensive TPS material that can be bonded to a substrate, that is unaffected by deflections, by differences in thermal expansion or by contraction of a TPS shield, and that is suitable for windward and leeward surfaces of conventional robotic and human entry vehicles that would otherwise employ a rigid TPS shield. This technology produces large areas of heat shields that can be relatively easily attached on the exterior of spacecraft.

The low density flexible ablator can be deployed by mechanical mechanisms or by inflation and is comparable in performance to its rigid counterparts of the same density and composition. Recent testing in excess of 400W/cm2 demonstrated that the TPS char has good structural integrity and retains similar flexibility to the virgin material, there by eliminating potential failure due to fluttering and internal stress buildup as a result of pyrolysis and shrinkage of the system. These flexible ablators can operate at heating regimes where state of the art flexible TPS (non-ablative) will not survive. Flexible ablators enable and improve many missions including (1) hypersonic inflatable aerodynamic decelerators or other deployed concepts delivering large payload to Mars and (2) replacing rigid TPS materials there by reducing design complexity associated with rigid TPS materials resulting in reduced TPS costs.

This unique approach modifies the phenolic polymer network by adding thermoplastic molecules with flexible segments such as aliphatic carbon and siloxane. The thermoplastic molecules are terminated with bifunctional groups that can directly react with the phenolic under the curing condition to form chemical bonds. Further incorporation of these segments can be facilitated by a relay reaction of a second molecular component which can bond with both the first flexible segments and the phenolic network. The selections of flexible, thermoplastic segments are based on desired properties, which include flexibility, ablative, an charring ability, heat resistance, and low catalycity. The modified phenolic is a truly molecular composite in which flexible segments are connected with the phenolic network through strong chemical bonds and are uniformly distributed among the networks. This feature renders a uniform toughening/strengthening effect without compromising the lightweight nature of the materials. The process is also feasible to scale up and amenable for manufacturing.

The graded Thermal Protection System (TPS) offers a lower density than comparable state-of-the-art TPS systems operating at similar maximum heating conditions. This approach is straight forward in terms of processing and surface-treatment application and can be applied to machine PICA materials without having an effect on the final tolerance. The process results in increased usability and handling since standard uncoated PICA is relatively weak. Surface-densified PICA provides an approach for improvements in the robustness for the baseline CEV heat shield. A graded approach eliminates the need for joints and/or bonding agents between material plies. PICA surface densification offers robust mechanical protection against transit damage, handling damage, and in-flight object damage.

NASA's technology is the first successful 3D-printing of high temperature carbon fiber filled thermoset polyimide composites. Selective Laser Sintering (SLS) of carbon-filled RTM370 is followed by post-curing to achieve higher temperature capability, resulting in a composite part with a glass transition temperature of 370 °C. SLS typically uses thermoplastic polymeric powders and the resultant parts have a useful temperature range of 150-185 °C, while often being weaker compared to traditionally processed materials. Recently, higher temperature thermoplastics have been manufactured into 3D parts by high temperature SLS that requires a melting temperature of 380 °C, but the usable temperature range for these parts is still under 200 °C. NASA's thermoset polyimide composites are melt-processable between 150-240 °C, allowing the use of regular SLS machines. The resultant parts are subsequently post-cured using multi-step cycles that slowly heat the material to slightly below its glass transition temperature, while avoiding dimensional change during the process. This invention will greatly benefit aerospace companies in the production of parts with complex geometry for engine components requiring over 300 °C applications, while having a wealth of other potential applications including, but not limited to, printing legacy parts for military aircraft and producing components for high performance electric cars.