Cladding and Freeform Deposition for Coolant Channel Closeout

The one-piece multi-metallic composite overwrap thrust chamber assembly is centrally composed of an additively manufactured integral-channeled copper combustion chamber. The central chamber is being manufactured using a GRCop42 or GRCop84 copper-alloy additive manufacturing technology previously developed by NASA. A bimetallic joint (interface) is then built onto the nozzle end of the chamber using bimetallic additive manufacturing techniques. The result is a strong bond between the chamber and the interface with proper diffusion at the nozzle end of the copper-alloy. The bimetallic interface serves as the foundation of a freeform regen nozzle. A blown powder-based directed energy deposition process (DED) is used to build the regen nozzle with integral channels for coolant flow. The coolant circuits are closed with an integral manifold added using a radial cladding operation. To complete the TCA, the entire assembly including the combustion chamber and regen nozzle is wrapped with a composite overwrap capable of sustaining the required pressure and temperature loads.

The advent of additive manufacturing makes available new and innovative integrated thermal management systems, including integrating an oscillating Heat Pipe (OHP) into the leading edge of a hypersonic vehicle for rapid dissipation of large quantities of heat. OHPs have interconnected capillary channels filled with a working fluid that forms a train of liquid plugs and vapor bubbles to facilitate rapid heat transfer. Multiple additive manufacturing techniques may be used, including powder bed fusion, binder jetting, metal material extrusion, directed energy deposit, sheet lamination, ultrasonic, and electrochemical techniques. These high performance OHPs can be made with materials such as Refractory High Entropy Alloys (RHEAs) that can withstand high temperature applications. The structure of the OHP can be integrated into the constructed leading edge. The benefits include a heat transport capacity of 10 to 100 times greater than before. Integrated OHPs avoid the bends or welds in traditional heat pipes, especially at the locations where the highest thermal stresses might cause thermal-structural failure of a leading edge. Alternating the diameters of the OHP channels alleviate start-up issues typically found in liquid metal oscillating heat pipe designs in high temperature applications by aiding in the instigation of a circulating flow due to multiple forces acting upon the working fluid.

For extremely high-temperature sealing applications, Glenn researchers have devised novel methods for fabricating single-crystal preloaders. NASA's high-temperature preloaders consist of investment cast or machined parts that are fabricated in various configurations from single crystal superalloys. Machined preloaders include a variety of spring configurations, compressed axially or radially, fabricated from single crystal slabs. Before machining, the slabs are carefully oriented in a special goniometer using x-diffraction techniques. This helps to maintain proper crystal orientation relative to the machined part and the applied loads. For more complex geometry components which cannot be easily and economically machined, an investment casting approach would be used. Complex preloader geometries include wire coil springs of various configurations. These single crystal preloaders would be designed with the appropriate stiffness for the intended thermal barrier/seal application and placed underneath, or integrated within, the seal/barrier. At extrememly high temperature, the preload device keeps the seal/barrier mated against the opposing surface as the gap between the two surfaces changes, maintaining contact between surfaces and preventing convective heat transfer.

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.

A paraffin-based hybrid fuel formulation for low-temperature cycling has been developed. Cracked or flawed paraffin fuel grains can pose a safety risk during testing and are unuseable. Cracking of the fuel grain typically occurs during the cool-down process. The paraffin-based fuel grain contracts by as much as 15% while cooling from a liquid at 230°F to a solid state and eventually room temperature, not only causing visible cracking, but residual stresses that can damage the grain throughout processing. The new process is an improvement to others that were tried and found unworkable including spin casting and some additive manufacturing techniques. To remedy the issues with previous manufacturing efforts, the innovators at MSFC developed an oven program mimicking the intrinsic cooldown process of paraffin wax for use in monolithic casting. Innovators monitored paraffin wax cooling in a stainless steel vessel with four thermocouples to develop the program. Rather than cooling quickly, the oven program cools the grain incrementally, allowing the temperature to equilibrate along the entire grain before cooling further, resulting in greater temperature uniformity. This process produces intact full-scale paraffin fuel grains (11" diameter by 35" length) capable of surviving cold temperatures for use in propulsion systems with hybrid engines.