Advanced carbon-based heat exchangers (carbon/carbon composite or graphite) used in solar-thermal propulsion engines must be protected for extended periods from the ≥3000 K hydrogen propellant. In previous work for NASA and DoD, Ultramet successfully fabricated and demonstrated the hot hydrogen survivability of rhenium-based solar-thermal engines, and of refractory metal carbide coatings on open-cell foam components for nuclear-thermal propulsion. The potential exists to combine these technologies by using ductile rhenium as an interlayer between a carbon heat exchanger and a high melting point outer metal carbide layer with well-established survivability in high temperature hydrogen. Relative to rhenium, the carbides are lower in density and cost. This combination has the potential to alleviate issues related to thermal expansion mismatch between the substrate and protective coatings, maximize adhesion, minimize the potential for cracking, and maximize component use temperature and lifetime. In this project, Ultramet will fabricate a matrix of coated coupon test specimens, including various coating materials and layered coatings, and perform initial hot hydrogen performance testing in the Compact Fuel Element Environmental Test (CFEET) facility at NASA MSFC.
Solar-thermal propulsion, with its high specific impulse, is attractive for interplanetary scientific missions and orbital maneuvering of satellites. NASA interests include both deep-space and near-Earth exploration and operations. Solar-thermal propulsion is considered an enhancing technology for crewed missions to a near-Earth asteroid (Design Reference Mission 6) and the Martian moons (DRM 8). Small satellites provide low-cost platforms for robotic missions but require compact, lightweight, high-performing, and affordable propulsion systems.
Non-NASA government and commercial applications for solar-thermal propulsion (as an alternative to chemical/electric) include spacecraft for communications, reconnaissance/surveillance, imaging, and remote sensing. Compact ultrahigh temperature heat exchangers developed for solar-thermal propulsion may find dual use in high-efficiency terrestrial power plants.