Description: Efficient nuclear-thermal propulsion requires heating a low molecular weight gas, typically hydrogen, to high temperature and expelling it through a nozzle. The higher the temperature and pressure, the higher the thrust and specific impulse. For ground test facilities that will be heating the gas to temperatures up to 4400 F (2425 C), the number of materials that can be used is severely limited. The need for compatibility with hot hydrogen limits the field even further. In this project, Ultramet will design, fabricate, and test a system for heating high-pressure hydrogen up to 2400 C. The system will comprise a resistively heated open-cell refractory carbide foam heating element and an oxide-lined rhenium tube to contain the heater. By passing an electric current through the foam ligaments, they can be heated to arbitrarily high temperatures, and the carbides are stable in hot hydrogen. In previous work, Ultramet has demonstrated the ability to make robust electrical connections to foam heaters that will tolerate high temperature operation. Unlike most other refractory metals, rhenium is both ductile and non-reactive with hydrogen. It is also very strong and creep-resistant at high temperature. Consequently, rhenium is the material of choice for the pressure vessel and other hydrogen-washed surfaces such as the convergent/divergent nozzle. In the immediate vicinity of the heater, a thin layer of zirconia or hafnia (both of which are compatible with rhenium up to their melting point) will be used to electrically insulate the rhenium from the foam heater.

Benefits: Beyond ground test hardware for nuclear-thermal propulsion technologies, the resistively heated foam technology can also be used as an ignition system for ionic liquid advanced monopropellant rocket engines, as well as non-hypergolic bipropellant engines. The former application is being actively pursued by the Air Force.

The most lucrative commercial application for this technology would be ignition systems and catalyst preheaters for turbine engines used for terrestrial power generation. Other applications include ignition systems and catalyst preheaters for aeropropulsion turbine engines, gas and water heaters where high efficiency is critical, and air heaters for hypersonic wind tunnels similar to the Aerodynamic and Propulsion Test Unit at the Arnold Engineering Development Center.