Propulsion System Components
Advanced Propulsion Concepts
Solar Thermal Propulsion
The Air Force and NASA chose rhenium for their solar thermal propulsion systems because of its high melting point and excellent ductility. In these systems, sunlight is focused into a rhenium blackbody heat exchanger, and the solar energy heats the hydrogen. The heated hydrogen propellant is then exhausted through a nozzle to produce thrust with a specific impulse of up to 1000 seconds.
Solar thermal rocket engine
Rhenium foam was used for the heat exchanger because of its excellent ductility, strength at temperature, high surface area, and ease of fabrication. Furthermore, to ensure that the cavity had the optical performance of a true blackbody, the interior of the cavity was coated with black rhenium.
Solar thermal thrusters. Rhenium foam is used for support structures to accommodate thermal expansion and minimize parasitic heat loss to the main structure.
Nuclear Thermal Propulsion
Early efforts with nuclear thermal propulsion used Ultramet open-cell niobium carbide foam to contain the fuel particles yet allow the hydrogen propellant easy access to the bed. Later work, instead, focused on ceramic foams as the fuel.
Ultramet ceramic and metal foams have excellent heat transfer properties because of their high surface area. For this reason, Ultramet is pioneering the use of open-cell foams as the fissionable material for nuclear thermal propulsion systems. Coating the foam ligaments with a mixture of uranium carbide and other refractory metal carbides generates an open structure in which the propellant gas has immediate and unimpeded access to all the fuel-coated ligaments. A similar design can be used for terrestrial power generation by replacing the hydrogen propellant with helium in a closed-loop system.