The development of hybrid propulsion concepts at the Naval Ordnance Test Station has created the requirement for a lightweight nozzle system capable of withstanding gas temperatures in excess of 6200°F for burn times longer than 30 sec. A series of 40 nozzle-material test firings illustrated that tungsten nozzle insert systems provided the best operation. The inserts tested included tungsten metal-oxide compacts and tungsten infiltrated with metals selected on the basis of their latent heats of vaporization and boiling points. Temperaturetime profiles taken during firings indicated that heat transfer rates were lower with the tungsten-metal systems than for unmodified tungsten, which indicated the possibility that transpiration or mass transfer cooling was being realized. A laboratory program was initiated to subject infiltrated tungsten samples to a plasma-arc heat source. It was found that, during exposure to high heat fluxes, temperatures were reduced by inducing flow of the molten infiltrant by pressurizing the back side of the specimen. This technique, in combination with ablative sacrificial materials, offers an excellent nozzle system for long-time firing durations in hybrid and solid propellant rockets.