metallic liner within the carbon-carbon shell. The metallic liner is required for compatibility reasons as well as to provide a leak tight vessel (carbon-carbon is porous). Several approaches to provide the leak tight metallic liner are currently being investigated, including chemical vapor deposition, chemical vapor infiltration and freestanding thin walled metallic liners. Thermacore is currently fabricating a carbon fiber reinforced silicon carbide matrix/tungsten-lithium heat pipe. This heat pipe will have a CVD applied tungsten liner as the inner containment envelope. Also being investigated are graphite-aluminum [10] and graphite-copper [11] composites for advanced low temperature radiator heat pipes. The use of the graphite aluminum composite in conjunction with the high performance aluminum-ammonia heat pipe previously mentioned in this paper, would result in a very high performance -low mass low temperature radiator element. For further mass reduction metallized plastic films [11] and configuration pumped wick structures [12] show some promise for room temperature radiators and multilayer metal-ceramic composites for high temperature radiators [13]. In general, the unique ability of the fiber-formed composites to provide mechanical and thermal properties which are customized for an application offers significant potential. Through selection and orientation of the fibers, specific thermal expansion and thermal conductivity characteristics can be achieved. By customizing the thermal expansion rates of the material, success in application of both the inner liner material and any outer coating may be achieved. In addition, advanced fibers offer very high thermal conductivities, up to four times that of copper. This makes possible high heat flux applications where wall/interface delta-Ts were previously limiting. In addition, since the fibers can be woven into a porous wick structure which is then coated by CVI, extremely high thermal conductivity is theoretically achievable in this type of wick structure which will help in achieving high heat flux capability. Recent developments in bonding aluminum powder wicks without high temperature sintering are leading to improved performance and reduced mass. Increase in permeability by a factor of four can be achieved for the same wick pumping capability. This improves heat pipe performance and, with appropriate redesign, lighter heater pipes will be possible. In addition, low temperature bonding avoids annealing pressure boundaries. This, in turn, allows the use of heat treated high strength materials, thus resulting in additional reduced heat pipe weight. This process will allow fabrication of the space station main radiator heat pipes at about one-third of the weight of the current design, with an increase in performance from 1400 W to 2000 W. Space-related Applications It may be that heat pipes can be designed that will be little affected by long immersion in the hostile environment of a nuclear reactor core. Whether the void volume represented by even the highest performance heat pipes could be made acceptable to the reactor designer is beyond the scope of this paper. Most space-born applications under consideration involve out-of-core uses: input to Stirling engines, main radiators for the solar or nuclear powerplant, shield cooling and cooling of on-board electronics. Electronics cooling includes the need for thermal buses, cold plates, flexible thermal ‘extension cords’ and radiators. An application whose potential was recently successfully demonstrated was the cooling of the throat of a long burning rocket engine. An improvement of 8% was realized by increasing the specific impulse from 279 s to 301 s for a 5 lb thrust engine, resulting in heat fluxes of
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