This report discusses the design implications for spacecraft radiators made possible by the successful fabrication and proof-of-concept testing of a graphite-fiber-carbon-matrix composite (i.e., carbon-carbon (CC)) heat pipe. The prototype heat pipe, or space radiator element, consists of a C-C composite shell with integrally woven fins. It has a thin-walled furnace-brazed metallic (Nb-1%Zr) liner with end caps for containment of the potassium working fluid. A short extension of this liner, at increased wall thickness beyond the C-C shell, forms the heat pipe evaporator section which is in thermal contact with the radiator fluid that needs to be cooled. During the fabrication process the C-C shell condenser section was exposed to an atomic oxygen (AO) ion source for a total AO fluence of 4x1020 atoms/cm2, thereby raising its surface emissivity for heat radiation to a value of 0.85 to 0.90 at design operating temperatures of 700 to 800 K. The prototype heat pipe was extensively tested from startup at ambient conditions, with the working fluid initially in the frozen state, to a condenser temperature of nearly 700 K. Post-test inspection showed the heat pipe to be in excellent condition after several thermal cycles from ambient to operating temperature.
The report also discusses the advantage of segmented space radiator designs utilizing heat pipe elements, or segments, in their survivability to micrometeoroid damage. This survivability is further raised by the use of condenser sections with attached fins, which also improve the radiation heat transfer rate. Since the problem of heat radiation from a fin does not lend itself to a closed analytical solution, a derivation of the governing differential equation and boundary conditions is given in appendix A, along with solutions for rectangular and parabolic fin profile geometries obtained by use of a finite difference computer code written by the author.
From geometric and thermal transport properties of the C-C composite heat pipe tested, a specific radiator mass of 1.45 kg/m2 can be derived. This is less than one-fourth the specific mass of present day satellite radiators. Using composites with ultra-high conductivity would further reduce the area density of spacecraft radiators, and utilizing alternate heat pipe fluids with compatible liner materials would extend the C-C heat pipe technology to a wide range of temperatures and applications.