Design Considerations for Lightweight Space Radiators Based on Fabrication and Test Experience With a Carbon-Carbon Composite Prototype Heat Pipe

Abstract

Summary This report discusses the design implications for spacecraftradiators made possible by the successful fabrication andproof-of-concept testing of a graphite-fiber-carbon-matrix com-posite (i.e., carbon-carbon (C-C)) heat pipe. The prototype heatpipe, or space radiator element, consists of a C-C compositeshell with integrally woven fins. It has a thin-walled furnace-brazed metallic (Nb- 1%Zr) liner with end caps for containmentof the potassium working fluid. A short extension of this liner,at increased wall thickness beyond the C-C shell, forms theheat pipe evaporator section which is in thermal contact withthe radiator fluid that needs to be cooled. During the fabricationprocess the C_ shell condenser section was exposed to anatomic oxygen (AO) ion source for a total AO fluence of4×102o atoms/cm 2, thereby raising its surface emissivity forheat radiation to a value of 0.85 to 0.90 at design operatingtemperatures of 700 to 800 K. The prototype heat pipe wasextensively tested from startup at ambient conditions, with theworking fluid initially in the frozen state, to a condensertemperature of nearly 700 K. Post-test inspection showed theheat pipe to be in excellent condition after several thermalcycles from ambient to operating temperature.The report also discusses the advantage of segmented spaceradiator designs utilizing heat pipe elements, or segments, intheir survivability to micrometeoroid damage. This survivabil-ity is further raised by the use of condenser sections withattached fins, which also improve the radiation heat transferrate. Since the problem of heat radiation from a fin does not lenditself to a closed analytical solution, a derivation of the govern-ing differential equation and boundary conditions is given inappendix A, along with solutions for rectangular and parabolicfin profile geometries obtained by use of a finite differencecomputer code written by the author.From geometric and thermal transport properties of the C-Ccomposite heat pipe tested, a specific radiator mass of 1.45 kg/m 2 can be derived. This is less than one-fourth the specific massof present day satellite radiators. Using composites with ultra-high conductivity would further reduce the area density ofspacecraft radiators, and utilizing alternate heat pipe fluids withcompatible liner materials would extend the C-C heat pipetechnology to a wide range of temperatures and applications.