By virtue of their inherent parallel redundancy, heat pipes (HP) are logical elemental building blocks for the construction of spacecraft radiators. In pumped loop space radiators, a micrometeoroid puncture of a cooling-fluid carrying tube would cause eventual loss of cooling fluid, thus leading to failure of the radiator. In contrast, space radiators composed of a large number of heat pipes would be relatively immune to puncture from micrometeoroids or small space debris because loss of an individual heat pipe, whose function is completely independent of that of its neighbors, would result only in the loss of that small fraction of total radiating area represented by the punctured heat pipe’s radiating surface. Thus, overall radiator reliability can be significantly enhanced, even with lower wall thickness of its heat pipe elements, which also would reduce radiator mass.
Increased survivability coupled with reduced mass is of strategic importance in spacecraft power system radiators, since past studies of power systems with either solar or nuclear heat sources have shown (Juhasz and Jones, 1986, Brandhorst et al., 1991) that radiator weight accounts for a significant portion of overall spacecraft launch mass. This is especially true for dynamic energy conversion systems utilizing the Brayton or Stirling thermodynamic cycles, since these systems have relatively low mean effective heat rejection temperatures. Thus, application of graphite-carbon composite technology to space radiator heat pipes will lead to even greater savings in the total Earth-to-orbit mass that needs to be launched for a given mission, and thereby contribute to the “low-cost access to space” initiative, which is a goal to be implemented during the early decades of the 21st century.
The purpose of this paper is to review, expand and extrapolate the results of space radiator research conducted by the NASA Glenn Research Center (GRC) and its contractors under the Civil Space Technology Initiative, (see Juhasz and Peterson (1994), Juhasz and Rovang (1995), and summary paper by Juhasz (2002)). The approach taken in extrapolating prototype C-C heat pipe test results to the design of full-scale space radiators is analogous to that taken by Groll in proposing the use of heat pipes as elemental units, or building blocks, for various thermal applications in industry (Groll, 1973). Such an extrapolation is especially appropriate, because the development of very high conductivity composites with the required mechanical and optical properties for space radiator use makes possible some highly attractive design options. Of these, the most impressive is one offering a radiator whose specific mass is less than one fourth that of a pumped loop design (<1.6 kg/m2 compared to 6 to 10 kg/m2) based on near term technology, while long term material development breakthroughs may lead to 1.0 kg/m2 as an asymptotic low limit for radiator area specific mass expressed in kg/m2, given that strength and durability of the fins during simulated launch conditions can be demonstrated. At the same time, heat pipe radiators provide significantly higher survivability to micrometeoroid damage, owing to the parallel redundancy of heat pipes.
Space limitations prevent an in-depth discussion of all aspects of the C-C heat pipe design, fabrication, and testing carried out under a joint program between NASA Glenn and the Rockwell International Division of the Boeing Company; however, this paper addresses the highlights and briefly analyzes radiating fin heat transfer, which is an essential feature of the proposed “integral fin” C-C heat pipe concept. Also included is a discussion of several potential radiator designs, based on the C-C heat pipe as an elemental unit of typical radiator panels, for space power systems ranging from multi-kilowatt to megawatt levels for space and lunar base applications.
Download
PDF Ebook High Conductivity Carbon-Carbon Heat Pipes for Light Weight Space Power System Radiators
