Analysis of a flexible polymeric film with imbedded micro heat pipes for spacecraft radiators

Abstract

In response to the pursuit of interplanetary travel and a continuous human presence in space, there is increasing focus within the space industry on spacecraft designs that change configuration within the space environment. Flexible thermal radiators are being developed to accommodate deployment mechanisms. An analytical model suggests that a lightweight polymeric material with imbedded micro heat pipe arrays can meet heat dissipation requirements while contributing less mass than competing flexible materials. The heat pipe capillary limit is evaluated as a function of temperature using two candidate working fluids. Using water, maximum heat transport is 18 mW per channel at 140° to 160° C. Maximum heat transport using methanol is 2.2 mW at 120° C: an order-of-magnitude difference. A thermal circuit model translates heat transport per channel into total radiator capacity as a function of heat source temperature and environmental sink temperature. Using water as the working fluid, radiator capacity varies from 6.0 kW to 12.2 kW for source temperatures from 20° to 50° C. For source temperatures 40° C and higher, capacity meets or exceeds the dissipation requirements of a reference spacecraft design. Methanol is not recommended as a working fluid because it produces radiator capacities two to three times lower than when water is used. Although thermal control system specifications constrain the micro heat pipe operating range, design changes directed at alleviating capillary limitations should increase radiator capacity. Technical issues for further investigation include effects of film billowing; performance limitations related to vapor viscosity; working fluid diffusion; and chemical reactivity between the case and working fluid. Compared to a competing graphite fiber weave, the polymeric material has an effective conductivity over ten times higher. Its area power density (kW/m²) is 18% to 60% lower than the graphite weave, but its mass power density (kW/kg) is several times higher. Greater flexibility and lower mass make it more amenable to structural integration than the graphite material. Recently developed space-stable polymers offer resistance to harsh temperature and radiation environments, helping to clear the path toward a more extensive use of polymers within the space industry.