An improved radiator has been designed to provide inexpensive and highly reliable thermal control for an extravehicular mobility unit (EMU) [basically, a space suit equipped with a portable life-support system (PLSS)]. The amount of cooling fluid (which could be water) needed to obtain a given degree of thermal control by use of this radiator is smaller than the amounts consumed by prior EMU radiators. For NASA, this radiator will reduce the cost of extravehicular activities at the Space Station by reducing the amounts of equipment and materials that must be lofted to outer space, reducing the volume of equipment carried on the wearer's back, and reducing the consumption of battery power. The improved radiator will benefit not only the space industry but also those terrestrial industries in which water is used to transport heat into radiators or external heat exchangers in order to thermally control equipment and/or living spaces in very cold climates.
One novel aspect of this radiator is that the tubes within it accommodate, and are not damaged by, the expansion of water or other cooling fluid upon freezing. Consequently, the cost of heating or of burying pipes to prevent freezing can be eliminated. Another, related novel aspect is that selective freezing in the tubes is utilized to control the rejection of heat: At low thermal load, water or other cooling fluid flows in only one tube and the fluid in all the other tubes is frozen. At high thermal load, the fluid flows in all the tubes.
The radiator is divided into four panels plumbed in parallel to minimize the pressure drop that must be overcome in pumping. Each panel (see figure) occupies part of the back and part of one side of a PLSS, which is carried on the back of an EMU. The rates of flow through each radiator tube and the radiator surface area attached to each tube are chosen so that the fluid in one tube in each panel will not freeze under any circumstances. The side of the radiator facing the EMU is separated from the EMU by a layer of insulating material to minimize heating of the PLSS in an environment hotter than 80 °F (27 °C) when the radiator is off and to minimize loss of heat in a cold environment at low metabolic load. The outer (radiating) surface of the radiator is coated with a low-thermal-absorptance, high-thermal-emittance material to minimize heating in a hot environment and to maximize the rejection of heat by radiation.
The tubes in each radiator panel are held in channels in an extruded plate that constitutes, in effect, one large radiator fin. In each panel, the tube in which the fluid always flows is the shortest and innermost one, located at the junction of inlet and return manifolds. The rate of flow through the innermost tube is high, relative to the rates of flow through the other tubes in the panel. Very little radiator-fin area is connected to this tube; this provision minimizes the rejection of heat at low thermal load. Larger radiator-fin areas are connected to the other tubes in the panel. Except for the innermost and outermost tube, the rate of flow through each tube is governed by an orifice sized so that the ratio between the fin area and the heat conveyed by the flow is the same for all tube/fin combinations on the panel.
For the outermost tube, this ratio is larger than it is for the others; consequently, the outer tube and fin area tend to remain cooler than the others, so that when the metabolic rate (and thus the heat load) decreases, the fluid freezes in the outer tube before it freezes in the others. After the fluid in the outermost (which is also the longest) tube freezes, the next longest tube supplies heat not only to its own fin area but to the fin area normally served by the outer tube. As a result, the next longest tube runs cooler and becomes the next one in which the fluid freezes if the heat load decreases further. This pattern is repeated until the effective radiating area drops sufficiently that the heat supply and the heat loss are in balance. However, the radiator is designed so that even in the coldest possible environment and the lowest possible heat load, the supply of heat to the innermost tube is sufficient to prevent freezing in that tube. During a thaw, the progression of freezing is reversed: heat is transferred into a frozen tube from an adjacent shorter unfrozen tube. Once a tube thaws, it transfers heat across the radiating surface to the next frozen tube until it also thaws, and so forth, progressing outward from the shorter to the longer tubes.
This work was done by Robert J. Copeland of TDA Research, Inc., for Johnson Space Center. For further information, contact the Johnson Patent Office: (281) 483-0837. MSC-22813