The waste heat from electronics must be removed to keep them from over-heating. On Earth, the ultimate heat sink is typically either a liquid coolant or the atmosphere. In space, the waste heat is typically transported by grooved-aluminum, Constant Conductance Heat Pipes (CCHPs) or Loop Heat Pipes (LHPs) to a radiator, and radiated into the environment. Constant Conductance Heat Pipes (CCHPs) represent an effective way to transport the heat over several meters, however, they have two limitations:
1. Maximum operating temperature
The maximum operating temperature is roughly 60°C for ammonia Constant Conductance Heat Pipes (CCHPs) and Loop Heat Pipes (LHPs), before the power carrying capability drops off. It is desirable to operate at as high a temperature as possible, since the thermal radiation scales with T4. Operating at a higher temperature allows a smaller and therefore lighter radiator.
2. Ground Testability
Grooved Constant Conductance Heat Pipes (CCHPs) have a very high permeability for flow, but a very low pumping capability. CCHPs are normally tested on Earth with an adverse elevation (evaporator above condenser) of 0.1in (2.5 mm). The power carrying capability drops to zero with an adverse elevation of about 0.4 in (1 cm). During ground testing of the spacecraft, the CCHP must be gravity aided or level. LHPs do not have this constraint but are considerably more complex and expensive.
Heat is generally transferred by conduction from the electronics to the aluminum/ammonia CCHPs. Two additional devices have been used to improve the heat transfer to the two-phase device: flexible thermal straps, and encapsulated conduction plates. Flexible thermal straps are usually used to transfer small amounts of power when the electronics move relative to the heat sink. Encapsulated conduction plates have an effective thermal conductivity of around 550 W/m-K, with two-dimensional spreading. In some encapsulated conduction plates, the effective thermal conductivity can decrease with thermal cycling. They are extremely expensive since they are formed by hot isostatic pressing. They also require thermal vias located below the electronics, so the electronics locations are fixed.
ACT has recently worked with NASA Johnson Space Center and NASA Marshall Space Flight Center to demonstrate flight heritage for two additional spacecraft thermal control devices: copper/water heat pipes and High Conductivity (HiK™) plates. Copper/water heat pipes are commonly used in many military and consumer electronics, including almost all laptops, typically in heat pipe assemblies. The benefits of copper/water heat pipes include their ability to operate at temperatures up to about 150°C, operate against adverse elevations of up to 25cm, and tight bend radius. Their major limitation is that the heat pipes carry only low powers at temperatures below ~20°C, and only transfer heat by conduction when the water is frozen. However, by controlling the water inventory so that no free liquid is available, copper/water heat pipes have been shown to withstand thousands of freeze/thaw cycles during terrestrial testing.
HiK™ plates use copper heat pipes that are flattened and embedded in an aluminum plate to increase the effective thermal conductivity. The heat pipe layout is tailored to most efficiently conduct heat from the electronics to the area where the plate is cooled. Water is the most common working fluid, but methanol can be used when the heat pipe needs to operate at lower temperatures. The benefits of HiK™ plates over encapsulated conduction cards include higher effective thermal conductivity (500 to 1200 W/m K), lower cost, no degradation with thermal cycling, and the ability to conduct heat around corners.
ACT’s copper-water heat pipes and HiK™ plates are commonly used for thermal management of electronics equipment on earth and aircraft but have not been used in spacecraft thermal control applications to date (2017), due to the satellite industry’s requirement that any device or system be successfully tested in a microgravity environment prior to adoption. ACT, NASA Marshall Space Flight Center, and the International Space Station office at NASA’s Johnson Space Center, demonstrated flight heritage in Low-Earth Orbit. The testing was conducted aboard the International Space Station (ISS) under the Advanced Passive Thermal experiment (APTx) project. In the ISS test as shown in Figure 2, the heat pipes were embedded in a HiK™ aluminum base plate, and subject to a variety of thermal tests over a temperature range of -10 to 38 ºC for a ten-day period. Results showed excellent agreement with both predictions and ground tests. The HiK™ plate underwent 15 freeze-thaw cycles between -30 and 70 ºC during ground testing as shown in Figure 3, and an additional 14 freeze-thaw cycles during the ISS testing. The following was demonstrated during 10 days of testing on the ISS:
- Successful operation of the copper/water heat pipes and HiK™ plate
- Ability of the copper/water HPs and HiK™ plate to survive multiple freeze/thaw cycles
- Copper/water heat pipes can carry the required power
- Copper/water heat pipes and HiK™ plate can start up from a frozen state
In addition to ACT’s current aluminum/ammonia constant conductance heat pipes (CCHPs), ACT can now offer a broader toolbox for the spacecraft thermal control engineer: spot cooling of electronic devices with copper/water heat pipes, effective heat spreading of electronic boards and enclosures with our HiK™ plates, and efficient heat transport outside the electronics control box to dissipate the heat with our CCHPs. ACT is the only company in the world that can currently offer these capabilities.