How do phase change material heat sinks keep small satellites cool?

The short answer: melt during the burst, refreeze during the rest

A phase change material heat sink keeps a small satellite cool by storing heat instead of getting rid of it right away. Many small satellite parts, such as high-power radio amplifiers and optical terminals, run in short, intense bursts and then sit idle for much of the orbit. A phase change material, or PCM, packed against that hardware melts while the part is working, absorbing large amounts of heat at a nearly fixed temperature. When the part goes quiet, the PCM slowly refreezes and hands the stored heat to the radiator.

The payoff is sizing. Without storage, the radiator has to be big enough to dump the peak heat load. With a PCM buffer, it only has to keep up with the average load across the orbit, which can shrink its area considerably. The PCM also narrows the temperature swings the electronics see, which may reduce fatigue in solder joints and bonded layers.

Which materials are used: paraffins versus low-melting alloys

The most common choices are organic paraffins such as n-octadecane and eicosane. They store a lot of latent heat, behave stably over repeated cycles and are light, which matters on a spacecraft where every gram counts. One study of PCM heat sinks for small satellites put octadecane and eicosane through more than 10,000 melt and freeze cycles to check that they hold up.

Low-melting-point metal alloys are the other option. They conduct heat far better than wax but are much denser, and that extra mass has limited their use on mass-constrained vehicles. Choosing between them is a trade between how fast heat can get in and how much the heat sink weighs.

What makes it hard: wax is a poor conductor

The central problem with paraffin PCMs is thermal conductivity. Solid n-octadecane conducts only about 0.358 W/(m·K), and the liquid about 0.13 W/(m·K). Heat enters at the hot wall and then crawls through the wax, so the material nearest the electronics melts while the rest stays solid and unused.

Engineers therefore build a conductive skeleton inside the wax. A 2022 numerical study in the journal Aerospace compared copper, aluminum and graphite foams with topology-optimized fins under microgravity conditions. Graphite foam melted the PCM fastest, but its low porosity left little room for wax, so it ranked last overall. At equal mass of conductive material, metal foam beat fins, and copper foam beat aluminum foam. A thin 5 percent copper fin case took about 1237 seconds to melt, against 68.6 seconds for a 79 percent porosity aluminum foam. The best balance of melting speed, weight and storage space came from combining a few percent of aluminum fins with a 98 percent porosity copper foam, and the authors note a plain straight fin could stand in for the optimized shape.

How these heat sinks are tested and qualified

Because a heat sink has to survive launch and then work in vacuum, testing happens in both regimes. A 2026 CubeSat study filled heat sinks for roughly 4 inch by 4 inch battery modules with a paraffin melting near 37 degrees C, enhanced with carbon foam, expanded graphite or copper foam, and ran them in a vacuum chamber and at atmospheric pressure. Composites combining carbon foam and copper foam performed best in both conditions compared with plain paraffin.

A separate 2026 design framework presented at the Small Satellite Conference showed that a simplified thermal model, without full computational fluid dynamics, could predict vacuum test results within 0.5 degrees C. It was demonstrated on a prototype for a 50 W optical terminal and a qualification model for a 36 W transmitter, which was tested to rideshare launch standards for pressurized hardware and passed vibration testing.

An old idea in a smaller package

None of this is new in principle. NASA published a phase change materials handbook in 1971 that already covered metallic fillers in PCM composites, material compatibility and the problems the space environment poses. What has changed is scale: small satellites now carry bursty, high-power payloads in tight volumes, and cheap foams, simplified models and vacuum testing make a mission-specific PCM heat sink something a small team can design and qualify.