Thermal Vacuum Testing


Space is cold. But your spaceship’s biggest fear in space is overheating.

While space is “cold”, heat works very differently in space than here on Earth. On Earth, there are lots of kinds of hot and cold: summer and winter, muggy and windy, sunny and shady, and so on. Each of those conditions feels different, which reflects the physics of how the heat is moving; conduction, convection, and radiation, respectively. On Earth, all of them play a part in regulating temperature thanks to the atmosphere.

But not in the vacuum of space, where nothing is touching your ship; there’s nothing to carry the heat in or out of it except radiation. And since radiation is much less effective than the other two kinds of heat transfer, temperatures in space are patchy instead of smooth gradients. While in total space is very cold, the parts exposed to lots of radiation are quite hot.

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It's this seeming contradiction that's one of the toughest engineering challenges in aerospace. On Earth, having shade might keep you 10 degrees Celsius cooler; on the Moon the difference is more like 100 degrees. This puts an incredible design constraint on the electronics of a robotic Moon mission. Not only will Andy be doing a lot of computing - generating heat - but the sun shines very brightly on it with no atmosphere to diffuse that heat. Andy has to get rid of it somehow.
Past missions have done a lot to manage heat, just as Andy will. One of the favorite methods is to prevent heat from being absorbed in the first place. Many of the space-faring parts on the Apollo missions, for example, are highly reflective, to keep heat from ever being absorbed.

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Andy's gold color serves a similar purpose, reflecting away heat from the sun and the heated ground.

But, unlike Apollo, Andy will generate power with solar panels. Because absorbing infrared radiation is how solar panels function, they can't reflect it away. And even if they could, the electronics generate as much as seventy Watts of waste heat; a lot for such a small rover.

So Andy will also employ the second common strategy, radiating the heat away. All of the electronics will fit into a common "card carrier", which conducts the heat to the big radiator on Andy's back, right below the solar panels. This radiator will be painted white because that color reflects and emits away the most radiation.

It’s clear that thermal management, centered around the radiator, is crucial to Andy's success. Since that system is so mission-critical, the team is paying extra attention to verifying its performance. There’s only one way to do that without actually sending it to the Moon: throw it in a vacuum chamber. Vacuum chambers, able to keep the atmosphere inside thin enough to prevent conduction and convection, are a common testing tool in aerospace. Add in high-powered thermal lamps and a cold shroud, and it’s ready to prove if the Protoflight avionics can survive those harsh Lunar conditions.

An even bigger vacuum chamber at the Johnson Space Center.

An even bigger vacuum chamber at the Johnson Space Center.

But, there’s quite a lot of tests before that one. Each Protoflight component will be tested on its own. Once there’s a good understanding of each part individually, then the test of the whole system will be meaningful. 

This semester the team began those tests, with a smaller vacuum chamber here at Carnegie Mellon. An expendable test board was put in a modular avionics “card carrier”, and used resistors to generate heat. The test determined how efficiently the card carrier actually conducted heat to the radiator under various conditions, like a long drive or when shutting down to prevent an overheat.

In the vacuum chamber, a steady-state test. On the left, the resistive heaters on a copper plate. A thermistor is visible in the upper-right corner. The other board, previously tested, is a motor controller.

In the vacuum chamber, a steady-state test. On the left, the resistive heaters on a copper plate. A thermistor is visible in the upper-right corner. The other board, previously tested, is a motor controller.

With these initial tests successful, Andy is one step closer to stepping out and exploring our celestial neighborhood.

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