Planning a space mission is difficult. Planning a high-altitude balloon flight is arguably just as difficult. Sure, you don’t have to deal with orbital mechanics when your payload is on board a balloon, 40km above the ground, with close to no conduction or convection to cool your payload, and bombarded with radiation. But you need to solve a whole new set of problems.
At high altitudes of circa 30km your payload is effectively in space. The official boundary for space is at 100km, but this is rather arbitrary: at an altitude of 30km the atmosphere is more space-like than ‘ground-like’.
You might think space (or near-space) is cold. And you’re right: it is very cold. That’s because the air pressure is very low. Simply, there aren’t enough air particles to transfer heat energy to your payload. When you’re on the ground, the air pressure is high enough that there’s enough particles and molecules to cool you down or warm you up. In the upper atmosphere, the pressure is so low that the normal physics of heat exchange don’t really apply.
Due to the vacuum environment in space or the upper atmosphere, it’s hard to get rid of heat generated by the payload computer. It’s not unlike having a flask of tea or soup, and keeping it warm for hours.
Because of the very low pressure in the upper atmosphere, conduction and convection are minimised: this means that the payload will experience a heat build-up while flying, because there are not enough atmospheric particles up there to take away the heat energy. The obvious solution to this is to use heatsinks.
Heatsinks are like stationary fans that have a large surface area. Heat is transferred from a chip or a module to the heatsink, and then that heat energy is taken away from the sink (and also the chip/module) by convection. This keeps the payload computer cool.
In an ideal physical situation, the number of gas particles hitting and bouncing off the heatsink at near-space altitudes should be equal to the number of particles bouncing off the chips when the payload computer is being tested on the surface of Earth. That means dissipation of heat energy stays cool and the payload can function. There’s a good bit of physics here, but it simply means that your payload’s heat generating components need to be heatsinked effectively.
The PiSAT Project is currently solving these problems, by learning from other flights and applying things we have learned. At the moment, we’re looking at off-the-shelf materials – mainly aluminium – as an effective heatsink for balloon flights, suborbital flights, and ultimately orbital flights.