Why aren’t heat shield tiles on rockets and space shhuttles white since white paint reflects heat better?


Why aren’t heat shield tiles on rockets and space shhuttles white since white paint reflects heat better?

In: 5

white works to reflect energy that is coming in as light, because white absorbs less light than other colors.

However, heat from re-entry is due to rubbing (friction) of air against the ship and compression (air gets hotter when you squeeze it… and it’s being squeezed really hard by being in front of the ship)

because of these things, the color wouldn’t have that big of an effect. for example, it makes no difference what color your teapot is. it heats up more or less the same on the stove.

instead, they focus on making the best insulation they can – whatever color it is.

Because heat shields on rockets/space shuttles are not trying to reflect light. White does not reflect heat better, it reflects light (EM radiation) better. The heat that is dangerous to these crafts when entering/exiting the atmosphere is collision with the gases in our atmosphere at high speeds. So the shield tiles need to be able to withstand the thermal and mechanical impacts, not deal with EM radiation.

White reflects *light* better, but the heat there isn’t coming from bright light, it’s coming from the compression of the air in front of the shuttle.

Painting them white wouldn’t really help, they’d be unpainted in seconds

The tiles on the heatshields aren’t trying to reflect heat, there’s too much heat for that to be feasible, they’re trying to insulate. They’re a thick block of material that doesn’t conduct heat well so the side facing downwards during reentry gets stupid hot but it takes soooo long for the heat to move through the tile that its through the atmosphere before the shuttle/capsule itself can get damaged.

The blunt shape of the heatshield also helps as it generates a little cushion of air between the shield and the stupid hot compressed air right in front of it.

You’re not going to develop a paint that can withstand 20 seconds at 1600C so painting it ends up being a bad idea as that paint will quickly vaporize and turn into hot gas right up against your tile rather than a safe distance in front of it and would start to eat into it.

There aren’t really any ways to cheat reentry, you just have to tank your way through it

On the space shuttle, the tiles on the back are in fact white to reflect away heat. The tiles on the bottom side however are black because black tiles are better at radiating away their heat. The side of the spacecraft facing the incoming atmosphere and the side facing away are both heated in entirely different ways.

I should add that the space shuttle is somewhat unique because its heat shielding was intended to be reusable. The bottom tiles protected the craft by being a very good thermal insulator. Most other reentry capsules have used an ablative heat shield. This style of heat shield is far more effective and easier to make, but it is burned away during reentry, meaning it’s single use only. It doesn’t matter so much what color an ablative heat shield is.