wrote:

I think I understand why objects heat up when going through the
atmosphere and therefore why re-entry vehicles need heat shielding. My
question is why we don't need heat shielding when launching. Aren't we
going through the same atmosphere?

BigKhat


Yes. But there's a difference between going up, when speed is
increasing, as air density is decreasing, and coming down, where air
density is INcreasing, and you're tryibg to use that atmosphere to
decelerate (rather than rockets, which would be impractical in this
case), and that kinetic energy can only be converted to heat.

This is one of the problems with the idea of flying hypersonic
airbreathers to orbit. By definition, it has to stay in the meaningful
atmosphere to keep picking up oxidizer at increasing speed. As close to
orbital speed as your materials will allow you, with a small rocket kick
the rest of the way to orbit.

Pure rockets can take the fastest path into less dense air. Not just
for termodynamic reasons, but because rocket engines are more efficent
in decreasing atmospheric density.

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