) writes:
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?

Heres why. When you launch from the ground, you start moving through
the thickest air you are going to encounter, but you are moving through
it very *very* slowly, relative to the speeds when coming back in.

When the shuttle, or any re-entering spacecraft coming in from orbit,
starts to hit air, granted very thin air very high up, the spacecraft
is flying at about 17,400 MPH, or about *five miles per second*.

Wheras, on the way up, you don't hit such speeds until you're far more
out of the athmosphere. And, on the way up, as you accelerate, the
air is thinning out even faster, while on the way down, you need that
air drag to slow you down, so you don't stick your pointy nose through
it, but you point the thickest and widest part of your spacecraft
frontwise so that you can catch more of the air, so that it's drag can
slow you down.

And, all that air drag does slow you down, from orbital speeds of
five miles per second, to the 180 MPH speed of when the shuttle's tires
contact the runway. Thats a lot of slowing down, and the air does it
all for you, but in the process, it makes a LOT of heat as a result.

Does that help ?

Andre

--
" I'm a man... But, I can change... If I have to... I guess. "
The Man Prayer, Red Green.