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?

Apart from the smug reply that the heat shield will probably already
be in place on launch... consider this:

Heating comes roughly from velocity times drag, with drag coming
roughly from density of the air, which decreases rapidly with
altitude. In this sense, it is not the same atmosphere, as the
upper layers (100km height and more) are almost a vacuum
compared to sea level.

On launch, energy from chemicals is used to accellerate from 0 to
mach 23 or something - orbital speed. You want to use the least
amount of chemicals to get at the required speed. Any heating
cum drag will WASTE chemicals, by putting their energy into
heating the air, instead of increasing the speed of your vehicle.
Thus, the engineers are told to avoid drag cum heating as good
as possible. Solution: go up almost vertically, at not-so-high
speed, until most of the air is below you. Then start to really
accellerate.

On reentry, it is the other way round. The vehicle must be
decellerated from mach 23 to 0 (or you won't survive landing...),
so all that energy must be shed. Now, it's a nice thing to have
this atmosphere cum drag - you don't need any more chemicals
to burn for decelleration! So, in this case, the engineers are
told to search for drag cum heating (within the limits of the
heat shielding, of course), because this drag is exactly the
thing you use as a brake.

I hope this layman's understanding was both correct and understandable.

best regards
Patrick