system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't they use jet engines for the first stage of rockets?

I just learnt about specific impulse and how the specific impulse of jet engines is much higher because they don't have to carry their own oxygen. And that made me think; why don't they use jet engines for the first stage, when there's still oxygen around? Wouldn't that be much more efficient?
gpt
Some of the problems:

Jets are only efficient at a specific designed air pressure and density. This is bad for vertical movement, where those parameters constantly change.

A rocket will quickly leave all altitudes where there's enough oxygen to burn. This means that the first stage would barely be used, which is a horribly inefficient thing to do.

Air consists primarily of nitrogen, so you need to separate the oxygen from the rest of the air - while in flight.

To get enough specific impulse from oxygen, you need to liquify it first, otherwise it's density is far too low to carry enough momentum. That means you need *a lot* of oxygen, which is difficult to come by if you fly very slowly (which you *do* at launch).

All of that under very cold conditions, which makes the processes even more complicated. For example your heat exchangers might freeze.

Tackling all these problems usually makes the rocket way heavier than just taking the oxygen and a normal rocket engine with you. It's also expensive as hell, considering your tech will be lost after one use. Unless you are SpaceX that is.