[El Gato]

Just so everybody understands, it's the "push against" that you are (rightly) objecting to. The propeller or the jet don't "push against" the air, rather they move the air backwards and, by conservation of momentum, that causes the engine to move forwards. More properly it causes a force in the foward direction. The same as the recoil of a gun, which would "kick" just as much (well close enough) in a vacumn as it does down here at the bottom of the atmosphere.

[spunkets]

Re: propellers

"they move the air backwards"

The English word push, describes that action perfectly.

[MonroeDNA]

Exactly right.

If you throw a bseball backeards, you get moved forwards. Conservation of mommentum.

If the baseball hits a trampoline, and comes back and hits you, you move forward sitll, though the trampoline moves backwards.

Rockets fly in space (vacuum) because they throw things backwards (hot particles). Nothing to push against, but nothing needs to.

"For every reaction, there is an equal, but opposite reaction." Jet engines, rocket engines, and propellers work, giving a nod to this statement.