Question

a rocket can move in air free space, but a jet plane cannot. why

Answer 1

A rocket is self-reliant, as inside the rocket both the fuel and the oxygen needed to burn the fuel are available.

Therefore, a rocket can move in air free space.

On the other hand, jet has fuel only.

It takes oxygen from the atmosphere to burn the fuel.

Hence, a jet plane cannot operate in air free space.

Answer 2

When the rocket is fired its fuels burn and exhaust gases are released (action force) and these exhaust gases apply a same force on rocket (reaction force). This helps it to move in air free because it doesn't need the air (oxygen) for burning fuel. But jet constantly burns it's fuel and it needs air (oxygen) and so a rocket can move in air free space, but a jet plane cannot.