Question

A rocket does not have an engine fitted in it. How does it accelerates up wards?

Answer 1

Sure, air has nothing to do with it...
Rockets work by application of Newton's Third Law of Motion: For every reaction, there is an equal and opposite reaction. It doesn't matter whether that initial action happens in air, in water, in a vacuum, or, technically, in a solid. The gases shoot out the back of the rocket, the rocket is pushed forward by the reaction. That's how space vehicles maneuver in space (e.g., the space shuttle adjusting its orbit, a lunar lander orienting properly for landing, or a spacecraft braking to drop out of orbit toward a body).

No rocket actually needs atmosphere to work at all. Jets do, because they compress the gases coming in and combine them with the fuel. Rockets basically take a fuel and an oxidizing agent (liquid oxygen, most commonly with the big rockets) and mixes them in a combustion chamber with one exit, out the back in the direction opposite the one you want the rocket to move in.

So, once your hypothetical rocket leaves earth's gravitation field, it will continue to move on pretty much at the speed it was going when the rocket burn ended, as you said at the end. However, any additional force applied to it, by a conventional rocket, a solar sail, controlled fusion explosions, or any other propulsion technique, will push it one way or another.

Answer 2

thnx