The force that propels a rocket in a vacuum is Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction.

Here's how it works:

1. Rocket Engine: The rocket engine burns fuel, creating hot, expanding gases.

2. Exhaust: The gases are expelled out of the rocket's nozzle at high velocity. This is the "action".

3. Reaction: As the gases are expelled, they push against the rocket in the opposite direction. This is the "equal and opposite reaction".

No Air Needed: This process does not require air. The rocket carries its own fuel and oxidizer, so it can operate in the vacuum of space.

Key Point: The rocket's motion is not caused by pushing against anything (like air). It's caused by the momentum of the exhaust gases being transferred to the rocket.