How do rockets accelerate in space (or vacuum) in absence of any material to provide them the required reaction force?

– Xochielt Sanchez

Answer by Robert Frost:

Inside a rocket, there is a combustion chamber in which we ignite a fuel and oxidizer.  They burn, converting into a very hot gas that wants to expand, rapidly.  But the chamber is rigid and there is only one small hole, so the gas is ejected through that hole, out of the back of the rocket.

Newton's third law tells us that,

For every action there is an equal and opposite reaction.

Imagine you are on roller skates and you are holding a heavy cannon ball.  What happens if you throw the cannon ball in front of you?

If there is a force propelling the cannon ball forwards, there must be an equal and opposite force propelling you backwards.

But, you won't move backwards as quickly as the cannon ball is moving forwards, because you are more massive.  This concept involves momentum.

Momentum (P) equals the mass of an object (m) times its velocity (v).

Momentum of a system is conserved.  That means that without outside influence, the total momentum of a system is constant.  So, if you throw the cannonball and change its momentum by giving it a velocity in a particular direction, your change in momentum will be equal and opposite.

Tsiolkovsky came up with a rocket equation based on all of this:

The change in speed of a rocket is equal to the exhaust velocity of the fuel times the natural log of the initial mass of the rocket divided by the final mass.  The final mass is the initial mass minus the fuel that was ejected out of the rear of the rocket.

So, we can make a rocket go faster by either increasing the exhaust velocity (make the exploding fuel hotter) or by ejecting the fuel out of the back of the rocket faster.

How do rockets accelerate in space (or vacuum) in absence of any material to provide them the required reaction force?

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