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Union University Department of Physics

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Science Guys > September 2002

September 2002

How does a rocket work in space where there is no air to push against?

The Wright brothers probably never thought about flying in space where there was no air. They designed a propeller that pushed on the air to make their plane move forward. A rocket could have also moved the plane forward but rockets were not yet invented. Just what is the difference in a propeller and a rocket driven device. It is fairly common for people to have a misconception regarding the principle on which a rocket operates.

The force of friction propels ordinary objects such as cars and trains. The train pushes on the track and the car pushes on the road because of friction between the wheels and track or road. However, a rocket in space has nothing to push against. Therefore, the force of propulsion must be something other than friction. The rocket works because of the law of conservation of linear momentum.

The law of conservation of linear momentum is very important in physics. Momentum is defined as the mass of an object times its velocity. Simply stated the conservation law says that in a closed system (one without outside influences) the total momentum of the system remains constant. Now the momentum of various parts of the system may change, but the total momentum must always be constant.

Consider a machine gun mounted on a lightweight cart. If the gun is fired, the bullets go in one direction while the cart recoils in the other. The magnitude of the momentum of the bullets equals the momentum of the cart but the directions are opposite. Thus, one momentum is positive and one is negative, making the total change (their sum) zero. Although things are now moving, the total momentum of the gun-cart system has not changed.

In a similar manner, a rocket moves in space because the gases are given momentum as they are expelled by the rocket engine. Consider the rocket resting in space. There is no momentum in the system. Next, the engine ignites. As the exhaust gases go in one direction, the rocket goes in the other to keep the total momentum of the system constant. This momentum change of the gases gives the rocket the "push" to go forward. We call this push, the thrust of the rocket, i.e. the force exerted on the rocket.

This thrust depends upon the speed of the exhaust gases and the mass of gas being expelled each second, sometimes called the burn rate in pounds of fuel per second. On Earth, air tends to inhibit the exhaust gases getting out of the engine. This reduces the thrust. However, in space since there is no atmosphere, the exhaust gases can exit much easier and faster, thus increasing the thrust. Therefore, the rocket engine actually works better in space than here on Earth.