How Rockets Work

In the late 1600s, English scientist and mathematician Isaac Newton developed laws that explained how and why things move the way they do. Although he never mentioned the rocket, the third of his three laws of motion describes how rockets work. It states that for every action there is an equal and opposite reaction. So if something is pushed forward, something else must move backward by an equal force.

When a gun is fired, the burning gunpowder forces a bullet forward from the barrel. Meanwhile, the gun moves backward, or in the opposite direction. The opposite movement is called recoil. This raises a question, however. If the opposite reaction is supposed to be equal, why doesn't the gun fly backward with the same force and speed as the bullet? The answer to this lies in Newton's second law of motion. This states that the more massive an object, the slower it is to accelerate. So it is harder to get a heavy object to move than a light one. Since the gun weighs many times more than the bullet, it reacts much less to the force.

If a gun shot out a continuous stream of bullets—as a machine gun does—the recoil would be continuous. This is what happens in a rocket. The gas molecules produced by the burning fuel act like trillions of tiny, individual bullets. As each one is ejected from the rear of the rocket, the rocket is moved in the opposite direction according to Newton's third law.

rocket nozzle combustion chamber rocket nozzle combustion chamber

The reaction of the gases rushing from the nozzle...

causes the rocket to move in the opposite direction.

The reaction of the gases rushing from the nozzle...

causes the rocket to move in the opposite direction.

Rockets are the simplest of all motors. A fuel is burned, and the gases produced are allowed to escape from a narrow opening. The reaction of this escaping gas causes the motor to move in the opposite direction.

In this painting from the early 1800s, a British warship is firing Congreve rockets at land-based targets. The British discovered that rockets were very effective when used against cities and forts.

This is not desirable in battle. The lurching causes wear and tear to the structure of the ship. Rockets, on the other hand, require only light, thin-walled launching tubes with collapsible wooden frames to support them. Unlike a gun, the rocket is not attached to the boat in any way. In a sense, it takes its recoil with it when it takes off. The British navy realized that this meant a very small boat could carry firepower equivalent to that of a large ship. The upstart new nation called the United States learned this very soon.

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