Basic Physics of Rocket Propulsion
How Newton's Third Law and Conservation of Momentum Propel Rockets
Dec 16, 2008
There is an old myth that rockets need something to push against to move forward. The fact that rockets work in space far from anything to push against provides convincing evidence that this myth is incorrect. What then propels rockets forward?
Rocket propulsion can be explained equally well with either of two fundamental laws of physics: Newton's third law or conservation of momentum.
Newton's Third Law
Newton's third law states: "For every action, there is an equal and opposite reaction." The key to understanding the concept behind these words is action reaction pairs. If A applies a force on B, the reaction must be that B applies an equal but opposite force on A. There can be no third object involved.
In a rocket engine some type of (usually) chemical reaction spits the burned rocket fuel out of the back of the rocket. Via this chemical reaction, the rocket exerts a strong backward force on the burned rocket fuel. According to Newton's third law the required reaction is that the burned rocket fuel exerts an equal forward force on the rocket. This force accelerates the rocket forward.
Newton's third law explains the rocket's forward propulsion. Because Newton's third law says nothing about pushing against something, the rocket does not need to push against anything to accelerate forward.
Law of Conservation of Momentum
An object's momentum is its mass multiplied by its velocity: momentum equals mass times velocity. Momentum, like velocity, is a vector quantity. It includes direction. If an object changes the direction of its motion, its velocity and momentum both change.
The law of conservation of momentum applies only to isolated systems, which have no external forces acting on them. Momentum conservation states that the total momentum of an isolated system must remain constant. Physicists say that any quantity, such as momentum or energy, that must remain constant is conserved.
A rocket sitting on a launch pad or at rest in space has a zero velocity and total momentum. If the rocket and the fuel inside the rocket is an isolated system, then the total momentum of the rocket and fuel must remain zero as the rocket launches.
When the rocket ignites, violent chemical reactions in the rocket fuel thrust the burned rocket fuel out the back of the rocket at a high rate of speed. This burned rocket fuel has a large backwards momentum. However the total momentum of the rocket fuel system must be conserved and remain zero. If the burned fuel has a backwards momentum, the rocket must have an equal forward momentum. The rocket must accelerate forward to get the needed forward momentum.
The backward and forward momenta add up to zero because momentum is a vector. The two momenta have opposite signs. If the forward momentum is positive, the backward momentum is negative. The equal positive and negative numbers add to zero.
The rocket has a forward momentum so that the rocket and fuel system keep the zero total momentum when the burned fuel has a backward momentum.
These principles apply to any rocket from a toy water rocket to the launch of the space shuttle. They also apply to more than rocket propulsion. For example, a gun recoils when fired because of the same principles.
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