# The rocket principle

Figure 1: The rocket principle is Newton’s third law in action: Pushing an object backwards will push you forwards.

Before we go into the technical details of how a rocket motor works, let us see how a rocket or a satellite accelerates.

Let us assume that a satellite has a powerful chemical motor, which expels hot gasses in a certain direction when it fires. Newton’s second law states that the motor has exerted a force on the gas molecules to accelerate them, and that the size of this force is given by .

Newton’s third law states that for every action, there is an equal and opposite reaction. Hence there must be a counter force equal in size, but oppositely directed, force on the satellite from the hot gas molecule.

So the gas molecule, which has a very small mass compared to the satellite, is accelerated to very high velocities in one direction, and then the satellite experiences an acceleration in the opposite direction. Since the mass of the satellite is so large however, the acceleration the satellite experiences is very small.

When the force is summed up on all the gas molecules, the acceleration can be substantial on the order of many thousands of kilo-newtons. Currently (anno 2020) the most powerful rocket motor ever flown were the solid boosters of the Space Shuttle, clocking in at 13 800 kN each. However, more powerful motors are in development.