We have now seen why rockets have a force acting on them when an onboard motor fires.
Let us take a closer look at the momentum of the system and how it is transferred. Imagine the same rocket with the motor firing. The rocket has a mass of . It releases a small part of that mass, called , over a short time span of . The momentum of the system at time is , where is the velocity of the rocket. When the small mass is accelerated from the rocket to a velocity of , the momentum of the system, that is the rocket with momentum and the momentum of the small mass , must be the same as at . This means that the rocket has lost some mass but gained velocity.
The change in momentum of the system, that is, the rocket and the small expelled mass, is
Since there are no external forces on the system, the change of momentum on the system must be zero, and hence . For small time periods , we kan write d’s instead of deltas:
Note that since the overall mass of the rocket decreases when is ejected.
Remembering Newton’s second law, and that the sum of the forces acting on the rocket equals the change in time of momentum, we can write
which gives us
Integrating both sides of this equation,
we have the following relation between change in velocity and mass:
where is the initial mass before the rocket motor starts to burn and is the final mass of the rocket after the burn stopped. The difference between the two, , is the fuel burned. This equation is often called the (ideal) rocket equation, or also sometimes the Tsiolkovsky rocket equation, after one of the scientists who first derived it.
In the integration of the equations we assumed to be a constant in time. It turns out that this is usually a very good approximation. The rocket motor takes in a certain amount of fuel and oxidizer and accelerates the molecules from the chemical reaction to a given velocity. Note that the actual exhaust velocity has a second term that comes from the pressure difference from the rocket pressure and the ambient pressure. We have neglected in a moment.
We must take a look at other simplifications we have done. We assumed that there were no other forces acting on the system. This means that the rocket equation is not valid on a launch vehicle since the gravitational and aerodynamically forces is non-negligible. Also we assumed that the burn happens in a short timespan. This is a good approximation for orbital manoeuvres of satellites, which we will discuss in the next chapter.
This article is a part of a pre-course program used by NAROM in different courses, for example Fly a Rocket!