Ioonos

Falling and moving forward at the same time

A launcher does not stop and let its cargo float back down again. It is just as much a catapult as it is a means of transport. It gives the payload both potential energy (altitude) and kinetic energy (speed). It is these two energies combined that enable a satellite, for example, to establish its orbit or trajectory. The point at which the payload is released by the launcher is called the “injection point”. For this reason the launcher’s trajectory must be extremely precise.

The launcher, combining power and finesse

Let’s take the example of a geostationary satellite that is given a speed that is one percent less than that planned: this corresponds, at a speed of 10,000 metres per second, to a difference of 100 metres per second. The satellite will not be able to keep to the trajectory required by its orbit, thereby falling around the earth, as planned. It will have to compensate by using its own tanks of propellant. However, this propellant is there to correct minor orbital perturbations throughout the satellite’s life. By using some of it to correct the launcher’s “mistake”, it will shorten its life expectancy by two years: this is unlikely to go down very well with whoever is paying for the satellite!

Target and arrow

You could compare the launcher to an arrow and the injection point to its target. However, it is not like firing a bow and arrow The launcher has to start with a vertical trajectory to gain height and then gradually tip over to give enough speed to the satellite to be “injected”. At the same time the ejection of the various stages and the cap covering the satellite has to be planned: not as simple as you might think! For this reason, the ideal trajectory, known as the nominal trajectory, is calculated before the flight. As actual conditions are sometimes different, the launcher is capable of making calculations to constantly readjust to the best trajectory. All of this to ensure that the satellite has a good start in life!