How does it work?
Did you say nozzle?
There is something about hosepipes that you probably already know: if you do not hold the nozzle firmly then you are likely to get wet! A rocket is somewhat similar: you have to control its thrust or, like a balloon that is released without tying a knot, it will zoom around all over the place.
nozzle aerospike
Hourglass or funnel?
There is a key element in a rocket engine called the nozzle. It is a kind of funnel that can be seen at the very end of the thrusters, made of very resistant material, which enables the energy from the burning gases to be transformed into kinetic energy* or, in other words, movement. In fact, this funnel has more of an hour glass shape: first it becomes narrow and then opens out. It is said to be convergent-divergent: this enables gases to accelerate, but to remain below the speed of sound (1,000 km/hour) in the first part, and then to exceed the speed of sound in the divergent part. Being less compressed, the gas reduces its pressure and gains in speed and power.
Much more than a rudder…
As well as playing an important role in propulsion, the nozzle also enables the rocket to be steered. It is usually equipped with very powerful hydraulic jacks which enable the rocket to be directed in relation to two axes: left/right and straight/tilted. This is how the rocket is made to go straight up into the sky for the first few minutes after lift-off and then begin a more horizontal trajectory once perturbations associated with the atmosphere have disappeared with altitude.
Rockets are not the only things in space
A nozzle is a vital element of any object that has to move in space independently of the forces associated with the orbital trajectory. So satellites also have nozzles because, as we have seen, they have to regularly correct their trajectory in order to remain in their orbit plane. Usually much smaller, their nozzles enable them to adjust their direction, although steering a satellite is not a simple matter! (see Do satellites need steering?)
