Airbus Defence and Space

How Ariane escapes Earth’s pull

How is it possible for a launch vehicle, weighing over 750 metric tonnes and measuring over 45 metres tall, to break away from the Earth’s gravitational pull? How powerful are the engines? At what speed does the rocket travel and how is it guided? Let’s take the example of Ariane 5to explain this technical feat.

An incredible amount of force is required to prise the rocket from the clutches of the Earth in order to send the satellites it is carrying into orbit. At lift off, Ariane 5 generates 1,300 metric tonnes of thrust. This power is provided by liquid and solid fuel engines. The liquid fuel engine functions according to the same principles as an aeroplane engine. The only difference is that oxygen, which becomes scarce at high altitudes and eventually disappears in space, must be compensated with liquid oxygen. Liquid oxygen is then mixed with hydrogen in the combustion chamber, producing a very high pressure gas. In this case, we are dealing with cryogenic engines like the Vulcain on Ariane 5’s cryogenic main stage (EPC). However, this stage is insufficient for lift off. It needs the help of powder acceleration stages (EAP). These auxiliary thrusters, simple metallic tubes attached either side of the rocket, contain powder, a solid fuel quite similar to fireworks. These actually supply over 90% of the thrust needed for lift off. Outfitted in such a way, the rocket can leave the ground.

After just two minutes in flight, the launcher has gained about 65 km in altitude. The auxiliary thrusters, having consumed 480 metric tonnes of powder, are then cast off and fall to the ocean where they eventually sink. This leaves only Vulcain propelling the launcher. One minute later, at an altitude of 105 km, the rocket fairing that protects the first satellite is in turn cast off. At this height, air friction is negligible and the rocket loses almost one metric tonne. Ten minutes after ignition, Vulcain switches off and the main stage detaches. All that remains is the last part of the rocket, the payload compartment, housing the satellites and the cryotechnic upper stage (ESC) containing the navigation computer and all the trajectory data. Rockets are not in fact guided from the ground, but rather autonomously. In the event of an unsolvable problem, an autodestruct system can be activated, either automatically or manually.

The final stage continues its progression. The rocket is propelled by a single engine that has yet to be used during the launch vehicle’s climb into the atmosphere. For over 15 minutes, it climbs to up to 650 km in altitude. Its speed reaches over nine km/s (roughly 32,000 km/h) before cutting out. Once underway, it reaches 1,000 km and finally releases its first satellite. Five minutes later, or about 30 minutes after lift off, a second satellite can be released in the case of a double launch. Everything has gone according to plan. Mission accomplished.

Ariane 5