Two years. On average, there are two years from the start of construction to ignition for a launcher like Ariane 5. 24 months during which every move must be carefully performed and each part manufactured with pinpoint accuracy. At the end of the line, an initial error of just one micron (0.001 mm) in a seal adjustment can lead to a launch failure – it really is as easy as that.
Suffice to say that 24 hours from launch, tension is already riding high. Transported and assembled in French Guiana over the last month and a half, the launch vehicle arrives at the launch pad after a final inspection of its vital functions. After an hour and a half of hauling, it takes about 6 hours to connect the launch vehicle to the launch pad. This is a critical moment: at this very minute, Ariane 5 begins its estimated service life.
On the ground, the preparations gain momentum. Twelve hours from lift off, the electronic monitoring systems are operational and the teams have begun notably to implement the fluids. At T minus eight hours, new electrical equipment is brought to life. T minus seven and a half hours: inspection of the electrical system begins.
Next comes a critical step, unique to cryogenically propelled launch vehicles: the transfer of fluids (fuel and oxidisers) from a storage tank to the launch vehicle. There are numerous constraints. First of all, liquefied gasses must be kept at extremely low temperatures (as low as -250 °C). Then, since the launch vehicle’s engines cannot tolerate any impurities, it is necessary to take all sorts of measures to ensure absolute cleanliness during transfer. This means performing exchange circuit preparation procedures that are long and complex, yet essential nonetheless.
During the last few minutes before lift off, verification procedures intensify, under a four-part inspection by several separate teams: the on-site launch operators, the backup teams verifying the proper running of the operations, the technical authorities and experts who are on hand, ready to intervene if need be.
Satellites, telemetry tracking stations, air and maritime authorities give the green light. Gradually, the launcher is isolated from ‘ground support equipment’ and becomes independent. One by one, the safety barriers are lifted.
T minus six seconds: the transfer of controls. From now on, the launcher’s control unit takes over and gives the ignite command to the Vulcain engine. T minus zero: the rocket does not lift off. Not yet. The power generated by Vulcain is not enough for Ariane 5 to leave the ground (How Ariane escapes Earth’s pull). It is still necessary to verify satisfactory engine functioning for seven seconds. At this moment, there is still time to stop the whole process.
T + seven seconds: the powder boosters take over. They are responsible for lifting the 750 metric tonne launch vehicle out of the Earth’s gravitational pull. The rocket assumes its trajectory; the launch has been a success. But the mission is not over: It is still up to the launch vehicle to position its satellites correctly in space. But that’s another story...