3, 2, 1...blastoff!

by Jeanette Cain

Countdown. The final stage begins when separate components reach the launch center and are assembled into a launch vehicle. Ariane 5 schedule begins 21 days before final scheduled liftoff. The last countdown begins 6 hours before the scheduled liftoff time. Engineers stay steadily busy preparing for the final countdown. Their priorities are to make the site ready, and to make certain that all personnel had been removed from the launch area. The preparations needed to synchronize event sequences begins about an hour before liftoff. This is the beginning of the well-known 10, 9, 8...phrase.

In Kourou of French Guiana, the Jupiter control room oversees the liftoff of Ariane 5. This task belongs to 3 teams, which monitor the launcher status, the payload, and the tracking station. When weather and safety personnel work in other areas, the tracking station will follow the liftoff. The director of operations will only authorize final countdown when all reports show a status of green.

The upper stage of the Ariane 5 rocket began its journey in Bremen, Germany. It was transported by a ship carrying its parts, while other components were shipped along the rivers of Europe. From Europe's rivers they were sent to Rotterdam (Holland), or to Le Harve (France). At one of these destination, they unite with the ship headed for Kourou. It takes about 11 days for the crossing from Le Harve to Kourou.

Ariane 5's payload integration is begun when the parts and components arrive. Satellites and the protective nose are mounted on the launcher. Final assembly only begins about 8 days before the scheduled liftoff. Satellites are linked to Ariane 5 and then to the Jupiter control room, which assures careful monitoring of the payload during the final stage of countdown.

A launcher is supported by a 959 ton launch table during the assembly. A truck begins towing the launcher and table the day before scheduled liftoff. It is moved to the launchpad on railroad tracks. The combined weight of the truck and table is 1,650 tons, or equal to the weight of 1,500 cars. Once reaching the pad, the propellant, composed of fuel and oxidant, will be piped into the launcher.

The launch area has three trenches that provide room for the flames coming from the boosters and main engine during liftoff. Water, supplied at 30 cubic meters per second, is from a tower. It is also needed to reduce the noise at liftoff and to cool the trenches. If the water were absence from this stage, the vibrations produced by the noise could cause serious damage to the launcher and its payload.

The launch pad area will be ready 6 hours before scheduled blastoff. During this time, the flight program will be loaded into the two computers onboard. The program will be tested to check radio links between the ground and the launcher. The main stage tanks will be filled with propellant five hours before liftoff. The synchronized sequence of events leading to liftoff will begin six minutes before launching.

Countdown follows the steps below:
-360 second - synchronized sequence to main ignition is initiated;
-30 second - opening of values to flood the flames with water;
-13 seconds - authorization given to onboard computers to take over;

Main ignition - ignition sequence is begun for main engine; its operating status is checked; lastly, the two solid-fuel rocket boosters are ignited;

Main ignition +7 seconds - Liftoff.
Sources:
1. Editors. Secrets of the Universe. International Master Publishing: US. 1999

2. Couper, Heather and Nigel Henbest. Space Encyclopedia DK Publishing Book: NY, 1999