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Home | Update | LATEST ISSUE | Gallery | FR Profile | Datafiles | FR 8/98 ARIANE 5 PROBLEMS SEEM TO BE SOLVED by Christopher Hess The most important event in the recent history of European spaceflight is coming closer. The third qualification flight of of Ariane 5 will decide about the future of the European launch service Arianespace. Should the flight fail, all of the company's improvement programs for the launcher will be academic. It would take years to restore the customersÕ trust into Ariane 5. The evaluation of the second flight from 30 October 1997 gives all the reasons for optimism, even though there were some anomalies encountered during the flight. The flight went according to plan until the solid fuel boosters separated from the main stage at an altitude of 55 km. At that point, the launcher started clockwise rotatio. The torque was later measured at around 900 Nm. The attitude control system which is installed at the vehicle equipment bay was not strong enough to counteract this torque. The rotation of the rocket stabilized at 33 degrees per second (six rotations per minute). Today, this anomaly can be traced to the function of the Vulcain main cryogenic stage engine. The inside of the nozzle consists of 256 small spiral cooling pipes which are welded onto the inner surface of the nozzle. Obviously, the roughness from the welding effected the main jet of the engine thrust. This effect was proven on the test stand of engine manufacturer SEP in Vernon, France. A specially designed torquemeter registered approximately 775 Nm torque generated by the main engine jet. The reason why this force was only effective after booster separation is that, during lift-off, only ten percent of the total thrust are generated by the Vulcain engine. 90 percent of the thrust are coming from the solid fuel boosters. The engine tests in Vernon also revealed that that the oxygen and hydrogen turbine exhausts next to the Vulcain's nozzle are also generating a considerable amount of torque, depending on their position. This torque can reach up to 1000 Nm. However, this force will be used in the next flight to counteract the torque coming from the roughness inside the nozzle. Furthermore, the launcher will be fitted with two extra sets of the attitude control system with two roll control nozzles each. All together, Flight 503 will have enough attitude control authority to counteract three times the torque encountered during 502. Should flight 503 prove that the turbine exhaust positioning is enough to counteract the roll torque, the extra attitude control system components will be omitted in later flights. According to Andr_ van Gaver, head of ESAÕs Ariane programm office, the rolling motion of the rocket also led to a premature shut off (approximately seven seconds) of the main stageÕs Vulcain engine which could not be fully compensated later by the flight of the upper stage. This problem led to an apogee of only 26600 km instead of the required 36000 km for GEO. The roll anomaly made the fuel spin in the tank accordingly, forming a concave surface within the tank. The centrally located quantity sensor most probably sensed a fuel low condition too early, consequently leading to the premature engine shut-off. For this anomaly, van Gaver does not see the necessity for any corrective measures since the problem will be solved accordingly when the roll torque problem is solved. The booster separation showed yet another problem. Originally the two boosters were to be recovered after a controlled reentry and landing with a parachute recovery system. However, the recovery system failed and the booster were lost. Probable reason for this incident was a cover on the forward skirt of the boosters which is not designed strong enough to withstand the loads during reentry. It looks like the covers got damaged, subsequently exposing wiring, electrics and pyrotechnic equipment of the recovery system to the strong aerodynamical loads. To correct this situation in the next flight, the position of certain electric, pyrotechnic as well as transmission lines will be relocated to protect the recovery system. Two other flight abnomalies concern the above specification acoustic levels under the nose fairing and a higher than anticipated impact level at the payload interface. The first of the two will be corrected by an additional water injection manifold on the launch pad during the ignition phase. An additional damper ring will be installed in the vehicle equipment bay of future flights to dampen the impact levels until a redesigned VEB is ready in 2000 under the Performance 2000 improvement program. It seems that the acute technical problems of Ariane 5 are under control. The tension before the third flight is still high, also because of the long pending decision for a commercial payload. This decision has now been made. Due to a fire is the W2 satellite, which flight 503 was supposed to launch for Eutelsat, not available any more. A mock-up, built by the German supplier Kayser-Threde, will be used instead as the second payload along with ESA's Atmospheric Reentry Demonstrator. This means that there will be no commercial payload on board flight 503. André van Gaver from ESA knows: "We are qualifying Ariane 5 under bad conditions." From page 40 of FLUG REVUE 8/98 Home | Update | LATEST ISSUE | Gallery | FR Profile | Datafiles | FR 8/98 Copyright 1998 by Motor-Presse Stuttgart. All rights reserved. Last updated July 13, 1998 FLUG REVUE, Ubierstr. 83, 53173 Bonn, Germany