When an LE-7A engine suffered a hydrogen leak during a test firing in July 2000 necessitating a redesign that would postpone the demonstration flight of Japan's H-2A to July 2001,38^9 the European Space Agency reassigned its Artemis communications satellite to the Ariane V for launch in the summer of 2001. This did not pose any problems as Artemis had actually been designed for the Ariane V.40 Unfortunately, when it was launched on 12 July 2001 the Aestus upper stage malfunctioned and stranded the satellite in the wrong orbit. This was a near-replay of the test in 1997 that failed to achieve the desired transfer orbit. A combusion instability at engine ignition had reduced the thrust, and resulted in the early
depletion of the propellant. The strong pressure variation that was responsible for this instability was attributed to a dynamic coupling between the propellant feed and the internal parts of the combustion chamber. The most probable cause of the high-frequency instabilities was the presence of water vapour in the propellant lines. The investigation recommended that the hydraulic conditions be dynamically modelled mathematically and the ignition phase made steadier and smoother.41 A new ignition system was developed, and the procedures revised to improve the regulation of humidity within the engine during its preparation.42*43 On the positive side, the exhaustive tests confirmed the robustness of the engine in nominal conditions. By the end of 2001, it was hoped to resume Ariane V operations in early 2002, and on 1 March 2002 one placed Envisat into Sun-synchronous orbit, marking the first time that this vehicle had aimed for polar orbit.44
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