These from the ESA info pack prepared a few days ago - we had a discussion last night on the Ariane launch capacity; the thread starts here. -- Daniel
Why is the ATV launched on an Ariane 5ES instead of an ECA?
The ATV missions require several ignitions of the upper stage, first to circularize the orbit and then achieve a safe re-entry of the stage. At this time, such multiple ignitions cannot be done with the ESC-A stage as the HM-7B engine cannot be restarted. A re-ignition test on a similar upper stage is planned on an Ariane 5 mission prior to ATV launch.
Are there new requirements on the launcher (hardware and software) as the ATV is more than twice heavier than any payload composite ever flown on Ariane 5?
The main improvement is the introduction of a reinforced equipment bay structure to withstand the loads. The lower part of the launcher remains unchanged. In addition, numerous tests have been conducted on the EPS upper stage’s Aestus engine. As usual for any Ariane 5 mission, mission-specific flight software will be used to take in consideration the different centre of gravity and other specifications.