XMM has its ears full of noise
8 July 1999
How might it feel to stand but a few metres behind a 747 Jumbo jet with its reactors at full thrust for takeoff ? A human being would probably not survive - at the very least his eardrums would be shattered !This is the kind of ordeal that XMM has just experienced. On the 6th July, ESA's X-ray space observatory successfully passed its last major and crucial environmental test before launch. The fully assembled spacecraft standing 10 metres tall, now bearing its solar arrays and telescope sunshield, was submitted to a barrage of sound, simulating the conditions it will encounter during the first 60 seconds after liftoff.The test took place in the LEAF (Large European Acoustic Facility) test chamber at ESA's ESTEC technical centre in Noordwijk, Netherlands, where the spacecraft modules had, over the past weeks, completed other environmental tests (vibration and thermal) before being assembled on 26th May.
The noise produced at liftoff by an Ariane 5 is already significantly reduced by different means. A deluge of water floods the launch table and pad, preventing the sound from bouncing back up towards the rocket as it rises. The inside of the fairing, in which the spacecraft is positionned, is also lined with acoustic insulation. Nevertheless the noise levels are considerable. Satellite design and construction have to account for this phenomenon or else face the consequences of potential damage to a satellite's structure and onboard equipment.
For the acoustic test, XMM had been + wired up ; with some 80 accelerometres over its surface to measure the slightest effect of the intense noise it will really experience in only a few months time. Once the thick concrete doors of the test chamber were closed - and from which for obvious safety reasons not a whisper emerged - XMM suffered the onslaught.
Operations were directed from an adjacent control room, where technicians first needed a short trial run to set the correct noise levels, measured inside the chamber by microphones. When the installation was perfectly calibrated, the huge horn-shaped loudspeakers in the chamber walls projected a spectrum of low and high frequencies (30-2000 Hz) of increasing intensity upon the spacecraft. For about a minute, XMM had its ears full with 140dB of noise.
In a few minutes, it was all over. The spacecraft could breathe again ! The data obtained from the accelerometres indicated that XMM had passed this last ordeal with flying colours - and a day ahead of the original schedule.
Technicians from prime contractor DaimlerChrysler Dornier and the ESA XMM project team can now proceed to the next stage of the spacecraft's preparation. Further tests will be conducted on its electrical systems and a full deployment test of its solar arrays will take place in August. There will also be ground system trials when the spacecraft, still in the ESTEC clean room, will be controlled from the XMM Spacecraft Operations Centre in Darmstadt.
Meanwhile in Vevey Switzerland on the shores of lake Geneva, the APCO company has formally delivered the spacecraft's giant container in which it will be shipped to French Guiana. Next week the container will be leaving APCO by road for Basle where it will be transferred to a barge upon which it will travel down the Rhine to Holland. The container should be arriving at ESTEC on 19th July.
After final verifications, it will be time for the spacecraft to pack its bags. XMM is due to leave Rotterdam on 13th September and liftoff is still scheduled for 15th December next.
