XMM spreads its wings for the last time on Earth
18 August 1999After a successful second System Validation Test between 20 and 30 July - during which all aspects of the XMM spacecraft including its science instruments were controlled in real-time from the XMM Mission Control Centre at ESOC in Darmstadt as if the satellite was already in orbit - XMM has deployed its solar arrays, another important milestone before the spacecraft is packed for shipping to the Kourou launchsite.
After a satellite is released into space by its launcher, project teams are often apprehensive when the craft's solar panels are about to be deployed. It is a crucial moment: without the power generated by these arrays, the satellite could die starved of power once the batteries are drained.
XMM is equipped with two solar arrays, provided by Fokker Space BV, Leiden, the Netherlands. They are of an advanced design (ARA Mk-III), using silicon solar cells, which have been well proven on previous successful space missions.
Folded for launch but with a deployed span of 16.16 m. the arrays are often referred to as "wings" because they extend symmetrically each side of the satellite at the level of its service module. Each is composed of three hinged panels covered with solar cells, small chips that convert sunlight into electrical energy. Each panel is the size of a comfortable king-size bed (1.81 by 1.94 metres) and bears 1240 solar cells arranged in four sections.
The deployment test taking place at ESTEC on 9 August followed many that were carried out at Fokker but was the first ever conducted with the solar panels fully integrated onto XMM, with all their interfaces connected to the spacecraft and after the acoustic test to which the complete satellite was submitted on 6 July.
Each wing has a mass of 40 kg. To simulate the conditions of weightlessness, the solar arrays were extended using two 'Zero-G' deployment rigs on which the panels were suspended. These compensate for gravity with calibrated spring supports, moving on air cushion bearings to eliminate friction.
The test reproduced the release of the folded panels triggered by the automatic on-board sequence. Pyrotechnic units, provided by Sener SA. Madrid Spain, first activate four thermal knives on each wing which cut the tensioned Kevlar cables holding the panels in their stowed configuration. Spiral springs and delicate mechanisms then allow a smooth and shockless deployment.
The deployment sequence is in two steps: counting from the separation signal after the satellite is release by Ariane, the first wing will deploy 31.5 minutes later, followed by the second wing after 39 minutes. Each wing takes 4 minutes to be fully extended.
Members of the XMM project team and prime contractor Dornier watched in the clean room as XMM gracefully deployed its wings. The entire test was closely scrutinised and filmed for later analysis whilst all the test data was recorded. Everyone imagined the day, now not so far away, when XMM will next stretch its wings but for real, in orbit.
Both solar arrays will feed the satellite with a total power of 2200 Watts after launch and at least 1600 Watts at the end of an extended mission of 10 years. The power is delivered to the spacecraft via 24 independent power lines, corresponding to the 24 sections of the arrays. XMM's power subsystem then provides 28 Volts to the different equipment aboard the satellite. To cover the eclipse seasons, when XMM will pass in the shadow of the Earth, the satellite is equipped with two Nickel-Cadmium batteries (24 Ah) procured from SAFT Poitiers, France.
Fokker Space BV has over the past 30 years been Europe's leading manufacturer of solar arrays for telecommunications, meteorology and scientific spacecraft, for example ECS, Marecs, Olympus, Hipparcos and SOHO. On-going ESA projects to which Fokker is supplying solar arrays include Envisat, Integral, Rosetta and the ATV. The first ever satellite to be equipped with solar panels by Fokker was ANS, the first Dutch national satellite, launched in August 1974 which carried an ultra-violet and an X-ray astronomy payload.