When NASA's Orion capsule made its maiden flight in 2014, it was easy to forget that only half of the spacecraft actually went into orbit. A dummy version of the European Service Module (ESM), which is still undergoing development, sat behind the the unmanned capsule as it lifted off from Cape Canaveral. That development has just passed a major milestone according to the main contractor, Airbus Defence and Space, with a model of the ESM's solar array performing "flawlessly" in a deployment test this week.

Solar arrays have been common on manned spacecraft and unmanned cargo ships since the first Soviet Soyuz and Progress missions of the 1960s and '70s. The purpose of these arrays is to provide primary electrical power to the craft and the inclusion of such a system on the Orion ESM, which is derived from ESA's Automated Transfer Vehicle (ATV), seemed like a no-brainer.

However, things changed when Orion went from being a replacement for the Space Shuttle to a deep-space craft. Instead of going into low-Earth orbit, Orion will be capable of executing a Trans Lunar Injection (TLI) boost to send it to the Moon and beyond. This puts much more stress on the solar array and the engineering has to take into account that the array deflects during TLI by as much as 106 cm (48 in) at the tips.

"A crewed mission to the Moon involves challenging requirements for the design and development of a solar array," says Arnaud de Jong, head of the Airbus Defence and Space Solar Array team. "To limit heavy stresses on the solar array due to the boost to the Moon orbit and back to Earth, the wing must be capable of angling 60 degrees forward and backward, like that of a bird. That broad movement meant we had to design the wing with thickened solar array panels and reinforced hinges and beams, which required extensive testing."

To test the effectiveness of this strengthening along with its resilience to acoustic, vibration, and shock, Airbus is subjecting a model of the array to a series of qualification exercises, including the successfully completed deployment test at NASA's Plum Brook Station facility in Sandusky, Ohio. The test array consisted of a yoke and three panels, and three dummy wings integrated with the test model of the Orion ESM.

When delivered in 2017, the solar array will consist of three panels with 1,242 gallium arsenide cells per panel. It will weigh in at over 260 kg (573 lb) and the near 15,000 cells will generate 11.1 kW.

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