The challenges of spacecraft assembly, integration and test (AIT)

Every SSTL satellite undergoes meticulous inspection and thorough testing before it is launched into space, in order to withstand the tough conditions outside of the Earth's atmosphere.

SSTL's spacecraft are designed around a stack of module trays with each module performing a different function for the mission, such as power management, navigation, on-board computing, communication and propulsion.

Modules are assembled in the Flight Assembly cleanroom, one of a suite of cleanrooms housed in SSTL’s Kepler Building, and then tested for functionality and performance by design engineers in the huge first floor Laboratory, before being delivered into the Assembly Integration and Test Hall. Here the module trays are handed over to the AIT engineers and the process of stacking them in the structure, connecting them together, and attaching the solar panels, to form the flight-ready spacecraft begins.
  When Space Blog visited SSTL, a DMC3 spacecraft and the Kaz-MRES spacecraft were visible in the AIT Hall, undergoing assembly and testing.

Space Blog spoke to Steve Forster, Senior Assembly Integration and Test Engineer, to get a better understanding of the procedures that lead to a spacecraft being given SSTL’s ‘Space-ready seal of approval’. In this Blog we’ll cover the process up to what’s known as the ‘soft stack’ phase.
 

Steve explains that the Assembly, Integration and Test (AIT) campaign for every SSTL spacecraft starts with the flight harness - the craft’s complex series of wires, interconnections and connectors - and the Electrical Ground Support System Equipment (EGSE) that forms the ‘mission control’ for the testing process. To manufacture the harness to an exact fit, a wooden replica of the spacecraft, affectionately referred to at SSTL as ‘WoodSat’, is made. Then the thousands of wires and connectors that make up the spacecraft’s flight harness and which will ultimately link the spacecraft’s individual modules together are overlaid onto WoodSat. This process safely allows the wires to be cut to exact lengths and routed correctly around WoodSat in readiness for transferring the flight harness to the real spacecraf.

 

 Once the module trays have been assembled and delivered into AIT, the ‘soft stack’ phase can begin where the spacecraft sub-systems are assembled into a ‘Flat Sat’ configuration and functional and performance tests are conducted to verify the interfaces between the modules. Tests include ambient pressure thermal cycling, conducted within SSTL’s 125 cubic metre walk-in thermal chamber, which can be used to check the workmanship, function and performance aspects of the spacecraft by subjecting it to temperatures ranging from -30 to +70 degrees centigrade in less than 90 minutes if required. During the final stages of the ‘soft stack’ phase the team then assemble the spacecraft using the flight structure, all the way up to mating the solar panels. 

 However, this is just a dress rehearsal and makes the spacecraft strong enough to handle but not fully ready for flight : the fasteners are only partially tightened and don’t have yet have adhesive applied – a process known as head staking.