At recent induction day I was shown around the labs at a satellite company. The academic giving the tour was talking about the James Webb telescope (they were working on testing some component that is going to be part of it) and mentioned in passing the difficulty faced designing a telescope that unfolded in space. I didn't question it at the time (stupidly) and have since been wondering what difficulties they face? Is the 'unfolding' a particularly complex process? I understand that they have to get it right before it launches as they will be unable to carry out maintenance.
Moving parts are always an issue in space. With no gravity things don't move how you expect. With no air things stick together - two very precisely machined surfaces will actually weld into a single piece of metal. Small amounts of air left behind will expand and cause mechanisms to stick. Extreme temperature differences between parts cause them to stick as do extreme temperature changes as parts go into sun/shadow
It is also impossible to test many of these structures on the ground since they can't support their own weight in gravity.