Is there any classic aviation/space engineering moment where the engineers had been working on a concept for months and then realised that their approach to the problem wasn't quite right and so had to discard all their hard work and re-start from scratch?
In 2018 SpaceX determined that carbon fibre composite material was not going to be suitable for their new BFR rocket due to difficulties with material resilience in different temperature environments and cost.
They switched to a Stainless steel design instead and abandoned 2 years of work and tens of thousands of manhours. This had involved building and testing a huge carbon fibre pressure vessel and some large carbon fibre test segments. The giant carbon fibre winding tool which had been brought in specially for BFR was also scrapped.
The Gemini inflatable wing concept took around 4 years and 165 million dollars, and was discarded right at the end, just as they were ironing out the very last issues. The parachute system was used instead, and the wing never flew.
The manned manouvering unit was only flown three times before being replaced by the robotic arm due to safety concerns.