Maybe not open source, but an open hardware project. The particular problem for SpaceX is that a lot of the IP is covered by ITAR: https://en.wikipedia.org/wiki/International_Traffic_in_Arms_... See this FAQ entry: https://www.reddit.com/r/spacex/wiki/faq/company#wiki_what_p...

But in theory it's possible, e.g. Facebook's Open Compute Project, it just needs the right kind of funding model.