One of the most impressive long term hobby projects is Robert's Rocket Project. He started building a 100 lbf liquid engine in 2001, fired a regeneratively cooled version in 2007, started building a regen 250 lbf in 2008. watzlavick.com/robert/rocket/…
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He built a test stand, he did regen design on a small aluminum engine from scratch, did most of the machining, designed the vehicle and recovery system and QDs. Just gradually made progress over decades and discussed decisions on the mailing list ARocket. freelists.org/list/arocket