super late to the thread but i hope you read this. I'm in the high power rocketry scene and liquid engines are out there in the amateur world but uncommon.

You don't see many flights though but there's plenty of test footage. I think one main reason for lack of flights is that Tripoli and NAR ( the two main self-governing bodies for high power rocketry ) currently do not insure bi-propellant rockets at their events. Another reason, is configuring the engine for flight is complicated because of the tanks and pressurization required. It seems like most amateurs run out of effort or money once they have a working injector, combustion chamber, and nozzle. The last mile ( tanks and pressurization ) seem to be out of reach mostly.

btw, if anyone has an engine they want to fly I will happily build you an airframe! :)