It sounds like this is a regen cooled engine and the pressure in the coolingWhy seal the injector? The pressure will build up on the backside of the injector regardless (key point for good atomization). Yeah, you may lose a bit of pressure through the cooling channels until you reach the injector, but OTOH having ambient pressure in the chamber/nozzle vs. your operating pressure, will only increase the pressure differential. The nozzle and cooling jacket must obviously be designed to withstand any transients during engine start-up (like water hammering effect), as well as softening from the pre-heat stage. Do we have any design info? This is an open-source project they claim ...passages caused the chamber to collapse inward. It's the differentialpressure between the cooling passages and chamber that causes this so in order for a hydro test to catch this you would have to seal the injector somehow. A normal hydro test where you plug the nozzle wouldn't catch it. It's a hard test to do and they are not the first group this has happenedto.
/Henrik