Tested at NASA-MSFC, a LOX/LH2 rocket of 20,000 lbs thrust with a 3d printed injector head.
Using traditional manufacturing methods, 163 individual parts would be made and then assembled. But with 3-D printing technology, only two parts were required, saving time and money and allowing engineers to build parts that enhance rocket engine performance and are less prone to failure.
I can see pluses and minuses here. The obvious plus is that a printed part can be hideously complex, geometrically, and thus performance can be high and weight very low. But the parts count thing *might* work against it. Especially if the goal is a reusable engine. Because if there’s any damage… well, ya gotta toss the whole thing.
Years ago I designed and tested a series of increasingly capable liquid rocket engines that used off the shelf spray nozzles in the injector. Weighed more, sure. But the injector head itself was, eventually remarkably simple to design and machine out of simple aluminum, and the injectors could be simply threaded in and out. Easy.
But had I been able to simply 3d print a regen-cooled nozzle and/or combustion chamber… wow, would that have been handy!