How to build a transparent hybrid rocket engine using acrylic rod and a canister of oxygen, with a breakdown of how various other types of rocket engines work along the way. Consider supporting my ...

Model rockets, in contrast, generally use solid rocket engines that mix the oxidizer and fuel into a single package, and they are ignited with heat.

So they used wax. You can see a video about the process, including a build of a model rocket engine, in the video below. The issue is the creation of tiny cooling channels in the combustion chamber.

One of the first issues Mueller had to address was how to build a turbopump. In the late 1990s, NASA had developed a rocket engine nearly as powerful as the proposed Merlin engine called Fastrac.