>All liquid rocket engines are reusable. As always there are exceptions - rs-68 has an ablative nozzle that is rated for a single flight, so you need to test without that I guess.

Other engines could be similarly rated for jinutes of runtime by default for maximum performance. SpaceX aldo repeatedly ststed Merlin and Raptor were designed for cheap and fregvent reuse.

Yes, Merlin was designed to be cheap. That's why it's cheap. Being reusable was not an impediment to it being cheap. Being reusable is not an excuse for being expensive.

A cheap rocket engine needs a cheap development process, which means you want your test articles to last a long time. Far from being an obstacle to cheapness, reusability is a requirement for an affordable engine development program.

Let's go way back to Usenet and listen to Henry Spencer on this subject.

https://yarchive.net/space/rocket/rl10.html

"To reiterate my original point: almost any regeneratively-cooled liquid rocket engine is reusable, even if it was built for expendable vehicles. The design requirement for the F-1 was 20 starts and 2250s of firing; as part of the test program, six of them accumulated over 5000s each."

The Merlin engines don't run hydrogen, though. Because of hydrogen's propensity to embrittle metals it comes into contact with over time, it's much harder to design a hydrogen engine for long runtimes (vs. say, a one-minute static fire plus a eight-minute flight).

Any hydrocarbon engine will have hydrogen in the thrust chamber, and in the gas generator. They all run a bit fuel rich to maximize Isp, and that will mean the hot partially burned gas has some molecular hydrogen in it.

What makes hydrogen engines more expensive is the low density of LH2, which greatly increases the pumping power needed to bring the propellant up to pressure for injection into the thrust chamber.

Reusability is not binary, saying 'all liquid rocket engines are reusable' is pointless. It's like saying surely a Formula 1 car is equivalent to a Tacoma in terms of 'reusability', because both can be restarted. Spoiler: they're not.