[pfdietz]

Reusability doesn't make engines costly. All liquid rocket engines are reusable. They have to be, so they can be tested before being put on the vehicle, and so during development you can keep reusing test articles.

The Merlin engines on the Falcon 9 are reusable (and on the first stage, they are reused). They are reported to cost SpaceX just $400K per engine to manufacture.

[m4rtink]

>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.

[skykooler]

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).

[pfdietz]

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.

[bernulli]

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.