[hypercube33]

Possibility of corruption aside, these are clones of the Saturn 5 rocket engines and tech just scaled up - the whole thing is built for hydrogen which is clean burning (water is the byproduct) but you'd think SpaceX could take the design and make something for 20mil

[cratermoon]

The RS-25 is related to the J-2 used in S-II and S-IVB upper stages - a very successful engine. The STS program came up a new design based on a high-pressure combustion chamber running at 3,000 psi (21,000 kPa) for higher performance.

The first SLS flights will use available Block II RS-25D engines left over from the shuttle program, and when those run out (and if SLS is still flying) the rocket will switch over to the RS-25E, a cheaper, expendable version.

The F-1 engines used on the first stage of the Saturn V were built to burn Kerosene, and the RS-25 series has no common heritage.

[dotancohen]

  > The RS-25 is related to the J-2

How so? Other than sharing a fuel type, and thus surely things learned during J-2 development and operation influenced the RS-25 development and operation, so far as I know the two engines are completely different. Different cycles, different power packs (e.g. turbopump), wildly different packaging, different head pressure, different chamber pressure, throat, etc etc etc. I'm pretty sure that the J-2 did not cool the bell with the fuel, though I could be mistaken. Are the combustion chambers similar? What makes the two engines related, other than the fuel type and of course manufacturer?

[cratermoon]

I defer to http://www.astronautix.com/h/hg-3.html

A development of the J-2 using a high-performance high-pressure chamber engine. "Technology led to Space Shuttle Main Engines".

Edit to add: "In essence, the HG-3 concept eventually became the space shuttle main engine."

https://web.archive.org/web/20051115064042/http://history.ms...

[woooooo]

SpaceX is on record with not wanting to use hydrogen for the "pain in the ass factor".

It boils at a lower point than oxygen, so you have to insulate the tanks from each other, it's very.. undense (sparse?) per volume so you need bigger tanks, and it's the smallest molecule that exists and makes leaks and shipping harder.

Methane is slightly less efficient but way easier logistically.

[rblatz]

Don’t forget hydrogen embrittlement, a major risk in reusable rockets.

[wkat4242]

Yeah I bet if they could do without the LOX they'd do it too. But for the oxidiser there is no choice but to use a compressed gas, obviously.

[dotancohen]

There are some alternatives to O2, various molecules that contain oxygen which react readily. But those same properties that make them good oxidizers (namely, reactivity) also make them toxic and difficult to handle.

[woooooo]

Liquefied, not compressed. Pressure vessels are heavy, just liquefy it by getting it really cold. It only needs to perform for 10 minutes in booster stages.