[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...