This answer mentions that the J-2 engines used by the Saturn V's second and third stages were substantially more resilient than the "extremely complex and sensitive" Space Shuttle main engines. What led to the difference between the two engines? Was it just the nature of going from a gas generator cycle to staged combustion?

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    $\begingroup$ I noticed that we have a j-2 tag for a few questions with no description, so I submitted an edit to add a description. $\endgroup$ – DylanSp Jul 8 '19 at 23:26
  • $\begingroup$ And as a related question, How would the STS have differed if it used J-2's as the main engine? $\endgroup$ – DylanSp Jul 8 '19 at 23:33
  • $\begingroup$ good, edit approved. now it won't be mistaken for J₂ again. $\endgroup$ – uhoh Jul 9 '19 at 0:26
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    $\begingroup$ I don't think "fragile" is a good description of a machine that withstood a 6000 deg F temperature gradient and 6000 psi pressure gradient, and flew 135 x 3 flights without a mechanical problem.. How often did J-2s get re-used? This seems like a false premise to me. $\endgroup$ – Organic Marble Jul 9 '19 at 1:04
  • $\begingroup$ @OrganicMarble I'm definitely open to suggestions on better wording. I thought about "brittle", but that doesn't really fit either... $\endgroup$ – DylanSp Jul 9 '19 at 1:06

A number of factors contributed to the complexity. Staged combustion was definitely a big one. The SSME also ran at a much higher chamber pressure than the J-2, 3000 psi vs 760 psi. This seems to have required two turbopumps for each of the propellants instead of the usual one -- there are low- and high-pressure turbopumps for the fuel (LPFTP, HPFTP) and low- and high-pressure turbopumps for the oxidizer (LPOTP, HPOTP). Since turbopumps are one of the biggest sources of headaches in large engines, doubling their number had a predictable effect on maintenance.

  • $\begingroup$ What advantages did the higher chamber pressure give, better Isp, more thrust? $\endgroup$ – DylanSp Jul 8 '19 at 23:41
  • $\begingroup$ The net result was better Isp. Thrust to weight was about 73:1 for both the J-2 and SSME. SSME was twice the thrust and twice the weight. $\endgroup$ – Russell Borogove Jul 8 '19 at 23:47
  • $\begingroup$ @DylanSp higher chamber pressure generally gives you better thermal efficiency. With any internal combustion engine, basically, you can only extract the ideal amount of energy from the fuel if you burn it at infinite pressure – of course that never happens, but the RS-25's pressure of 200 bar gets you pretty close. As a result of that extra energy, the exhaust can be made to exit faster, which is what gives you the better Isp. (For a given engine, increasing the pressure will also increase the thrust, but this is more a function of density.) $\endgroup$ – leftaroundabout Jul 9 '19 at 8:39

It's really an apples to oranges comparison.

  • J-2s are upper stage, gas generator, single-use engines.
  • SSMEs are booster, staged combustion, reusable engines.

Trying to compare them in that way is really fruitless. They were not designed to perform the same function.


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