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Reading an article about new engines for the SLS (Summary: There is no plan what to do once the current SSME's are expended), they mention that the J-2X is too powerful for the upper stage of SLS.

Ultimately‭, ‬the 294,000-lb‭.-‬thrust J-2X engine may be too powerful for the SLS upper stage‭, ‬at least in the 105,000-ton Block IB‭ ‬intermediate variant en route to the 130-ton version needed for Mars missions‭. ‬“Essentially‭, ‬that stage wants to be about‭ [‬120,000‭ ‬lb‭. ‬thrust‭] ‬total‭,‬”‭ ‬May says‭, ‬and the agency is considering four RL-10s or two‭ ‬“60k-class”‭ ‬engines instead‭.

The original J-2 was about 234 Klbs of thrust. The J-2X seems to be uprated to 294 Klbs thrust.

Regardless, the J-2 was the second stage engine on Saturn V. There was a cluster of 5 J-2's. The third stage used a single J-2.

So this leaves me wondering, if the upper stage on Saturn V was appropriate at 234 Klbs thrust, why is SLS, which is meant vaguely to be in the same class, more likely to need only 120 Klbs thrust?

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    $\begingroup$ In both the Saturn V and SLS case, the upper stage is used both for circularization and for trans-destination injection. If SLS's booster and core leaves the upper stage in a faster trajectory than the first two Saturn V stages, less thrust (= acceleration) would be needed for the circularization burn (which is time sensitive), so that's my best hypothesis. The injection burn is fairly insensitive to thrust, and the RL-10 has better Isp than the J-2. $\endgroup$ – Russell Borogove Nov 23 '14 at 0:09
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    $\begingroup$ I'm too lazy to do a more detailed analysis but a quick glance at thrust x burn time suggests that SLS's booster/core stages pack more punch than the first two Saturn V stages. $\endgroup$ – Russell Borogove Nov 23 '14 at 1:32
  • $\begingroup$ I agree with the first 2 comments and just want to add that the SLS core likely is able to achieve a higher core burnout velocity because the RS-25 hydrolox engines have much better Isp than the F-1s. $\endgroup$ – user8269 Jan 5 at 1:35

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