Recently the engines of the SLS Block II has become a hot topic around. Block II will be the monster of tomorrow and it is likely that many high-performance engines may be used. And here are my ideas of what possible hardware can be used
Core: 1. 6xSSMEs(come on, why are you always sticking on 5?)
2. Plug-nozzle cryogenic engines of 1-million-lb range were considered for Novas during the Space Race, and they only exist in blueprints. Will it be possible to bring them alive? 2 of these engines would be equivalent to 5 SSMEs.
Boosters: 1. Advanced SRBs: ASRMs or 156/160 inch SRBs(shuttle SRBs: 148 inches)
2. Our classic favorite: Big kerolox LRBs. Considered engines may include the F-1A, RD-172(a proposed variant of the RD-170), and the RLA-600(a big 2-chamber engine considered for the UR-700 series). I recommend upgraded F-1s up to 2 million lbs, 2 of each LRB, producing 4 million lbs and could also serve as the core of our next gen of EELVs(hence reviving Jarvis).
3. Some cryogenic giants are being developed.The RS-100(intended for Teamvision's Jupiter III upgrades) produces a thrust 1.5 times of the RS-68. The RS-XXX(for next gen Deltas) produces something similar to the F-1. These engines are not not given much attention, so I bring it up to arouse your interest.
4. Large hydrolox boosters. I'm not very familiar with this kind of propulsion. Can somebody please tell me more?
So, what are your ideas? Come, share it!
4x RS-25 engines are actually optimum for that length and width/capacity of corestage. 5x engines gives a modest payload increase, but probably not worth the extra cost and weight of the fifth engine. 6x RS-25 engines in the corestage would drain the propellants too quickly, shifting more of the lifting burden to the Upper Stage (on Block II especially), forcing a redesign. Similar compromises happened during Ares V design cycles.
Comparatively speaking, 6 engines doesn't give much extra thrust compared to 5 engines and its' extra 7500 pounds approximate mass will make a detrimental hit to the payload performance, along with - as I said - the extra propellant consumption. Making the corestage longer again to compensate with extra propellants will force a redesign of the Solid or other boosters to fit. Also, a sixth engine will increase costs by tens of millions per launcher.