I've heard of Nitrogen 5+ and that potentially there's higher molecular weight nitrogen compounds that would make good monopropellants, but they haven't even been able to make a 6 member nitrogen ring. There are some nitrogen versions of cubane that look interesting, though. CL-20 is being looked at as a solid fuel additive (and explosive, of course). However all of these would have the severe drawback that they're high explosives and very expensive to manufacture and store.
Getting back to the SLS, the design has inherent limitations from the requirements to keep the same people building the same things. At what point in the SLS program will someone ask if using the same basic SRB for 50 years is the opposite of cutting edge? The core stage is already over 200 feet tall (due to using a narrow tank diameter for heavy lift
). Add a second stage and a little capsule (the Block II Crew configuration) and it's 385 feet tall (22 feet more than a Saturn V). The minimum height of the crawler is 20 feet, so the capsule
configuration only clears the top of the VAB door by 60 feet.
Needless to say, the SLS will never launch anything really long like a huge space-station section because a low-density first stage and narrow tank diameter has already constrained the growth potential to the Saturn V class of HLV, capable of putting two men on the moon and returning them safely to Earth, but still not enough to send up deep-space missions in one shot. That would be okay if the SLS had a high flight rate and low cost where you could spread a mission across half a dozen launches, but the SLS won't have a high-flight rate nor a low cost.