PaperLemon

I believe SpaceX has discussed plans for HLLVs larger than Falcon Heavy should a market for them open up. Given all the hype currently surrounding Mars and Elon's own passion for making flights to Mars a reality, I think it's likely that they've at least put it through an analysis and talked to their engineers about what kind of systems will need to be developed to make it happen. I remember a mention in an article about a potential successor to FH that would use Merlin 2 engines and be capable of lifting around 120-140 mT, which is right in the ballpark of SLS's capabilities. Of course I'll be the first to admit that I'm biased in favor of SpaceX and I'm pretty skeptical of the defense and aerospace industry's ability to deliver SLS on time and within the original budget. For what it's worth, neither vehicle exists anywhere but on paper in the present, so what will or won't happen is still up in the air and priorities change quickly when politics get involved.

You might want to check out Taverio’s Pizza and Aerospace mod on the spaceport. His pack includes a couple of scramjets that actually function like real ones (i.e. you need to be going really fast for them to produce any appreciable thrust). I made an X-43 replica using his parts that flew pretty well, but you've got to bring along another propulsion system to get it up to speed first. I've managed well over 2000m/s using this combination.

I'll give credit where credit is due. The STS was visionary that incorporated a lot of forward thinking engineering and scientific goals which in turn advanced the breadth and depth of our knowledge about space and space travel. You could argue that it inspired generations of boys and girls to take up an interest in STEM fields. That said I have a hard time justifying it's existence in retrospect. They promised a space truck which was massively oversold on capabilities that never quite materialized for a market that wasn't even there. It wasn't very cost efficient and it wasn't so much reusable as it was refurbishable. To be fair a lot of overhead was added due to the dozens of groups with conflicting goals trying to get a piece of the action. There were a lot of requirements from different factions, such as the Air Force's insistence that it be able to launch spy satellites into a polar orbit from Vandenberg AFB. Because of this it ended up really having to pack a lot of complexity and capability into one machine on a budget and certain safety considerations ended up being thrown out. For what it's worth, I think we would have been better off sticking to a Soyuz-like craft for carrying people to and from LEO. Existing Titan and Delta launchers could have been used to send up the big space station bits like the Russian Protons. This way we have a clear separation of mission roles and less complexity allowing for things to go wrong. After all there's no sense in taking a multi-tool to do a specific job when all we needed was a screwdriver. I'm also a big fan of Big Gemini and think that it might have been a good choice for crew and resupply missions to stations and even orbital construction yards, serving in a similar capacity as TKS for Salyut. It would have had that capability to carry 9 or 12 people depending on configuration and would be roughly the same mass as an Apollo capsule, but would carry twice the cargo to LEO. And this was without the proposed cylindrical cargo module, which would've increased cargo capacity even further.