SpaceX has outlined its concepts for the next generation of Heavy-Lift Launch Vehicles, the Falcon X and Falcon XX…
…notice the jumps in capability by replacing the bank of 9 Merlin 1 engines by a single Merlin 2. That’s going to be a HUGE engine, akin to the Saturn V I-C stage’s main engines. The single-engine concept is a step-down in reliability because an engine-out is fatal, but with the right engineering that might be a manageable risk.
Right now I’m not so convinced that SpaceX needs a Falxon X. The Falcon 9 Heavy is large enough to deliver most satellites to space that the market wants.
Also, two Falcon 9 Heavies can deliver 64,000 kg to LEO. That’s enough to deliver any single component of a Sustainable Space Development (SSD) architecture which ultimately involves bringing lunar water ice to LEO. At that point, who even needs a super heavy lift rocket for any purpose?
It seems to me as though the purpose for a Falcon XX is for a single shot for manned missions beyond LEO. Again, if you can fuel an Earth Departure Stage (EDS) using an LEO fuel depot, then the rest of the stack can be lifted using Falcon 9 Heavies. However, if NASA insists on building a heavy lift vehicle, then I’d rather SpaceX & other commercial companies build it due to the proven cost-effectiveness of the COTS/CRS/(CRS)-approach. But my perspective is, “Develop SSD first – then we’ll realize what we do and don’t need next”.