I don't have a citation but believe the Shuttle SRBs hit the water at a high enough speed that they could not be refurbished. Instead parts were removed and re-used, presumably with rework.

Hydrogen has the highest energy per mass, but that doesn't make it the most efficient fuel. Efficiency is measured by total system requirements, not energy/mass. The extra dry mass from much larger tanks, cryogenic cooling systems and lower thrust causes Hydrolox to be less efficient than dense fuels for first stages. Once in space low thrust is less of a problem, but it still has to accelerate enormous, heavy tanks, and long trips have a lot more leakage. A fuel with twice the energy per Kg, that loses more than 50% due to leaks is less efficient without even considering dry mass.

It took 100 years for commercial aviation to get to this point, so in 100 years space launch may have similar economics. Not as cheap, but similar cost ratios. According to statista.com/statistics/591285/aviation-industry-fuel-cost fuel costs are between 19-32% of airline costs, making total costs are 3-5x fuel costs. That would mean future Starships would cost $3M-$5M per launch.

I couldn't really think of physical limitations, the limitations are really economic. For example if volunteers staffed SpaceX Starship launches for free it would lower costs, but not be economically realistic. if you can imagine unobtanium materials that never need refurbishment, then what remains is mostly the cost of LOX and Methane.

@uhoh Good point, I'll add a constraint.

Isn't total annual energy generated/used by humans growing at an exponential rate? And if thats true would it be driving Heller's potential rocket velocities over time? For an absurd example, if we reached the point where we generate enough energy to produce storable anti-matter it would enable anti-matter powered space ships. Less absurd might be generating positive energy out of Fusion reactions, etc.

But at ground level XRS only 64% & Raptor is 72%. XRS launch ISP only 339 using LH2, barely more than Raptor's 330 from Methane, while XRS also imposes the much heavier tanks that LH2 requires. So it seems far more effective to use the base Raptor for launch, and switch to Vacuum Raptors as the atmosphere thins. You save significant mass on engines and tanks, have no efficiency losses in Mach 1-3 regime, and have a fuel less prone to boil-off on long journeys. There is no useful space mission that requires Aerospikes to be able to be performed.

Raptors are designed to land on Earth, and Vacuum Raptors on Mars. You don't need Aerospikes which have always suffered fromf poor thrust to weight ratios, and poor efficiency in low (1-3) Mach realms. The XRS-2200 had a 35-1 ratio, with a production goal of increasing that to 80-1 at a similar thrust to Raptor. Even if it had met those audacious goals it still meant that using two Raptors, ground level and vacuum versions, would mass less and offer higher performance. For comparison, the XRS-2200 & Vacuum Raptor are both at 83% of theoretical max efficiency in space

I had to downvote you because your source is almost 5 years making it totally inapplicable, and your point is inaccurate and speculative. NASA has no schedule or plans for any Mars landings because as yet it has no launch systems or interplanetary vessels capable of delivering humans to the surface and back. The answer to bone density loss is exercise, which has been proven to work well on ISS.