Having played several hundred hours of KSP I can confirm you shouldn't underestimate momentum. It's amazing that it's even possible. I think the number I've heard is if the earth's gravity was something like 10% stronger this wouldn't be feasible.
If gravity was 10% stronger rockets might be much more powerful and been reusable much sooner. Having the military need to produce nuclear powered rockets for effective ICBMs would have produced a very different nuclear power development path than the one on this Earth.
With a modest increase in Earth's gravity, none of the rockets we have ever built would have been able to put even a small payload into even low earth orbit. Even a Saturn V wouldn't be able to launch Sputnik. Increase gravity a little more and no chemical rocket we could feasibly construct given known materials and methods would be able to do the job.
It's not about the amount of thrust. It's about being able to carry enough fuel to sustain that thrust long enough, without the weight of the fuel making the rocket too heavy to lift high enough to reach orbit.
Possibly not able to carry enough chemical fuel to lift the fuel into space and return and do a controlled landing without refueling? That seems more plausible.
Fuel is heavy, and most of it is spent on... lifting fuel.
But the way the rocket works is creating a continuous explosion behind it. The propellant explodes, pushing the rocket. It's not just the rocket equation, but the explosion from a central point BEHIND the rocket that pushes it further.
Having a nuclear explosion for example behind the rocket would surely be enough to push it into space... even on Jupiter.
Yes, this is what I was referring to. Though there's probably another percentage that makes most rocket fuel also insufficient for anything more than LEO.
Even in an ASDS landing, it exceeds the Kármán line. I think the typical apogee is around 120-130 km.
Stage sep happens below the line, however.