At that point you start getting into "why not just build a space elevator?" territory. If it's just trailing into LEO, the first stage doesn't even need to mak

At that point you start getting into "why not just build a space elevator?" territory. If it's just trailing into LEO, the first stage doesn't even need to make orbit to get to GTO. That makes recovery way easier as well as increasing the payload massively. A space elevator of that type is within current technology, and while the cost is obviously higher than a "taxi", the benefits are as well. In answer to your question you could just eat the cost problem and use a really big battery + a large number of taxis, and accept that they will have slow recharges. NB that there will be a complication with launch windows with both a taxi and a space elevator. You have to be limited more closely to orbital times without a second stage. My personal favorite is still an orbital loop- put a huge superconducting filament in orbit around the earth, then hang tethers from magnetic bearings. There is almost no tensile stress on the loop, so no need for fancy carbon nanotubes- it just needs a sufficient superconductor and enough mass to stay in a stable orbit. The downsides are of course that the power requirements are staggering, the cost of getting that mass to space is staggering (although much cheaper than a space elevator), and if power fails the tethers fall down (safe but inconvenient). Edit: as for the RTG- the power, even with these improvements, is waaaaaaaaaay too low. This is a 25% improvement, to RTGs which are in the hundreds of watts and weight hundreds or thousands of kg. VASIMR systems use tens of kW. It also is not possible to make many more RTGs, and certainly not very large ones- they are remnants of cold war nuclear power-building. It is immensely expensive to produce these isotopes. Each one is a work of international diplomacy and bureaucratic art. It would not be possible to make them hundreds or thousands of times larger, which would be required to be comparable to rockets. Not to mention, these are still filled with radioactive material.