Barring some huge breakthrough in specific impulse, the best trick I can see for simplifying a launch to orbit is to make use of the atmosphere somehow. But orbital speeds are in the ballpark of Mach 27 or so, whereas the best anyone has demonstrated in the way of using air to assist in the thrust rather than merely be an obstacle is perhaps Mach 10, and that's a stretch. A simple ramjet is good up to somewhere around Mach 3.
So, unless we can devise scramjets (or other devices that seem even more dubious to me, like the notion of ramming in hypersonic air,
liquefying it
, and then using that for oxidant/reaction mass (80 percent of its mass will be nitrogen after all) in a rocket engine--other people here take it seriously but I can't imagine how one could shed that heat that fast, and dumping the heat means dumping energy which translates as drag) that can have us cheerfully breathing air all the way up to orbital speeds, at some point we have to go over to using a rocket, in the sense that we have to haul along the oxidant as well as fuel, anyway. If that stage can be far enough along then we might consider hauling the fuel tankage for the lower, fuel-only, stage, plus the doubtless heavier airbreathing engines, though this means carrying more fuel and more oxidant and heavier engines to deliver more thrust. But even if we could get up to say Mach 15, which would very significantly reduce the mass of propellent needed to go from there to full orbital velocity, it would seem more sensible to separate the upper stage from the atmospheric lower stage and thus minimize the weight of the upper stage. Thus even if we did have really nifty airbreathing engines that could take us more than halfway to orbit, we'd still probably want to have two stages. I'd think that the lower stage could fly itself back to base as a reusable stage, so both stages could at least be reusable, though you might want to actually make only the actual return module of the upper one reusable since fuel tankage as such is probably pretty cheap to replace. Or, leave it in orbit and use it as building material for a space station.
If we could have some sort of airbreathing solution that went all the way up to full orbital velocity, then that might be viable as a single stage system I guess.
But given that in reality even pushing a scramjet up to Mach 10 (if it even went that high) was a stunt and trying to push the envelope of useful airbreathing into higher hypersonic speeds is probably going to be tough sledding--well, all the back-of-the envelope figuring I've ever done assuming that say Mach 3 is the effective upper limit of jet propulsion, or even stretching it to say Mach 5, shows that the benefits of gaining in effect much higher specific impulse for a small portion of the flight are very easily offset assuming the engines or the aerodynamic compromises one has to make weigh a lot more than the simple rocket alternatives that try to simply brute-force their way past the atmosphere as a pure nuisance. So I'm not so surprised that in real life all space missions have in fact gone on pure rocket drive without even an attempt to make an airbreathing lower-atmosphere booster stage or two.
Given that, the wisdom of staged operations is as karl2025 said; staging helps keep the overall propellent/payload ratio down.
I should point out that one of your list of "single-stage" craft, the MiG, was a prototype for the Soviet Spiral concept, and it was definitely not single-stage--it was one of those ambitious airbreathing lower stage, rocket upper stage things. It never flew into orbit, possibly because it was impossible then and is still impossible to get the sort of hypersonic airspeeds the lower stage of the concept needed to make sense. And even if it could have worked we'd be looking at one very gigantic hypersonic airplane, if it were going to boost any significant mass into orbit. I suppose off the top of my head one might get something the mass of the SSO into orbit with maybe half or even a third of the total all-up launch weight, if the hypersonic airplane part could go fast enough, but even 1/3 of a Shuttle's all up launch weight is still 700 tons! It would be up the Russian alley to make an airplane that massive, conceivably. To make it hypersonic, with all the severe compromises of low-speed performance that entails and correspondingly high takeoff speeds required and corresponding long runways....
Especially bearing in mind that except for that NASA scramjet test no one has operated any sort of aircraft at the sorts of airspeeds we'd need to get the upper stage weight down as dramatically as I've supposed, except returning reentry capsules aerobraking--these aren't "airplanes", even the Shuttle being known to its pilots as the "Bionic Brick." To make a launch stage capable of fairing the orbital upper rocket stage into itself and surviving the separation/launch of it and flying its insanely expensive self back to base for reuse--well, it's harder than making a subsonic jet of the same mass, which is itself a challenge the aeronautical world has not yet risen to.
Could those wacky Soviets have actually done it in the 1970s? I have my doubts the Russians, or we, or anyone, could do it now. And it would not be cheap!
Anyway Spiral and Mig-105 were definitely not single stage!
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Mind you, if we could only make some kind of airbreathing engine that could take us all the way from takeoff to orbital speeds, we could get tremendous improvements. Using the heat from hydrogen-oxygen burning with high efficiency, bearing in mind that for every oxygen molecule we intake we get 4 nitrogen molecules along for the ride so the exhaust speed will be something like half what a pure oxy-hydrogen rocket's would be but the mass is four times as great, bearing in mind we aren't hauling oxygen so all we need to carry is hydrogen--the effective impulse is in the ballpark of an equivalent velocity of our propellant of 14,000 instead 3000, and we only need to burn up about half the launch mass instead of 95 percent of it! 100 tons to orbit needs a takeoff mass of maybe 200 tons then.
But I've made so many handwaves it's practically ASB. For one thing I neglected the weight of the engines, for another the structural weight of an airplane that can take flying in atmosphere (and taking in air for the engines) at Mach 27 and somehow avoid getting vaporized, with a fuselage full of hydrogen. For another of course no one has come close to demonstrating engines that can possibly operate in that regime, and if we could make them they almost certainly would not work at takeoff so we'd need a whole other engine system to get up to say Mach 3 (if scramjets can even light up at that speed! I picked 3 because that's the sort of speed an SR-71 could reach on its engines) or more. Even at a petite 200 tons its still a lot heavier than any supersonic aircraft that has ever flown and of course it has to be even worse at subsonic performance than the SR-71 or B-70 ever was, and they were bad enough in that regime.
