Heinlein famously said that once you are in low Earth orbit you are 'halfway to anywhere.' In strict terms of kinetic energy he was wrong. LEO is merely halfway to an independent solar orbit that escapes Earth, but then goes nowhere in particular. With nominal additional delta v, great ingenuity, and enormous patience, you could indeed exploit the 'interplanetary superhighway,' but in spite of the name that is the slowest way to get anywhere.
In more practical respects a case could be made that low orbit is a good deal more than halfway to anywhere. Once in orbit you can use high specific impulse propulsion to reach other worlds, then single-stage rockets to land on and take off from them. None of these maneuvers is as difficult as the brutal lift from Earth surface to orbit.
I have said relatively little on this blog about reaching Earth orbit, precisely because it is so difficult. Getting to orbit is the elephant in the room of space travel, an access ticket that costs about $10 million per ton, give or take.
This cost structure, I would argue, is due as much to modest traffic volume as to purely technical limitations. (More precisely, low traffic volume is one of the technical limitations, perhaps the most important one.) In the current era there are about 60-70 space launches each year, with total payloads probably equivalent to a few hundred tons to LEO.
And for this low traffic volume the familiar expendable multistage rocket is the optimal solution. The launch vehicles are (relatively!) simple and cheap, since they don't need the features - heat shields, wings or parachutes, landing gear or cushions, and so forth - that would be needed to recover them. (The associated ground support facilities can also be skipped.)
Moreover, expendable rockets are built on a production line, achieving some modest manufacturing efficiencies, albeit much less than true mass production (like cars) might allow.
Another way to look at this is that if we had a classic reusable orbiter, capable of being turned around for re-launch every few days, it would be grossly under-utilized. Total world traffic could be handled by one or two vehicles - prototypes, in effect - eliminating all the efficiencies of series construction and fleet operation.
Moreover, those one or two orbiters could not be optimized to match varied payloads and insertion orbits. Thus, most launches would fail to take full advantage of the orbiter's capabilities - meaning that those launches would cost more than, ideally, they should.
The Shuttle, now on the verge of its final flight, is in large part a testimony to misjudged traffic demand. For all of its design compromises and operational shortcomings - which, among other things, cost the lives of two space crews - it was not an outright failure. It proved that a semi-reusable spacecraft is technically and operationally possible.
If we were to build a second generation version we could do a considerably better job. (For one thing, it would not carry astronauts for routine payload launches.) But we will not build a second generation version, and nor will anyone else, at least not in the near future. There is not the traffic demand to call for developing and deploying it.
Note the contrast between SpaceX's Falcon and the 'alt space' proposals of earlier decades. Rather than try to revolutionize space lift, SpaceX has aimed at modest, incremental streamlining of familiar launch technologies. The Falcon doesn't look like the future. It just looks like another two-stage expendable rocket. Which is pretty cool in its own right.
A flip side of this discussion is that I am unpersuaded by the various radical launch alternatives - elevators, launch loops, even laser launch - that have been proposed in recent years. My gut reaction is that they all reek of desperation. Because the Shuttle failed to provide routine biweekly space flights, the thinking seems to go, we should abandon chemfuel rockets entirely in favor of almost purely speculative technologies. I doubt that this is either necessary or viable.
At some point in the plausible midfuture we may reach the point of wanting to put thousands rather than hundreds of tons into orbit each year. At that point - not earlier, and probably not later - we will have reason, and financing, to develop reusable orbiters.
I'm highly doubtful of classic SSTO rockets. The design would have to be too extreme, even with major improvements in materials technology. On the other hand, reusable TSTO is almost certainly technically achievable. It would have a much smaller payload fraction than expendable rockets, since the stages must come back for recovery. But if the traffic is sufficient, there is at least a fair chance of streamlining operations to the point where re-use of the launcher pays off.
Discuss. Be civilized.
Via APOD, the final Shuttle rollout.