Going Up?
When thinking about how to get into orbit, most of us probably default to the image of rockets soaring upwards on columns of billowing exhaust. And that method certainly works. But it's also the case that chemical propulsion has gotten just about as sophisticated as it possibly can. For various reasons that are rooted in such charming subjects as chemistry and thermodynamics, there is an upper limit to the efficiency one can achieve using combustion-driven rocket propulsion. And, as it happens, that limit to efficiency is basically where we are with the hydrogen/oxygen engines of the Space Shuttle orbiter. Oh, better engines are possible and are being built as I write this; but no great leap forward awaits us in the field of chemical rockets. It's all fine-tuning from here.
To me, this is no great worry. Many possible advances in materials and structures still await, so that the elusive dream of SSTO (Single-Stage-to-Orbit) flight may finally be achieved. SSTO would reduce launch costs dramatically--high launch costs being the big obstacle to accomplishing everything you could possibly want in space--and at present the real challenge is that the required mass fraction is about one-tenth. In other words, the vehicle that could reach orbit with only a single propulsion stage, carrying its entire empty mass into orbit and bringing it back again, would have to be ten times lighter when empty than when fully loaded and fueled. Presently, this is structurally unachievable in a vehicle sturdy enough to be reusable. (This is essentially the problem that brought down the X-33.) But, as I say, I doubt that it will always be so.
Some folks, though, aren't content with the prospect of chemical propulsion forever. I'm not, either, frankly; leastways, not for chemical propulsion on interplanetary missions. To get into the really exciting business of space travel, we're going to have to head into the realm of nuclear propulsion in the near term, and who knows what systems in the longer term-thermonuclear fusion, perhaps. But for access to orbit, on a pure cost basis, and even with all the additional considerations of environmental protection and so on, I don't see abandoning chemical propulsion. Not in the near future, certainly, and not even in the long term. But that's me.
More dedicated futurists than myself like to talk about something called a space elevator. It has turned up in science fiction from time to time; the idea is simply to run a cable from the surface of the Earth right up into orbit tens of thousands of miles out. What you're after is a length such that the inertia of the cable and its tangential velocity component (due to the Earth's rotation) will keep it from falling, while its tip will remain stationary over the same point on the Earth's surface. And assuming you build the cable so as to run payloads up and down it at will, then you do indeed have an elevator that goes into space.
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