I've said before and will probably say again--air launch interests me, but only at supersonic speeds. The Mach 3 speeds we know pretty large planes can reach are just starting to make a significant dent in the delta-V. I'm generally unimpressed by the argument that merely saving a bit of it because rockets are more efficient in near-vacuum than in the thick soup of the lower atmosphere makes a worthwhile difference. You have to compare the cost of making a really really gigantic airplane that can lift a rocket weighing all-up something like 500 tonnes to that altitude against the cost of simply making that rocket a bit bigger and then launching it from the ground, choked nozzles and all.
Now if your really really big plane can also go really really fast, we can talk about serious reductions of the rocket lower stage to offset that gigantic and hugely challenging and expensive airplane. I'm a sucker for grandiose aircraft, I'll admit! Any excuse to make a thousand tonne airplane to rival the speed of an SR-71, and you've got my attention!
Even so, whenever I've set about to do the math, it comes out pretty marginal at best. Your great big supersonic plane is best viewed as a fly-back reusable first stage, but the sorts of speeds proven engines can reach--about Mach 3, 1000 meters/sec if we push it, only half-matches the job a typical first stage booster can do; you still need quite a lot of delta-V to fill the gap for typical second stage separation speeds.
It's why I'm a sucker for Skylon, which proposes to integrate the whole thing into an SSTO and reuse all of it, and proposes to use engines that are as yet unproven in actual flight to do so. I'm sold on the theory; can anyone bring it into practice, and if they do will it quite match the ambitious performance targets they count on?
Skylon proposes to reach Mach 5 and beyond in airbreathing mode, using a strategy of using the heat-sink properties of hydrogen quite prodigally--hydrogen is light stuff so from a mass point of view they can be generous with it, but it is bulky--the craft they propose looks pretty big for the light mass they think they can make it. Sort of like a hypersonic Zeppelin!
So I'm a sucker for it.
But I have thought about the wisdom of using a version of the SABRE engines that is purely airbreathing--that would be what Reaction Engines is calling a SCIMATAR--and launching the actual rocket as a separate stage. It goes against REI's philosophy of saving costs by keeping the vehicle integrated, and their hope that integrating rocket engine mode in the SABRE is easy compared to the airbreathing part, and they can get plenty of thrust from the same engines they used to get to 1600 m/sec in atmosphere. Also I believe REI is counting on significant aerodynamic lift during the early rocket boost phase, before the speeds become so great the thermal challenge is too much--by which time they will have ascended on earlier excess lift and momentum, modestly sustained by a portion of the rocket thrust. Thus in rocket mode only a small fraction of the thrust goes to fighting gravity which helps with the economy of reaction mass to reach orbit. We'd throw that away by separating the plane from the rocket. And that separation has proven problematic in tests; I gather it would be pretty important to drop the rocket from the plane rather than try to launch it from the top of it, and even then the aerodynamic forces would be rather wicked and treacherous--a problem Skylon's integrated structure sidesteps completely.
Skylon may be pie in the sky, but if so probably all serious air launch is.
Of course in addition to the quite marginal advantage of launching in thinner air that subsonic air launch offers, there is the apparent operational advantage of being able to simply take off from any major runway in the world.
That's
apparent. It works great for little rockets, launching miniature satellites or even more miniscule payloads to GEO. Such payloads are the bread and butter of what commercial space business we have, which boils down pretty much to comsats and related tech like GPS constellations.
To launch something serious, like a manned spacecraft in the 10 tonne range or a big satellite or a space probe into deep space, you would need bigger airplanes than are currently built. For something like Apollo, or even a Soyuz, you'd need something
a lot bigger. Such a big airplane would
not be the sort of thing any run of the mill airport could handle; pretty soon we're in the ballpark of needing to build a very special runway indeed, very very long, reinforced like no one's business to handle the massive footprint.
Even Skylon, it turns out, would need a specially reinforced runway to take off of, though they plan to be able to land anywhere. (To what purpose they'd land at a field they can't take off from I don't know--maybe to be lightly filled with hydrogen fuel only for ferrying itself to a proper base?) To exceed Skylon's rather paltry 15 tonne payload to orbit (optimally; from a high latitude base to a tough orbit it can rapidly fall below 10 tonnes) we'd need some Gargantuan airplanes indeed and would probably have to design them to operate off of water.
If we can't have Skylon operational as advertised, and we can't have more exotic mass transportation into orbit via launch loops or orbital rings or the like, I fear we're pretty much stuck with launching rockets from the ground. And the extra rocket to punch up to the flight regimes the airplanes might reach is not that hard to do--it tends to be massive and fuel-guzzling as all hell, but apparently the fuel and mass is not the big cost item. Specialized white elephant hardware like a 2000 tonne airplane is probably going to cost far more than a launch pad and a succession of multihundred, even thousand, tonne booster stages--unless the frequency of launch turns out to be orders of magnitude more. If we wanted dozens of missions to the Moon every year, something like airlaunch on a mega-scale might pay off, if we could lick the problems--but then, systems like a Lofstrom Loop might get a hearing too, if we were that obsessed with getting lots of mass into space.
And in some ASB scenario (which I tend to follow avidly!) where all of a sudden alien derelict starships turn up in orbit around Venus or some such and there's a space race between superpowers to get an investigating team there first--in their haste, they won't take time to develop such elaborate infrastructure, no matter how massive the savings--they'll go with the first damn thing they can lash together to get the necessary mass up into orbit. That is, rockets, lots of them. They'll build dozens of new launch pads, on the old model, rather than sit back and wait for the Godot of a giant supersonic booster airplane, or build launch loops, or any such.
Then if the damn alien artifact turns out to be comprehensible and useful, like as not we'll have gravity drives and warp drive before you know it
It would be nice to have a non-ASB motive to persuade governments and big corporations to spend a really significant amount of resources consistently for serious space operations; I'm afraid I come up empty, I just want space travel to be happening because it's what I think human beings ought to be doing with our capabilities. That persuades no budget committees.
The closest I can come to it is admittedly ASB scenarios where there is a habitable second planet in the solar system and thus the goal is big and attractive and exciting to a broader public; also the fear that a rival superpower will get there and claim all the real estate is operational too. There are those who think that same motive ought to count for the Moon and Mars we have, but if we could have explorers landing there who can live off the land indefinitely, not in airtight little modules and spacesuits but breathing the open air, I'd think it would have more traction--as would the more noble motives too.
But even then--if a habitable Mars or near-Earth world in some kind of close resonance orbit existed, it would be easy for politicians to kick the can of getting there down another decade or generation or three...
Unless the Other Guys looked to be doing it no matter what; then it would be necessary to keep pace with Them to keep an eye on 'em. But that "problem" might be resolved by a summit meeting in which both parties agree neither will go.
