Ick. Air-launch has never made a ton of sense for true orbital in my book. ...
It's always seemed to me that the advantage, if any, would mainly accrue if one can launch at seriously supersonic speeds--Mach 3 was actually attained in the 1960s with both the SR-71 and the B-70 Valkyrie bomber, and that seems about the limit of straightforward jet-type engines. I've been fascinated by Skylon lately, with its cooled-air-intake airbreathing strategy it can plausibly get up to Mach 5 in airbreathing mode. 1600 meters/sec rocket ignition speed makes a serious reduction of propellant required to reach orbit. Even Mach 3, call it a round 1 kilometer/sec, is a real help. And of course in these proposals* the airplane launch platform is completely reusable, as could be the orbital return stage and depending on how ambitious one wants to be, possibly the engines and even some or all of the fuel tanks. If we ignore those possibilities and pursue the "cheap numerous disposable rocket" concept (we really ought to be reusing the entry vehicle though) we have a rocket that is scaled down considerably from a ground launched one.
However, unless one envisions a lot of launches, clearly a launch airplane that can achieve even Mach 3 (and then maneuver at that speed, at those altitudes, to give the rocket a good clear separation and a good trajectory for orbital launch) is a very specialized item. Simply adding a booster stage to the same rocket for ground launch costs a lot of mass but it's mostly propellant; I think it was in the SSTO thread some months back I asked what fraction of the cost of a launch is the fuel, and was told it's darn little. If we are mass-producing ground-launch boosters, and have an efficiently running launch site, then I can see why the economics of this kind of high-speed air launch has never been appealing.
The more realistic proposals that rely on existing types of subsonic jet seem to offer little advantage. It's very nice to be able to choose one's effective launch site by flying there from any of the very many big airports there are in the world. But on the other hand using even something as big as the C-5 or Ruslan we can't lift all that much rocket system mass.
Looking over your canonical posts on the Wiki, I don't see any detailed breakdowns of the system masses of the Saturn 1C or Saturn Multibody series, nor even a lump sum launch mass. I think somewhere or other you've said how much the F-1A engine is upgraded over the OTL F-1, which I believe delivered 600 kiloNewtons of thrust (with a mass of about 7 tonnes). I am guessing a standard Saturn 1C with typical 20+ tonne payload stack (including the escape system) masses somewhere between 540 and 600 tonnes on the pad. Say we can knock about 30 percent of that off by air-launching it at Mach 3--the reduced stack (mostly reduced by downsizing the first stage considerably--but that's a hit at the economics of just using the standard 1C or upgraded Multibody stage, now we need a new one specialized for this mission!) still is a payload for what amounts to an SST/stunt plane of over 400 tonnes! The fuel load for the airplane doesn't have to be as much as it would for a serious intercontinental bomber or SST, because we don't want to cruise for hours to travel long distances, we just want to climb to full speed and altitude fast and return a much lightened airplane (that can take its sweet time about it) to base. Still even an aggressively limited fuel load will mass something between 5-10 percent of the takeoff weight, and it doesn't seem realistic for the airplane itself to weigh less than its payload. So all up we have a takeoff mass of something in the ballpark of 800-1000 tonnes, which puts even the Mriya Ruslan derivative to shame.
By launching subsonic we can make the airplane simpler, with much more efficient subsonic aerodynamics. But we will be saving very little rocket propellant mass, even if the rocket engine efficiency can be made better for a stratospheric than a sea-level launch. So I think it comes out even heavier than the supersonic version (though it might need a shorter runway).
But a zero airspeed, zero altitude launch costs relatively little extra pad weight compared to a subsonic launch.
I'm all for making giant monster airplanes, so much the better if they can zip along at Mach 3 or even higher supersonic speeds. (I'm assuming we are talking over-ocean flight paths here of course!
) But fun is fun, and serious business is something else I guess.
Here's something NAA though might be serious business in the mid-60s though.
Conceptually it's like Spiral, but I like that 11 tonne lenticular reusable vehicle! And the article says the system was fully reusable, including the winged second stage rocket. Which I guess could propel the orbiter capsule into orbit, or a near-orbit the spacecraft could circularize with onboard OMS, but survive reentry mainly because it would be a lot lighter empty of fuel, so the re-entry heating phase would be brief (due no doubt in part to very high accelerations, so it had better be robust). Then of course the second stage has a long flight back to a base, or needs to land at a second base far downrage, and somehow be brought back to the launch base.
All up takeoff weight is remarkably similar to a Saturn 1B. But of course the 1B could launch close to twice the mass into orbit! To match the minimal capabilities of the Saturn Multibody with no boosters on the sides, it would have to be twice that mass...
Still bigger than Spiral though, and earlier. I don't know if the estimates of the structural masses of the airplane and the winged second stages were any more realistic than those the Spiral program claimed for their respective stages.
If not, realism might raise the take-off weight considerably, and persuade ditching the idea of re-using the orbital rocket stage.
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*Not Skylon of course, which is SSTO and lands the whole thing for 100 percent reusability--but that's why it's an advanced concept that remains to be proven! And who knows how the economics would really work out.
