Even if it can lift a launch vehicle to the same altitude as a rocket first stage, velocity is the more important component of getting to orbit. A Falcon 9's first stage gets it to 11,000kph, about 3 times the top speed of an SR-71, the fastest jet aircraft.
The Tsiolkovsky equation suggests that's going to be a big saving:
http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation
And yes, my only qualification is Kerbal as well ;-)
Sure, but the altitude gain is almost "free" - you need to accelerate by something like 6 km/s laterally (number pulled out of somewhere, it's 2.3 in kerbal space program but the planets are smaller there) and 1 km/s vertically (depending on ascent profile, 0 for an instantaneous ascent and infinite for a slow hover upwards, but it ends up something like that), so by Pythagoras it's sqrt-37 = really not a lot more than the 6 kms/s you'd have to spend if you could start at 100km altitude.
Intuitively we assume rockets go "upwards" partly because that's the part we see, but actually the gravity turn occurs very early and most of the acceleration is closer to horizontal. E.g. the less-crazy of the space shuttle abort plans if something went wrong early in flight was to land at Shannon.
If you look at the patents, towing is the big difference. Other spaceplane concepts have involved the big plane carrying the little plane, but it is easier to tow another plane than it is to carry it.
The "prior art" that hasn't been mentioned yet is the Japanese "Baka Bomb" of WWII which flew as a towed glider until it got near a target, then the pilot would switch on rocket engines for the terminal part of the flight.
And in a towed/mated application, you're not forced to carry the subsequently useless mass of air-breathing engines with you either.
Also, the design constraints of air-breathing engines tend to be a lot looser than rocket engines. Which buys you increased reliability.
I don't know how to estimate the behavior of a jet engine, but plugging in the numbers for a Cessna 172 shows that the mass of air flowing through the propeller per second is several hundred times higher than the amount of air actually used by the engine.
For props, I imagine this would be a fundamental difference, as they're not ejecting mass, no? They're expecting energy to pull themselves through something more fluid-like.
In a rocket, and also in the various derivatives of a jet engine, forward thrust is generated by the expansion and acceleration of mass away from you. Either fuel + oxidizer in the former, or fuel + compressed air taken from the atmosphere in the latter. And the corresponding effect of that fuel+ acceleration on your mass.
In a prop, forward thrust is generated by linking a powerplant to a blade that interacts with air. The exhaust mass from the powerplant burning fuel in this isn't directly connected to the forward thrust. In that, if you had a non-propeller method of propulsion, you could hook the powerplant to that and still have effectively the same system.
1) Rocket engines work a lot better at low ambient pressure. You get more thrust for the same fuel flow. You can fly with a bigger nozzle, increasing this benefit. This is probably bigger than the altitude and speed benefits in subsonic airlaunch.
2) Air launch is flexible since you can fly a long distance to a secluded spot over the ocean and can launch in any direction. You could fly polar flights from Florida. Or low inclination orbits from California. Both of which are impossible for fixed launchers. You can fly towards an orbit's ground track to get flexibility in launch time as well.
3) If you have a problem with the early rocket flight stage, you have enough altitude that gives you more emergency options. For a ground takeoff rocket, the early part of the flight is very dangerous.
Reaching orbit is really hard. It is extremely hard with a single stage vehicle. The problem is exponential in nature. The required mass ratio is proportional to e^(required delta vee / exhaust velocity)
For a reference design, with 3500 m/s exhaust velocity and mission delta vee of 10 km/s, you'd need a mass ratio of 17. That means the vehicle is 16 parts fuel and 1 part rocket and payload.
Helping a single stage with an aerotow to 0.3 bar ambient pressure, 10 km altitude and 300 m/s speed might just help it enough. Say if delta vee is reduced 10% and average exhaust 10% as well, the required mass ratio suddenly drops to 10!
To demonstrate what that means: if you could make the ground launched reference rocket plane to fly to orbit with a 3,5 ton empty weight and 0,6 ton payload, (70 ton wet mass), the aerotow version could replace 2,9 tons of propellant with payload, meaning 6 times the useful payload.
These calculations are not very accurate, and are very sensitive to the reference design's payload but they give you an idea of the nonlinearity.
And since you're towing with presumably a big airplane you could probably loiter for a few hours (maybe more) which would mean that the 30 minute good weather window doesn't need to be all that precise. Even less so if you're willing to use a tanker to refuel the tow aircraft.
Further, since you're flying -- not stationary -- you have a lot of freedom to shift the launch point a bit if your weather window moves too far forwards or backwards. If it's two hours too early you can take off early and fly east. If it's too late you can take off late and fly west.
6x the payload for the same rocket is pretty darn attractive!