Small rockets spend a larger proportion of their fuel to overcome air resistance in low altitude, due to the fact that drag scales with square of dimension while fuel and mass scale with cube of dimension. So it is more advantageous to use an airplane to help a small rocket, like pegasas launched from b-52 mentioned above, to overcome parts of lower atmosphere resistance that to help a larger one. Any rocket that can puta six men capsule into orbit is very large indeed, on the scale of a delta or atlas rocket at very least. These don't burn much of their fuel overcoming lower atmosphere resistance, so is not helped per se tremendously by being launched from air.
Then the air launcher turns into simply a reusable first stage. But a subsonic air breathing launcher is a very weak first stage, giving a very modest amount of energy to the rocket compare to a proper rocket first stage. So it doesn't save the rocket a great deal to be launched from 50,000 feet but Mach 0.7, then ground level standing still. Your typical first stage ground launched rocket would have be traveling at Mach 3 and have gained maybe 5-6 times more energy by this altitude, and would have expended only something like 1/2 of its fuel. So this air launcher would save the first stage maybe 10 percent.
The economics of air subsonic launch of large rocket is thus it doesn't save a great deal of fuel weight for the rocket, but does add the cost of large, unique, possibly one off airplane, in addition to adding cost and weight by requiring the fully laden rocket to withstand unusual ( for a lunch rocket) stress of bing slung horizontally while fully fueled.
Then the air launcher turns into simply a reusable first stage. But a subsonic air breathing launcher is a very weak first stage, giving a very modest amount of energy to the rocket compare to a proper rocket first stage. So it doesn't save the rocket a great deal to be launched from 50,000 feet but Mach 0.7, then ground level standing still. Your typical first stage ground launched rocket would have be traveling at Mach 3 and have gained maybe 5-6 times more energy by this altitude, and would have expended only something like 1/2 of its fuel. So this air launcher would save the first stage maybe 10 percent.
The economics of air subsonic launch of large rocket is thus it doesn't save a great deal of fuel weight for the rocket, but does add the cost of large, unique, possibly one off airplane, in addition to adding cost and weight by requiring the fully laden rocket to withstand unusual ( for a lunch rocket) stress of bing slung horizontally while fully fueled.
