I don't think your suggestion is feasable.
The main problem with reaching orbit is not as much the altitude [250+ km], but more the speed required to maintain orbit [32,000 km/h+].
I believe that the shuttle combination weighs in at something like 3 million pounds on lift off. More than 85% of that weight is fuel [feel free to correct me if you might have more specific data]. All that fuel is required to accelerate the shuttle to the required orbit speed. Higher altitudes require increased orbit speed, thus more acceleration, thus [much] more fuel. Remember that velocity energy is:
= 0.5 x mass x velocity x velocity [velocity squared]
= 0.5 x mass x 8,888 x 8,888
= mass x 39.5 MN
[Mega Newton] [mass in kg]
Altitude energy is:
= mass x altitude x gravity
= mass x 250,000 x 9.8
= mass x 2.4 MN
I assumed orbit speed @ 32,000 km/h [= 8,888m/s] @ 250km [= 250,000m]. Not sure if this combination is correct, but it should be quite close.
So a total of 42 million Newtons are required to get every kilogram [2.2 lbs] into a 250 km circular orbit.
If you would lauch a shuttle type vehicle from a 747 type vehicle, the speed gain [250 m/s --> 0.03 MN
x mass] and altitude gain [13,700 m --> 0.13 MN
x mass] is superficial, so you'll need to carry maybe 2.5 million pounds of rocketfuel to 45,000ft to get a shuttle into orbit . . . not likely!
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