[Bring_Back_Shantz]

Originally Posted by karl262

Probably has to do with weights.

A quick Google says the launch weight of the shuttle with fuel and everything was 4.4 million pounds plus up to 65,000 pounds payload capacity. The max takeoff weight of a 747 says 735,000 pounds.

When they carried the shuttle on the 747 it was minus the booster rockets and fuel tank. Granted it would probably require less fuel weight to get the shuttle into orbit from 40,000 feet and moving 500 mph, but that is for smarter people than me to calculate.

Somewhat related, one cold war idea for launching icbms was to put them in cargo planes and push them out of the rear during flight. I think the concept did advance with tests but never reached active service if I'm not mistaken.

For reference, to orbit with the ISS you're looking at an altitude of ~400 km, and a speed of ~17,000 mph, so starting from 40,000 ft, and 500 mph, makes essentially no difference.