X15Blue

Air-launched orbital launch vehicle. Family:

In March 1962, NASA proposed an orbital launch vehicle using the B-52/X-15 combination. A Blue Scout booster would be fitted to an extendable launch rail on the belly of the X-15. The X-15 would be air-launched from the B-52, and then itself air-launch the Blue Scout well above the earth's atmosphere.

The proposal was rejected due to concerns over safety and economics (despite it being promoted as a low-cost launch system).