It doesn't help much. Most of the work in achieving orbit is in gaining speed "sideways". Your average orbital launch achieves high airplane altitudes in the fi

It doesn't help much. Most of the work in achieving orbit is in gaining speed "sideways". Your average orbital launch achieves high airplane altitudes in the first 15-20 seconds out of a 500 second flight. It's easier to build a slightly bigger rocket than to add a giant airplane to the system.