I was going to post this. The issue I have with the concept is that the delta in velocity is not that much. This plane will fly, at best, 600MPH. Orbit is 18,000. Now I am going to assume that you do gain some benefits in getting over the air resulting in less drag and that the fuel needed to overcome the inertia of starting at 0 is non-trivial. But even the Pegasus was a commercial flop and they were aiming for an under served market segment willing to pay just about anything to get to orbit. When, not if WHEN, SpaceX can launch, recover, refurb and relaunch their first stages in less than 60 days, the economics of this plane will collapse. What I see happening? Mount a telescope on that center pylon and fly to 70,000 feet. Sort of like SOFIA With the better engines and flight profile it has to be cheaper than flying a 747 with a giant hole in the side of its fuselage.