Musk's Falcon 9 separates at 50 miles, SR-71 tops out at round 18 miles.
Does launching at 18 miles and Mach 3 add enough DeltaV to change a suborbital shot to orbital?
The Falcon 9 second stage weights 93 tons though. Then again, it also delivers about 23 tons to LEO. Reduce that requirement and the rocket can be significantly lighter
The issue is that Mach 3 is nothing compared to orbital speed of Mach 25. What makes things worse is that the rocket equation includes a logarithm, so Mach 2.5 is not 10% of Mach 25 but much less.
Yes, but as Marathag said, you save the complexity of having a first stage that must punch through the troposphere. In other words, while dV numbers savings from sea level/static to 26km/Match 3 aren't that different, how much is that in terms of fuel mass?
Also, air launch means a fully reusable first stage (the aircraft), which makes launches significantly cheaper, as well as the flexibility of having plenty of "launch sites" - basically, anywhere with a long runway and depots of the fuel the F-12B would have used.
And a heavy enough air launched rocket would make it to LEO. The problem is what is "heavy enough" and whether the aircraft can support it and accelerate it to match 3 or not. The other question is whether the F-12B could make a quick dash, in order to give some additional starting speed to the rocket, without damage to the engines and airframe