Re: Why air launch?
For a once off launch, ground launch is clearly cheaper, but at least 98% of the take off mass is lost. (Saturn V took off weighing 3,000 tons to get 40 tons on the moon and 10 tons back to earth). For repeated launches, once the cost of the bits lost getting to the air launched altitude match the cost of air launching, you are at break even, but after that is all profit. Potentially 20 or 30% lower costs per launch add up to a lot of money when you consider the current costs per kilo are (roughly) $5k to LEO and $30k to GEO.