World's Biggest Aircraft Will Ferry Passengers To Space In Stratolaunch Vision

What people aren't saying is that getting to the edge of space is only about 10% of the work of getting to orbit.

Simplistically, potential energy is mgh. g is about 10, we want to get to about 300 km, which is 3x10^5 meters. So potential energy per kg is about 3x10^6.

Orbital velocity is about 8000 m/s, & KE is mv^2/2, so orbital energy per kg is 8000^2/2 which is 32x10^6 which is around 3x10^7.

Orbital energy is about 10 times just getting high enough.

Sub-orbital flights are a good start, but getting into orbit is much, much harder.

I hadn't realized that there already is an air-launch-to-orbit system called "Pegasus" (http://en.wikipedia.org/wiki/Pegasus_(rocket) ).

It comments that air launches reduce the costs to orbit not (as ColinWright rightly points out) through energy savings but because it 1) doesn't have as many weather-related launch delays, 2) it can insert into almost arbitrary orbits, 3) there's no need for a blast-proof pad and related ground equipment, 4) launches over an ocean save on insurance costs, and reduce collateral damage should an explosion occur, and 5) the higher altitude means the first stage engine bell doesn't need to optimized for higher pressures, and the lack of high cross-winds means there's no need for gimbals; lighter fins suffice.

On the other hand, it also says that Pegasus is "one of the most expensive "launch-to-orbit" vehicles" (I assume per-kilogram to orbit), but that the flexibility in choosing the orbit makes up for it for small payloads which otherwise must piggyback.