[drgath]

That's pretty much the approach of Virgin Galactic. Reasons why it works for them, but not for NASA would have to be explained by someone more qualified. I do know that VG barely gets into LEO, whereas NASA's requirements are much higher. In any case, I presume the scientists at NASA (and SpaceX & co) have considered just about every alternative, and the current approach still remains the most efficient for their needs.

[danielweber]

Orbital Sciences is doing something similar, IIRC.

They are the two big approaches to cheap space being done now. 1) Be cost-effective at making rockets, 2) launch from jets. My money is on #1, which SpaceX is doing, but #2 has some things going for it that could prove me wrong. #2 is also strictly limited in just how big you can make a payload, while for #1 you can get up to around 200 ton payloads before things start working against you.

[danieldyer]

The issue is that people confuse getting into space with getting into LEO. Virgin Galactic are not even close to getting into LEO – their plan is to fly a ballistic trajectory which takes them just above 100km before falling back to Earth.

Since they don't need to achieve anywhere near the required velocity to enter LEO, they can use a much smaller solid-fuel rocket engine and launch from a jet.

[dragonwriter]

I think there are strength/scaling issues with winged aircraft that, given the fuel requirements for any sizable payload to orbit even from the speed/altitude a jet can achieve, makes this extremely challenging, but its something people keep working on.

[sehugg]

I'm sure this has been studied to death, but my guess is that the extra risk during flight/separation, and the extra weight of designing the rocket to handle the different forces encountered when attached to the mothership are not compelling enough to make it worth the extra few % in delta-V. (SpaceShipOne went to Mach 3, not 25, so the boost was greater by percentage)

[teraflop]

Not even LEO; so far, all of Virgin's flights have been strictly suborbital.