SpaceShipTwo Second Powered Flight

[magnemoe]

I don't think you really need to do all the maths to know the difference between a conventional plane's flight and orbit is so vast you're not really saving a huge amount.

If somebody was to design a 2 craft system that's practical for getting into orbit, you're looking at some of the original space shuttle concepts.

http://www.nss.org/resources/library/shuttledecision/chapter08.htm#intro

That thing would work in theory...... but that's what it'd have to be. A big rocket strapped to an enormous rocket, and that kills most of the useful aspects of Virgin's system such as the ability to land on standard runways, and the fast turnaround of the launch vehicle due to it having simple jet engines. Not to mention it was deemed to expensive even for NASA.

Note that that design is more like Falcon 9 reusable than Spaceship 2. Takeoff is standard rocket but landing like an glider not powered landing.

Some spaceplane ideas use plane+ upper stage, this probably work if you have an hypersonic plane. Drop upper stage at mach 7 at edge of atmosphere.

At this performance level an plane at lower stage makes sense. However we don't have any hypersonic planes.

It might even be an good idea to put an rocket engine on the plane, that way you drop payload at perhaps 100 km, can use an oversized transfer stage with twr <1 to both reach orbital speed and put the payload in correct orbit.