Isn't that Thunderbird 3...?

I still don't get why we launch rockets from the ground, it seems horribly inefficient. We have planes that can basically reach the edge of space, why not just fire a vastly smaller rocket engine once you're already up there to get the final push (kinda like what virgin galactic is doing)? Seems way cheaper and easier and I'm surprised more space programs aren't trying it.

Oh I think I know this one 

Attaining an orbit is about speed, rather than height. It doesn't matter how high you are in the atmosphere or even above it when you start, your craft still needs to achieve a velocity of about 7 km/s or 15,000 mph to achieve an orbit around earth. Otherwise you just fall back down again. That's a lot of thrust, and if you're launching a lot of weight (which is usually mostly fuel), it's actually easier to start from zero speed on the ground rather than half way up at X speed, where you'd need a very powerful engine to take off and get you going, as it were. The current airspeed record of a jet is the SR-71 blackbird, and that's a 'mere' 2000 mph (according to wiki mind) without carrying something else like a payload. That's a big difference in speed which rockets then need to accomplish, which require substantial fuel, weighing down the plane further. Unfortunately, the business of getting into space is this awful race between engine and weight and the ratio between fuel used and actual payload put in orbit is almost sickening. Unfortunately, our currently most efficient method of thrust needs all that fuel weight to function, as rockets work on the principle of ejecting all that mass to provide acceleration (reaction thrust). It's all horribly inefficient but it's all we have for the moment and in the grand scheme, the 2,000 mph speed boost you'd get from a plane isn't worth it.

Remember, SpaceShipTwo (Virgin's plane) is only suborbital, so it's never attaining the necessary speed to go into orbit (wiki says it's top speed is 4000 km/h) and it's also reasonably lightweight so it doesn't need to hitch a ride on a monster to get into the air for the first leg. The speed difference in this case does make it worthwhile.

I think at the start of the thread I posted about the Skylon spaceplane, which is still in development and involves a special hybrid engine which gets the plane up to around Mach 6 and then starts burning like a rocket. The idea of a SSTO spaceplane has been around for decades, and no-one has really figured out how to make it work. The Skylon people are closest, but it's still years away with many more hours of testing and engineering. It is, in my view, the future and could dramatically change the way we do space.

edit - DAMN YOU TREY!!

They keep trying air launches, but the plane part basically just replaces the disposable first stage, and in the end it's more complicated and expensive than just starting with rockets on the ground. The best you can hope for is a reusable rocket first stage, like Space X is trying to build.

If anyone DOES manage to do it, it will probably be the military, as the ability to put something small into orbit without a large, easy to detect ground launch might have uses worth the price. Hell, the small unmanned mini-shuttle they're playing with might be air launched at some point.