This is so funny, I was just watching "Modern Marvels - Weird Weapons of WWII (The Axis)". Turns out, a similar approach was discussed by Werner von Braun (the father of modern rocket science, basically the most important guy other than Goddard in that whole field) to create a "space plane" with the capability of quickly dropping a (radioactive, but not atomic) bomb on a target anywhere in the world, but more specifically major US cities. One major difference was that the track to launch the space plane was straight, and not curved. Given it was curved, would they have still been able to launch the lightweight space plane with just enough fuel it needs to get up to LEO, or would they still have run into the issue that they can't get the plane to go fast enough and point upward enough to break the atmosphere?

Source: http://youtu.be/EHZC9Hab05s?t=2m26s