Get ready to put that dilapidated VW bus out to pasture hippies, because Reaction Engines's A2 concept promises to reach speeds as high as Mach 5 on hydrogen fuel. That is around 2x the speed of the now defunct Concorde with zero carbon emissions. That means you could be out with Greenpeace bitching about Apple products on one side of the country, and back in your hometown on the other side well inside an hour—without feeling like you have harmed mother Earth.
The A2 is designed to carry up to 300 passengers and it will feature a a 2-mode engine (turbojet and ramjet propulsion systems) that will make it efficient at low speeds and able to shift into wicked high speeds with ease. Unfortunately, before the A2 can become a reality, engineers must tackle the problem of producing large quantities of hydrogen fuel without producing carbon emissions. This is certainly no simple task, so it appears that the hippies will have to hold on to those vans a little while longer. [PopSci via Green Blog via Treehugger]