The trick is, and it is a trick, to "instantly cool the air". Think about that for a moment. Generally the way most 'air breathing' engines work is that they take air, heat it (which expands it (thanks Boyle!)) and toss it backwards which imparts a forced in the forward direction (thanks Newton!). The difference in temperature between the inlet and exhaust temperature of a jet engine is a component of its efficiency. (see <a href="http://en.wikipedia.org/wiki/Jet_engine#Energy_efficiency" rel="nofollow">http://en.wikipedia.org/wiki/Jet_engine#Energy_efficiency</a> for some math :-)<p>But basically if you're suddenly cooling air it gets smaller, which creates a lower pressure, and was used effectively in steam pumps to create the vacuum needed to lift water out of mines.<p>So the Reaction Engines concept takes really hot air that is compressed because it is being pushed out of the way by the air frame, then super cools it which causes it to become much smaller, and then dumps that into the engine. Which then heats it up again and pushed out.<p>If I understand the theory correctly, it takes heat from the air, removes it and puts it into liquid hydrogen (which then becomes vaporous(sp?) hydrogen) and then burns that hydrogen with residual oxygen to put the heat back into the air plus what ever is extra from the hydrogen. What bugs me about it is that it seems to double count the energy in the air rather than single count it. But I'm interested in seeing one of their machines in action. If someone knows whether or not there is a violation of thermodynamics in there I'd like to understand that.<p>That said a mach 5 vehicle with a range of 12,000 miles and a payload capacity of 15 tons has much more interesting uses than carrying passengers I think. Would make a heck of a cruise missile.