One answer to this question can be found at a test site 2,900km (1,800 miles) away from Kitty Hawk at the foot of the Rocky Mountains in Colorado. The facility at the Colorado Air and Space Port was purpose-built by Reaction Engines to run "hot tests" of the technology that the company"s revolutionary new rocket engine depends on. Tests that were backed by the US government"s secretive Defence Advanced Research Projects Agency (Darpa).