In the long term, scramjets offer the possibility of cheap access to space.

ECONOMIST: Hypersonic aviation

The X-43A, or "Hyper-X" was built specifically to test scramjets, or supersonic ramjets, which burn fuel in air travelling at supersonic speeds.

BBC: Mach 10 scramjet prepares for launch

However, this process only starts happening efficiently at Mach 5, which means conventional rocket technology is required to get the scramjets to their critical operating speed.

BBC: Experimental jet 'a success'

Scramjets are much lighter than conventional engines of equivalent power.

BBC: Experimental jet 'a success'

Whilst much of the attention surrounding scramjets has focussed on the shorter journey times they could bring to long-haul passenger air travel, the first applications are likely to be in the space delivery business - launching small payloads, such as communications satellites, into orbit.

BBC: Experimental jet 'a success'