A UK company which hopes to build a re-usable space plane has won an important endorsement from the European Space Agency (ESA).
Reaction Engines believes its novel Sabre engine, which would operate like a jet engine in the atmosphere and a rocket in space, could displace rockets for space access and transform air travel by bringing any destination on earth to no more than four hours away.
That ambition was given a boost this week by ESA, which has acted as an independent auditor on the Sabre test programme.
"ESA are satisfied that the tests demonstrate the technology required for the Sabre engine development," the agency's head of propulsion engineering Mark Ford told a news conference.
"One of the major obstacles to a re-usable vehicle has been removed," he said.
"The gateway is now open to move beyond the jet age."
The space plane, dubbed Skylon, only exists on paper.
What the company has right now is a remarkable heat exchanger that is able to cool air sucked into the engine at high speed from 1,000 degrees Celsius to minus 150 degrees in one hundredth of a second.
This core piece of technology solves one of the constraints that limit jet engines to a top speed of about 2.5 times the speed of sound, which Reaction Engines believes it could double.
With the Sabre engine in jet mode, the air has to be compressed before being injected into the engine's combustion chambers.
Without pre-cooling, the heat generated by compression would make the air hot enough to melt the engine.
The challenge for the engineers was to find a way to cool the air quickly without frost forming on the heat exchanger, which would clog it up and stop it working.
Using a nest of fine pipes that resemble a large wire coil, the engineers have managed to get round this fatal problem that would normally follow from such rapid cooling of the moisture in atmospheric air.
They are tight-lipped on exactly how they managed to do it.
"We are not going to tell you how this works," said the company's chief designer Richard Varvill, who started his career at the military engine division of Rolls-Royce.
"It is our most closely guarded secret."
The company has deliberately avoided filing patents on its heat exchanger technology to avoid details of how it works - particularly the method for preventing the build-up of frost - becoming public.
The Sabre engine could take a plane to five times the speed of sound and an altitude of 25 km, about 20 per cent of the speed and altitude needed to reach orbit.
For space access, the engines would then switch to rocket mode to do the remaining 80 per cent.
Reaction Engines believes Sabre is the only engine of its kind in development and the company now needs to raise about £250 million to fund the next three-year development phase in which it plans to build a small-scale version of the complete engine.
Scramjets on test vehicles like the U.S. Air Force Waverider, also use atmospheric air to create thrust but they have to be accelerated to their operating speed by normal jet engines or rockets before they kick in.
The Sabre engine can operate from a standing start.
Sabre produces thrust by burning hyrdrogen and oxygen but inside the atmosphere it would take that oxygen from the air, reducing the amount it would have to carry in fuel tanks for rocket mode, cutting weight and allowing Skylon to go into orbit in one stage.
If the developers are successful, Sabre would be the first engine in history to send a vehicle into space without using disposable, multi-stage rockets.
Skylon is years away, but in the meantime the technology is attracting interest from the global aerospace industry and governments because it effectively doubles the technical limits of current jet engines and could cut the cost of space access.
The heat exchanger technology could also be incorporated into a new jet engine design that could cut 5 to 10 per cent off airline fuel bills.
That would be significant in an industry where incremental efficiency gains of one per cent or so, from improvements in wing design for instance, are big news.
Sabre will be a new engine class that can operate in both air-breathing and rocket modes.
* Alan Bond, a former British aircraft engineer, began his career with Rolls-Royce's Rocket Division, and went on to design the HOTOL space plane.
The HOTOL or "Horizontal Take Off and Landing" project was to be fitted with an air-breathing engine, the RB454, which would extract the air's oxygen to burn.
Once the air became too thin, a small supply of Liquid Oxygen would be used to place the craft in orbit.
Around 7 tons of cargo could have been orbited for an estimated cost of £5 million, less than for a Space Shuttle launch.
The HOTOL study was launched in 1986, but two years later the government refused to fund it further.
In 1989 Bond helped form Reaction and designed its new concept craft, Skylon.
* Designing a single stage to orbit propulsion system has been unsuccessful till now, largely due to the weight of the on-board oxidiser including liquid oxygen, needed by conventional rocket engines.
A possible solution was to reduce the weight by using oxygen in the atmosphere in the combustion process just like an ordinary jet engine.
The saving in weight would enable single stage launch vehicles to be re-usable.
Ultra-lightweight heat exchangers are the key enabling components in the engines for Mach 5 cruising speed and aircraft-like access to space.
* This new approach would enable Sabre-powered vehicles to save carrying over 250 tons of on-board oxidant on their way to orbit, and removes the necessity for massive throw-away first stages that are jettisoned.
* Sabre could be the first engine to achieve this goal by operating in two modes: initially in air-breathing mode and then in conventional rocket mode.
The rocket engine sucks in atmospheric air as a source of oxygen (as in a typical jet engine) to burn with its liquid hydrogen fuel in the rocket combustion chamber.
Secondly in conventional rocket mode - the engine is above the atmosphere and transitions to using on-board liquid oxygen.
* The Skylon space plane only exists on paper. It would use the Sabre engine, could operate without a pilot, and be capable of transporting 15 tonnes of cargo into space.
Lapcat is a concept passenger aircraft that could use the Sabre engine to cut flight time from Brussels to Sydney to less than 4 hours.