UK engineers have begun critical tests on a new engine technology designed to lift a spaceplane into orbit.
The proposed Skylon vehicle would operate like an airliner, taking off and landing at a conventional runway.
Its major innovation is the Sabre engine, which can breathe air like a jet at lower speeds but switch to a rocket mode in the high atmosphere.
Reaction Engines Limited (REL) believes the test campaign will prove the readiness of Sabre's key elements.
This being so, the firm would then approach investors to raise the £250m needed to take the project into the final design phase.
"We intend to go to the Farnborough International Air Show in July with a clear message," explained REL managing director Alan Bond.
"The message is that Britain has the next step beyond the jet engine; that we can reduce the world to four hours - the maximum time it would take to go anywhere. And that it also gives us aircraft that can go into space, replacing all the expendable rockets we use today."
To have a chance of delivering this message, REL's engineers will need a flawless performance in the experiments now being run on a rig at their headquarters in Culham, Oxfordshire.
The test stand will not validate the full Sabre propulsion system, but simply its enabling technology - a special type of pre-cooler heat exchanger.
Sabre is part jet engine, part rocket engine. It burns hydrogen and oxygen to provide thrust - but in the lower atmosphere this oxygen is taken from the atmosphere.
The approach should save weight and allow Skylon to go straight to orbit without the need for the multiple propellant stages seen in today's throw-away rockets.
But it is a challenging prospect. At high speeds, the Sabre engines must cope with 1,000-degree gases entering their intakes. These need to be cooled prior to being compressed and burnt with the hydrogen.
Reaction Engines' breakthrough is a module containing arrays of extremely fine piping that can extract the heat and plunge the intake gases to minus 140C in just 1/100th of a second.
Ordinarily, the moisture in the air would be expected to freeze out rapidly, covering the pre-cooler's pipes in a blanket of frost and compromising their operation.
But the REL team has also devised a means to stop this happening, permitting Sabre to run in jet mode for as long as is needed before making the transition to a booster rocket.
Sabre engine: How the test will work