The promise of hypersonic flight sending us halfway around the world in a matter of hours is being bandied about again, this time by a British company that declares, with all due humility, that it has made "the biggest breakthrough in aerospace propulsion technology since the invention of the jet engine."

Reaction Engines Limited says its hypersonic engine will send us streaking across the sky at speeds well over Mach 5, allowing us to have bagels for breakfast in New York and sushi for lunch in Tokyo. The hypersonic engine design reportedly includes new ways of cooling the air for an engine that will use oxygen in the atmosphere up to Mach 5.5 before switching to rocket power for the ride in space.

Hypersonic flight has long topped the list of dreamy aerospace ideas. The military loves the idea of super-fast missiles and even bombers, while the rest of us dream of flying from the Big Apple to Tokyo in just a few hours. The big problem has been propulsion. At speeds beyond Mach 2 or so, a jet engine has trouble getting the oxygen needed for combustion. It's sorta like trying to take a deep breath by sticking your head out the window at 200 mph.

Solving this problem is not impossible, but neither is it terribly practical. Kelly Johnson's SR-71 Blackbird design used very creative ways to handle the incoming air needed to achieve a record-setting Mach 3+ speeds. But past that it gets really tough, and dealing with all that heat poses another challenge.

So far, only rocket engines have been capable of practical hypersonic flight, but the vehicles that use them require multiple stages to reach space. To avoid the cumbersome need to carry a supply of oxygen, as is done on rockets and was used by the space shuttle, engineers have struggled with an air-breathing design that can operate in the hypersonic speed range as a first stage.

A look at how the engine works. Photo: Reaction Engines Ltd.

Reaction Engines claims it's cracked the problem with a design that could allow a vehicle to take off, reach orbit using a combination of an air-breathing engine and rocket, then return to Earth. The secret is cooling the air as it enters the hypersonic SABRE engine.

"[The] pre-cooler technology is designed to cool the incoming airstream from over 1,000 Celsius to minus 150 Celsius in less than 1/100th of a second, without blocking with frost," the company claimed in its press release.

It's a promising design that tackles one of the bigger problems facing hypersonic engines: the enormous amount of heat generated when you compress air at extremely high speeds. The air-breathing engine will accelerate a vehicle to about Mach 5.5, according to the company, after which a liquid oxygen tank will supply a rocket engine for the portion of the flight in space. But unlike current space vehicles, there will only be one stage involved for the entire flight thanks to the boost from the SABRE design.

The European Space Agency says it has evaluated the pre-cooler design and says it is satisfied that the design should move forward. The agency is negotiating a contract to help support the further development of Reaction Engines' design.

Reaction Engines said it has completed more than 100 test runs of the cooling system and it hopes to have a sub-scale ground engine running by 2015. But as the X-51 Waverider team has discovered, it's a long road, er, flight from a new component breakthrough to hypersonic flight.