Guy Norris Los Angeles

Seeing is believing, as the saying goes. That's certainly what Reaction Engines hopes as it moves deeper into advanced tests of a novel pre-cooler that it believes could revolutionize air-breathing access to space.

Designed to chill engine inlet air by more than 1,150C (2,100F) in less than one hundredth of a second, the pre-cooler is one of the key enabling technologies at the heart of the Sabre (synergetic air-breathing rocket engine). The hybrid propulsion system is designed for either atmospheric transport at speeds around Mach 5 or as the air-breathing element of a single-stage-to-orbit spaceplane dubbed Skylon.

Having fought for credibility for more than two decades, the small U.K.-based team behind the development of Skylon and its combined-cycle rocket engine are seeing growing interest in the technology, as crucial tests of the pre-cooler element continue at its site near Oxford, England.

The pre-cooler functions as a heat exchanger, cooling air from the atmosphere to cryogenic temperatures as it enters the engine via an axisymmetric inlet. The denser air passes through a turbo-compressor and into the rocket combustion chamber where it becomes an oxidizer to be burned with sub-cooled liquid hydrogen fuel. A closed-cycle helium loop forms part of the heat exchanger system and uses hydrogen fuel as a heat sink before it enters the combustion chamber.

“We breathe air for as long as we can up to M 5.5,” says Reaction Engines' Roger Longstaff. After this point, the Sabre transitions to pure rocket mode, burning a combination of liquid hydrogen and liquid oxygen from internal tanks. Throughout most of the atmospheric flight, the intake captures more air than required. Excess air passes down a spill duct that incorporates a burner to recover some of the drag losses.

While the concept itself is not new, no one has so far succeeded in implementing a rocket-based combined-cycle propulsion system that derives the oxidizer from the atmosphere. This is why the main focus for the initial testing is on the pre-cooler and anti-frost systems; the basic feasibility of both was confirmed during independent audits carried out in 2011 by experts from the European Space Agency.

“To date, we have undertaken over 200 test runs of varying duration and purpose,” says Longstaff, who adds that earlier issues with the test rig that slowed the pace of work this summer appear to be resolved. “I can confirm that the test rig is performing nominally and testing continues.” Initial tests of the pre-cooler to date “have confirmed aerodynamic stability and uniformity, structural integrity, freedom of vibration across a wide range above and beyond the flight envelope.”