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.
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