“We are on the threshold of a major step change in aerospace propulsion,” said Nigel Whitehead, BAE Systems managing director according to the Financial Times after the British Aerospace company confirmed a £20.6 million ($13.5 million) investment in the jet engine research firm Reaction Engines.
Originally developed by an ex-Rolls Royce engineer, Alan Bond, and two partners, Reactions’ SABRE engine is said by the FT to have passed technical assessment by the US Air Force and the European Space Agency, and BAE’s stake will allow the firm to move forward to a ground-based demonstrator engine by 2020 by unlocking an additional £60 million ($39.38 million) of UK government grants plus the undoubted benefit of BAE’s aerospace expertise.
So what’s so special about the SABRE that justifies the “major step change in aerospace propulsion” claim?
SABRE is an acronym for Synergetic Air-Breathing Rocket Engine. Reaction claims the “air-breather” can achieve speeds of 2,500 mph, sufficient to reach low-earth orbit and hence launch satellites or to achieve passenger transport from London to Sydney in around 4 hours. Okay, that would be a major step change.
It does this with a reusable hybrid jet propulsion/rocket engine that is more neatly illustrated on Reaction Engines’ website but broadly combines a relatively conventional jet turbine with a rocket engine to achieve the best of both worlds.
Taking off conventionally, the jet engine burns air and hydrogen to achieve flight up to the edge of space from where the jet engine takes over to power the craft to hypersonic speeds. The secret is the provision of liquid oxygen for the rocket stage while it remains within the atmosphere. Conventional rockets carry hundreds of tons of liquid oxygen for the whole flight, but the Sabre compresses atmospheric oxygen to 140 atmospheres before injecting it into the combustion chamber saving hugely on fuel carrying capacity.
The air has to be pre-cooled to a near liquid state to prevent it from melting the engine components but, in the past, attempts to do this have floundered on the size and weight of the heat exchangers.
Heat Exchanger Breakthrough
With SABRE, this cooling of the airflow is done with small ultra-lightweight heat exchangers that cool the airstream from 1,000 degrees C to minus 150 degrees C in less than 1/100th of a second without adding substantial weight and complexity to the engine.
SABRE’s heat exchangers are said to be 100 times lighter than any current technology allowing them to be used in this way for aerospace applications for the first time. The firm claims using these miniaturized heat exchangers will have an impact on aerospace propulsion systems comparable to the impact of the silicon chip on computing: creating new products, new markets and new capabilities.
The Skylons Are Coming
Reaction Engines’ hypersonic aircraft/spacecraft design is called Skylon and is expected to be capable of up to Mach 5.5 in air-breathing mode up to an altitude of 25 kilometers – which is 20% of the speed and 20% of the altitude needed to reach orbit, before switching to rocket mode capable of achieving Mach 25 and low-earth orbit. The initial mission will be to challenge the likes of Space X and Orbital Sciences in the satellite launch market but the technology also opens up the possibility of passenger transport and, of course, defense, a market wherein Northrup Grumman has had a long-term project to build a hypersonic bomber.