SKYLON spacecraft's engine passes critical test


November 30, 2012

A rendering of SKYLON in flight, showing the SABRE engine

A rendering of SKYLON in flight, showing the SABRE engine

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Reaction Engines Ltd. announced on Wednesday the completion of a critical round of testing of its SABRE engine’s precooler system. The SABRE is a radical type of hybrid jet/rocket engine capable of propelling a spacecraft into orbit or an aircraft in the atmosphere, at a velocity of Mach 5 (3,800 mph, 3,300 knots, 6,115 km/h). It’s intended for Reaction Engines’ SKYLON spacecraft and its airliner derivative, the LAPCAT A2 hypersonic aircraft.

The company had carried out its own tests earlier this year at its facility in Oxfordshire, UK and this series was done under the supervision of the European Space Agency (ESA) on behalf of the UK Space Agency to provide official validation of the technology. According to Reaction Engine’s press release, "the pre-cooler test objectives have all been successfully met and ESA are satisfied that the tests demonstrate the technology required for the SABRE engine development."

Skylon itself is something of a radical departure as well. It’s a planned Single Stage To Orbit (SSTO) spacecraft that's intended to take off and land at conventional airports. Unlike many hypersonic craft, SKYLON will take off and accelerate to hypersonic speeds under its own power using the SABRE engine, without the need of a mothership or rocket boosters. The engine is thermodynamically simple, but extremely complex in engineering. It is designed to be extremely lightweight, with the skin of some components being thinner than a human hair.

Conventional rockets require special launch facilities and need to carry along the oxygen needed to burn their fuel. This adds weight and reduces the payload that can be delivered to orbit. If the rocket is a SSTO designed to return to Earth, the payload is even smaller. SKYLON’s SABRE engine reduces the need for carrying so much oxygen by using air like a jet, to burn its fuel for part of the ascent.

SKYLON's internal layout

SABRE is basically a rocket engine that uses a precooled compressor for part of the ascent. It acts as an air-breathing jet until it reaches Mach 5 and an altitude of 25 kilometers (15.53 mi). By this time, it’s already 20 percent on its way to space. For the other 80 percent, SABRE converts to a pure rocket mode using its onboard store of liquid oxygen instead of air to loft into orbit at a speed of Mach 25 (19,000 mph, 16,500 knots, 30,600 km/h).

Since it works at everything from a dead stop to escape velocity, the SABRE engine not only needs to be able to switch from jet to rocket mode, it also needs to be able to reconfigure its geometry in flight to accommodate the constantly-changing pressure and temperature of the air blasting into it. Imagine being caught in a wind 25 times stronger than that of a Category 5 hurricane, and you can see the magnitude of the problem.

The SABRE engine in air-breathing mode

The SABRE engine requires the incoming air to be compressed to 140 atmospheres. This compression makes the air so hot that it would melt any known material, so the incoming air needs to be pre-cooled until its nearly a liquid. Previous experimental engines used the cryogenic hydrogen fuel to cool the heat exchangers, but this wasted fuel and caused all sorts of problems, such as making metals brittle.

SABRE gets around this by using a closed-loop helium cycle. This is designed to cool the incoming airstream from over 1,000ºC (1,832ºF) to -150ºC (-238ºF) in less than 1/100th of a second without blocking with frost. It’s less wasteful of the hydrogen, which keeps the helium loop cold and avoids hydrogen brittling, so more heat-resistant materials can be used. Once cooled, a “relatively conventional” turbo compressor using jet engine technology can be used to compress the air to the required pressure – “relatively” being the operative word for an engine designed to go into space.

Reaction Engines is still years away from a completed engine and the construction of SKYLON is years after that, though the company remains optimistic and is currently seeking additional funds to continue development.

Source: Reaction Engines, LTD

About the Author
David Szondy David Szondy is a freelance writer based in Monroe, Washington. An award-winning playwright, he has contributed to Charged and iQ magazine and is the author of the website Tales of Future Past. All articles by David Szondy

I always miss the application of the Whitcomb area rule in the Skylon airframe.


re; kwarks

Whitcomb's area rule only applies in high speed atmospheric flight. On launch the SKYLON spend very little time in atmosphere thick enough for it to matter, on reentry and landing the additional drag helps slow the craft down and not sculpting the fuselage simplifies design and manufacture and very well might make it stronger.


cautiously optimistic, however, remember when we had those ultra efficient aerospike engines and the whole project was brought down because the fuel tanks weren't strong enough?

Michael Kruger

@ Kraft, Pikeman, the fuselage design of Skylon is actually very close to a Sears-Haack aerodynamic body - look it up. With the exception of the control surfaces and the wings/engines, it's aerodynmically quite well sculpted, and conversely to Pikemans comment about drag, I think the most challenging aspect of this design for achieving orbit is deceleration on re-entry, it looks quite slippery to me, it's bound to need some sort of air brake.


All very well, but why is the engine banana shaped? No explanation given.



Quote from Mark Hempsell of Reaction Engines.

"Why a Curved nacelle? – the most frequently asked technical question. The answer is: the air intake on the front of the nacelle needs to point directly into the incoming airflow whereas SKYLON’s wings and body need to fly with an angle of incidence to create lift, so the intake points down by 7 degrees to account for this. The rocket thrust chambers in the back of nacelle need to point through the centre of mass of the vehicle so are angled down; again by 7 degrees but it is a coincidence the angle is the same."


To Mutley: Many thanks for the explanation. This curvature is not without precedent, of course. The McDonnel F-4 Phantom also had markedly curved down tail pipes for its twin turbojets. However, the Lockheed F-104 Starfighter had the tailpipe slightly curved UP! I wonder what the explanation was there.



Starfighter pilots were all hotshots who spent most of their flight-time in the "inverted position". With this in mind... (grin)

Edwin Wityshyn

To: Edwin - Ah, now I understand. Furthermore, whilst inverted the Starfighter would have marked dihedral in the the wings, leading to increased stability. No wonder everything went to their heads!


In order to cool the air as fast as claimed, the tubes or plates must be very close together. This requires tiny air flow passages. The heat exchanger must necessarily be in the path of the oncoming air, so it will produce significant drag. It will also be subjected to enormous turbulence, which will require substantial weight. Will the gain in performance overcome this burden?

David Mayer

Glad to see new technology in flight! as an A&P I understand the heating problems at these speeds Really hope it works out,be a major leap in aviation

Dave Ussery
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