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The Engineer Q&A: SABRE

This is your chance to ask the team behind one of the UK’s most exciting engineering projects, Skylon, about the technology behind its revolutionary air-breathing rocket engine, SABRE.

One of the UK’s most exciting engineering projects, the effort to develop and build Skylon, a single-stage-to-orbit spacecraft, has captured the imaginations of people both inside and outside the sector.

The team behind Skylon, at Culham-based Reaction Engines, has agreed to answer questions from readers of The Engineer about the propulsion system that will send Skylon into space.

The SABRE engine is unlike any technology that has been previously used for spaceflight.

Conventional launchers carry both their fuel and oxygen on-board, mixing and burning them to provide thrust.

SABRE, by contrast, uses the oxygen in the atmosphere, while it is available, to burn its hydrogen fuel, switching to on-board liquid oxygen only when the air is too thin to provide enough.

To achieve this, the air must be cooled drastically and rapidly as it enters the engine, and Reaction Engines has developed a new type of lightweight heat exchanger, combined with a frost control system, which uses the cold liquid hydrogen fuel as a chilling agent.

In air-breathing mode, SABRE will accelerate Skylon to MACH 5.4; once switched into rocket mode, it will take the craft up to orbital velocity of MACH 25.

You can read The Engineer’s 2010 feature about Skylon here, and visit Reaction Engines’ website here.


The Sabre engine, with its heat exchanger system in place

Use the comment box below to send us your questions about the SABRE engine and how it works; the heat exchanger system; and the possible applications of the SABRE engine, by noon tomorrow (28 September). We will pass the best ones on to the Reaction Engines team, and publish the answers in a feature next week.

Thanks for your questions. Comments are now closed.

Readers' comments (21)

  • As an Aerospace student I am an big follower of Skylon and the space exploration as a whole.

    Since you intend to answer questions on the Sabre engines, I am interested in how can Reaction Engines protect their Frost Control and heat exchangers technology's once Skylon begin entering comercial service?

    Also is it possible to carry out ground test to simulate the conditions of Mach 5 flight of a sabre engine?

    Anyway best of luck for the future and I hope that all goes well for this phase of testing.

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  • I should have asked a sabre question so here are a couple:

    * Are there applications for Sabre or its general technology beyond LAPCAT and Skylon? For example could one use it in the first stage of a 2-stage rocket? I know this might be anathema but I'm just wondering if there are ways to spread the cost of development or reach the point of generating revenue sooner.

    * what are the pain points/complexities in the engine after the precooler? The inlets come to mind as you have mentioned them before.

    * What other industries might the precooler find a use in? If, magically, it could be made a lot cheaper then how would it benefit aircraft engines? I'm not up to much with physics so the answer which is obvious to you isn't to me.

    * What about uses completely outside engines?

    * Forgive my ignorance but what about unstarts like the SR-71s experienced?

    * Have you decided what kind of nozzle to use? The information on the net about the e/d/ nozzle tests seemed to indicate something broke during testing and it was never clear if e/d was a step too far.

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  • Would love to hear more about the barriers that have/will occur to the development from govt, finance, eco doomsters, regulators - but sticking to the technical....

    What other applications do you see for the heat exchanger technologies and other technologies, manufacturing techniques and processes that are being developed?

    How much has the use of computer modelling/simulation etc aided this project - would it have been possible without it or just taken a lot longer?

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  • Could your revolutionary heat exchanger design, or aspects of it, be used for more mundane cooling applications, such as air conditioning or refrigeration? Or would the fridge also have to travel at MACH 5?!

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  • Are there plans to commercialise Sabre technology prior to full Skylon development in order to generate revenue to fund further Skylon development? Seems to me that Sabre is quite advanced in its development while Skylon is still only in the planning stages.

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  • I have some questions:

    1) once a SABRE engine is built and working, will it be possible to upgrade and improve the design to yield even greater efficiency?

    2) the exhaust from the SABRE engine is water vapour, which I understand is a powerful greenhouse gas. Has any thought been given to the possible effects on Earth's climate?

    3) assuming LAPCAT A2 is ever built, how expensive are flights likely to be? Will it be cheap enough to be mass transportation?

    4) The LAPCAT A2 is similar to a Russian supersonic bomber design of the late 50's - the Myasishchev M-50. Is this a coincidence, or was the earlier design an inspiration?

    5) will it be possible to build a bigger and more powerful version of Skylon which can carry more cargo?

    Thanks in advance

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  • Could the ramjet be fitted with flap valves to enable it to work at lower air speeds? as in the WW2 v1?

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  • What additional technical challenges are presented by the deep cryo precooler tests, as opposed to the "warm" cryogenic tests that have been performed to date? What modifications if any were needed to the Viper engine to allow it to run at low intake temperatures?

    How much margin on air breathing specific impulse do you have on the SABRE, whilst still being able to meet the payload specifications on the SKYLON D1?

    Have you considered using the NTV as a platform for other high-speed/high-altitude scientific or engineering research? Could e.g. a ballistic parachute be added to an NTV to allow for vehicle intact recovery?

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  • So is this thing going to have plasma engines as an add-on (space docked) for interplanentary travel later? I like the 100% re-usable craft concept, and the engine design appears sound. Looks a bit like the SR-71 Blackbird. Once you get above Mach 2, is the compressor really necessary below 70,000 ft?

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  • This is more of an airframe question, than Sabre-related.

    In my simple mind, dorsal re-entry seems to have advantages. Imagine re-configuring skylon slightly so that the vertical stabilizer is removed and replaced by two on the nacelles, much like SR-71. The aircraft would take off apparently "upside down", with the vertical stabilizers doubling as main gear struts. The craft would re-enter on its back, with the stabilizers, gear, and payload door out of the main heating zone. Advantages: a contiguous TPS with no holes at all, a TPS that is out of FOD danger during take-off and landing, and a lighter airframe because of dual-duty in the vertical stabilizers. Disadvantage: obviously, the payload has to handle loads from two directions, unless the suggestively circular cross-section of the payload bay could allow the hard points to rotate during flight.

    Apologies for being a bit late with the question: I'm on PST :-).


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