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They have details on their web site:

http://www.reactionengines.co.uk/sabre_howworks.html

My reading of their explanation is that in a full system they would "pre-burn" some of the hydrogen and oxygen also used to power the main rocket motor to power the cooling of the helium that then cools the intake air.

[NB I have no idea whether that would actually work or not - just trying to interpret what their own explanation is!]

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Thanks for the link, this clears up a lot: http://www.reactionengines.co.uk/images/sabre/sabre_cycle_10...

There is no nitrogen in the system (not as consumable, anyway). Shoddy BBC reporting, as usual.

The heat exchangers use chilled hydrogen fuel they have to take up anyway, before it is burned in the rocket motor.

That makes a ton more sense.

To be fair to the BBC, what they describe is the test setup that has actually run, not the design of the full engine - which they haven't built yet.

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