[tobiasu]

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.

[arethuza]

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.