[tobiasu]

How and where does the nitrogen boiler(?) get its energy too cool helium to -170C at a constant rate? Is it effective to chill what must be ginormous amounts of air to -150C? It's all nice that the heat exchanger works, but where does that heat go?

There's a large discrepancy between the inlet air temperature (1000C vs 20C) in the article.

Somehow my sense for violation of the laws of thermodynamics is tingling.

[arethuza]

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!]

[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.

[pja]

In the tests it's presumably just a continuous flow of pre-cooled liquid nitrogen from a tank & the heated N2 is probably dumped into the environment.

The design for a flight-capable engine is supposed to dump the waste heat into the fuel I believe.