In sci.space.history William Mook wrote:

The ones that reveal you do not understand how the momentum of
moving air becomes a drag force when you stop air to liquefy it to
extract oxygen from it.

http://www.reactionengines.co.uk/sabre.html suggests they are not
actually liquefying the air, only taking it down to the vapor boundary
(whatever that is . They present this as a difference from previous
"LACE" efforts. How much that really chances things I cannot say.

rick jones
--
The computing industry isn't as much a game of "Follow The Leader" as
it is one of "Ring Around the Rosy" or perhaps "Duck Duck Goose."
- Rick Jones
these opinions are mine, all mine; HP might not want them anyway...
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...