Hi all,

As a long time observer of the Sci.Space.* groups, and of the space and aerospace industry I'm have become tempted to call Scram jets and other air-breathing solutions (including Sir Richard's noble attempts out in the US Midwest) as SCAM-jets. They're a neat way to keep engineers and scientists funded, but highly unlikely to ever produce an actual useful product. See NASA X-33 program, and possibly Orion and SLS as examples of this.

They all demand incredible technologies and enormous flight rates just to make them look even vaguely economical. AS David Spain pointed out, that Henry Spencer regularly points out, LOX is cheap, and TSTO operations and technology is relatively well understood.

Now the heat exchanger tech that Reaction Engines Sabre is supposed to use is incredible. Someone stated that it had basically shrunk the technology and machinery that used occupy an entire (large) building into a shipping container. But its still nowhere near light enough, or efficient enough to actually supply the Sabre's thirst for oxidizer.

And the sticky question remains: if you've cooled and compressed enough atmospheric oxygen to power your upper and out-of-atmosphere flight phase what did you do with the other 80% of the atmosphere you processed. Note if you are turning atmospheric Oxygen into LOX you can't avoid producing Liquid nitrogen on the way. You make LOX by compressing and cooling. Unfortunately LN always happens before LOX because N2 liquidifies at higher temperature and lower pressure than O2. Back on terra-firma the LN generated from LOX production is shipped off to the dozens (if not hundreds) of different industries that use it for making useful products, or cooling stuff down real fast or real low. On your way to LEO you have to ditch it, and that is just WAY TO MUCH WASTE ENERGY being thrown away.

Of cause if you could find a method to turn the O2 and N2 into one of the nitrous oxides and use that as an oxider instead... That might be interesting, and fun to watch from a minimum safe distance. And the technology, if light enough and efficient enough to go into an aerospace vehicle would be a very welcome addition to all sorts of manufacturing here on earth.

REgards
Frank