In article ,
says...

http://what-if.xkcd.com/38/

An interesting "what if".

(And Bob, this is what makes some of your plans for "retrieve this or that"
so economical.)

Yes, Bob should read this.

Jeff
--
"the perennial claim that hypersonic airbreathing propulsion would
magically make space launch cheaper is nonsense -- LOX is much cheaper
than advanced airbreathing engines, and so are the tanks to put it in
and the extra thrust to carry it." - Henry Spencer