Skylon SSTO is a an ongoing venture from England built around a combined cycle engine called "SABRE" http://www.reactionengines.co.uk/space_skylon.html The program has been around for a long time, and , although it is definitely not vaporware, I am a bit cautious. Although the SABRE engine has been receiving funding last year, the amounts are dedicated to "studies" and they are not sufficient to develop the entire program. http://www.reactionengines.co.uk/press_release/Press_Release_17July2013_SABRE.pdf