Studies recently completed for NASA show significant improvements in duct heating turbofans and low-bypass turbojets for an advanced supersonic transport. Dual-valve variable-cycle engines (VCE) prove to be noncompetitive; however, an emerging double-bypass dual-cycle VCE concept is reviewed which does indicate range improvements that are possible. It is 1985 technology and would require extensive development.Engine availability in the United States,as important to program success as technology or economics,is discussed. A fan derivative of an improved-technology Rolls Royce Olympus engine is also considered.An interesting case is developed showing that an Olympus fan derivative on a Douglas AST design offers an earlier U.S. SST possibility than heretofore thought possible. This might be important if airline pressure should mount as a result of overwhelming public demand for Concorde supersonic travel.