This paper considers issues relevant to propulsion design for Single Stage To Orbit (SSTO) vehicles. In particular two air-breathing engine concepts involving precooling are considered, these being the SABRE (Synergetic Air-Breathing and Rocket Engine) as designed for the SKYLON SSTO launch vehicle, and the LACE (Liquid Air Cycle Engine). It is shown that through entropy minimization the SABRE has made substantial gains in performance over the traditional LACE precooled engine concept, and has shown itself as the only credible means of realising a SSTO vehicle. Furthermore, it is demonstrated that the precooler is a major source of thermodynamic irreversibility within the engine cycle and that a further reduction in entropy can be realised by increasing the heat transfer coefficient on the air side of the precooler. If this may be achieved, an increase of between 5 and 10% payload mass delivered to orbit by the SKYLON launch vehicle is possible.