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.

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While expansion deflection (ED) nozzles have traditionally been considered primarily for use as altitude compensating devices to improve the performance of single stage to orbit vehicles, they also offer the potential for enhancing high altitude propulsion systems. If intended to only operate in near vacuum conditions, the complexity of analysis and inherent risks involved in the ED concept are greatly reduced. An integrated approach to the design and performance analysis of such nozzles is presented, comprising a mixture of computational fluid dynamics, the method of characteristics, and a semi-empirical model to allow full analysis of the closed wake flow-field of an ED nozzle. While it is demonstrated that the influence of the parameters used to define the throat region is critical to the successful application of the ED nozzle, it is also shown that with careful design the weight savings possible are significant. The analysis method itself is flexible and rapid, and lends itself well to incremental improvements in accuracy as the flow under consideration becomes better understood.

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