Saturn Application Single-Stage-to-Orbit (1966)

NASA image

Even as early as the mid-1960s, improved propulsion concepts and reduced costs via reusability were being explored. In 1966, a team at Douglas Aircraft company pursued a series of launch vehicle concepts using a plugged nozzle or aerospike engine. They proposed an evolutionary development of existing hardware that would gradually incorporate and test this technology. The final step in this phased development was a piloted Single-stage-to-orbit (SSTO) vehicle based on S-IVB topped by a Gemini. 

This penultimate crewed design was called the Saturn Application Single-Stage-to-Orbit (SASSTO). The Gemini spacecraft design was the best fit for this concept, and it could be utilized with minimal changes from the NASA mission design. Their analysis suggested this reusable design could be lighter and cheaper than conventional launch vehicle designs with equivalent capabilities. This proposal never saw hardware and SSTO capabilities and plug-nozzle rockets are still mostly a dream.