Single-stage-to-orbit
The Skylon is a proposed SSTO that was first revealed in 1993. The Skylon would be propelled by two hybrid-cycle rocket engines. They are called SABRE engines. The SABRE engines would have two modes of operation: in atmosphere, they would act as airbreathing engines; at very high altitudes (approaching the edge of Earth's atmosphere), they would switch to a closed cycle rocket mode, using on-board liquid oxidizer to continue fuel combustion and propel the craft.