Reaction Engines has passed the first development milestone for a hybrid rocket engine designed to power its single-stage-to-orbit Skylon spacecraft.

Passing the preliminary requirements review has kept the programme on track to launch a demonstration of a full-scale Sabre test engine before 2020, Reaction Engines says.

The UK-based start-up calls Sabre potentially the “greatest advance in propulsion since the jet engine”.

Unlike two-stage-to-orbit designs, the Skylon is designed to take-off from a runway and launch into space without a booster stage. At high altitudes, the air-breathing engine uses a pre-cooler system to dramatically raise the pressure ratio of oxygen entering the ramjet inlet. This cooled, compressed air is then mixed with liquid fuel and combusted in a rocket engine.

The latest review of the full-scale engine comes about three years after Reaction Engines completed a series of component-level tests.

The Skylon development project began nearly six years ago, after Reaction Engines received a development contract from the European Space Agency.

Reaction Engines is recruiting workers and buying special equipment to begin building Sabre pre-coolers in-house and to prepare for full-scale rig tests in 2019.

The existing design staff have relocated to larger offices at the Culham Science Centre near Oxford, while two manufacturing units have been consolidated in nearby Didcot.

The UK government has committed £60 million ($91 million) to the project, which is supplemented by an undisclosed amount of private capital.