I did some digging, reaction engines are jet engines that can transition into a rocket propelled mode. It could enable a single stage to orbit plane.
Within the atmosphere the jet engine will burn atmospheric oxygen which would theoretically reduce the amount of fuel needed to reach a certain delta-V. Once outside of the atmosphere the engine switches to a rocket propelled mode, usually by injecting stored oxygen into the engine in addition to jet fuel.
So far there hasn't been a working prototype of a reaction engine. A British company SABRE closed down due to lack of funding but it did prove several pieces of the reactive engine design could work independently.
Within the atmosphere the jet engine will burn atmospheric oxygen which would theoretically reduce the amount of fuel needed to reach a certain delta-V. Once outside of the atmosphere the engine switches to a rocket propelled mode, usually by injecting stored oxygen into the engine in addition to jet fuel.
So far there hasn't been a working prototype of a reaction engine. A British company SABRE closed down due to lack of funding but it did prove several pieces of the reactive engine design could work independently.