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.

I have a friend whose dad finished his engineering career at SABRE (he retired some years before it shut down). According to him, the original concept wasn't far-fetched, but that they didn't crack some manufacturing / materials issues. He thought they should have continued to work on those, instead of redesigning to avoid them. He had retired by the time we spoke about it, and I don't know the particular model / design to which he was referring. If that's accurate, then possibly materials technology / manufacturing techniques have matured to the point that the "simpler" design he preferred could be feasible. That's all second-hand speculation, so take it with a dose of salt, but a short timeline might be reasonable.