SABRE was a concept under development by Reaction Engines Limited for a hypersonic precooled hybrid air-breathing rocket engine. The engine was designed to achieve single-stage-to-orbit capability, propelling the proposed Skylon spaceplane to low Earth orbit. SABRE was an evolution of Alan Bond's series of LACE-like designs that started in the early/mid-1980s for the HOTOL project. Reaction Engines went into bankruptcy in 2024 before completing the project.