The Theseus was the Bristol Aeroplane Company's first attempt at a gas-turbine engine design. A turboprop delivering just over 2,000 hp (1,500 kW) was chosen rather than compete with companies that were already developing turbojets. A heat exchanger to transfer waste heat from the exhaust to the compressor exit was necessary to meet a requirement for a fuel consumption comparable to a piston engine. However, the heat exchanger was abandoned after tests showed that it had a high pressure loss and only produced a fuel saving of 8%. Furthermore overall performance was reduced

The Bristol Proteus was the Bristol Engine Company's first mass-produced gas turbine engine design, a turboprop that delivered just over 4,000 hp (3,000 kW). The Proteus was a reverse-flow gas turbine. Because the second turbine drove no compressor stages, but only the propeller, this engine was classified as a free-turbine. It powered the Bristol Britannia airliner, small naval patrol craft, hovercraft and electrical generating sets. It was also used to power a land-speed record car, the Bluebird-Proteus CN7. After the merger of Bristol with Armstrong Siddeley the engine became the Bristol Siddeley Proteus, and later the Rolls-Royce Proteus. The Proteus was to have been superseded by the Bristol Orion which would have given a Britannia a 75% increase in power for cruising faster.