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.

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 [1] [2] As well as being one of the first engines to feature a free propeller turbine, the Theseus was the first turboprop in the world to pass a type test in January 1947.

[3] Following 156 hours of ground runs and the receipt of a test certificate from the Ministry of Supply on 28 January 1947, two Theseus engines were fitted in the outer positions of a four-engined Avro Lincoln for air tests.