New British Engine Will Raise Airliner Speeds To Over 400 Miles An Hour. The Lincoln bomber in flight powered by its two Theseus turbine-propeller engines. The two Merlin's have been put out of action. A new Bristol aero engine, the Theseus, which is likely to raise the speed of passenger airliners from around 250 miles an hour to over 400 miles an hour was demonstrated at Bristol by the Bristol Aeroplane Company. The engine, in which a gas turbine is made to drive a propeller, is designed to ma