In late 1925 and early 1926, the RAE published a series of papers on the sleeve valve principle. In short, the sleeve valve replaced the normal poppet valves in the engine with a rotating sleeve inside the cylinder. The sleeve rotated, and holes in the sleeve and cylinder lined up to open and close the valve. The advantages were primarily simplicity and that less energy is needed to run the system. However at higher powers and RPMs, when the engine needs to move considerably more air and so so more quickly, the sleeve design comes into its own. The sleeve is also much easier to "drive" than the poppet, there are no pushrods or rockers needed just a gear at the base of the cylinder, so it is a much better design to use in "dense" two-row engines where the pushrods otherwise take up considerable space. It was this "future expansion" capability that interested Roy Fedden when he first read of the work.
By 1927 Fedden had built a working two cylinder V as a testbed, with the idea of developing it into a V-12. However several problems cropped up on the design, notably that the sleeves tended to burst during the power stroke and strip their driving gears. This led to a long series of tests and materials changes and upgrades that required six years and an estimated 2 million pounds, but by 1933 the problems had been worked out.
The result was a Jupiter-sized engine adapted to the sleeve system, the Perseus, and it's smaller cousin, the Bristol Aquila. The first production versions of the Perseus were rated at 580 horsepower, the same as the same-year model Mercury, which shows that the sleeve system was being underutilized. However this was quickly uprated as improvements were introduced, and by 1936 the Perseus was delivering 810hp, eventually topping out at 930hp in 1939.
The Perseus saw limited use in the civilian field, notably on the Short Empire flying-boats, but was more common in the now-expanding military field where it was found on the Westland Lysander, Vickers Vildebeest, Blackburn Botha, Skua and Roc bombers.
The main contribution of the Perseus is that its mechanicals were used as the basic piston and cylinder for the "twinned" versions, the tremendously successful Hercules and Centaurus. It was in these designs that the advantages of the sleeve valve were finally put to good use, and by war's end the Centarus was one of the most powerful engines in the world.