The Bristol Siddeley BS.100 is a British twin-spool, vectored thrust, turbofan aero engine that first ran in 1960.

Based on the 300-series Olympus,[1] the BS.100 was similar in general arrangement to that of the company's Pegasus design, but with the addition of plenum chamber burning (PCB), to enable the projected Hawker Siddeley P.1154 VSTOL fighter design to accelerate to supersonic speed [2] and to allow the aircraft to hover.

PCB was quite difficult to develop since combustion had to occur on the bends between fan exit and entry to the two vectoring front nozzles and at much lower inlet pressure and temperature than a conventional afterburner.

The PCB flame-holders were highly complex sheet metal fabrications which were difficult to envisage on a 2D engineering drawing.

[3] A preserved Bristol Siddeley BS.100 is on public display at the Fleet Air Arm Museum, RNAS Yeovilton.