Bristol Proteus
Bristol Siddeley Proteus

Design work on the Proteus started in September 1944, during the course of development the gas generator section was built as a small turbojet which became known as the Bristol Phoebus. This engine was test flown in May 1946 fitted to the bomb bay of an Avro Lincoln, performance was poor due to airflow problems. The centrifugal compressor was redesigned but similar problems were encountered when the Proteus started ground testing on 25 January 1947.
 
The original Proteus Mk.600 delivered 3,780 hp (2,820 kW), and was going to be used on the early versions of the Britannia and the Saunders-Roe Princess flying-boat. The versions on the Princess were mounted in a large frame driving a single propeller through a gearbox, and were known as the Coupled Proteus. The Coupled Proteus was also intended to be used on the Mk.II versions of the Bristol Brabazon, but this project was cancelled. Only three Princesses were built, only one of which flew, and by the time the Britannia was ready for testing the manufacturer had decided to equip it with the later Mk.700 Proteus instead.

The Proteus was a two spool, reverse-flow gas turbine, similar to the original Whittle engine designs with an extra turbine stage. Because the turbine stages of the inner spool drove no compressor stages, but only the propeller, this engine is sometimes classified as a free turbine.

During development, there were severe problems with compressor blades, turbine blades and bearings failing at even low power output levels. This led to the famous quote of Proteus Chief Engineer Frank Owner to Chief Engineer of the Engine Division Stanley Hooker: "You know, Stanley, when we designed the Proteus I decided we should make the engine with the lowest fuel consumption in the world, regardless of its weight and bulk. So far, we have achieved the weight and bulk!"
 
At this point the Proteus proved to have troubling icing problems, causing the engine and aircraft projects to be delayed while solutions were found. The Mk.705 of 3,900 hp (2,900 kW) was the first version to see widespread production on the Bristol Britannia 100 and some 300 series. The Mk.755 of 4,120 hp (3,070 kW) was used on the 200 series (not built) and other 300's, and the Mk.765 of 4,445 hp (3,315 kW) was used on the RAF's Series 250 aircraft.

Variants:
As with most gas turbine engines of the 1940s, 50s, and 60s the Ministry of Supply allocated the Proteus a designation which was apparently little used. Officially the Proteus was named Bristol BPr.n Proteus

Proteus series 1
(BPr.1)Prototype and early production engines used for development and testing.

Proteus series 2
Initial production versions, renamed as Proteus 600 series engines.
 
Proteus series 3
The fully developed initial version, re-named as the Proteus 700 series.
 Proteus 600Initial production engines re-named from Proteus series 2.

Applications:
Bristol Britannia
Bristol Type 167 Brabazon I Mk.II
Saunders-Roe Princess

Specifications:

Proteus Mk.705
Type: Turboprop
Length: 113 in (2,870 mm)
Diameter: 39.5 in (1,003 mm)
Dry weight: 2,850 lb (1293 kg)
Compressor: Two-spool 12-stage axial, followed by a single centrifugal stage
Combustors: Reverse-flow
Turbine: Two-stage power (free turbine), two-stage driving compressor
Fuel type: Aviation kerosene
Maximum power output: 3,320 shp (2,475 kW) + 1,200 lb (5.33 kN) residual thrust giving 3,780 eshp
Overall pressure ratio: 7.2:1
Fuel consumption: 273 Imp gal (1,241 L) /hour
Specific fuel consumption: 0.495 lb/h/eshp
Power-to-weight ratio: 1.32 eshp/lb