The famous aero-engine firm of D. Napier & Sons of Acton, West London, though often thought reluctant to move with the times, had in the 1920s experimented with exhaust-driven superchargers and in the 1930s dabbled in diesels, but with little success. During World War 11 the firm was too busy getting the Sabre into production and sorting out its many problems to be concerned with less conventional though theoretically more economical engines. Napier was, however, well aware that the General Electric Company of America had developed heat-resistant steels that enabled turbo-superchargers to be in-stalled in operational aircraft such as the P-38 Lightning, P-47 Thunderbolt and B-29 Superfortress, and that other firms, notably Junkers, had steadily improved the aircraft diesel.
At the beginning of 1945 the Ministry of Aircraft Production issued a requirement for an economical 6000 hp engine, fully expecting tenders to propose the use of power-recovery systems employing exhaust gas turbines, possibly driving the crankshaft through fluid couplings, as was being proposed by the Curtiss Wright Company in America. However, Sir Harry Ricardo, one of Britainís greatest engine designers, had recently suggested that the most economical combination would be a diesel two-stroke allied to a gas turbine, propounding the theory that careful design would enable the virtues of one to compensate for the weaknesses of the other, and result in an engine of considerable power that would be economical throughout a wide altitude band.
Napier was impressed by his arguments and using pre-war experience with the Culverin and Cutlass (valveless two-stroke diesels) and more recent work on the Sabre (horizontally opposed cranks and centrifugal compressor) designed a 24-cylinder H-type engine of 75 litres capacity. It was not built, however, because the company decided that the commercial outlets for an engine of this size would be very limited. It was cut in half, a horizontally opposed 12cylinder two-stroke diesel driving the rear propeller shaft of a contra-rotating unit, while an 11-stage axial compressor and twin-turbine assembly (based on the Niaid), mounted below the main engine and driven by augmented diesel exhaust, drove a co-axial shaft and the front propeller. Both turbine discs were used for take-off, but during the cruise stage one was isolated and the fuel/air augmentation cut off. This engine was expected to produce at least 3000 shp - it was named the Nomad.
The compressor and turbine assemblies of the Napier E. 125 (Nm.3 Nomad 1) were tested during 1948 and in October 1949 the prototype engine was run as a complete unit. It was 10ft 6in. long, 4ft 10in. wide and 4ft 1in. deep. Weighing 4200lb and rated at 3125 ehp, the engine was installed with considerable difficulty in the nose of Lincoln SX973, which had been allocated to Napierís Luton Flight Test Department in November 1948. SX973 flew with the Nomad 1 in 1950, the engine making its only public appearance at the 1951 SBAC display at Farnborough. In total the Nomad 1 ran for 860 hours on the test rig, 270 hours driving test propellers and 120 hours in flight. It was temperamental, but when running properly it could produce 3000 shp plus 320lb thrust with an sfc of 0.36 lb/ehp/hr.
Long before tests on the Nomad 1were complete Napier had decided to re-design the engine to achieve a better gas flow and less complication. Designated the E145 Nomad 2, the revised version took advantage of development work on a much improved axial compressor designed for the Niaid, a new loop scavenge system which smoothed the gas flow through the diesel, and the Beier infinitely variable gearbox. The original centrifugal compressor was deleted, the 12-stage axial compressor being used to provide supercharged air for the diesel, the exhaust gases of which drove the three-stage turbine. The turbine in turn powered the compressor and assisted the diesel in driving the single 13 ft-diameter four-bladed Rotol or de Havilland propeller through the Beier gearbox, which reduced the output rpm to match the crankshaft. The design of the Nomad 2 was impressive and six engines were ordered by the MoS.
In June 1950 Avro produced a brochure for the Avro 717, a Lincoln fitted with two Nomad engines installed in the standard inner nacelles for use as an additional test bed and a possible long-distance record breaker. With the outer Merlins removed and the airframe generally cleaned up it was estimated that the aircraft would be capable of a still-air range of 13,382nm at heights above 30,000 ft. This was not practical from the crew aspect in an unpressurized aircraft; figures were also given for 15,000 ft, where a range of 12,687nm was suggested. As ever the MoS was less sanguine, estimating that 10,500nm was more likely, and the record-breaking idea was quietly dropped. Plans for the aircraftís use as a test bed continued, however, with Avro suggesting the use of Tudor wing assemblies from the six Mk I airframes about to be scrapped, and the fitting of Lancastrian-type nose and tail fairings. It was proposed that Air Service Training Ltd. should carry out the modifications at Hamble, although Napier, who wanted to do the engine installation at Luton, opposed this.
It had been expected that flight testing of the Nomad 2 mounted in the nose of SX973 would start in June 1952, but a year earlier it was already clear that the engine would be late, and so the plan was changed. The use of SX973 and the development of the Avro 717 was abandoned and it was proposed that a Lincoln or Shackleton should be used with Nomads installed in the outer nacelles. The airframe work required would be minimal and the MoS was satisfied that Napier could do the complete job. On 13 October, 1952, they authorized the transfer of the second prototype Shackleton, VW131, to Napier for conversion and subsequent flight development of the E145 Nomad, the aircraft arriving at Luton on 16 January, 1953, about a month after the engine had first run in the test chamber. The E 145 was 10ft 11in. long, 4ft 10in. wide and 3ft 4in. deep, weighed 3580lb and produced 3135ehp for take-off at an sfc of 0.345 lb/ehp/hr, well below that of its nearest competitor, the Wright Turbo Compound Cyclone.
The engine installation in the Shackleton was extremely neat. The power plant was slung from four vibration dampers carried on a pair of semi-cantilever bearers. The beautifully faired cowlings fitted tightly, with streamlined blisters over the cylinder heads. The cooling radiators were mounted within the wing leading edge between the inner and outer engines, reminiscent of the Mosquito. The installation was first checked with dummy engines, which were also used for vibration tests carried out in April 1954, by which time some 350 hours running time had been attained on the Nomad 2. At least one, possibly two, flight-cleared engines were installed soon afterwards and some reports suggest that the engines were run in the airframe. Unfortunately interest in the Nomad was waning, and following the demise of the R.112D flying boat, and the decision to order the Griffon-powered Shackleton MR 3 for Coastal Command, development of the engine slowed markedly and the project was finally cancelled in April 1955 after an expenditure of £5.1 million.
VW131 remained at Luton until the fuselage was allocated to Avro for research into the ditching characteristics of the aircraft early in January 1956. The aircraft was dismantled and the fuselage despatched to their Bracebridge Heath facility near Lincoln on 7 February 1956. The remainder of the aircraft was scrapped at Luton.