Bristol 188 - Design and Development

Design and Development

The aircraft had its genesis in Operational Requirement 330 for a high speed (Mach 3) reconnaissance aircraft, which eventually developed into the Avro 730. As the 730 was expected to operate at high speeds for extended periods of time, more data was needed on high speed operations, leading to Operational Requirement ER.134T for a testbed capable of speeds greater than Mach 2. The aircraft was expected to run at these speeds for extended periods of time, allowing it to study kinetic heating effects on such an aircraft. The aircraft was expected to spend a considerable amount of time with a skin temperature around 300 Celsius.

Several firms took interest in this very advanced specification and the eventual contract (6/Acft/10144) was awarded to Bristol Aircraft in February 1953.

Bristol gave the project the type number 188, of which three aircraft were to be built, one a pure test bed and the other two (constructor numbers 13518 and 13519) for flight testing. Under contract number KC/2M/04/CB.42(b) serial numbers XF923 and XF926 were given on 4 January 1954 to the two that would fly. To support the development of the Avro 730 Mach 2 reconnaissance bomber, another three aircraft were ordered (Serial Numbers XK429, XK434 and XK436). The follow-up order was cancelled when the Avro 730 programme was cancelled in 1957 as part of that year's review of defence spending. The 188 project was continued as a high speed research aircraft.

The advanced nature of the aircraft meant that new construction methods had to be developed. Several materials were considered for construction and two specialist grades of steel were selected: a titanium-stabilized 18-8 austenitic steel and a 12%-Cr steel used in gas turbines (Firth-Vickers Rex 448). These had to be manufactured to better tolerances in sufficient quantities for construction to start. The 12% chromium stainless steel with a honeycomb centre was used for the construction of the outer skin, to which no paint was applied. Riveting was a potential method for construction but the new arc welding technique using an Argon gas shield known as puddle welding was used. There were long delays with the method and was less than satisfactory. The W. G. Armstrong Whitworth company, provided substantial technical help and support to Bristol during this period; they produced major sections of the airframe as a subcontractor.

A fused-quartz windscreen and canopy and cockpit refrigeration system were designed and fitted but were never tested in the environment for which they had been designed.

The engine installations had to allow development of variable-geometry intake systems. These would permit maximum efficiency across a range of supersonic speeds. This had also led to the engines' position on the wings rather than within the fuselage. Rolls-Royce engines were initially selected to power the 188, but five engine combinations were subsequently evaluated: two with Rolls-Royce Avon 200s, two with the de Havilland Gyron Junior and one with an AJ.65, the last disintegrating on test. Finally, in 1957, the choice for the 188 was two 10,000 lbf (44 kN) thrust Gyron Junior DGJ10Rs, developing 14,000 lbf (62 kN) of thrust on reheat at sea level and 20,000 lbf (89 kN) at Mach 2 at 36,000 ft (11,000 m).

The Gyron Junior was then under development for the Saunders-Roe SR.177 supersonic interceptor and incorporated a fully variable reheat, from idle to full power, the first such application used in an aircraft. Unfortunately this choice of powerplant resulted in the 188 having a typical endurance of only 25 minutes, which was too little time for any serious high-speed research. Chief Test Pilot Godfrey L. Auty reported that while the 188 transitioned smoothly from subsonic to supersonic flight, the Gyron Junior engines were prone to surging beyond that speed, causing the aircraft to pitch and yaw.

In order to solve the aerodynamic and flutter problems, a large number of scale models were employed. Mounted on converted rocket boosters, a number of models were launched from RAE Aberporth, for free-flight investigation.