The Bristol Type 188 had its genesis in
Operational Requirement 330, which sought a high speed (
Mach 3)
reconnaissance aircraft, which eventually developed into the
Avro 730.
Air Ministry officials recognised that, as the Avro 730 was expected to operate at high speeds for extended periods of time, more data was needed on high speed operations; to fulfil this need, the follow-on
Operational Requirement ER.134T was issued in February 1953; it called for a flying
testbed capable of attaining speeds in excess of Mach 2. This aircraft was expected to run at these speeds for extended periods of time, allowing it to study
kinetic heating effects on such an aircraft. Furthermore, it was also expected to operate for a considerable amount of time with a
skin temperature around 300 Celsius. Several of Britain's aircraft manufacturers took interest in this relatively advanced specification, Bristol Aircraft being specifically invited to
tendered by ministry officials. Instead, a relatively cutting-edge technique of
arc welding using an
argon gas shield, known as
puddle welding, was used. Another British aircraft manufacturer, the
W. G. Armstrong Whitworth company, provided substantial technical help and support to Bristol during this period; they had also explored the use of puddle welding for advanced aerospace purposes. Armstrong Whitworth 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 pilot was provisioned with a conventional
ejector seat. All of the flight controls, along with elements such as the
undercarriage and four-part
air brakes, were
hydraulically powered. The Type 188 was originally intended to be powered by a pair of
Rolls-Royce Avon engines, however, these were substituted for a pair of
De Havilland Gyron Junior engines (each of which was half a ton lighter) in June 1957. This change of engines necessitated some configuration changes, including the mounting of the engines further forwards along with elongated nacelles and jet pipes. The Gyron Junior was then under development for the
Saunders-Roe SR.177 supersonic interceptor and incorporated a fully variable
reheat arrangement, which achieved a smooth variation in thrust between dry and full reheat, so being one of the first engines in the world to give continuous variation in thrust from idle to max reheat. This choice of powerplant resulted in the Type 188 having a typical endurance of only 25 minutes, not long enough to conduct the high-speed research tests that were required of it. Chief Test Pilot Godfrey L. Auty reported that while the Type 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 identified, a large number of scale models were produced and tested. Some of these models were mounted on converted
rocket boosters and used to conduct free-flight investigations, multiple such launches were conducted from
RAE Aberporth. As a result of these tests, along with
wind tunnel testing, various aerodynamic refinements and alterations were made to the Type 188's design. As a result, a largely rectangular wing plan was adopted between the engine nacelles alongside horn-balanced
ailerons on the compact outer wings; the
all-moving tailplane was also raised to the top of the
fin while the
chord was also extended to accommodate single-engine failure during take-off. It was ultimately decided that there would be three aircraft built, one being a static test frame while the other two (constructor numbers
13518 and
13519) would be used for flight testing. On 4 January 1954, under contract number
KC/2M/04/CB.42(b), the
serial numbers
XF923 and
XF926 were given to the two aircraft. To support the development of the Avro 730 Mach 3 reconnaissance bomber, another three aircraft were ordered (Serial Numbers
XK429,
XK434 and
XK436). However, this follow-up order would be eventually cancelled shortly after the Avro 730 programme was itself terminated during 1957 as part of
that year's review of defence spending. Despite this, the Type 188 project was continued for the time being as a high speed research aircraft. ==Operational history==