Tuesday, October 20, 2009

The Rolls-Royce Thrust Measuring Rig

The Rolls-Royce Thrust Measuring Rig (TMR) was a pioneering vertical take-off and landing aircraft developed by Rolls-Royce in the 1950s. The TMR used two Nene turbojet engines mounted back-to-back horizontally within a steel framework, raised upon four legs with castors for wheels. The TMR had no lifting surfaces (wings, blades, etc.) and was understandably nicknamed the Flying Bedstead.

The output of the jets was directed towards the centre of the rig with one jetpipe discharging downwards through a central nozzle while the other jet discharged downwards through two smaller nozzles on either side. Four outrigger arms extended out from the rig, one on either side and one each at the front and rear, through which compressed air was released for control in roll, pitch and yaw when in flight. The purpose of the rig was, as the name suggests, to test turbojet engines for lifting purposes and to develop techniques for controlling such an aircraft.

The man largely responsible for the development of the TMR was Dr Alan Arnold Griffith who had worked on gas turbine design at the Royal Aircraft Establishment in the 1920s and was a pioneer of jet lift technology. Griffith was employed by Rolls-Royce in 1939.

Two Thrust Measuring Rigs were built with the first taking to the air on 3 July 1953 at Hucknall Aerodrome, Nottinghamshire, England, though it remained tethered to the ground while airborne. The first free flight by the TMR was made on 3 August 1954 with R.T. Shepherd, Rolls-Royce's chief test pilot, at the controls. The TMR had only marginal excess power and flying was tricky due to this, combined with the slow throttle response of the engines, and a considerably degree of anticipation in the use of engine power was required in order to prevent overshooting of desired altitude, and to ensure a gentle touchdown when landing. As the TMR possessed no inherent stability, it incorporated an automatic stabiliser system.

Following successful trials of the TMR, Rolls-Royce began development of the Rolls-Royce RB.108 direct-lift turbojet, five of which were used to power the first true British VTOL aircraft, the Short SC.1.

The second Thrust Measuring Rig (Serial XK426) was destroyed in 1957 but the first (Serial XJ314) is preserved and on public display at the Science Museum in London, England.