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Concorde needed to fly long distances to be economically viable; this required high efficiency. Turbofan engines were rejected due to their larger cross-section producing excessive drag. Turbojets were found to be the best choice of engines.[48] The engine used was the twin spool Rolls-Royce/Snecma Olympus 593, a development of the Bristol engine first used for the Avro Vulcan bomber, and developed into an afterburning supersonic variant for the BAC TSR-2 strike bomber.[49] The aircraft used reheat (afterburners) at takeoff and to pass through the transonic regime (i.e., “go supersonic”) between Mach 0.95 and Mach 1.7. The afterburners were switched off at all other times.[50] Due to jet engines being highly inefficient at low speeds, Concorde burned two tonnes of fuel (almost 2% of the maximum fuel load) taxiing to the runway.[51] To conserve fuel only the two outer engines were run after landing for taxiing. The intake design for Concorde’s engines was especially critical.[52] Conventional jet engines can take in air at only around Mach 0.5; therefore the air has to be slowed from the Mach 2.0 airspeed that enters the engine intake. In particular, Concorde needed to control the shock waves that this reduction in speed generates to avoid damage to the engines. This was done by a pair of intake ramps and an auxiliary spill door, whose position moved in-flight to slow transiting air.[53] Engine failure causes problems on conventional subsonic aircraft; not only does the aircraft lose thrust on that side but the engine creates drag, causing the aircraft to yaw and bank in the direction of the failed engine. If this had happened to Concorde at supersonic speeds, it theoretically could have caused a catastrophic failure of the airframe. Although computer simulations predicted considerable problems, in practice Concorde could shut down both engines on the same side of the aircraft at Mach 2 without the predicted difficulties.[54] During an engine failure the required air intake is virtually zero so, on Concorde, engine failure was countered by the opening of the auxiliary spill door and the full extension of the ramps, which deflected the air downwards past the engine, gaining lift and minimising drag. Concorde pilots were routinely trained to handle double engine failure.[55] [edit]Heating issues Air friction on the outer surfaces caused the cabin to heat up during flight. Every surface, such as windows and panels, was warm to the touch by end of the flight.[56] Besides engines, the hottest part of the structure of any supersonic aircraft, due to aerodynamic heating, is the nose. The engineers used Hiduminium R.R. 58, an aluminium alloy, throughout the aircraft due to its familiarity, cost and ease of construction. The highest temperature that aluminium could sustain over the life of the aircraft was 127 °C (261 °F), which limited the top speed to Mach 2.02.[57] Concorde went through two cycles of heating and cooling during a flight, first cooling down as it gained altitude, then heating up after going supersonic. The reverse happened when descending and slowing down. This had to be factored into the metallurgical and fatigue modelling. A test rig was built that repeatedly heated up a full-size section of the wing, and then cooled it, and periodically samples of metal were taken for testing.[58][59] Owing to air friction as the plane traveled at supersonic speed, the fuselage would heat up and expand by as much as 300 mm (almost 1 ft). The most obvious manifestation of this being a gap that opened up on the flight deck between the flight engineer’s console and the bulkhead. On some aircraft that conducted a retiring supersonic flight, the flight engineers placed their caps in this expanded gap, wedging the cap when it shrank again.[60] To keep the cabin cool, Concorde used the fuel as a heat sink for the heat from the air conditioning.[61] The same method also cooled the hydraulics. During supersonic flight the surfaces forward from the cockpit became heated, and a visor was used to deflect much of this heat from directly reaching the cockpit.[62] Concorde had livery restrictions; the majority of the surface had to be covered with a highly reflective white paint to avoid overheating the aluminium structure due to heating effects from supersonic flight at Mach 2. In 1996, Air France briefly painted F-BTSD in a predominantly blue livery, with the exception of the wings, in a promotional deal with Pepsi.[63] In this paint scheme, Air France were advised to remain at Mach 2 for no more than 20 minutes at a time, but there was no restriction at speeds under Mach 1.7. F-BTSD was used because it was not scheduled for any long flights that required extended Mach 2 operations.[64] [edit]Structural issues Fuel pitch trim Due to the high speeds at which Concorde travelled, large forces were applied to the aircraft’s structure during banks and turns. This caused twisting and the distortion of the aircraft’s structure. In addition there were concerns over maintaining precise control at supersonic speeds; both of these issues were resolved by active ratio changes between the inboard and outboard elevons, varying at differing speeds including supersonic. Only the innermost elevons, which are attached to the stiffest area of the wings, were active at high speed.[65] Additionally, the narrow fuselage meant that the aircraft flexed.[36] This was visible from the rear passengers’ viewpoints.[66] When any aircraft passes the critical mach of that particular airframe, the centre of pressure shifts rearwards. This causes a pitch down force on the aircraft if the centre of mass remains where it was. The engineers designed the wings in a specific manner to reduce this shift, but there was still a shift of about 2 metres. This could have been countered by the use of trim controls, but at such high speeds this would have caused a dramatic increase in the drag on the aircraft. Instead, the distribution of fuel along the aircraft was shifted during acceleration and deceleration to move the centre of mass, effectively acting as an auxiliary trim control.[67]