The Development

While the Concorde fuel system had been designed as far as possible along the conventional lines, employing well tried principles and practices throughout, a number of new problems came to light during the development which had not been previously encountered with other civil transport aircraft at the time. These problems stemmed mostly from the following novel features of Concorde:—

(1) The environment was more severe, extending to high temperatures and very low ambient pressures.

(2) The high, performance of the aircraft lead to a proportionately high performance requirement for the fuel system.

(3) Besides the conventional function of supplying fuel to the engines, Concorde fuel system had to perform the additional functions of controlling and of absorbing surplus heat rejected by other aircraft and engine systems. A number of the particular problems, the research and the development that have gone into the system are described briefly below.

At high temperatures and low ambient pressures, such as experienced by Concorde when cruising at Mach 2.0 at 60,000ft, conventional kerosene may be both highly supersaturated with dissolved air and near its boiling point. It was therefore necessary to take measures to prevent loss of fuel due to boiling-off, to prevent high transient pressures in the tank and loss of fuel due to a rapid-de-aeration of the fuel, and to ensure that the tank pumps would give the required performance at these conditions. The fuel system usage sequence was arranged such as to minimise the intake of heat through the wing skin and thus keep temperatures to a minimum, but a considerable amount of research work was necessary to provide data on fuel vapourisation and aeration characteristics. More particularly, a large effort was necessary to design and develop the fuel tank pumps- which had to both start and give the required performance with a low inlet pressure and with fuel which is both near its boiling point and containing a high percentage of dissolved air.

Although the boundary layer and structure temperatures of Concorde are not such as to constitute an ignition risk, there was, as for subsonic aircraft, the risk of ignition by lightning strike of fuel vapours emitted from the vent outlets. A programme to demonstrate the probability and distribution of lightning strikes was carried out by the Lightning and Transients Institute in the USA and the evidence so derived was used in choosing the positions of the vent outlets. As an additional safeguard, an explosion suppression system was fitted in the main vent gallery adjacent to the outlet orifices.

Although subsonic aircraft have to be capable of sustaining their engines during short periods of negative g, Concorde had a rather special problem. In the case of subsonic aircraft, where both the fuel temperatures and the altitudes are relatively low, when a negative g condition is encountered the tank pumps become uncovered but the engines continue to run for a period on the fuel remaining in the supply lines. An amount of air enters the lines, but this becomes mixed with the fuel before reaching the engine and the effect upon the engine, after the negative g period is over, is small.

For Concorde the situation was quite different in that, at high altitude and high temperature, if the pumps become uncovered the pressure at the inlet end of the fuel line dropped to a mere llb/sq in, which is below the vapour pressure of the fuel at the engine inlet where the temperature is above 100°C. The fuel in the line would therefore boil, and the engine would go out immediately. To overcome this situation it was necessary to design a constantly pressurised fuel accumulator to maintain the fuel flow and pressure during any period when the tank pumps may become uncovered.

Another fuel characteristic which had demanded a good deal of research was its thermal stability. When subjected to high temperatures, certain elements in the fuel became unstable and a gum deposit was formed in pipes, heat exchangers and filters. A considerable amount of laboratory research was undertaken, mostly by the oil companies, to provide data upon which to establish what could be the maximum temperatures for the fuel in the tanks and throughout the aircraft and engine systems.

In addition to the laboratory tests, over 3,000hr testing had been carried out on a representative rig at the Shell Thornton Laboratories to confirm that the decisions taken were valid. Because of the additional functions, the Concorde fuel system had to perform, and because of the need to use the fuel in a certain manner to minimise temperature problems, Concorde fuel system is somewhat more complex, than the subsonic aircraft. It has a number of sub-systems for the transfer of fuel between tanks and there is a good deal of interdependence and interaction of the sub-systems. ; It was therefore necessary to prove the satisfactory functioning of the total system by means of a full-scale test rig which embodied all the tanks and the sub-systems and was capable of achieving all the attitudes, pressures (altitudes), temperatures and rates of temperature change which Concorde’s fuel system would experience in flight.

The Concorde Fuel System

Concorde, like most airliners, has multiple fuel tanks which have been detailed in the picture below. The only difference concerning Concorde is that during the flight, fuel is transferred from tank to tank to maintain trim and balance of the aircraft as Concorde does not have a full tail plane which other aircraft use on a subsonic flights to perform this task. Any swept wing designed to fly close to Mach 1; experiences changes in the pressure pattern over it, these always have the effect of moving the centre of lift rearwards as speed increases. As a result there is a tendency to pitch down, and on subsonic aircraft this is trimmed out by an upwards deflection of the elevators, or, more usually, by moving the tail-plane itself, as this cause less drag.

An aircraft such as Concorde flying at Mach 2 and above needs to do rather more, as the rearwards movement of the centre of lift is much greater. Some of the effects of the aerodynamic changes are countered by the gentle cambers and twists of Concorde’s wing, but there still remains a shift of about six feet to be accounted for. That may not sound much, but the forces involved (the lift) is opposing, and therefore equal to, the weight – which may be 170 tons at the time.

Moving the elevons up to compensate for this would obviously produce an appalling increase in drag, and would leave precious little further upward deflection available for control purposes. Instead, fuel is moved to change the internal weight distribution. In fact most of Concorde’s 95 tons of fuel is kept in tanks in the wings, but the forward two, and another in the tail cone, are used for trim as well as storage. Together they hold about 33 tons of fuel.

During supersonic flight, Centre of Gravity movement is a critical task, and therefore fuel is required to be moved around the aircraft to shift the Centre of Gravity for different speeds.

The fuel system and centre of gravity (CG) are inseparable, they are has one; CG and flight controls are also inseparable, they are as one. They are highly integrated in function but not automated in control.

The fuel is also used as a heat sink for cooling purposes. Surplus heat from the air conditioning and hydraulic systems from the constant speed drive and generator and also from the engine lubricating oil is rejected through heat exchangers to the fuel.