Fuels for high performance aircraft
Meeting demands of design specifications has led designers to re-think factors once considered to be "givens", fuel amongst them. Aircraft expected to sustain high speeds for long distances, for example, required powerful engines, which not only required carrying space for greater amounts of fuel, but produced temperatures which made conventional fuels dangerous.
When, in 1958, rumours leaked of a high-performance supersonic bomber proposal for the US Air Force, then referred to as WS-110A but ultimately to be designated XB-70, the common jet fuels of the time were recognised as being too heavy for the power they yield to push an aircraft beyond Mach 2 without constant use of afterburners. This would mean carrying huge amounts of fuel on board, leading to a heavier aircraft and ultimately to reduced performance.
Chemical fuel was the answer to the problem of creating a fuel lighter than kerosene or AvGas but delivering more power, making extended hypersonic performance achievable. The unlikely solution was Boron. As a powder mixed with ordinary fuel, it promised performance well beyond that available at the time.
But burning Boron fuel posed more problems. The fumes were toxic, not a problem in flight, but very risky for ground crew. Burning Boron fuel also tends to leave a heavy deposit on anything in its path, with turbine blades being particularly vulnerable.
It was proposed that the Boron fuel would therefore be used only for afterburning. Aircraft like WS-110A would cruise on normal fuel and use the special fuel for bursts over the target or to evade attack. It was predicted at the same time that a considerable amount of stainless steel honeycomb insulation would be needed to protect fuels from kinetic heating at high speeds.
Both the USAF and US Navy began investing huge amounts of money in developing these new fuels for the aircraft of the 1960s, known as "zip fuels". The XB-70 Valkyrie was developed in roughly the same period as the Blackbird, as a supersonic strategic bomber. It was initially to use an ethyl borane fuel which gave up to 10% greater range and a higher cruising speed. This expensive fuel project was cancelled in 1959.
By the time the Lockheed A-12 / SR-71 series materialised in the early 1960s, the problems of sustained performance and high temperatures became a very real problem. The Blackbird was fuelled by a fuel designated JP-7, with a low enough vapour pressure to resist ignition by a naked flame in normal circumstances. Its high toxicity demands special care in ground-handling. It is carried in special tankers for in-flight refuelling, the KC-135Q.
However, its temperature stability means that the predicted insulation is not required, despite the high kinetic heat factor. Fuel in the wings of the SR-71 is used first, since these large areas suffer the most from heating of the thin, flat tanks, but there is no insulation. The aircraft is, however, painted in a special very dark blue highly emissive material.
The engines of the SR-71 are J58s (JT11D-20B), single-spool afterburning turbojets, with high titanium content. Air is bled from the fourth compressor stage and ducted to the afterburner to reduce temperatures and improve thrust augmentation. Because the fuel is so heat-stable, ignition is achieved chemically, rather than electronically. Triethyl borane (TEB) is injected to produce instant combustion on contact with the JP-7.
Mixed-power or pure rocket designs, such as the French SNCASO Trident interceptor of 1953 produced fairly obvious fuel demands. It carried a pair of turbojet engines, one on each wing tip, on which the aircraft relied for most of its flight. For quick takeoffs and for supersonic speed bursts the aircraft used a rocket engine in the fuselage.
This was fuelled, after some experimentation, by a mix of Furaline (mainly aniline) and RFNA (red fuming nitric acid). Tanks and engine surfaces required special treatment to inhibit attack by this highly corrosive acid.
The two chemicals self-ignite on contact. Driven by electric valves to the water-cooled turbopump at the rear of the fuselage, the fuel was moved to the three thrust chambers, each having about 4,000 lb (1,814 kg) thrust in the stratosphere. These could be fired separately or together.
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