The ultimate fuel from a mass-to-performance perspective is hydrogen. However, gaseous hydrogen has very low density; liquified hydrogen has higher density but is complex and expensive to store. When combined with other elements, like carbon, the hydrogen can be rendered into the easily burnable hydrocarbon fuels. Other elements, like aluminum and beryllium have even higher energy content than carbon, but do not mix well to form a stable fuel that can be easily burned.
Of all the low-mass elements, boron has the combination of high energy, low weight and wide availability that makes it interesting as a potential fuel. Boranes have a high specific energy, about 70,000 kJ/kg (30,000 BTU/lb). This compares favorably to a typical kerosene-based fuel, such as JP-4 or RP-1, which provides about 42,000 kJ/kg (18,000 BTU/lb). They are not suitable for burning as a fuel on their own, however, as they are often prone to self-ignition in contact with air, making them dangerous to handle.
When mixed with conventional jet fuels, however, they add to the energy content while becoming somewhat more stable. In general terms, boron-enhanced fuels offer up to 140% the energy density of plain JP-4 in terms of both weight and volume. In the US a whole family of fuels were investigated, and generally referred to by the names they were assigned during the Air Force's Project HEF: HEF-1 (ethyldiborane), HEF-2 (propylpentaborane), HEF-3 (ethyldecaborane), HEF-4 (methyldecaborane), and HEF-5 (ethylacetylenedecaborane).
All zip fuels also have a number of disadvantages. For one, the fuel is toxic, as is its exhaust. This was of little concern "in flight", but a major concern for ground crews expected to service the aircraft. Additionally, the fuels burn to create solids that are both "sticky" and corrosive. In particular, boron oxides are chemically reactive (and thus toxic), and boron carbide is mechanically "sharp". This caused serious problems for turbine blades in jet engines, where the exhaust built up on the blades and reduced their effectiveness and sometimes caused catastrophic failure of the engine. Finally, the exhaust plume is filled with particulates, like coal smoke, allowing an aircraft to be spotted visually at long range.
In the end, the problem of burning HEF throughout the entire engine proved impossible to solve. Removing the buildup was difficult, and the wear on the metals it caused was something that materials science was unable to address. It was possible to burn it with relative ease in an afterburner, but this would only be effective on aircraft that used an afterburner for extended periods of time. Combined with the high cost of producing the fuel and the toxicity issues, the value of zip fuel was seriously eroded. In 1959 the Air Force eventually gave up and canceled the program, although some small-scale work as a rocket fuel continued. This too proved to be a dead-end; the solid boron oxides in the combustion products interfered with the expected thermodynamics, and the thrust advantages could not be realized.
Read more about this topic: Zip Fuel
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