Zip fuel, also known as high energy fuel (HEF), is any member of a family of jet fuels containing additives in the form of hydro-boron compounds, or boranes. Zip fuels offer higher energy density than conventional fuels, helping extend the range of jet aircraft. In the 1950s, when the short range of jet aircraft was a major problem for military planners, zip fuels were a topic of significant study. A number of aircraft were designed to make use of zip, including the XB-70 Valkyrie, XF-108 Rapier, as well as the BOMARC, and even the nuclear-powered aircraft program. The Navy considered converting all of their jet engines to zip and began studies of converting their aircraft carriers to safely store it. In testing, the fuels proved to have several serious problems, and the entire effort was eventually cancelled in 1959.
Description The highest energy density fuel (by weight) in common propellant combinations is hydrogen. However, gaseous hydrogen has very low (volume) density; liquified hydrogen has higher density but is complex and expensive to store. When combined with carbon, 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, because they are often prone to self-ignition in contact with air, making them dangerous to handle. When mixed with conventional jet fuels, they add to the energy content while becoming somewhat more stable. In general terms, boron-enhanced fuels offer up to 40% higher energy density than 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). Zip fuels 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 servicing the aircraft. The fuels burn to create solids that are both sticky and corrosive, while boron carbide solids are abrasive. 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, as with 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 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 reduced. After interest in boranes as jet fuel waned, some small-scale research into their use as 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.
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