Zip fuel

Zip fuel, also known as high energy fuel, is any member of a family of jet fuels containing additives in the form of hydro-boron compounds, or boranes. Zip fuels offered higher power than conventional fuels, helping extend the range of jet aircraft, a major problem for the military planners in the 1950s. 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. In testing, the fuels proved to have several serious problems and the entire effort was eventually canceled in 1959. It was later claimed that the Blackstar spaceplane uses zip fuel, but Blackstar is almost certainly mythical.

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Description

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).^[1] They are not suitable for burning as a fuel on their own, 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.^[2] 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).^[2]

All of these fuels had a number of disadvantages. For one, the fuel was toxic, as was its exhaust. This was of little concern "in flight", but a major concern for ground crews expected to service the aircraft. Additionally, the fuels burned to create solids that were both "sticky" as well as corrosive. In particular, boron oxides were chemically reactive, and boron carbide was mechanically "sharp".^[1] This caused serious problems for turbine blades in jet engines, where the exhaust built up on the blades and reduced their effectiveness. Finally, the exhaust was dark black, 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. 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.

History

Several studies were made into boronated fuels over the years, starting with the U.S. Army's rocket-related Project HERMES in the late 1940s, the U.S. Navy Bureau of Aeronautics's Project ZIP in 1952, and the U.S. Air Force's Project HEF (High Energy Fuels) in 1955.^[3] For much of the 1950s, zip fuels were considered to be the "next big thing" and considerable amounts of money were poured into these projects in an effort to bring them into service. The Navy's name stuck, and all of the boronated fuels became known as "zip fuels", although the Air Force's naming for the fuels themselves became common.

The main thrust of the Air Force's program was based on HEF-3, which seemed to be the most likely candidate for quick introduction. The General Electric J93 used on the B-70 and F-108 was designed to use HEF-3 in its afterburner section only. This avoided both of the main problems with HEF; by burning it only in the afterburners the problem with buildup on the turbine was eliminated, and since the afterburners were only used for takeoff and high-speed flight, the problems with the toxic exhaust were greatly reduced.^[4] However, this meant that the advantages of using HEF-3 were also greatly reduced; by using it only for one portion of the engine, and only during certain portions of the flight regime, the range was only extended by about 10% in total. There were plans to introduce a later version of the J93 that would burn HEF-4 throughout. Meanwhile there were also studies on using HEF-3 in the BOMARC ramjets, as well as studies about carrying it on the US Navy's aircraft carrier fleet to power future aircraft, but these programs both died out.

As the problems were proving intractable, the Air Force cancelled their program in 1959, and interest in zip basically disappeared. By this point the only design still considering using HEF was the XB-70 and its J93. North American Aviation and General Electric responded by redesigning the engine to run on a new higher-density form of jet fuel, JP-6, and filling one of the two bomb bays with a new fuel tank. In doing so the range was reduced from about 7,700 nautical miles (14,260 km) to 5,500 nautical miles (10,190 km).^[2] This reduced the selection of targets that could be attacked from the United States and required in-flight refueling to do it, one more problem that led to the project's eventual re-direction as a purely experimental aircraft. Pierre Trudeau later asked the Canadian Forces to consider converting the BOMARC to zip fuels in an effort to bolster the effectiveness of these expensive missiles. A report on the topic stated that the needed modifications would add weight that would offset any advantage. The Canadian BOMARC units were deactivated in 1972.

It is estimated that the US spent about $1 billion on the program, in inflation-adjusted dollars. At least five HEF production plants were built in the US, and two workers were killed in an explosion that destroyed one plant in New York.^[3] Most of the program was top secret while being carried out, but both the US and USSR independently declassified their research in 1964. There was a "leak" on the topic prior to this date, however, in the September 1958 issue of RAF Flying Review contained an article on the "Boron Bomber", referring to the B-70.

One potentially lasting relic of the HEF program is an abandoned dirt airfield outside Boron, California. Marked on sectional maps as "Air Force Plant #72", nothing but the airstrip and a water tank were even built on the site. It is speculated that this would have been a factory for HEF fuel, using the large borax deposits nearby (giving the town its name), where it could be easily shipped to Edwards Air Force Base.^[2]

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