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.
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.
[1] 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.
[3][4] 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).
[5][6] Finally, the exhaust plume is filled with particulates, as with coal smoke, allowing an aircraft to be spotted visually at long range.
This too proved to be a dead-end, as the solid boron oxides in the combustion products interfered with the expected thermodynamics, and the thrust advantages could not be realized.
[8] For much of the 1950s, zip fuels were considered to be the "next big thing" and considerable funds were expended on these projects in an effort to bring them into service.
HEF became part of the WS-110 efforts to build a new long-range bomber to replace the B-52 Stratofortress with a design able to dash at speeds up to Mach 2.
The initial designs from Boeing and North American Aviation (NAA) both used conventional fuels for takeoff and cruise, switching to HEF during the high-speed dash, burning it only in their afterburner sections.
[4] This reduced the selection of targets that could be attacked from the US and required in-flight refueling for every mission profile, one more problem that led to the project's eventual re-direction as a purely experimental aircraft.
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.