The second reason for zip fuel's abandonment was that the stuff itself proved disappointing. In jets, instead of burning completely, it produced a sticky residue which clung to turbine blades. Boron additives in rocket fuels also failed. The combustion process involved proved unexpectedly complex, and the promised additional energy was not forthcoming.
The navy's notice suggests, though, that boron is back. The boffins behind it think that new physical forms of the element, known as allotropes, may offer ways around both the partial-combustion and the toxicity problems. Allotropes of an element can have very different properties from each other (graphite and diamond, for example, are both allotropes of carbon). The organisers suggest a novel boron allotrope, perhaps interlaced at the molecular level with a suitable oxidising agent, might yield a completely combustible, non-toxic fuel, and they are asking the country's chemists to bring them one.
Whether such allotropes exist remains to be seen. But America is not the only place working on the idea of boron-powered jets and rockets. China is interested, too. A project involving gelled fuel that has particles of boron suspended in it is under way at the National University of Defence Technology, in Changsha. The objective is to develop fuel for ramjets, a type of engine that operates efficiently only above Mach 3. So far, the researchers involved have managed to produce one that is 40% boron and yet burns more or less completely. One way or another, then, it looks plausible that boron-based fuels may get the green light.