What is "zip fuel"

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 offered higher energy density 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.