The ease of breaking the chemical bonds of the compound is also taken into consideration. Interest in this substance began as a possible fuel for high-speed jets. The propellant mix that would produce the greatest specific impulse for a rocket motor is sometimes given as oxygen difluoride and pentaborane. During the early years of the space race and the missile gap, American rocket engineers thought they could more cheaply produce a rocket that would compete with the Soviets by using an existing first stage and putting an upper stage with an engine that produces thrust at a very high specific impulse atop it. So projects were begun to investigate this fuel. This pentaborane was considered for use as a fuel by North American Aviation when the XB-70 Valkyrie was in the planning stages, but the aircraft ended up using hydrocarbon fuel instead. Pentaborane was also investigated to be used as a bipropellant with nitrogen tetroxide. In the Soviet Union, Valentin Glushko used it for the experimental RD-270M rocket engine, under development between 1962 and 1970. Other boranes were evaluated as fuels, including propylpentaborane (BEF-2) and ethyldecaborane (REF-3). Diborane and decaborane and their derivates were also investigated. Problems with this fuel include its toxicity and its characteristic of bursting into flame on contact with the air. Furthermore, its exhaust (when used in a jet engine) would also be toxic. The US destroyed its last stockpiles of "Green Dragon" in 2000, long after the pentaborane had been discarded as unworkable. The destruction procedure hydrolyzed the pentaborane with steam to yield hydrogen and a boric acid solution. The long delay occurred in part because there are no industrial plants consuming pentaborane as a feedstock. Instead, army engineers constructed a bespoke system, nicknamed the "Dragon Slayer". Safety As one of the compounds that have a NFPA 704 (fire diamond) rating of 4 for every category, it is naturally extremely dangerous. Above 30 °C it can form explosive concentration of vapors with air. Its vapors are heavier than air. It is pyrophoric—can ignite spontaneously in contact with air, when even slightly impure. It can also readily form shock sensitive explosive compounds, and reacts violently with some fire suppressants, notably with halocarbons and water. It is highly toxic and symptoms of lower-level exposure may occur with up to 48 hours delay. Its acute toxicity is comparable to some nerve agents. Occupational exposure limits for pentaborane set