Pentaborane

Pentaborane, also called pentaboron nonahydride, stable pentaborane, or pentaborane(9) (to distinguish it from B5H11), is a chemical compound considered in the 1950s as a good prospect for a rocket or jet fuel by both the U.S. and Russian armed services, a so-called "exotic fuel". Its chemical structure is that of five atoms of boron compounded with nine atoms of hydrogen (B5H9); it is one of the boranes. Because simple boron compounds burn with a characteristic green flame, the nickname for this fuel in the U.S. industry was "Green Dragon".

Pentaborane is a colorless mobile liquid with a strong pungent odor (resembling garlic, acetylene, or sour milk). 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. Above 150 °C, it decomposes, producing hydrogen; when it occurs in a closed container, the consequent rise of pressure may be dangerous. It is highly toxic on inhalation, ingestion, and skin absorption; it is damaging to eyes and skin, can damage liver and kidneys, and can attack the nervous system; symptoms of lower-level exposure may occur with up to 48 hours delay. Its acute chemical toxicity is comparable to some nerve agents. It is much more stable in presence of water than diborane. It is highly soluble in hydrocarbons, benzene, and cyclohexane, and in greases including those used in lab equipment. Evaporation from skin may cause frostbites. In storage, it decomposes negligibly, yielding small amount of hydrogen and solid residue. It is manufactured by pyrolysis of diborane. In the USA, pentaborane was produced by Callery Chemical Company; in 1985, Callery repurchased some of the reserve fuel and processed it to elementary boron.

Fuel use
Because hydrogen makes the most energetic fuel with either oxygen or fluorine as oxidizer, but can only be liquefied at very low temperatures and is still cumbersome to work with due to its lack of density even in liquid form, compounds are sought to put the most hydrogen in the least volume and with the lightest non-hydrogen component. Pentaborane surpasses its equivalent carbon compounds because their self-linking element, carbon, weighs two unified atomic mass units more than an atom of boron, and some boranes contain more hydrogen than the carbon equivalent. 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.

Problems with this fuel include its toxicity and its characteristic of bursting into flame on contact with the air; furthermore, its exhaust would also be toxic. Long after the chemical was considered unworkable, the total United States stock of the chemical, 1900 pounds, was destroyed in the year 2000, when a safe and inexpensive means for doing so was discovered - hydrolysis with steam, yielding hydrogen and a boric acid solution. The system was nicknamed "Dragon Slayer".

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 1962-1970.

Other borane-derived fuels were eg. propyl pentaborane (BEF-2) and ethyl decaborane (REF-3). Diborane and decaborane and their derivates were also investigated.

In chemistry, pentaborane is used for synthesis of polyboron cages.