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In the late 1950s the U.S Air Force searched for a fuel with similar characteristics to jet fuel but with higher energy density allowing longer flight range for bombers. In 1947 the British had discovered elemental boron to be the ideal replacement to carbon as it had lower bond dissociation energy leading to higher heat of combustion, 68 MJ/kg vs 42.8 MJ/kg for JP-4.$^1$

Pentaborane B5H11 was found to be unstable, so B5H9 was used as it possessed much better stability. Pentaborane B5H9 performed well in turbojet combustors but it was found that emissions of solid boron particles upon combustion caused severe erosion of the high pressure turbine blades. Is it possible that new ceramic matrix composite turbines blade with higher density and abrasion resistance could solve this problem?

$^1$ Frietas 2018 handbook of energy density Robert A. Freitas Jr., “Energy Density,” IMM Report.

Christophe Pochari
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  • This answer suggests more disadvantages than just abrasion. –  Jan 26 '20 at 05:44
  • why would you assume that? 60% more flight range sounds desirable no? – Christophe Pochari Jan 26 '20 at 05:46
  • There's a hyperlink to a post here, I'm not referring to the energy density you mentioned. From that link: "Significant disadvantages such as a tendency to spontaneous ignition in the presence of air, and the buildup of solid combustion products on turbine blades, leading eventually to engine failure. The fuels are also toxic as are the combustion products." –  Jan 26 '20 at 05:49
  • that's what I've gathered, it's mostly negative. I had understood abrasion was the biggest issue, not buildup of solid material. Does this mean the fuel is only practical in rocket engines or ramjets? – Christophe Pochari Jan 26 '20 at 05:53
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    I'm afraid I don't know, I was only pointing out a related post with relevant information. –  Jan 26 '20 at 05:54
  • Thanks. I suppose borane could make a good fuel for the Reaction Engines Skylon, since it's a conventional rocket engine but with a pre-cooler rather than onboard oxidizer. – Christophe Pochari Jan 26 '20 at 05:58
  • what if two turbine sections were designed and high-pressure sand or some kind of fluid were injected to clean the turbine blades of material build-up and continuously switched? – Christophe Pochari Jan 26 '20 at 05:59

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Borane fuels are not practical for a multiple-use aircraft, primarily because there are no naturally occurring sources they could be easily derived from. This rules them out commercially, and gives a military disadvantage of being unable to utilize local supply chains.

Note that the XB-70's borane fuel was to be carried in addition to kerosene, for one high-speed dash. Even though capable of multiple flights, the military purpose of nuclear bombers would have been fulfilled in a single mission.

Modern turbines are built with tighter tolerances than those of the 1960s. This would make a fuel that deposits solid particles, as pentaborane does, more of a problem, not less.

These considerations essentially limit pentaborane to simpler and single-use engines that can tolerate this, such as rockets.

However, the fact that no space missions have used pentaborane, even when it cost nothing and had to be destroyed, suggests that its toxicity and near-impossibility of safe handling on industrial scale outweigh its advantages even in such applications.

To quote the paper,

When reading about pentaborane, a TLV of .005 ppm and indications that it is pyrophoric over a wide range of oxygen concentrations meant this was toxic material prone to ignite very easily. What is difficult to comprehend from these numbers and quantitative data is that pentaborane is as toxic as some of our Nation’s nerve warfare agents and will spontaneously burst into persistent flame in nearly oxygen deficient atmospheres.

Spacecraft are no strangers to exotic and highly toxic fuels, such as hydrazine. However, pentaborane turned out to be on another level of difficult. It's also not as high-performing as hydrogen or the record holder Li-F2-H2 tripropellant, also not used for similar reasons.

Therac
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