9 advanced molecules that could revolutionize jet and missile fuel

June 18, 2014 |


Based on current oil prices, there’s almost no chance that super high-performing aviation fuels can be made at mainstream aviation “everyday low prices” from fossil sources.

But these 9 high-density, high-power, high-performance fuel molecules could come from renewable resources, and revolutionize aircraft and missile design. The Digest investigates.

Boom! Boom! Boom! This morning, the heavy thud of artillery fire shakes the morning air. A live-fire exercise is on this week, eight miles to my east in the Whiskey and Zulu impact areas at Camp Pendleton, California.

It’s a relatively thin line between explosives and fuels — after all, the burning of fuel is a controlled exercise in explosion — and the current darling in explosives research, CL-20 (for purists, that’s 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane — try pronouncing that at the end of a long day at the office), was originally developed by the Navy as a rocket fuel. It has a 5% improvement in detonation pressure than HMX, and HMX in turn has more than twice the detonation pressure of TNT. In short, these are nasty explosives.

Out there on the horizon — at almost a 30% increase in detonation pressure (and significantly more dense, too), are the nitrocarbons, and specifically octanitrocubane, known as ONC. As an explosive, it has a predicted detonation pressure more than 25% greater than HMX. It’s as nasty as they come.

One man’s explosive is another man’s fuel

But ONC has some interesting properties as a fuel. First of all, it’s dense — you can pack quite a bit of it into a missile. It’s high heat of combustion makes it possible for missiles to travel farther or carry more explosive power, without changing missile design.

But here’s the most interesting aspect. As it has no hydrogen, there isn’t any water produced when it burns, and you don’t see that tell-tale vapor stream behind a rocket or jet that uses it. Making the rockets or jets harder to track — it’s a threat-multiplier, and improves stealth.

All of which reminds us that a renaissance in fuel development — for military or aviation purposes — may well be upon us. The arrival of synthetic biology, a new generation of catalysts, improved lab skill and the availability of cellulosic sources for fuels — this potent mix of new materials and strategies is being employed in some of the US’s leading labs to explore the new opportunities for high-performance fuels.

The new molecules, and the potential for aviation

Imagine, for example, taking a Boeing 777 and extending its (full passenger and cargo load) range from 9,000 to 11,500 miles  — and being able to fly non-stop between any two cities in the world. Or F-18A/F Superhornets being able to add significant weapon loads, or fly faster , or add up to 30% to flight range. It’s like getting a new generation of aircraft without the multi-billion price tag.

As a team of researchers from the Naval Air Warfare Center at China Lake and NIST, headed by Dr. Ben Harvey, observed a few months back in a journal article we summarized here:

“Renewable fuels with densities that exceed those of conventional jet fuels by up to 13% can be generated from multicyclic sesquiterpenes. This advance has the potential to improve the range of aircraft, ships, and ground vehicles without altering engine configurations. In addition, as strategies to efficiently convert lignocellulosic biomass into sugars improve and organisms are developed that can utilize these sugar mixtures and convert them to sesquiterpenes, these fuels can be produced on a scale that would help supplant significant quantities of petroleum.”

So, let’s look at the potential new molecules — there are more than 50,000 potential molecules amongst the terpenoids alone, so we’ll select the best that are known and undergoing testing or development today.

Sub 125,000 BTU biobased aviation fuels

Here, we have farnesane and SPK fuels (synthetic pariffinic kerosene, also known as HEFA). Both are expected to be used as blendstocks — both are now approved for commercial use under ASTM specs.

125,000 BTUs

Here are our baseline fuels: JP-5 and Jet-A kerosenes. These are standard aviation fuels used today and they have 125,000 BTUs of energy per gallon, or about 10 percent more energy per gallon than gasoline and 60% more than ethanol.

In here, also add in the 100% aromatics fuels such as ARA’s ReadiJet, now licensed by Aemetis and Blue Sun Biodiesel. We haven’t seen a specific BTU figure, but ARA advises that “Because ReadiJet contains high-density aromatic and cycloparaffin compounds, bulk fuel density and volumetric energy content is also comparable to petroleum jet fuel.”

1Another one in this range — possibly available through biology, is bisabolene. In 2011, we highlighted that the Joint Bioenergy Institute was seeking industry partners interested in licensing its technologies. In “Alternative Diesel Fuel from Biosynthetic Bisabolene” JBEI researchers had produced a chemical precursor that readily converts to bisabolane, a plant-derived hydrocarbon chemically related to turpentine that can deliver comparable performance to standard D2 diesel fuel.

Last year, researchers at Lawrence Berkeley National Labs developed a new technology that enables the engineering of host microorganisms suitable for biofuel processing using ionic liquid pretreatment. Laboratory tests indicated that an engineered strain of E. coli carrying the IL-tolerance genes was able to grow and produce the fuel precursor, bisabolene, in the presence of 4% 1-ethyl-3-methylimidazolium chloride.

130,000-140,000 BTUs

#1-#3: valencene, premnaspirodiene, and b-caryophyllene. Now, we get into an interesting range of molecules. Think, for example, valencene, premnaspirodiene, and b-caryophyllene, which Allylix and the Naval Air Warfare Center have been working on.

In their recent write-up in Physical Chemistry Chemical Physics (a journal with a title right out of Lewis Carroll, but a respected peer-reviewed entity all the same), Harvey and his team write:

Biosynthetic valencene, premnaspirodiene, and natural caryophyllene were hydrogenated and evaluated as high performance fuels. The parent sesquiterpenes were then isomerized to complex mixtures of hydrocarbons with the heterogeneous acid catalyst Nafion SAC-13. High density fuels with net heats of combustion ranging from 133-141 000 Btu gal(-1), or up to 13% higher than commercial jet fuel could be generated by this approach. The products of caryophyllene isomerization were primarily tricyclic hydrocarbons which after hydrogenation increased the fuel density by 6%. The cetane numbers for the sesquiterpane fuels ranged from 20-32 and were highly dependent on the isomer distribution. Specific distillation cuts may have the potential to act as high density diesel fuels, while use of these hydrocarbons as additives to jet fuel will increase the range and/or time of flight of aircraft.

In English, this generally means that the densities are excellent — the cetane number range suggests that the fuels would likely end up being blended with petroleum-based kerosene (say, a 50/50 blend) to stay in spec.

140,000 BTUs

#4: Exo-THDCP. Now, we’re in the range of the fuel spec known as JP-10. It’s also known, somewhat less euphonically, as exo-tetrahydrodicyclopentadiene, or exo-THDCP. Generally, this is rocket fuel, used in very small quantities because, made from petroleum it costs $25 per gallon.

#5: Neoclavane. One candidate molecule is neoclovane. Harvey and colleagues noted “a fuel composed of only neoclovane would beexpected to have a density of B0.92 g mL1, with a calculated volumetric NHOC of nearly 141 000 Btu gal.” That’s definitely in the JP-10 range, and this research is based on the afore-mentioned real-world fuels development at Allylix.

160,000 BTUs

But what about the world beyond the 140,000 BTU range and JP-10. That brings us to the mysterious and high-powered RJ-5 military fuel spec, which calls for a net heat of combustion of 161,000 BTUs.

#6: perhydroinorbornadiene. Generally, RJ-5 is missile fuel, and is composed of perhydroinorbornadiene, a norbornadiene dimer. Norbornadiene is difficult and expensive to make — but what a fantastic target for synthetic biology and advanced catalysis. Not only do you have a ready market at $25 per gallon — the kind of pricing structure (that is, $25+ per gallon in the early days, reaching for mass markets later, at scale) is well-suited to technology development — not dissimilar to the structure for, say, iPhones and iPads.

But you not only have a great cost opportunity, there’s a good performance opportunity. It doesn’t only have to do with payload or mission range — technology delivered via flat-top carriers come into range sooner. 500 miles in additional missile range means that a 35-knot carrier, steaming into a hot spot, can project air power 12 hours sooner than a carrier carrying conventionally-fueled missiles.

Dr. Harvey and colleagues at China Lake have been investigating the potential to convert b-pinene into fuels that meet the RJ-5 spec. Last October, they reported:

Pinene dimers synthesized with these heterogeneous catalysts have a density and net heat of combustion comparable to JP-10. High density fuel candidates have been synthesized in up to 90% yield from β-pinene, a renewable strained bicyclic compound, which can be derived from wood and plant sources. These novel syntheses are based on heterogeneous acidic catalysts (also referred to as heteropolyacidic catalysts) including Montmorillonite-K10 and Nafion NR-50 which promote selective isomerization and dimerization of pinenes under moderate conditions (e.g., 80- 120°C, such as about 100 °C, and about atmospheric pressure).

Pinene is pretty abundant, all in all — given that it’s found in, er, pine trees. That pine-tree smell? That’s pinene.

Beta-pinene, used in Harvey’s work, can be found in parks all over the world via Maritime pine (pinus pinaster), which is native to North Africa and is currently grown in Hawaii and North Carolina, which is good news for the carriers out of Pearl or Norfolk.

Beyond 160,000 BTUs

#7: Octanonitrocubane. Here, we move beyond the “work in some lab somewhere” level and into the world of “it’s possible, but no one’s yet working on it” . And right back to ONC, or octanonitrocubane. It’s been described as “the most powerful nonnuclear explosive known.”

Until the 1960s, it was not even believed that cubanes, which had previously been known strictly as a theoretical molecule, could be synthesized at all; then, a team led by Eaton and Cole at the University of Chicago managed to synthesize it in 1964. Cubane (C8H8) looks like a cube — with each of eight corners occupied by a carbon atom, to which is attached a protruding hydrogen atom. Cubanes are possessed of a level of “angle strain” that make them highly reactive, and highly explosive. Perfect for our purposes.

How explosive? As noted above, ONC is reported to have a 30% higher explosive presure than CL-20 explosives — it’s nasty. We haven’t seen NHOC figures for the molecules, used as fuel — but you can bet they are going to generate a tremendous amount of energy, and well above 160,000 BTUs.

Beyond ONC?

#8-#9: DNH and HNHAH. Well, consider DNH and HNHAH, known amongst their friends as dodecanitrohexaprismane (DNH) and hexanitrohexaazaprismane (HNHAH) have higher energy than ONC. Their part of a class of molecules known as “cage explosives” and you can read about them in Strategic Technologies for the Army of the 21st Century.

Last year in RSC advances, a team from the Institute for Computation in Molecular and Materials Science at Nanjing University reported “new strategies to design two novel and super-high energy cage explosives: dodecanitrohexaprismane (DNH) and hexanitrohexaazaprismane (HNHAH)…results indicate that DNH has much higher energetic properties than ONC…although HNHAH has lower energetic properties than DNH, it has higher energetic properties than ONC slightly.”

The Bottom Line

So, we may well see even higher energy densities available from more exotic molecules in the future — whether they are used for rocket fuel, missile fuel or in more everyday aviation applications.

Will these materials come from biobased sources. Synth biologists may well be training their magic bugs as we speak, or who knows what novel catalysts might spin out of a pine tee some time soon.

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