Can warplanes fly farther, carry more weapons, with advanced biofuels? More new data.

April 13, 2014 |


We’ve been following the development of advanced fuels that carry more, travel farther — and here’s a giant step forward.

A China Lake-based research team finds terpene fuels that add 13% to the performance, compared to fossil fuels.

We take a look at the new breed of renewable fuels — with a Q&A to explore their significance.

In California, a group of researchers from the Naval Air Warfare Center Weapons Division at China Lake have confirmed that “high density fuels with net heats of combustion ranging from 133–141 000 Btu gal, or up to 13% higher than commercial jet fuel could be generated” from biosynthetic materials.

What does 13 percent mean?

That means carrying 13 percent less fuel for a standard mission, or traveling 13 percent farther with a full fuel load. Considering that the F18-E carries around 14,400 pounds of fuel and an F18-F carries around 13,500 — that’s enough to add an extra Harpoon or SLAM missile (based on weight). Or, extend the range of an F/A 18 Super Hornet from 2069 miles to 2337 miles, or extend mission time accordingly.

That’s just on the fuel density. But wait, there’s more.

Longer engine life, less metal fatigue

Last May, the Digest reported that tests conducted at Wright-Patterson Air Force Base have revealed that renewable fuels were lowering US warplane engine temperatures by 135 degrees, owing to absence of impurities found in conventional fossil fuels.

These were SPK fuels. The ones that are under investigation at China Lake are even more dense, more attractive. But the performance of these new, biosynthetic fuels could be expected to share one important characteristic with SPK fuels. The lack of the impurities that are found in petroleum-based fuels.

Why do impurities matter?

When those impurities burn, he explained, it causes high temperatures to radiate throughout the engine, causing an acceleration in metal fatigue. “At the temperatures that military jet engine perform at, an additional 25 degrees in temperature can shorten the life of the engine by half,” Mendoza said. He added that the preliminary data showed that engine parts could last up to 10 times longer, if the new high performance fuels were employed in place of conventional fossil fuels.

Moreover, the impurities can cause the fuel exhaust to “spray”, reducing the direct thrust from an engine. The same way that, if a wind changes directions, you either have to tack a sailboat or suffer a loss in speed. Or why a concentrated “jet stream” of water from a hose exerts more pressure than a stream set in “spray” mode.

All of which causes renewable fuels, which have reduced tendency to “spray”, can create increased thrust with comparable fuel density.

At the time, Robert Freerks of RLF Enterprises, the father of SPK fuels dating back to 1999, explained to the Digest.

“There is nothing new in [SPK] fuel, only the lack of some things that are found in conventional fuels.  The major difference is the aromatics content, which impacts several performance attributes of the fuel including net heat content, temperature of combustion (and radiant heat release), density, and conductivity.

“A paper by Linda Blevins of Sandia NL (2003) that indicated that the radiative heat release from aromatics was higher than from paraffins,” explained Dr. Freerks.  ”Omar Mendoza mentioned [in 2005-06] that the hot section of the engine is of high concern for reliability and is inspected for failure periodically.  He indicated that a roughly 10C drop in wall temperature results in doubling of the life of the hot section.  As there have been flight accidents due to hot section failures, this is of interest. “In the end, Omar is correct.  This analysis goes against older thought, so it was resisted by some, [who don’t] didn’t fully understand the multiple issues going on here, the facts and data.”

Answers for your questions on these advances in fuel technology

Q. Any changes needed to the warplanes?

A. No, these are “exact-same” in terms of engine design, they just perform better.

Q. Why are renewable fuels achieving these 13% higher performance metrics?

A. The bottom line? Biotechnology can directly produce high energy-density molecules in higher concentrations than found in a barrel of petroleums (if found at all).

Q. How are these different from SPK fuels that were recently certified for flight use and are now being used in military and commercial aviation?

A. SPK fuels are renewable, and have attractive performance properties, but lower energy densities than this class of molecules.

Q. What are the molecules called?

A. The most interesting group are the terpenes. If you’ve smelt fresh pine of late — that’s a terpene known as pinene that’s giving off that rich, dense fragrance.

As China Lake researcher Ben Harvey and his team explain in their paper, “Terpenoid structures are an obvious choice based on the vast number of naturally occurring terpenoids (~50 000)and the structural diversity of these molecules. In particular, monoterpenes (C10) and sesquiterpenes (C15) are of interest for both jet and diesel fuels. Farnesane has already been commercialized as a renewable diesel fuel, while hydrogenated cyclic terpenes derived from limonene, pinenes, and sabinene are being considered as components of jet fuel.

Q. Ah, farnesane. Isn’t that the molecule that Amyris makes? Why not produce a whole bunch of that?

“Although farnesane can be used as a renewable diesel fuel and component of jet fuel,” the research team writes, “it suffers from the same moderate density as the SPK fuels. A logical way to improve the density of renewable fuels is to incorporate cyclic molecules that have acceptable combustion properties…To develop renewable fuels that have the potential to outperform conventional petroleum fuels in regard to density and net heat of combustion, the current work explores the synthesis and fuel properties of both pure hydrogenated multicyclic sesquiterpenes and complex mixtures of isomerized sesquiterpenes.”

Q. I’m sorry, I don’t speak Geek. In English?

A. Farnesane doesn’t have that high energy density, though it has other attractive properties and makes a great jet fuel. But in military use — exact-same is good, but higher performance has high-value. At least, that’s what machine-gun makers used to explain about the comparable performance of rifles.

Q. What’s the value of a high-density fuel?

A. Well, take for example, missile fuel. Missiles use what is called JP-10 fuels, which have far higher performance characteristics than conventional jet fuel — owing to their higher energy densities. The armed services generally pay around $25 per gallon for these fuels.

Q. $25 a gallon? Wasn’t that the scandalous cost cited by critics as a reason not to use advanced biofuels? You’re telling me that DoD already pays this for other high-performance fuels made from petroleum?

A. Well, yes. But, owing to the cost, only in fuel used by missiles.

Q. Now, would these fuels made by advanced biofuels producers for the Navy actually cost $25 a gallon?

A. Hardly. In test quantities, they cost a lot. For the same reason that a season ticket cost a lot less per game than a single-game ticket. But the Navy is requiring that advanced biofuels be “cost-competitive” with petroleum-based fuels, when they are purchased at scale.

Q. Why is the Navy worried about conventional fuel prices and volatility?

Like everyone, the Navy experiences pain at the pump. In 2011 the Navy was forced to slash training and fleet operations in order to make up for $1 billion in cost overruns, caused by the run-up in oil prices that year. Every $1 increase in the cost of a barrel of oil results in a $31 million increase in Navy fuel expenditures.

Q. Is it true that Congress tried to ban purchase of advanced biofuels, unless they are “cost-competitive” with conventional fuels, even if they have unique and superior performance characteristics?

A. Yes and no. Section 315 of the FY14 NDAA limits us to purchasing alternative fuels only if they are cost competitive:

(a) LIMITATION.-None of the funds authorized to be appropriated by this Act or otherwise made available for fiscal year 2014 for the Department of Defense may be obligated or expended to make a bulk purchase of a drop-in fuel for operational purposes unless the cost of that drop-in fuel is cost-competitive with the cost of a traditional fuel available for the same purpose.

In 2012, OSD also issued the same policy in their Alternative Fuels Policy for Operational Platforms:

“..alternative drop-in replacement fuel procured for DoD- wide use and distribution within the Class III (Bulk) supply chain will compete with petroleum products under the DLA Bulk Purchase and Direct Delivery Purchase Programs. Awards will be based on the ability to meet requirements at the best value to the government, including cost.”

So, advanced biofuels can’t be purchased in bulk at higher prices, but they can be purchased for testing purposes at higher-than-marklet prices — generally owing to the small quantities involved. But there hasn’t been a determination yet on “drop-in fuels” that are bot “exact-same” in terms of performance.

Q. Why would anyone establish such a ban that? Are we supposed to have best-equipped troops in the world?

A. Our ideas on equipment are evolving. For a long time, fuels have not been thought of as conferring a tactical advantage. They have been thought of as commodities — the cheaper, the better. Rather, they are thought of, if at all, for the strategic benefits relating back to energy security. Now that differentiation is emerging, thinking should change, and perhaps it will.

The Bottom Line.

Well, labs aren’t the real world, and identifying a great fuel is only one step, albeit a mighty one, in the journey towards economic production at scale. So let’s applaud as we would at the conclusion of a great first act of a five-act play.

But, that doesn’t diminish what has been demonstrated.

As Harvey and his colleagues explain: “In this work, we have demonstrated that 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.”

The full article is here.

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