Airplane accidents are especially dangerous because jet fuel is highly flammable under crash conditions. On impact, jet fuel is dispersed in the air as a fine mist, which triggers a sequence of events that can lead to a fire engulfing an entire plane.

Researchers at the California Institute of Technology and the Jet Propulsion Laboratory, which is managed by Caltech, have been working on additives that inhibit the formation of this highly flammable mist during collisions. These additives are based on long molecules called polymers.

"This research is about making fuel safer and saving lives," said Project Manager Virendra Sarohia, based at JPL.

A new Caltech-led study in the journal Science describes polymers that could increase the safety of jet fuel and diesel fuel, particularly in the event of collision or a deliberate attempt to create a fuel explosion as part of a terrorist attack.

"The new polymers could reduce the intensity of post-crash fires, providing time for more passengers to escape," said Julia Kornfield, a professor of chemical engineering at Caltech who mentored Ming-Hsin Wei, Boyu Li and Ameri David. Their doctoral research is presented in the study.

Fuel misting also happens in jet engines under normal operations. The engine repeatedly ignites a combination of a spray of fuel and compressed air, and this process thrusts the plane forward. The problem arises when a fuel mist is created outside the engine. For example when a plane crashes, the entire volume of fuel could be involved in misting. "Once we control the mist in a crash, this aviation fuel is hard to ignite," said Sarohia, who collaborated with JPL technologist Simon Jones. "It allows time to fight fires and time to evacuate people from the accident."

Various tests have been conducted in relation to the new study. Impact tests using jet fuel show that the polymers reduce flame propagation in the resulting mist. In other tests, the polymers showed no adverse effects on diesel engine operation, researchers say. Larger-scale production is needed to provide enough polymer for jet engine tests.

"Years of testing are required to achieve FAA approval for use in jet fuel, so the polymer might be used first to reduce post-crash fires on roadways," Kornfield said.

How the Polymers Work
A polymer is a large molecule that has regularly repeated units. The new technology consists of polymer chains that are able to reversibly link together through chemical groups on their ends that stick together like Velcro. If you link these polymers end-to-end, very large chains form, which the study authors call "mega-supramolecules."

"Our polymers have backbones that, like fuel, have just carbon and hydrogen, but they are much, much longer. Typically our polymers have 50,000 carbon atoms in the backbone," said Kornfield.

"Such long polymers, specially constructed for a fuel additive, are unprecedented. Many years of laboratory effort have gone into the design of their structure and the development of careful methods for their synthesis," said Jones.

Sarohia likens the mechanism of the fuel additive to the clotting of blood. While blood is in the veins, it should flow freely; clotting in the veins could be fatal. But blood is supposed to clot when it gets to the surface of skin, so that a person doesn't bleed out. Similarly, the jet fuel with the polymer added should flow normally during routine operation of the aircraft; it's only during a collision that it should act to control the mist.

JPL's Involvement
Sarohia has been working on this research since the 1970s. The Tenerife Airport disaster in the Canary Islands in 1977, in which 583 passengers aboard two planes were killed in a runway collision, demonstrated the need for safer jet fuel. An international collaboration resulted in successful sled-driven plane crash tests of a fuel additive in the early 1980s.

But the analyses of a 1984 full-scale impact test in California's Mojave Desert were mixed. There was no more activity in the research program for more than a decade.

It looked as though the program had ended for good. But Sarohia remembers that after the Sept. 11, 2001, attacks on the World Trade Center, his daughter asked him, "Where's your fuel?" That got him thinking about the polymer again.

Not long afterwards, Sarohia received the support of JPL to restart the investigation of a polymer to control fuel mist. In 2003, Sarohia and colleagues demonstrated in tests at China Lake, California, that the polymer could be effective even at 500 mph impact speeds. The results provided the impetus for the Caltech-JPL collaboration.

The fuel additive tested in the 1980s consisted of ultralong polymers that interfered with engine operation. Therefore each and every aircraft would need to be retrofitted with a device called a "degrader" to break the polymers into small segments just before injection in the engine. However, the new polymers can release their end associations during fuel-injection and disperse into smaller units that are compatible with engine operation.

"The hope is that it will not require the modification of the engine," Sarohia said.

Long-haul diesel engine tests also show that the polymer has the potential to reduce emissions of particulate matter by controlling the fuel droplet size. These megasupramolecules may also reduce resistance to flow through pipelines. Ongoing research is establishing methods to produce the larger quantities of the polymer required to explore these opportunities.

Read more about this study from Caltech. The Science study was funded by the U.S. Army Tank Automotive Research Development and Engineering Center, the Federal Aviation Administration, the Schlumberger Foundation, and the Gates Grubstake Fund.