A new polymer added to jet fuel can reduce the chance of an explosion after a crash. Recent tests show that it improves safety without affecting fuel performance.
Jet engines compress air and combine it with a fine spray of jet fuel. Ignition of the mixture of air and jet fuel by an electric spark triggers a controlled explosion that thrusts the plane forward. Jet airplanes are powered by thousands of these tiny explosions.
However, the process that distributes the spray of fuel for ignition—known as misting—also causes fuel to rapidly disperse and easily catch fire in the event of an impact.
The additive is a type of polymer—a long molecule made up of many repeating subunits—capped at each end by units that act like Velcro. The individual polymers spontaneously link into ultralong chains called “megasupramolecules.”
Julia Kornfield, a professor of chemical engineering at the California Institute of Technology (Caltech), whose lab developed the additive, says megasupramolecules have an unprecedented combination of properties that allows them to control fuel misting, improve the flow of fuel through pipelines, and reduce soot formation.
Megasupramolecules inhibit misting under crash conditions and permit misting during fuel injection in the engine.
Other polymers have shown these benefits but have deficiencies that limit their usefulness. For example, ultralong polymers tend to break irreversibly when passing through pumps, pipelines, and filters. As a result, they lose their useful properties.
This is not an issue with megasupramolecules, however. Although supramolecules also detach into smaller parts as they pass through a pump, the process is reversible. The Velcro-like units at the ends of the individual chains simply reconnect when they meet, effectively “healing” the megasupramolecules.
First Tested in 1984
When added to fuel, megasupramolecules dramatically affect the flow behavior even when the polymer concentration is too low to influence other properties of the liquid. For example, the additive does not change the energy content, surface tension, or density of the fuel. Also, the power and efficiency of engines that use fuel with the additive is unchanged—at least in the diesel engines that have been tested so far.
When an impact occurs and there’s a sudden elongation of the fluid, the supramolecules stretch out and resist further elongation. This stretching allows them to inhibit the breakup of droplets—thus reducing the size of explosions—as well as to reduce turbulence in pipelines.
“The idea of megasupramolecules grew out of ultralong polymers,” says Ming-Hsin “Jeremy” Wei, a research scientist and a coauthor of the paper published in Science. “In the late 1970s and early 1980s, polymer scientists were very enthusiastic about adding ultralong polymers to fuel in order to make postimpact explosions of aircrafts less intense.”
The concept was tested in a full-scale crash test of an airplane in 1984. The plane was briefly engulfed in a fireball, generating negative headlines and causing ultralong polymers to quickly fall out of favor, Wei says.
Molecular Velcro
In 2002, Virendra Sarohia at the Jet Propulsion Laboratory (JPL), which is based at Caltech, sought to revive research on mist control in hopes of preventing another attack like that of 9/11.
“He reached out to me and convinced me to design a new polymer for mist control of jet fuel,” says Kornfield, the corresponding author on the new paper. The first breakthrough came in 2006 with the theoretical prediction of megasupramolecules by Ameri David, then a graduate student in her lab.
