A common food emulsifier could solve one of rocketry's oldest fuel problems

In propulsion system design, engineers have long faced a trade-off between solid and liquid rocket fuels. Each has advantages and disadvantages, and in an effort to try to min/max the efficacy of different types of fuel, researchers have long placed an emphasis on a hybrid between solids and liquids—in other words, they've been working with gels. Unfortunately, gels have their own sets of problems to deal with.

But a new paper published by Pranay Mudaliar and Jerin John of Concordia in the journal Langmuir uses an uncommon ingredient—lecithin—to elegantly solve some of those problems, though it's typically used in a completely different area of human activity.

Solid rocket boosters, like those that powered the space shuttle, are very stable, durable and quick to ignite. However, they are also impossible to turn off—you have to burn through all the propellant at once.

Liquid fuel, on the other hand, is much more flexible, much like automotive fuel. Liquid-fueled engines can be throttled up and down, restarted multiple times, and generally are much more controllable than their solid cousins. However, liquid fuels also float and slosh around in tanks, which is a particular problem in zero-gravity environments, as that sloshing can cause vapor pockets that can break a turbopump if not filtered out beforehand. Liquids also have a tendency to leak or evaporate over long periods, making them less than ideal for multiyear journeys through the solar system.

On paper, gels offer a great solution to that. While sitting in a tank, they would act like a dense solid, but can also act like a liquid when forced into a combustion chamber. However, they suffer from their own challenges, such as shelf life and temperature-dependent phase changes.

The paper details a new type of fuel that uses an unexpected special ingredient. Lecithin might sound familiar, but not in the context of rocket fuel. It's commonly used as an emulsifier in food to keep chocolate smooth and salad dressing cohesive, and can be found on thousands of supermarket food labels. In the context of rocket fuel, lecithin acts simply as an organic gelling agent.

When combined with a trace amount of calcium chloride salt, lecithin molecules self-organize into microscopic wormlike structures called micelles. Chains of these form an invisible 3D mesh that can trap and hold kerosene molecules, a more traditional type of rocket fuel, in place. Lecithin also has the added advantage of being an organic compound made up of carbon and hydrogen, so it will burn right alongside the kerosene rather than acting as simply dead weight like many other noncombustible thickening agents previously used to create gel fuels.

But perhaps the most interesting feature of the new fuel is a physical phenomenon known as shear thinning. When the gel is sitting under normal conditions in a tank, it holds its shape firmly like a gelatin, eliminating the sloshing features that cause so many problems in liquid-fuel rockets. However, as soon as there's mechanical pressure or shear force applied—such as when the spacecraft pressurizes the fuel lines—the micellar mesh breaks apart, causing the gel to quickly collapse into a liquid temporarily.

Measurements detailed in the paper show the gel's viscosity dropping by a factor of more than 100, creating an easily pumpable liquid that can be sprayed through injector nozzles. But when the pressure stops, the material snaps back to its rest state as a gel.

There are also two other environmental stimuli that can cause this liquid transformation—if the gel is heated up to between 59°C and 68°C, or splashed with a small amount of ethanol. In other words, localized thermal or chemical triggers could easily intentionally force specific amounts of fuel to liquefy without requiring the bulk of the fuel to do so.

Another appealing feature of the gel, from an experimental standpoint anyway, is that it's clear. Therefore, researchers can watch it with optical high-speed cameras and lasers to analyze its structure as well as its spray patterns. And a final appealing feature is its shelf life—the gel stayed stable in a tank for over a year, with no visible signs of the phase separation that plagues many other gelatin-like fuels.

It is still a very long road from the benchtop to the launchpad, though. The new fuel hasn't actually been sprayed through the high-pressure injectors that would be used on an actual rocket engine. Nor has it been shown to be capable of withstanding the vibrations and temperature shocks that come from being launched into space. But as the Concordia researchers continue its development, this new gel is looking promising for potentially powering a future mission.