For Ignition, Mix Metal And Ice

Steven F. Son has a surefire recipe for stress: Set up a costly and labor-intensive experiment you've never done before and then invite your sponsors to come and see it. That's the nerve-racking position the Purdue University mechanical engineering professor found himself in this past August as he stood in an Indiana field, hoping a 9-foot rocket would lift off.

As he watched the slim black rocket race 1,300 feet into the sky, Son says, he mainly felt relieved. But there was also a hint of vindication at seeing a project succeed that many said would never get off the ground. The entire rocket assembly, from tip to tail fins, had been built by Son and his collaborators at Purdue and Pennsylvania State University, but it was the gray toothpastelike material loaded inside the rocket that brought representatives from the National Aeronautics & Space Administration and the Air Force. That paste, a blend of aluminum nanoparticles and ice, is a new kind of propellant known as ALICE.

"Propellants have not changed much in the past 40 years," says Timothée Pourpoint, an aeronautics professor at Purdue who collaborated on the project. "We have to start looking at different approaches if we want to improve their performance." But many who work with propellants had been skeptical that the aluminum-ice blend would ever fly, Son says. They thought there might be too much alumina slag for a successful launch, and they wondered whether the high-pressure gases needed to ignite ALICE might create a bomb rather than a rocket.

At the moment, ALICE's performance doesn't beat or even match rocket fuels currently in use. But the August launch was an important proof-of-concept demonstration, both Son and Pourpoint say. Using the reaction of aluminum and water to generate propulsion was first proposed in the 1940s, Son notes. Water oxidizes the aluminum to produce hydrogen gas and alumina. The aluminum burns at around 3,000 °C, forcing hot exhaust gases out of the rocket to propel it skyward. Until now, however, the reaction simply wasn't fast enough to use as a rocket propellant.

Researchers on the ALICE project were able to speed up the reaction by using aluminum particles just 80 nm in diameter. The increased surface area changes the reaction kinetics so that ignition is possible, Son explains. "Using microsized aluminum, the reaction would be too slow," he says. "The nanoaluminum enables the reaction to go fast enough to make it into a propellant."

Embedding the nanoparticles in a matrix of ice rather than liquid water prevents side reactions that deactivate the metal. It takes a lot of energy to ignite the frozen mixture, but that also makes it safer to handle.

Figuring out how to create the ALICE mix was one of the project's biggest challenges, Son tells C&EN. Ultimately, the researchers learned that they could add the nanoparticles to water chilled just above its freezing point. Mixing by hand or, for an easier job, with a resonating mixer creates a paste that can then be loaded into molds and frozen until firing.

"The mixing process they developed and the demonstration that this thing can be launched was a pretty big advance in terms of basic science," says Thomas L. Jackson, a senior scientist at the University of Illinois, Urbana-Champaign's Center for Simulation of Advanced Rockets. "But it's not likely to replace booster propellants. If there are applications, they're likely to turn up down the road."

Indeed, Son and Pourpoint acknowledge that, in its current formulation, it's unlikely ALICE will ever, to quote the 1950s television series "The Honeymooners," go "Bang! Zoom! To the Moon!" They are currently exploring additives and nanoparticle coatings to improve the propellant's performance. There is, they note, one down-the-road application that ALICE is uniquely suited for—a fuel that could be prepared on the moon, Mars, or anywhere else there's water. Space travelers wouldn't need to take all their fuel with them, but could mix ALICE at their destination.