Aluminium ice for rocket propellant

Researchers in the US are developing a more environmentally friendly rocket propellant made of a frozen mixture of water and ‘nanoscale aluminium’ powder.

The propellant and could be manufactured on the moon, Mars and other water-bearing bodies.

The aluminium-ice, or ALICE, propellant could be used to launch rockets into orbit and for long-distance space missions, and could also generate hydrogen for fuel cells, said Steven Son, an associate professor of mechanical engineering at Purdue University in Illinois, US.

Purdue is working with NASA, the Air Force Office of Scientific Research and Pennsylvania State University to develop ALICE, which was used earlier this year to launch a 2.7m-tall rocket. The vehicle reached an altitude of 396m over Purdue's Scholer farms.

The tiny size of the aluminium particles, which have a diameter of about 80nm, is key to the propellant's performance. The nanoparticles combust more rapidly than larger particles and enable better control over the reaction and the rocket's thrust, said Timothée Pourpoint, a research assistant professor in the Purdue’s School of Aeronautics and Astronautics.

‘It is considered a green propellant, producing essentially hydrogen gas and aluminium oxide,’ Pourpoint said. ‘In contrast, each space shuttle flight consumes about 773 tons of the oxidiser ammonium perchlorate in the solid booster rockets. About 230 tons of hydrochloric acid immediately appears in the exhaust from such flights.’

ALICE provides thrust through a chemical reaction between water and aluminium. As the aluminium ignites, water molecules provide oxygen and hydrogen to fuel the combustion until all of the powder is burned.

Researchers have used aluminium particles in propellants before, but those propellants usually also contained larger, micron-size particles. The new fuel contains pure nanoparticles.

The research team explained that the fuel needs to be frozen so that it can remain intact while subjected to the forces of the launch. It must also be frozen to ensure that it does not slowly react before it is used.

The fuel, which is initially a paste, is packed into a cylindrical mould with a metal rod running through the centre. After it is frozen, the rod is removed, leaving a cavity running the length of the solid-fuel cylinder. A small rocket engine above the fuel is ignited, sending hot gasses into the centre hole, which causes the ALICE fuel to ignite uniformly.

‘This is essentially the same basic procedure used in the space shuttle's two solid-fuel rocket boosters,’ Son said. ‘An electric match ignites a small motor, which then ignites a bigger motor.’

In the future, the research team plan to explore the possibility of creating a gelled fuel using the nanoparticles. Such a gel would behave like a liquid fuel, making it possible to vary the rate at which the fuel is pumped into the combustion chamber to throttle the motor up and down and increase the vehicle's distance.

A gelled fuel could also be mixed with materials containing larger amounts of hydrogen and then used to run hydrogen fuel cells in addition to rocket motors, Son said.