What, exactly, makes a rocket fuel environmentally friendly?

Automobiles aren’t the only vehicles turning to more environmentally friendly fuel sources. As we reported recently, NASA are testing a new type of rocket propellant made of a mixture of water and “nanoscale aluminum” powder they claim could provide a cleaner way to launch rockets, power long-distance space missions and generate hydrogen for fuel cells. A number of readers wondered, not unreasonably, what qualifies a rocket fuel as eco-friendly. We now have a few more answers.

The aluminum-ice, or ALICE, propellant is considered “green” because it produces essentially hydrogen gas and aluminum oxide. This is compared to current space shuttle flights, which consume about 773 tons of the oxidizer ammonium perchlorate in the solid booster rockets, with about 230 tons of hydrochloric acid appearing immediately in the exhaust from such flights.

ALICE provides thrust through a chemical reaction between water and aluminum. As the aluminum ignites, water molecules provide oxygen and hydrogen to fuel the combustion until all of the powder is burned. The key to the propellant’s performance is the tiny size of the aluminum particles, which have a diameter of about 80 nanometers. The nanoparticles combust more rapidly than larger particles and enable better control over the reaction and the rocket’s thrust.

Other researchers have previously used aluminum particles in propellants, but those propellants usually also contained larger, micron-size particles, whereas the new fuel contained pure nanoparticles. Manufacturers over the last decade have learned how to make higher-quality nano-aluminum particles than was possible in the past. The fuel needs to be frozen solid so it remains intact while subjected to the forces of the launch and also to ensure that it does not slowly react before it is used.

Initially a paste, the fuel is packed into a cylindrical mold with a metal rod running through the center. After it's 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 center hole, causing the ALICE fuel to ignite uniformly.

"ALICE might one day replace some liquid or solid propellants, and, when perfected, might have a higher performance than conventional propellants," said Timothée Pourpoint, a research assistant professor in Purdue’s School of Aeronautics and Astronautics. "It's also extremely safe while frozen because it is difficult to accidentally ignite," he said.

Future work will focus on perfecting the fuel and also may 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 also could be mixed with materials containing larger amounts of hydrogen and then used to run hydrogen fuel cells in addition to rocket motors.

The researchers also note that, with findings from spacecraft indicating the presence of water on Mars and the moon - and, potentially, on asteroids, other moons and bodies in space - the propellant could theoretically be manufactured in space, instead of being transported at high cost.

Researchers at Purdue University are working with NASA, the Air Force Office of Scientific Research and Pennsylvania State University to develop the ALICE propellant.

The video below shows the initial development and testing of the aluminum ALICE propellant.