Air Force, NASA Make Progress with Alternative Rocket Fuel
WASHINGTON — The U.S. Air Force is funding the development of a new rocket fuel that promises to be as powerful as current propellants, but safer and simpler to handle, possibly cheaper and environmentally benign. It is made from a frozen mixture of finely powdered aluminum and water.
In August, the Air Force, NASA and scientists from Purdue and Pennsylvania State, or Penn State, universities launched a 3-meter rocket 400 meters into the air during an initial test flight of the propellant ALICE — so named for its ingredients, aluminum and ice.
The rocket accelerated to 320 kilometers per hour in 0.8 seconds, said Timothee Pourpoint, an aeronautics and astronautics professor at Purdue.
That is a promising start, but not better than current propellants, he said. ALICE developers are convinced they can do better. “We have a lot of ideas on how to improve performance,” Pourpoint said.
If performance can be boosted, aluminum-ice propellant offers a number of attractive attributes to the Air Force.
It is safer than other propellants, said Mitat Birkan, program manager for space power and propulsion at the Air Force Office of Scientific Research. It is less prone to ignite accidentally, “you can drop it and it won’t explode, and the danger from electrostatic discharge is negligible.”
And it is environmentally benign compared with some of the rocket fuels in use today, Birkan said.
NASA, for example, “can’t launch the space shuttle until the wind is blowing off shore” because the shuttle’s engines “produce tons and tons” of hydrochloric acid, which is harmful to humans, Birkan said.
An engine fueled with ALICE emits hydrogen and aluminum oxide, which Birkan said is relatively innocuous.
“Aluminum oxide is not something we want to be coating everything with, but not as bad” as hydrochloric acid, Pourpoint said.
Perhaps the most intriguing ALICE attribute is that the propellant is simple enough to be produced in situ — meaning just about anywhere. All that is needed is a source for water, powdered aluminum and a mixer, Birkan said. Together, those attributes could greatly simplify logistics for the Air Force, he said.
To NASA, in-situ manufacturing suggests that it might be possible to produce ALICE propellant on the Moon, Mars or elsewhere. Space exploration may require being able to refuel without returning home.
“We know there is aluminum on the Moon and some on Mars, and we know there is water on Mars,” Pourpoint said. The idea of using aluminum and water as a propellant is not new.
“Aluminum-water combustion has been studied since the 1960s as a viable propellant,” Grant Risha, a Penn State professor, wrote in a paper on ALICE propellant in August.
But previous attempts to make aluminum and water fuels foundered on the size of the aluminum particles that were used, Birkan said.
What is new here is “nano aluminum particles. That’s completely different from anything that has been done before,” he said.
Earlier efforts with aluminum powder used micron-size particles. A micron is about 100 times smaller than the width of a human hair. But those particles were much too big. They must be heated to 2,400 degrees Kelvin — about 2,125 degrees Celsius — to ignite. Nano-particles are 1,000 times smaller and will ignite at half that temperature.
“The key in here is nano aluminum,” Pourpoint said. “If you do the same thing with micron-size aluminum, it will not work.”
Nano-aluminum is mixed with water to make a slurry that assumes the consistency of clay. The mixture is then packed into molds and cooled to about minus 28 degrees Celsius.
Freezing the slurry prevents the aluminum particles from reacting with the water, Pourpoint said.
To light the frozen propellant, hot gas is forced down its hollow core until the aluminum reaches its ignition point. To burn, the aluminum needs oxygen, and it gets that from the water — manifested as ice — in the frozen slurry.
One way to improve ALICE propellant performance might be to add a richer oxygen source, possibly hydrogen peroxide, which has twice as much oxygen as water.
Even though it is frozen, ALICE propellant is warm compared with some rocket fuels, such as liquid hydrogen, which must be kept at minus 217 degrees Celsius. That makes ALICE attractive as a fuel for satellites, Pourpoint said.
Heat from the sun makes it difficult to keep hydrogen liquid, even in space, he said. It would be much easier to keep aluminum ice frozen. That would give it a longer useful life as a propellant for satellites, he said.
Aluminum and water combustion may be used to power other vehicles besides satellites, rockets and missiles.
“This reaction produces hydrogen,” Birkan said. “In fact, it’s one of the most efficient ways to produce hydrogen.” It could be used at hydrogen stations to make fuel for a generation of hydrogen-powered automobiles, he said.