Rocket propellant has barely changed in the more than 50
years since the launch of the first artificial satellite Sputnik. But a new mixture
of nano-aluminum powder and frozen water could make rocket launches more
environmentally friendly, and even allow spacecraft to refuel at distant
locations such as the moon or Mars.
The aluminum-ice propellant known as ALICE gets its kick
from a chemical reaction between water and aluminum. Researchers hope that the
hydrogen products of that reaction might go beyond launching rockets, and also
feed hydrogen fuel cells for long
duration space missions.
"In the bigger picture, we're looking at technology
that can store hydrogen long term," said Steven Son, a professor of mechanical
engineering at Purdue University. "Water is a nice, stable way to store
hydrogen."
Both NASA and the Air Force Office of Scientific Research
have shown enough interest in the concept to fund initial rocket firing tests.
The research teams at Purdue and Penn State University used ALICE to successfully
launch a rocket to 1,300 feet during an August flight test.
Such technology may not see action for some years to come,
or at least until NASA sorts out its space exploration plans. But the recent
confirmation of water
sources on the moon and Mars may hint at a future where ALICE and similar
rocket propellants become highly practical.
Nano-aluminum arrives
Aluminum already represents a small but critical part of
many rocket fuels, including the propellants for the space shuttle's solid booster
rockets and NASA's next generation Ares
rockets. The metal's high ignition temperature of more than 6,920 degrees Fahrenheit
forces exhaust gases out at high velocity to propel rockets upward.
ALICE squeezes even more out of the aluminum by using
nano-scale particles with diameters of 80 nanometers, or 500 times smaller than
the width of a human hair. Such tiny particles combust more rapidly than larger
particles to give an additional kick, and may allow easier control over a
rocket's thrust.
"The nano-scale aluminum is really key to making system
work," said Timothee Pourpoint, a professor in the school of aeronautics
and astronautics at Purdue. "Using only micron-sized aluminum powder and
water ice would not have worked."
Researchers have tossed around theoretical calculations of
using just aluminum and water for rocket propellant in the past. But the Purdue
and Penn State teams took advantage of the relatively new nano-aluminum to
translate the concepts into reality.
"There have certainly been previous research efforts with
nano-aluminum and water," Son told SPACE.com. "This effort is
the first time that anyone has actually launched a rocket."
Ice might suffice
The aluminum burning at extreme temperatures represents just
one part of the ALICE equation. The other includes the oxygen and hydrogen
locked within water molecules that help feed the aluminum combustion.
That reaction produces products in the form of hydrogen gas
and aluminum oxide, which may prove greener than existing rockets. Current space
shuttle flights release about 230 tons of hydrochloric acid in the exhaust
left behind by their solid rocket boosters.
Creating the proper mix of ALICE propellant proved tricky,
but the researchers ended up with slurry that some describe as being like
toothpaste.
"To produce an adequate mix, we wanted to mix it using
a machine, not mix it by hand with a spatula," Son recalled. "If
you're going to scale up propellant processing, you up have to automate the
mixing."
The freezing helped keep the propellant intact during the
first test launch, as well as prevent any premature aluminum-ice reactions
caused by accidental sparks, or slow oxidation from occurring.
Moving on up
Success aside, the researchers have already turned their
thoughts to the new ALICE mixtures that can boost performance beyond that of
existing rockets.
"We're at or slightly below conventional solid
propellants in terms of overall performance," Son noted. But he added that
the team had made "conservative choices" to ensure that the first
flight showing proof of concept went smoothly. Now they can push the envelope.
Perhaps the most immediate idea involves mixing nano-aluminum
with larger aluminum particles. That could allow more efficient use of the
aluminum, and cut back on the amount of wasted aluminum oxide in the initial
aluminum.
The researchers are also working with more energetic
aluminum-water mixtures as they initially chose a mixture with extra ice to err
on the safe side. Their caution came at the cost of lower exhaust gas
temperatures during launch, which translated into lower performance and less
hydrogen produced.
"We are thinking about another launch with a different
composition of aluminum-ice propellant," Pourpoint said. "We know we
can adjust the ratio between the two components as well as add components to
improve performance."
Future work might even look at creating gelled propellant
that behaves like liquid fuel. New mixtures could also produce more hydrogen,
and take a step closer to helping run hydrogen fuel cells.
But both Son and Pourpoint emphasized a more immediate
benefit from ALICE — giving more than a dozen undergraduate and graduate
students the chance to translate scientific concepts into a real
rocket launch. Getting to help pioneer a future phase of space exploration
doesn't hurt, either.
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