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 January
29, 2003: Waiting inside his Mercury capsule for the
command that would start the countdown and make him the first
American in space, Alan Shepard yelled impatiently, "Let's
light this candle!"
Those words may turn out to be more prophetic than Shepard intended.
Since 2001, NASA's Ames Research Center has been testing a new
rocket fuel made from--believe it or not--candle wax.
Right: A material in household candles, paraffin, could
become the environmentally friendly rocket fuel of the future.
Image copyright © 2003 Comstock, Inc., all rights reserved.
"We actually ordered the wax for our test firings through
a commercial Web site that sells candle wax in bulk," says
Arif Karabeyoglu, who developed the theory behind paraffin-based
rocket fuels and is currently a research associate at Stanford
University.
"We use the exact same wax found in 'hurricane' candles,"
he says.
Safer to handle and better for the environment than today's
solid rocket fuels, this modern twist on an ancient fuel could
someday propel sounding rockets and commercial-payload rockets
into space. It could even form the heart of a new generation
of shuttle solid rocket boosters (SRBs) that would have a key
safety feature today's SRBs lack: an "off" switch.
This may seem a shockingly primitive fuel for 21st Century rocket
technology. After all, humans have been burning candles (today
often made of "paraffin" wax) for the last five millennia.
Why didn't someone think of using it for rockets before?
As anyone who's lit a candle knows, paraffin normally burns quite
calmly, and it's difficult to make it burn at all without a wick.
By all appearances, it just wasn't the kind of high-powered,
explosive fuel needed to blast a rocket off of the planet!
Working in collaboration with David Altman, currently president
of Space Propulsion Group, and Brian Cantwell, a professor at
Stanford, Karabeyoglu figured out a way to make paraffin burn
three times faster than had ever been achieved before--fast enough
to serve as rocket fuel.
 In their design, the paraffin burns in the presence
of pure oxygen gas. This alone causes it to burn much hotter
than it does in air, which is only about 21% oxygen. That much
had been done before. Karabeyoglu's innovation was to blow the
oxygen past the melted surface of the paraffin fast enough to
"whip up" this surface, like the ocean's choppy surface
on a windy day. The "sea spray" of paraffin droplets
that this kicks up burns very rapidly, tripling the combustion
rate of the fuel.
Above: That's no candle flame! This test of the paraffin-based
fuel was conducted at NASA's Ames Research Center. Image courtesy
NASA.
More than 40 test firings by the Stanford-Ames collaborative
project have shown that the idea works as advertised. That's
good news for the rocket industry, because this paraffin fuel
would be much simpler and safer to work with than the toxic,
explosive fuels used today.
Just think of a household candle. You can safely carve it, melt
it, and mold it. If it's free from artificial colors or perfumes,
you could even lick it or chew on it. You could burn dozens of
them in a room with no fear of toxic gases making you sick.
Don't try any of these things with conventional solid rocket
fuels!
One reason for the benign nature of candle wax is that the oxidizer
needed for it to burn is separate from the wax itself: air in
the case of candles, and pure oxygen for rockets. (Chemically
speaking, combustion is the rapid "oxidation" of the
fuel, usually by combining with the oxygen gas in the air. That's
why fires go out when deprived of air.) This kind of rocket with
a solid fuel and a separate gaseous or liquid oxidizer is called
a "hybrid" rocket.
 In contrast, today's solid-fuel rockets
use solid materials such as perchlorate compounds as oxidizers,
and the fuel and oxidizer are mixed together before being packed
into the rocket. In other words, the fuel is "charged"
and ready to explode ... not a friendly material to work with.
It's not friendly for the environment either. When today's solid
fuels burn, they produce the acidic compound hydrogen chloride
and other noxious chemicals. When it rains, these compounds find
their way into lakes and soils, and the increased acidity can
harm plant and animal life.
Paraffin, in contrast, burns cleanly. The only gases left behind
are water vapor and carbon dioxide. Rocket launches are still
so rare that the total pollution they emit is tiny compared to
that from cars, airplanes, and coal-fired power plants. But in
the future, as more countries and private companies begin launching
people and payloads into space, clean-burning rocket fuels will
become an increasingly important environmental issue.
Above: The space shuttle Columbia (STS-107) leaves Earth
on Jan. 16, 2003. Photo credit and copyright: Becky
Ramotowski.
Using hybrid rockets would make all these rocket launches
a bit safer as well.
By controlling the flow of the oxidizing gas, "hybrid rockets
... can be throttled over a wide range, including shut-down and
restart," Cantwell said in a prepared statement. "That's
one reason why they could be considered as possible replacements
for the shuttle's current solid rocket boosters that cannot be
shut off after they are lit."
"A hybrid rocket equivalent to the space shuttle's solid
rockets would be about the same diameter, but would be somewhat
longer," Cantwell continues. "One design concept being
considered is a new hybrid booster rocket that is able to fly
back to the launch site for recharging," he says, which
would save considerable cost and time in preparing the boosters
for the next launch.
 Left: NASA and Stanford scientists and engineers
work on the testing rig for the new paraffin-based solid rocket
fuel. Pictured are (clockwise from bottom-left): Brian Cantwell,
Arif Karabeyoglu, Shane De'Zilwa, Rusty Hunt, Dave Yaste, Kent
Shiffer, Greg Zilliac. Image courtesy NASA.
However, we won't be seeing paraffin-based shuttle SRBs for
many years to come, if ever, Karabeyoglu says. The technology
is still in the demonstration phase, and would likely be used
for years on smaller rockets before being considered for NASA's
flagship launch vehicle.
But if the tests continue to go well, the launch director at
Mission Control may one day mean it quite literally when she
or he says, "All right, enough waiting around ... let's
light this candle!"
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