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NASA Science News for January 29, 2003

Candlestick Rocket Ship
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The high-tech rocket fuels of the future could be made from a
surprisingly low-tech material: candle wax! 

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. 

"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. 

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. 

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. 

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!" 

Credits & Contacts
Author: Patrick L. Barry 
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy

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