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May
4, 2007: On January 16, 2007, a dazzling blue flame
blasted across the sands of the Mojave desert. In many respects,
it looked like an ordinary rocket engine test, but this was
different. While most NASA rockets are powered by liquid oxygen
and hydrogen or solid chemicals, "we were testing a methane
engine," says project manager Terri Tramel of NASA's
Marshall Space Flight Center (MSFC).
Click
on the image to play the movie:
Above:
Test firing of a 7,500 pound-thrust LOX/methane engine. Image
credit: Mike Massee/XCOR Aerospace. [Press
release] [Movie]
The
main engine, built and fired by the NASA contractor team Alliant
Techsystems/XCOR Aerospace, is still in an early stage of
development and isn't ready for space. But if the technology
proves itself, methane engines like this one could eventually
be key to deep space exploration.
Methane
(CH4), the principal component of natural gas,
is abundant in the outer solar system. It can be harvested
from Mars, Titan, Jupiter, and many other planets and moons.
With fuel waiting at the destination, a rocket leaving Earth
wouldn't have to carry so much propellant, reducing the cost
of a mission.
Perhaps
surprisingly, this flammable gas has never powered a spacecraft
before. But now scientists and engineers at Marshall, the
Glenn Research Center and the Johnson Space Center are developing
LOX/methane engines as an option for the future. "Several
efforts are underway, including a rival LOX/methane main engine
design by KT Engineering," notes Tramel.
"This
work is funded by NASA's Exploration Technology Development
Program and shows how technologies being developed for exploration
may one day assist in future science missions," says
Mark D. Klem, manager of the Propulsion and Cryogenics Advanced
Development Project at the Glenn Research Center.
"Methane
has so many advantages," continues Tramel. "The
question is, why haven't we done this before?"
Consider
the following: Liquid hydrogen fuel used by the space shuttle
must be stored at a temperature of -252.9°C—only about 20
degrees above absolute zero! Liquid methane, on the other
hand, can be stored at the much warmer and more convenient
temperature of -161.6°C. That means methane fuel tanks wouldn't
need as much insulation, making them lighter and thus cheaper
to launch. The tanks could also be smaller, because liquid
methane is denser than liquid hydrogen, again saving money
and weight.
Methane
also gets high marks for human safety. While some rocket fuels
are potentially toxic, "methane is what we call a green
propellant," Tramel says. "You don't have to put
on a HAZMAT suit to handle it like fuels used on many space
vehicles."
But
the key attraction for methane is that it exists or can be
made on many worlds that NASA might want to visit someday,
including Mars.
Although
Mars is not rich in methane, methane can be manufactured there
via the Sabatier process: Mix some carbon dioxide (CO2) with
hydrogen (H), then heat the mixture to produce CH4
and H20--methane and water. The Martian atmosphere
is an abundant source of carbon dioxide, and the relatively
small amount of hydrogen required for the process may be brought
along from Earth or gathered in situ from Martian
ice.
 Traveling
further out in the solar system, methane becomes even easier
to come by. On Saturn's moon Titan, it is literally raining
liquid methane. Titan is dotted with lakes and rivers of methane
and other hydrocarbons that could one day serve as fuel depots.
Imagine, a methane-powered rocket could allow a robotic probe
to land on the surface of Titan, gather geological samples,
refill its tanks, and blast off to return those samples to
Earth. Such a sample-return mission from the outer solar system
has never been attempted.
Above:
This false-color radar image shows what researchers believe
are lakes of liquid methane on Titan. Credit: NASA/ESA/Cassini.
[More]
The
atmospheres of Jupiter, Saturn, Uranus and Neptune all contain
methane, and Pluto has frozen methane ice on its surface.
New kinds of missions to these worlds may become possible
with methane rockets.
This
first series of desert test firings of the 7,500 pound-thrust
main engine was a success, but challenges remain before methane
rockets will be ready for use in a real mission. "One
of the big questions with methane is its ability to ignite,"
Tramel says. Some rocket fuels ignite spontaneously when mixed
with the oxidizer, but methane requires an ignition source.
Ignition sources can be hard to make in the outer solar system
where planetary temperatures drop to hundreds of degrees below
zero. Tramel and her colleagues at Marshall and Glenn are
currently working to assure that the rocket will ignite reliably
in all conditions.
Such
challenges will be surmountable through NASA's continued efforts,
Tramel says, and she believes LOX-methane engines will be
used in rockets of the future. The blue flame in the desert
was a beautiful first step.
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Author: Patrick Barry | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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