Engineers have found a way to boost the performance of liquid
fueled rockets. Their secret: innovative plumbing.
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October 14, 2005: When you think of future rocket
technology, you probably think of ion propulsion, antimatter
engines and other exotic concepts.
Not
so fast! The final chapter in traditional liquid-fueled rockets
has yet to be written. Research is underway into a new generation
of liquid-fueled rocket designs that could double performance
over today's designs while also improving reliability.
Liquid-fueled
rockets have been around for a long time: The first liquid-powered
launch was performed in 1926 by Robert H. Goddard. That simple
rocket produced roughly 20 pounds of thrust, enough to carry
it about 40 feet into the air. Since then, designs have become
sophisticated and powerful. The space shuttle's three liquid-fueled
onboard engines, for instance, can exert more than 1.5 million
pounds of combined thrust en route to Earth orbit.
Right:
Robert Goddard and a 1920s-vintage liquid-fueled rocket. [More]
You
might assume that, by now, every conceivable refinement in
liquid-fueled rocket designs must have been made. You'd be
wrong. It turns out there's room for improvement.
Led
by the US Air Force, a group consisting of NASA, the Department
of Defense, and several industry partners are working on better
engine designs. Their program is called Integrated High Payoff
Rocket Propulsion Technologies, and they are looking at many
possible improvements. One of the most promising so far is
a new scheme for fuel flow:
The
basic idea behind a liquid-fueled rocket is rather simple.
A fuel and an oxidizer, both in liquid form, are fed into
a combustion chamber and ignited. For example, the shuttle
uses liquid hydrogen as its fuel and liquid oxygen as the
oxidizer. The hot gases produced by the combustion escape
rapidly through the cone-shaped nozzle, thus producing thrust.
The
details, of course, are much more complicated. For one, both
the liquid fuel and the oxidizer must be fed into the chamber
very rapidly and under great pressure. The shuttle's main
engines would drain a swimming pool full of fuel in only 25
seconds!
This
gushing torrent of fuel is driven by a turbopump. To power
the turbopump, a small amount of fuel is "preburned",
thus generating hot gases that drive the turbopump, which
in turn pumps the rest of the fuel into the main combustion
chamber. A similar process is used to pump the oxidizer.
Today's
liquid-fueled rockets send only a small amount of fuel and
oxidizer through the preburners. The bulk flows directly to
the main combustion chamber, skipping the preburners entirely.
One
of many innovations being tested by the Air Force and NASA
is to send all of the fuel and oxidizer through their
respective preburners. Only a small amount is consumed there--just
enough to run the turbos; the rest flows through to the combustion
chamber.
This
"full-flow staged cycle" design has an important
advantage: with more mass passing through the turbine that
drives the turbopump, the turbopump is driven harder, thus
reaching higher pressures. Higher pressures equal greater
performance from the rocket.
Such
a design has never been used in a liquid-fueled rocket in
the U.S. before, according to Gary Genge at NASA's Marshall
Space Flight Center. Genge is the Deputy Project Manager for
the Integrated Powerhead Demonstrator (IPD)--a test-engine
for these concepts.
Right:
A rendering of the Integrated Powerhead Demonstrator, showing
its innovative plumbing for routing fuel and oxidizer to the
combustion chamber. [More]
"These
designs we're exploring could boost performance in many ways,"
says Genge. "We're hoping for better fuel efficiency,
higher thrust-to-weight ratio, improved reliability--all at
a lower cost."
"At
this phase of the project, however, we're just trying to get
this alternate flow pattern working correctly," he notes.
Already
they've achieved one key goal: a cooler-running engine. "Turbopumps
using traditional flow patterns can heat up to 1800 C,"
says Genge. That's a lot of thermal stress on the engine.
The
"full flow" turbopump is cooler, because with more
mass running through it, lower temperatures can be used and
still achieve good performance. "We've lowered the temperature
by several hundred degrees," he says.
IPD
is meant only as a testbed for new ideas, notes Genge. The
demonstrator itself will never fly to space. But if the project
is successful, some of IPD's improvements could find their
way into the launch vehicles of the future.
Almost
a hundred years and thousands of launches after Goddard, the
best liquid-fueled rockets may be yet to come.
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Author: Patrick
L. Barry | Editor:
Dr. Tony Phillips | Credit: Science@NASA
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