Engineers have found a way to boost the performance
of liquid fueled rockets. Their secret: innovative plumbing.
by Dr Tony Phillips
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
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.
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.
Goddard and a 1920s-vintage liquid-fueled rocket.
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.
rendering of the Integrated Powerhead Demonstrator, showing
its innovative plumbing for routing fuel and oxidizer
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.
"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.