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July
16, 2007: Accelerating from 0 to 60 then slowing
down for a stop light is no problem for an ordinary automobile.
But if you were piloting a rocketship, it wouldn't be so easy.
Most rocket engines are designed to burn full-on (liftoff!)
or full-off (coasting through space) with no in-between. And
that can be a problem--namely, how do you land this thing?
Throttling
is crucial for a planetary lander. Descending
from orbit is a unique balancing act, cutting engine power
as the lander losses mass through the engine exhaust that
slows it, until landing pads just kiss the surface. For a
lunar landing, velocity drops from almost 4,000 mph to 0 in
about one hour.
Right:
Apollo 11's Lunar Module, the Eagle.
The
Apollo Lunar Module (LM) descent engine, the all-time throttling
champ, did it perfectly on six landings in 1969-72. It could
throttle from 10,125 lbs down to 1,250 lbs. It was also a
simple engine, burning corrosive fuel and oxidizer that ignited
on contact, and fed by pressurized tanks, eliminating the
need for pumps.
NASA
is heading back to the Moon in the next decade, and "we
want to put more mass down on the lunar surface than Apollo
did. That means we need a higher-performing engine,"
says engineer Tony Kim of NASA's Marshall Space Flight Center.
"The Apollo Lunar Module descent engine was very good,
very reliable, but it doesn't have the performance we need
for future exploration."
To
investigate technologies for a next-generation lunar lander,
engineers at two NASA centers--the Marshall Space Flight Center
in Alabama and the Glenn Research Center in Ohio--are supporting
Pratt & Whitney Rocketdyne in developing the Common Extensible
Cryogenic Engine--"CECE" for short.
At
CECE's core is the RL10 engine that boosted seven Surveyor
robot landers to the Moon in 1966-68, then flew dozens of
other missions for more than 2.2 million seconds of operations
(almost 26 days) and 718 in-space firings. The RL10 is a far
more powerful and complex beast than the Apollo LM engine.
It burns hydrogen and oxygen that are stored as supercold
liquids in insulated tanks. These are not only high-energy
propellants, but also environmentally friendly compared to
the corrosive fuel of the original LM.
Now the engine is being asked to demonstrate something new:
throttle from 100 percent of its 13,800-lb thrust to 10 percent
on command for a human-rated spacecraft. But making it throttle
is not as simple as pushing the gas pedal in and out. Like
most rocket engines, the RL10 was designed for full power.
Almost like a living organism, changes in one area are felt
through the entire body. For example, at low power, liquid
hydrogen can slow and vaporize in the coolant lines, possibly
stalling the engine.
Above:
Multiple images on the left are pictures of the CECE engine
at different throttle levels.
In
Phase 1 Demo 1 tests, "we were able to get the engine
modified and show that throttling is possible, though cautiously,"
Kim says. CECE racked up 932 seconds of firing time in eight
tests, though some were cut short "because we are experimenting."
The
principal challenge was "chugging." Something was
causing the engine to vibrate 100 times per second. Pratt
& Whitney Rocketdyne conducted a "Demo 1.5"
to investigate and isolate the problem: It turns out oxygen
vapors were forming on the injector plate and inhibiting normal
flow at lower throttle levels.
"We're
considering modifications to the injector and valves to improve
performance," Kim says. Already, CECE has demonstrated
stable combustion (no chugging) down to 5-to-1 and operability
(some chugging) at 11-to-1 throttle ratios.
CECE's
not ready for space, Kim emphasizes, but it is an important
testbed to develop technology. "This work has the potential
to influence design of the next lunar lander."
CECE
is one of a number of candidate technologies being investigated
under NASA's Exploration Technology Development Program.
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Author: Dave Dooling | Editor:
Dr. Tony Phillips | Credit: Science@NASA
More
Information |
NASA's
current throttling champ is the Space Shuttle Main Engine,
with a range of 65 to 109 percent of normal power. An
RL10A-5 used on the DC-X demonstrator in 1993 can throttle
3:1, but that was not a space engine.
NASA's
Future: The
Vision for Space Exploration
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