Perhaps a bit off-topic, but interesting aerospace stuff:
From NASA:
Intense Testing Paved Phoenix Road to Mars
When NASA's Phoenix Mars Lander descends to the surface of the Red Planet
on
May 25, few will be watching as closely as the men and women who have
spent
years planning, analyzing and conducting tests to prepare for the dramatic
and nerve-wracking event known as EDL -- Entry, Descent and Landing. For
after all their hard work, they know that landing on Mars is not a walk in
the park. Less than 50 percent of all previous lander missions have made
it
safely to the surface.
Like all missions, Phoenix was motivated by the potential science rewards.
With its robotic arm, Phoenix will be the first mission to reach out and
touch water ice in Mars' north polar region. The mission will study the
history of the water in the ice, monitor weather of the polar region, and
investigate whether the subsurface environment in the far-northern plains
of
Mars has ever been favorable for sustaining microbial life.
Much of the Phoenix spacecraft already sat in secure storage when, in
2003,
NASA selected it over other proposals to fly to Mars. Phoenix's main
systems
were designed and built for launch as the Mars Surveyor 2001 Lander, but
that mission was canceled in February 2000, after the loss of a similar
spacecraft, the Mars Polar Lander, during its arrival at Mars in 1999.
The team that proposed the Phoenix mission, led by Peter Smith of the
University of Arizona, Tucson, developed a plan to bring the spacecraft
out
of storage, thoroughly analyze and test it, resolve all known problems,
and
add upgrades so it could pursue a new set of science goals. The spacecraft
heritage of the 2001 lander, derived from the "faster, better, cheaper"
era,
brought with it op****tunities, along with several challenges.
Phoenix Project Manager Barry Goldstein of NASA's Jet Propulsion
Laboratory,
Pasadena, Calif., discussed the team's approach to adapting a pre-built
spacecraft for this mission, instead of developing one from scratch: "One
consequence of having so much of the hardware in place from the start was
that we could focus our resources into testing and analysis. We evaluated
the robustness of the vehicle to perform the mission we designed, most
notably the entry, descent and landing."
The team first focused on correcting all the vulnerabilities identified by
earlier investigations into the loss of the Mars Polar Lander. "That
wasn't
enough," Goldstein said. "We eventually identified and mitigated more than
a
dozen other potential issues with the spacecraft that could have had dire
consequences." Extensive testing and analysis also identified concerns
that
could have affected the lander, solar array deployment, and its science
instruments after arrival on the Martian surface. However, an acceptable
amount of risk still exists--for example, most hardware is at least 8 to
10
years old, and certain subsystems have no redundancy during the entry,
descent and landing.
Goldstein said, "We've done everything we can to lower the risks of this
mission to acceptable levels, but in no way does that mean we've
eliminated
all risk. Planetary exploration is risky by its very nature, and there are
numerous challenges ahead of us, the first of which is entry, descent and
landing."
Here are descriptions of five examples of problematic hardware and
resolutions resulting from the extensive work done by the Phoenix
engineering and science team.
Radar
Phoenix uses a radar system initially designed as an altimeter for fighter
jets. During the final minutes before landing, after the spacecraft has
jettisoned its heat ****eld, Phoenix will rely on the radar for information
about not just the altitude, but also the descent velocity and the
horizontal velocity. The onboard computer will use that information
several
times per second to adjust the firing of 12 descent thrusters.
Using the radar for this novel purpose required a tremendous amount of
testing, "We did more than 60 hours of flight testing, including 72
different drops at three sites with different geological characteristics,"
said David Skulsky, a JPL engineer on the Phoenix team. That's more radar
flight testing than all previous NASA Mars missions combined."
Radar tests also included custom-developed simulations of performance
under
Martian conditions. Running one of those simulator tests just four months
before the spacecraft was due to be delivered to Florida for launch,
Curtis
Chen, a JPL radar engineer, noticed some strange behavior. Analysis
confirmed that, under some cir***stances, the radar could be confused by
the
jettisoned heat ****eld.
JPL's Dara Sabahi, chief engineer for Phoenix, said, "If this occurred in
flight, the spacecraft would think it was much closer to the ground than
it
actually was. It would be a guaranteed failure."
Once the testing had revealed the potential problem, engineers designed a
relatively simple solution using adjustments related to the timing of
radar
pulses. However, the schedule was tight, and additional flight tests were
needed to be sure that fixing that issue had not created others. "We
worked
all the way to launch on the testing, and even did more testing after
launch
to be sure we understand the performance," Sabahi said.
In addition, NASA formed a Radar Independent Review Team of key radar
experts to evaluate the activities of the Phoenix team working with the
radar. The review team was chartered to determine if the radar had been
properly characterized, if the im****tant risks associated with its
performance have been identified, mitigated, and that unmitigated residual
radar risks represented a low risk to the mission. The Phoenix team
followed
all recommendations from the Independent Review Team. The review team
endorsed the approach taken by the project to resolve all anomalies. They
concluded that the probability for a successful landing on Mars under
radar
guidance was comparable to or better than that of prior missions.
Parachute
The lander will separate from its parachute about 40 seconds before
reaching
the ground. Thrusters will begin firing half a second later and continue
pulsing all the way to the surface, controlling both vertical and
horizontal
velocity, plus the spacecraft's orientation.
"We did some analysis that showed there was a three-to-five percent
chance,
depending on wind conditions, that the lander would have some kind of
re-contact with the parachute," said Rob Grover, chief of the Phoenix
entry,
descent and landing team at JPL. "The worst situation would be to have the
parachute come down right on top of the lander and prevent deployment of
the
solar arrays."
Rather than rely on the odds against such an occurrence, engineers
designed
a maneuver for the lander to avoid the parachute. Horizontal motion
identified by the radar while the lander is still connected to the
parachute
will indicate wind direction and speed. If the wind is strong, the
parachute
will blow away on its own. If the wind is weak, the lander will use its
thrusters after separating from the parachute to push itself upwind, away
from the falling parachute.
Motors
The robotic arm on Phoenix uses four electric motors from the same lot of
211 motors originally purchased for NASA's Mars Exploration Rover project.
Fifty of the motors were sent to Mars on rovers Spirit and Op****tunity. Of
the remaining motors, later testing identified two whose brushes were
broken. Motor brushes provide electrical contact between moving and
stationary parts of the motor. The brushes in these motors are solid
pieces
of a special mixture of copper, graphite and molybdenum made for Martian
conditions.
The motors installed on the Phoenix spacecraft had been tested and showed
no
trouble. In addition, their counterparts on Spirit and Op****tunity have
far
outperformed their design life under stressful real-Mars conditions. For
the
Phoenix team, the issue was how to *****s whether the two broken brushes
were enough reason not to rely on the motors in the robotic arm.
Goldstein,
the Phoenix project manager, said, "We did not rest on these motors'
excellent track record with Spirit and Op****tunity. We did our own
testing."
The Phoenix project put the arm motors through additional testing and also
turned to the NASA Engineering and Safety Center, a resource created for
providing just such assistance with independent analysis of engineering
issues related to risk for NASA projects. The Phoenix team followed
recommendations from a review team formed by the center. These
recommendations included using sensors to monitor any jarring of the
motors
during trans****tation of Phoenix from Denver, where it was built by
Lockheed
Martin Space Systems, to Florida for launch.
Scoop
Central to the design of the Phoenix mission is the intent to dig to an
icy
layer under the surface and deliver some of the ice-rich soil to a small
laboratory on the deck of the lander. That icy soil will probably be as
hard
as concrete.
The original design for the scoop at the end of the arm had three sets of
metal blades for cutting and scraping to loosen enough icy soil to sample.
The Phoenix team ran tests using sample materials as tough as those
expected
on Mars.
JPL engineer Lori ****rai**** said, "We found it took four to six hours to
get
enough material, but you are also fighting sublimation of the ice. The ice
would be disappearing by the time you are trying to pick it up."
In 2005, the team began working on an alternative design to loosen and
collect an icy sample more quickly. JPL's Gregory Peters came up with the
idea of a motorized rasp to replace one of the sets of blades. Honeybee
Robotics Spacecraft Mechanisms Corp., New York, built and tested the
redesigned scoop. The rasp uses a tile-cutting bit lowered at an angle
through a slot in the bottom of the scoop. Tests indicate the system can
loosen and lift and deliver an icy sample in about half an hour, which is
believed to be quick enough to outrun sublimation of the exposed ice under
Martian atmospheric conditions.
Stowaway carbon
The Phoenix team has tested all of the lander's science instruments
extensively. One that sniffs va****s generated from heating samples of soil
and ice will be checking for organic molecules. Most carbon-containing
chemicals are called organics. Organic chemicals can be present without
life, but they are an essential ingredient for life as we know it. Testing
made clear that this instrument -- the Thermal and Evolved-Gas Analyzer --
is sensitive enough to detect the trace amounts of organics that are
likely
to come from Earth aboard the lander.
"We want to be able to determine whether we're just seeing organics we
brought along with us," said William Boynton of the University of Arizona,
Tucson, lead scientist for this instrument.
The university assembled a meeting of organic chemists from around the
country in 2005 for a discussion of how to prepare for analyzing the data
from Phoenix. From that workshop came a recommendation for Phoenix to
carry
"blank" material specially made to be as free of carbon as possible, for
use
as an experimental control for comparison with samples of Martian soil and
ice.
The Phoenix team *****sed various possibilities for the blank material.
The
lander is carrying a block of a custom-made, very-low-carbon ceramic
product
from Corning Inc. During operations at the landing site, the powered rasp
will be able to produce shavings from the blank for analysis. The results
will help scientists interpret whether any organics found during analysis
of
Martian samples actually came from those samples.
There are many other examples of how the Phoenix mission has identified
concerns through testing and analysis, and then resolved them.
Goldstein said, "I can't guarantee success. We are in the business of
taking
risks, doing things that are very difficult. However, I am confident that
we
have a world-class team that has dug as deep as it could to find any
problems."
--
Best Regards,
Mike
http://photoshow.comcast.net/mikenoel
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