|How to Land Softly on a Hard Planet
Just one of the many problems in landing on another planet, after it's
been determined where to land and the method to get there, is landing
safely. For NASA'a Jet Propulsion Laboratory, a safe landing is
"the name of the game," as engineers work to prepare
two rovers for the journey to Mars.
The Mars Exploration Rovers scheduled for launch in 2003 are using
the same type airbag landing system that Mars Pathfinder used in
1997. The airbags must be strong enough to cushion the spacecraft if it
lands on rocks or rough terrain and allow it to bounce across Mars' surface
at freeway speeds after landing. To add to the complexity, the airbags
must be inflated seconds before touchdown and deflated once safely
on the ground.
"The 2003 rovers have a different mass [than Sojourner, the
Pathfinder rover], so we've made changes in the airbag design,"
said John Carson, cognizant engineer. "Our requirement is
to be able to land safely on a rock extending about a half-meter
(about 18 inches) above the surface. Extensive testing gives us a
process for trial and error before the final design."
How to Build a Better Airbag
While most new automobiles now come with airbags, spacecraft
don't. The fabric being used for the new Mars airbags is a synthetic
material called Vectran that was also used on Mars Pathfinder. Vectran
has almost twice the strength of other synthetic materials, such as Kevlar,
and performs better at cold temperatures.
Extensive testing of airbags is necessary before engineers arrive at a final design.
Denier is a term that measures the diameter of the thread used in
the product. There will be six 100-denier layers of the light but tough
Vectran protecting one or two inner bladders of the same material in
200-denier, according to Dara Sabahi, mechanical systems architect.
Using the 100-denier means there is more actual fabric in the outer
layers where it is needed, because there are more threads in the weave.
Each rover uses four airbags with six lobes each, which are all
connected. Connection is important, since it helps abate some of the
landing forces by keeping the bag system flexible and responsive to
ground pressure. The fabric of the airbags is not attached directly to the
rover; ropes that crisscross the bags hold the fabric to the rover. The
ropes give the bags shape, which makes inflation easier. While in flight,
the bags are stowed along with three gas generators that are used
Testing, Testing, Testing
Since the airbags are composed of many layers, some tearing in the
outer layers is acceptable and even expected. Engineers test the bags to
make sure there will be no catastrophic problems that would prevent a
Ropes on the airbags are called "tendons." They give the bags shape.
Mars airbag testing is done in world's largest vacuum chamber at the
Plum Brook Station of NASA's Glenn Research Center in Ohio. "The
Plum Brook facility is pretty impressive, along with all the people who
operate it," said Carson.
The test chamber used for the tests is a little over 30 meters
(100 feet) across and about 37 meters (120 feet) high - big enough
that three railroad tracks go through it. A test spacecraft and airbag
system weighing about 535 kilograms (about 1,180 pounds) are
accelerated with a bungee cord system onto a platform with rocks that
approximate the Mars surface. The drop is at landing speed, about
20 to 24 meters (yards) per second.
Tests are documented thoroughly with high-speed and video cameras,
in addition to visual inspections. Engineers even built a clear dome,
studded with rocks, that has a camera that documents tests from a
rock's-eye view. During testing, a crew from ILC Dover, the airbag's
manufacturer, stands by to make quick repairs and to note any
"We do extensive testing," said Tom Rivellini, deputy
mechanical systems architect. "We want to break the bag on
Earth, not on Mars. If we see a tear that is unexpected or goes too
deep, we can make changes now [before the final design]."
Carson added, "We'll go over all the data we've accumulated
so far, do some more testing, and decide on a design configuration."
And then on to Mars in 2003!