Pathfinder travels for eight months, arriving at Mars on July 4, 1997.
Then the lander, enclosed by front and rear protective heat shields, will begin
punching through the Martian atmosphere at 7.65 km/second, pop parachutes at 425
m/second, fire solid-propellant retrorockets for a final slowing, and bounce,
skid, and roll to a stop, courtesy of Rivellini's airbag.
The airbag subsystem consists of four bags, each composed of six 1.8m-diameter spherical lobes arranged in a billiard-rack configuration on 1m centers. Each bag attaches to one of the tetrahedral-shaped lander's four flat, triangular faces. Internal vents share the 441-cu-ft volume of each airbag, allowing all of the volume to be acted on during an impact. A system of internal and external ropes and tendons restrain membrane loads, stiffen the airbag, and connect it to the lander. With inflator, the sturdy subsystem weighs 98 kg.
ILC Dover (Frederica, DE) manufactures the airbag from four layers of Vectran®, a liquid crystal polymer fiber produced by Hoechst-Celanese. Early prototypes were made of Kevlar®, but Kevlar's low resistance to flex cracking caused failures along folds. To retain gas pressure, silicone coats the innermost bladder layer of Vectran.
Custom gas generators (from Thiokol Corp.) fill the 1,764 cu-ft of airbags to 1.0 psi in 1.5 seconds. Essentially small solid-propellant rockets, their exhaust passes through a coolant chamber containing a proprietary endothermic compound that reduces fill-gas temperatures to an acceptable level. A slow grain continues burning for an additional 20 seconds to account for the dramatic atmospheric cooling that would otherwise deflate the bag as Pathfinder rolls to a stop.
Rivellini's job doesn't end after landing. Hundreds of pounds of fabric draped around the lander must now be retracted. "Can you imagine getting there and not being able to get the lander out of the airbag?" Rivellini asks rhetorically.
He studied this problem for weeks, building models of garbage bags, ribbon, and string, before conceiving a sound method. It consists of five thin Vectran cords in each bag (four in the base) threaded in a zigzag pattern through loops sewn in the fabric. The cords merge together at a winch that generates several hundred pounds force via high-ratio gearing. Full retraction takes approximately two hours.
At this point, the significance of the lander's tetrahedral shape becomes apparent. With the airbag retracted, the panels open like a flower one by one, automatically righting the lander no matter what its original orientation.
Testing the airbag under realistic conditions proved extremely difficult. Early tests occurred at Sandia National Lab and resulted in changes from a large three-lobed design to the more compact six lobes. Engineers then produced a full-size example for impact testing in the United States' largest vacuum chamber, a 100-ft-wide by 120-ft-tall cavern at NASA Lewis' Plum Brook Station. "Mars has 1/100th the atmosphere of earth, and to simulate the gas dynamics and physics of the impact we had to do it in near vacuum," says Rivellini.
* Collision protection systems
* Safety device for balloonists
Propelled by huge elastic shock
cords against a 60-degree tilted platform strewn with jagged boulders, at speeds
as high as 27 m/second (60 mph), the airbags tore and failed. Dozens of
different fabric combinations were tried. Ultimately, Rivellini calculated that
they could reduce the fill-pressure from 1.5 psi to 1.0 psi, and he selected a
design with four thin layers of Vectran (200 denier), which, surprisingly,
outperformed single layers of 750 denier fabric. "Making the bag withstand a 60
mph impact with extreme horizontal velocity over rocks that cut your hands when
you just picked them up was definitely the biggest challenge," he says.
Though this will be the first use of an airbag landing system, engineers based their work on a JPL study performed in 1966. Extensive supercomputer modeling at Sandia, JPL, and Rockwell using ABAQUS, ADAMS, and DYNA-3D aided the airbag sizing, but proved insufficient to precisely model the harsh landing conditions. "We are at the lowest temperature limits of the fabric and gas generator, and hitting the most extreme terrain practical," says Rivellini, "We pushed the limits on everything."
Tom Rivellini holds an undergraduate degree from Syracuse University and an MS from the University of Texas at Austin, both in aerospace engineering. He has worked at JPL since 1991, and has designed aspects of several flight and technology programs, among them: MSTI Scout, an early earth-orbiting instrument testbed launched in 1992; the Mars Pathfinder; the New Millennium Mars Microprobe; the Rocky IV microrover demonstrator for the Pathfinder; and the Pluto Fast Flyby mission.
Additional details...Contact Tom Rivellini, Jet Propulsion Laboratory, 4800 Oak Grove Dr., M/S 158-224, Pasadena, CA 91109, (818) 354-5919.
Learn more about Pathfinder at JPL's Web site: http://mpfwww.jpl.nasa.gov/