|An artist's rendition illustrates MER's descent-rate limiter. Located in the spacecraft, it unspools a Kevlar rope tether attached to the Rover after a parachute slows the craft to about 200 mph. The Kevlar lowers the Rover 20 m below the craft so that three small retrorockets can fire without damaging it.|
Key to the Rover's soft landing is a descent-rate limiter that will lower the Lander 20 m below the spacecraft so three downward pointing retrorockets can slow descent from 200 mph to touchdown without damaging it. A similar device was used during the Pathfinder mission in 1997.
Problem with the commercial descent-rate limiter is that the device's steel strap wasn't strong enough to carry the heavier MER payload. To beef-up the design, NASA looked at swapping the steel strap with a different alloy but a six to seven week lead-time didn't fit the project's tight schedule.
Synthetic rope such as Kevlar was a second option. But rope spooled around a large shaft can't take a heavy load because the top winding starts to bury itself in the underlying layers. NASA didn't give up on the idea, however. Engineers there devised a two-spool design for storing the Kevlar rope. The rope would also wrap around a separate shaft to engage a braking system.
For the design to work, the brake shaft needed a high-friction surface so that the Kevlar rope wouldn't slip. NASA teamed with coating maker General Magnaplate, Linden N.J., (www.magnaplate.com) to develop the coating. NASA's Jet Propulsion Lab had previously worked with the company to solve a similar problem that plagued the support pads used during MER testing. "Like the design of the brake shaft, the pads needed high-friction surfaces to keep the equipment from moving as it sat on the pads," says Magnaplate spokesperson Mike Prager.
Normally Magnaplate provides dry-lubricant, low-friction coatings with high wear and corrosion resistance at temperatures to 1,300°F. For the NASA project, it modified an enhanced thermal-spray composite coating called Plasmadize with 0.003 to 0.005-in.-diameter metal particulates. This transformed the coating surface texture to that of about 100-grit sandpaper.
MER engineers tested the descent-rate-limiter braking system at the China Lake naval testing facility. "The high-friction coating on the two-spool set-up worked perfectly," Prager said. MER engineers refined the descent-rate limiter until it could carry the Lander and was about the size of a box of tissues.
To view a computer simulation of MER's entry, descent, and landing on Mars visit: http://mars.jpl.nasa.gov/mer/gallery/video/animation.html
-- Jean M. Hoffman