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A Mars Exploration Rover Update
       
Using an Earth Wind Tunnel to Test a Parachute Bound for Mars
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By Melanie Melton Knocke
The Planetary Society
29 January 2003

In May and June of this year, NASA will launch two Mars Exploration
Rover (MER) spacecraft to the Red Planet. The process of getting these 
rovers ready for launch involves a complicated series of tests, trials 
and tribulations - all designed to insure a successful mission.

One such test was completed on January 16, when engineers used the
world's largest wind tunnel and an explosive mortar charge to
successfully test MER's final flight-ready parachute design.

Using the 80x120 ft. wind tunnel, a part of the National Full-Scale
Aerodynamics Complex located at NASA's Ames Research Center, engineers 
from the Jet Propulsion Laboratory (JPL) and Pioneer Aerospace 
conducted a series of experiments designed to test the strength of 
various parachute design options. 

For MER, like the 1997 Pathfinder mission, a parachute will deploy
automatically during the spacecraft's descent through the Martian
atmosphere. The testing of this system is vital to make sure the
parachute can withstand opening at supersonic speeds (960 miles per
hour or 1,550 km per hour) without sustaining damage.

The mission can't just re-use the Pathfinder parachute design in part
because MER is a heavier spacecraft. The parachute loads experienced
by Pathfinder were approximately 8,000 pounds. MER loads will be over
twice that, at 17,600 pounds. So, while the MER parachute may look
similar to Pathfinder's, it is larger, made up of stronger materials
and has slightly different dimensions - all of which require testing.

Why a Wind Tunnel Test?
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In order to properly test the parachute design, it is critical to
deploy the parachute at the pre-determined speed. Earlier parachute
tests were conducted by dropping a parachute and attached mass out of
a hovering helicopter. Tricky aerodynamics, winds, and other factors
during these drop tests made it difficult to deploy the parachute at
exactly the right speed. 

To better test the system, engineers needed an environment where they
could more accurately control the wind speeds and forces working on
the parachute - and that environment would have to be large enough to
handle a 27 foot (8.2 meter) diameter parachute and its 79 foot (24
meter) lines. This is where the 80x120 ft. wind tunnel came into play.

In the wind tunnel, engineers could dial in the desired wind velocity
and be greeted with a steady stream of air at the exact speed required 
- the perfect setting for a parachute deploy test.

Earth Wind vs. Mars Wind
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But can you simulate conditions found high up in the Martian
atmosphere here on Earth? Yes. The pressures experienced by the
parachute as it deploys at supersonic speeds in the rarified Martian
atmosphere can be duplicated at much lower speeds on Earth because of
Earth's much thicker atmosphere. As a result, it is not necessary to
subject the parachute to supersonic speeds. Instead, the same
pressures can be experienced using wind speeds around 80 miles per
hour (130 km per hour).

The Final Strength Test
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For the final strength test, the polyester and nylon parachute and
its 48 suspension lines were carefully folded and then pressure
packed into a canister just like the one it will be traveling in to
Mars. 

The canister was then mounted on a tall pedestal inside the wind
tunnel. The winds were set to the proper speed and then an explosive
mortar charge released the parachute into the wind stream.

The parachute deployed to its full size in a matter of seconds without 
a hitch. Even though the deployment time at Mars will be much faster, 
the test proved that the parachute will be able to withstand the 
forces it encounters. 

Stay tuned as The Planetary Society brings you more updates on the
upcoming MER mission. Be sure and visit the Society's Red Rover Goes
to Mars site, where you have a chance to be a part of the MER team.

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