Sandia conducts tests at Solar Tower to benefit future NASA spaceexploration (Forwarded)

Sandia National Laboratories

Sandia media contact:
Michael Padilla, (505) 284-5325

FOR IMMEDIATE RELEASE: September 7, 2005

Sandia conducts tests at Solar Tower to benefit future NASA space
explorations

Missions of interest include Saturn's Titan

ALBUQUERQUE, N.M. -- For the last two years, tests have been conducted
at Sandia National Laboratories' National Solar Thermal Test Facility to
see how materials used for NASA's future planetary exploration missions
can withstand severe radiant heating.

The tests apply heat equivalent to 1,500 suns to spacecraft shields
called Advanced Charring Ablators. The ablators protect spacecraft
entering atmospheres at hypersonic speeds.

The test facility includes a 200-ft. "solar tower" surrounded by by a
field of hundreds of sun-tracking mirror arrays called heliostats. The
heliostats direct sunlight to the top of the tower where the test
objects are affixed.

Under a work agreement, researchers at Sandia and Applied Research
Associates, Inc. are conducting the tests for NASA Marshall's In-Space
Propulsion/Aerocapture Program. The R&D effort is tied to NASA's plan
for a future Titan mission with an orbiter and lander. Titan is Saturn's
largest moon.

The tests are led by Sandia solar tower expert Cheryl Ghanbari and Bill
Congdon, project principal investigator for Applied Research Associates,
Inc.

The tests are designed to simulate atmospheric heating of spacecraft
that enter Titan, including low levels of convective heating combined
with relatively high levels of thermal radiation.

The primary ablator candidates for the Titan mission are low-density
silicones and phenolics, all under 20 pounds-per-cubic-foot density.

To date, more than 100 five-inch diameter samples have been tested in
the solar environment inside the tower's wind tunnel using a large
quartz window.

Congdon says because of Titan's relatively high radiation environment,
some initial concerns had to be put to rest through testing. He says
radiation might penetrate in-depth within the ablator, causing an
increased "apparent" thermal conductivity and degrading insulation
performance.

"Radiation could also generate high-pressure gasses within the ablator
leading to spallation," Congdon says.

"We have been testing at the solar tower to see how the candidate Titan
materials can withstand the expected range of heating conditions,"
Ghanbari says. "Titan has a nitrogen-rich atmosphere and nitrogen is
used in the tests to similarly reduce ablator oxidation, while energy
from the sun-tracking heliostats is focused on the samples."

Congdon says ground tests are necessary to understand and model surface
ablation of the materials that will be severely heated during Titan entry.

During thermal radiation testing conducted in the solar tower, all of
these concerns were addressed and found not to be a problem for the
ablators of interest.

About the tests

The National Solar Thermal Test Facility consists of an eight-acre field
of 220 solar-collection heliostats and a 200-ft.-tall tower that
receives the collected energy at one of several test bays. A single
heliostat includes 25 mirrors that are each four feet square. Total
collection area of 220 heliostats is 88,000-square feet.

Because the heliostats are individually computer controlled, test
radiation can be a shaped pulse as well as a square wave in terms of
intensity vs. time, says Ghanbari.

Test samples are mounted high in the receiver tower, and the heliostats
direct the sunlight upward to irradiate the sample surface. The samples
are mounted in a water-cooled copper plate inside the wind tunnel with a
quartz window that allows entry of the concentrated radiation.

Exposure is controlled by a fast-moving shutter and by pre-programmed
heliostat movement. Radiation flux is calibrated before and after each
test by a radiometer installed to occupy the same position as the test
sample. Cooling effects from imposed surface flows are calibrated via a
flat-plate slug calorimeter.

The materials are subjected to square pulse environments at flux levels
of 100 and 150 W/cm2 for time periods that far exceed predicted flight
durations for such high heating. They are also tested to "exact" flux
vs. time environments (simulating actual flight conditions) using
programmed heliostat focusing at the solar tower facility.

The material samples are installed in the tower's wind tunnel and
exposed to the solar beam at flux levels up to 150 W/cm2, which is
approximately 1,500 times the intensity of the sun on earth on a clear
day. During exposure, air blows past the sample at about mach 0.3 with a
high-speed nitrogen sub-layer close to the sample surface.

Ghanbari says tests can be conducted only during about four hours midday
bracketing solar noon. Haze, clouds, and high winds that affect the
heliostats can degrade test conditions.

Current results

"All of the candidate materials showed no spallation and very good
thermal performance to these imposed environments," Congdon says.
Recently, five 12-inch by 12-inch panel samples were tested on top of
the tower. Up to 20 additional 12-inch panels will be tested late in the
summer followed by testing of 2-foot by 2-foot panels later in the year.

Additional tests for convective heating have been conducted on identical
material samples at the Interaction Heating Facility (IHF) at NASA's
Ames Research Center.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a
Lockheed Martin company, for the U.S. Department of Energy's National
Nuclear Security Administration. Sandia has major R&D responsibilities
in national security, energy and environmental technologies, and
economic competitiveness.

IMAGE CAPTIONS:

[Image 1:
http://www.sandia.gov/news-center/ne...solar-test.jpg
(776KB)]
Sandia’s Cheryl Ghanbari and Steve Moon of Gray Research, look at a
5-inch-diameter ablator sample that was tested in Sandia's solar wind
tunnel. (Photo by Randy Montoya)

[Image 2:
http://www.sandia.gov/news-center/ne...solar-heat.jpg
(376KB)]
Solar power heats NASA space shield material. The tests apply heat
equivalent to 1,500 suns to spacecraft shields. (Photo courtesy of Bill
Congdon, Applied Research Associates, Inc.)