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The Columbia disaster was caused by damage sustained during launch when
foam insulation broke off the main propellant tank under the aerodynamic
forces of launch. The debris struck the leading edge of the orbiters
left wing and damaged panels 8 and 9 of the RCC very near the point at
which the highest temperatures are experienced during re-entry. Each of
the orbiter’s wings has 22 RCC panels that are 0.250 to 0.500 inches
thick. RCC is a laminated composite material made from graphitized rayon
cloth impregnated with a phenolic resin; a three-stage pyrolysis process
converts the phenolic resin to the carbon matrix. The outer layers of
the RCC are converted to silicon carbide to provide oxidation resistance
for reuse capability. While the other components of TPS serve as
insulators, the areas protected by the RCC are too hot to insulate so
the RCC is designed to radiate heat from the hot lower surfaces to the
cooler upper surfaces and to the internal portion of the wing.
Development of
repair method
NASA is
attempting to develop a method to repair the critical RCC panels.
Scientists have developed a pre-ceramic polymer sealant impregnated with
silicon carbide which is referred to as Non-Oxide Adhesive eXperimental
(NOAX). Damage in the protective outer silicon-carbide coating of the
RCC may be repaired with NOAX, which itself, converts to silicon carbide
during re-entry. They have also designed a special space caulking gun
that Astronauts will use to apply a small amount of repair material to
the damage site. They will work the material with a putty knife and use
it to fill in cracks and holes in the RCC panels. If this procedure
should ever be required on an actual Shuttle mission, the lives of the
crew will depend on the integrity of the repair. So NASA has embarked on
an extensive testing program during which it has planned several arc jet
tests (re-entry simulation) as well as thermal-vacuum tests prior to
certifying the new material for use in an actual flight. Most of these
development and verification tests have been performed on the ground,
either in a laboratory setting or in vacuum chambers, but Astronauts
Piers Sellers and Mike Fossum conducted repairs of arc jet samples
during a spacewalk on STS-121 in July 2006. All of the arc jet samples
repaired during the on-orbit evaluation were tested in the arc jet and
successfully completed the entry simulation without burn-through.
Evaluating the
new repair method requires extensive use of 3D measurement methods.
First of all, researchers damage RCC samples in preparation for applying
the repair and the geometry of the samples after damage must be measured
to ensure they represent realistic damage scenarios and so that they can
be compared to the geometry after repair. The panel must then be
measured very accurately after the repair has been completed. This is
because the height of the repair can be no higher than .100 to .150
inches above the outer mold line of the panel or the repaired section
will penetrate the boundary layer and generate turbulence that increases
the heat flux and the shear load on the repair causing them to rapidly
fail. Finally, the repaired section is measured one more time after it
is tested in arc jet designed to simulate the flows and temperatures
associated with re-entry to determine its margin of safety.
Evaluating
alternative measurement methods
NASA engineers
considered several possible methods of performing these measurements
during the testing program. They looked at the structured light scanners
that project a pattern of light on the subject and look at the
deformation of the pattern. A camera looks at the shape of the projected
line and uses a method similar to triangulation to calculate the
distance of every point on the line from the camera. Structured light
scanners can scan multiple points of the entire field view at once.
Laser scanning was the other alternative that was considered. Laser
scanning systems work by projecting a line of laser light onto surfaces
while cameras continuously triangulate the changing distance and profile
of the laser line as it sweeps along, enabling the object to be
accurately replicated. The laser probe computer translates the video
image of the line into 3D coordinates, providing real-time data
renderings that give the operator immediate feedback on areas that might
have been missed.
NASA engineers
carefully considered both of these alternatives and determined that
though both provide the required .005 inch accuracy the laser scanner
technology offered better mobility and was less costly. The Laser Design
SLP-330 probe with a digitizing arm met all of NASA’s requirements.
Mobility is important because NASA’s three arc jet testing facilities
are located around the country where RCC repairs are being tested. The
ability to transport the scanner from one facility to another eliminates
the need to purchase a scanner for each facility.
The SLP-330
scanning laser probe provides a dual detector that captures much more
geometry per pass than single receptor detections and is available with
the Romer Cimcore Infinite arm. The package scans parts as long as 12
feet without moving the arm yet can easily be packed up and moved to
another location. The SLP-330 probe captures up to 50,000 points per
second and has dual receptors to see the laser line from opposed angles
for faster scanning with fewer passes over the part. It weighs less than
a pound. Laser Design’s laser scanning probes use Geomagic Studio as
their scanning interface, eliminating the need to go back and forth
between the scanning software and Geomagic Studio. The laser probe
offers manual point and shoot type scanning with a spray painting type
display that helps ensure complete coverage where scan data is needed.
Geomagic Studio provides an inspection comparison of scan data to
previous parts scanned or color models with differences mapped in color.
Romer CimCore Infinite articulating arms are available with measuring
ranges of 6, 8, 9, 10, and 12 feet. Infinite series arms feature Romer's
patented infinite rotation of the principal axes that allows inspection
of hard-to-reach areas and avoids damaging the arm against rotational
hard stops.
How the repair
kit is being tested
So far NASA has
performed ~50 tests on the RCC repair kit including the 4 performed
after repair during the spacewalk on the Shuttle Discovery (STS-121).
These tests simulated the repair of damage ranging from a crack to a 2
inch diameter silicon-carbide coating loss. The samples of approximately
4.5 x 5 inches in length are prepared by first intentionally damaging
them and then applying the repair over the damage. In each case, after
the damage was generated, NASA engineers scanned the panels to record
the damaged areas. They uploaded the resulting point cloud in Geomagic
Studio software which generated a surface model of the panel profile.
Then the crew member applied the paste, which has a consistency of
peanut butter, to the damaged panel being careful to keep the contour of
the repair as close as possible to the outer mold line of the panel. An
engineer scanned the repaired section and again imported the point cloud
into Geomagic. The engineer used Geomagic Qualify software to compare
the difference between the original damaged panel and the repaired
panel.
Then the repaired
panel was tested in the arc jet at temperatures up to about 3000 degrees
Fahrenheit. The samples were put in the arc jet and heated through one
of a set of design reference case profiles that is designed to simulate
the re-entry time-temperature history of the Shuttle at various portions
of the wing. For the hottest design reference case profile, the
temperature rose from room temperature to 2960 degrees Fahrenheit in 500
seconds then held at the condition for 500 seconds before ramping down
and terminating at 1200 seconds. The test ran for a total of 20
minutes. The panel was again scanned after the tests were completed and
compared in Geomagic Qualify to the repaired section in order to see how
much erosion had occurred.
Laser scanning
has proven invaluable in providing the high accuracy inspection required
for validating the Shuttle’s new RCC repair kit. Geometry of the
repaired area is critical. If the repaired panel cures to a shape that
interrupts the smooth flight surface the resulting turbulence can create
a hot spot on the surface of the wing that could burn through the
surface of the shuttle. Laser scanning lets NASA engineers quickly and
accurately measure the surface of the panel. They can use software to
easily compare the resulting surface model to the original panel
geometry, to the damaged panel, to the repaired panel, and finally to
the repaired panel after it has been tested in the arc jet. The Laser
Design SLP330 scanning laser probe has met all of NASA’s requirements
including accuracy, mobility and economy. |
