Impact Testing Of The Orbiter Thermal Protection System.pdf

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Impact Testing of the Orbiter
Thermal Protection System
Justin Kerr, Johnson Space Center
Ascent video and photography captured during the STS-107
mission showed debris from the external Tank (ET) striking the
Orbiter Columbia’s lower left wing 82 sec after launch (T+82).
During the mission, photo analysis teams led by the Image
Analysis Group in Astromaterials Research and Exploration
Science (ARES) estimated an upper limit of approximately 50
cm for the size of the debris liberated with an uncertainty of
±25 cm. The visual evidence implicated the ET left bipod ramp
as the source of the debris. Calculations of the debris velocity at
impact ranged from 190 m/sec to 256 m/sec, depending on the
various methods and assumptions used, with the most probable
velocity estimated to be approximately 200m/sec.
The Columbia Accident Investigation Board (CAIB) considered
the T+82 impact a likely initiating event of the accident
requiring further evaluation. To replicate the impact, the Orbiter
Vehicle Engineering Office chartered an Orbiter Thermal
Protection Impact Test Team. The scope of the task undertaken
by the team included design and construction of test article
hardware, modification of facilities to conduct the tests, and
detailed planning of test parameters. The impact test program
was conducted in close coordination with the CAIB and was led
by the Hypervelocity Impact Group of ARES.
Because the impact occurred on the lower left wing of the
Orbiter Columbia, test articles were developed for unique areas
on the Shuttle surface; i.e., wing acreage tile and structure, the
main landing gear door (MLGD), and the leading edge. The
materials in these areas would have differing reactions to an
impact by foam debris; likewise, a breach in these areas would
yield varying Orbiter system responses during entry plume
impingement. By the time testing began, the investigation teams
determined the debris impacted the lower leading edge between
panels 8L and 9L.
Engineers used the unique capabilities of the Southwest
Research Institute (SwRI) to conduct the tests. The foam
projectiles were launched to velocity using a large compressed-
gas gun. The outdoor test site included a stand on which to
mount targets and an 8-m-high curtain to catch ejected debris.
As many as 13 high-speed video cameras were used to image
both the projectile flight path and the impact event. Engineers
used the data from these images to determine the projectile
velocity and orientation at impact as well as the detailed motion
and potential failure of the target after foam impact. The targets
were equipped with strain gauges, accelerometers, and deflection
gauges that enabled engineers to measure the strains in the
Thermal Protection System (TPS) and the underlying Orbiter
structure. As many as 275 channels of instrumentation data
were collected during each test.
To prepare for the leading edge test program, five tests were
conducted on LI-900 TPS tiles bonded to a left MLGD. These
tests were used to evaluate the response of tile impacted by foam
at representative velocities and angles and to verify test facility,
instrumentation, and high-speed camera readiness. BX 250
foam at sizes (14,000, 20,000, 30,000 cm3), velocities (200 to
235 m/sec), and impact angles (5 to 13 deg) representative of the
Impact Testing of the Orbiter Thermal Protection System
Engineering
debris were shot at the MLGD with a large compressed-air gun
facility at SwRI. The results demonstrated very little damage to
the tiles for the range of parameters tested. None of the damage
could be considered critical.
initially impacted the panel several centimeters closer to the
middle of the panel and was clocked at an angle such that most
of the leading edge of the block loaded the panel. The foam
block produced a hole 41 cm by 42.5 cm at the widest point.
Investigators noted cracks along the lock and slip sides of the
panel. The panel 8L T-seal was cracked at its lug, which opened
a slight gap between the panel and the seal. Investigators found
the RCC materials displaced from the panel inside the test
article. The largest two fragments were approximately 20 cm
by 33 cm and 19 cm by 29 cm. Joint scenario testing of TPS
materials ended upon completion of this test.
A leading edge test article of representative structural response
was manufactured to enable impact testing of Reinforced
Carbon-Carbon (RCC) panels, RCC T-seals, and TPS tile
carrier panels. Tests on the RCC components were the priority.
Due to the scarcity of available RCC flight panels, analysis
techniques and impact tests on fiberglass panels of outer mold
line geometry that were identical to that of the RCC panels were
used to optimize the test conditions for each RCC test. Two
RCC panel tests were ultimately conducted, one each on panels
6L and 8L. In total, five tests were conducted on fiberglass
panels.
For the panel 6L test, a 20,000-cm3 BX 250 foam projectile
impacted the panel at a velocity of 234 m/sec. A lower corner
of the foam block first impacted the panel near the slip-side
rib. The resulting crack, which measured approximately 14 cm,
traversed the entire rib and lock and 1.9 cm onto the lower panel
face. In addition, the panel 6L T-seal exhibited a crack 6.4 cm
along its web.
The test conducted on panel 8L yielded results that were
consistent with estimated damage inferred from onboard
sensor data and forensic evidence. A foam block of the same
mass and approximately the same size as that tested for panel
6L was launched to a velocity of 237 m/sec. The foam block
Impact Testing of the Orbiter Thermal Protection System
Engineering

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