Infrared Leeside Temperature Sensing
The SILTS experiment
will obtain high-resolution infrared imagery of the upper (leeward)
surface of the orbiter fuselage and left wing during atmospheric
entry. This information will increase understanding of leeside aeroheating
phenomena and will be used to design a less conservative thermal
protection system. SILTS provides the opportunity to obtain data
under flight conditions for comparison with data obtained in ground-based
components make up the SILTS experiment system: (1) an infrared
camera, (2) infrared-transparent windows, (3) a temper ature-reference
surface, (4) a data and control electronics module, (5) a pressurized
nitrogen module and (6) window protection plugs. These components
are installed in a pod that is mounted atop the vertical stabilizer
and capped at the leading edge by a hemispherical dome. (The SILTS
pod replaces the top 24 inches of the vertical stabilizer.) Within
this dome, the infrared camera system is mounted in such a way that
it rotates to view the orbiter leeside surfaces through either of
two windows-one offering a view of the orbiter fuselage and the
other a view of the left wing. The camera is sensitive to heat sources
from 200 to 1,000 F.
indium-antimonide detector is cooled to cryogenic temperatures by
a Joule-Thompson cryostat. The camera's field of view is 40 by 40
degrees. Its rotating prism system scans four 100-line fields each
second, with a 4-1 interlace, resulting in a 400-line image.
Each of the
two infrared-transparent window assemblies consists of dual silicone
windows constrained within a carbon-phenolic window mount. The windows
and window mount assemblies are designed to withstand the entry
thermal environment to which they would be subjected without active
cooling. They are, however, transpiration cooled with gaseous nitrogen
during experiment operation so that they do not reach temperatures
at which they would become significant radiators in the infrared.
A small thermostatically controlled surface between the two window
assemblies provides an in-flight temperature reference source for
the infrared camera.
nitrogen system comprises two 3,000-psi gaseous nitrogen bottles
and all associated valves and plumbing. The pressure system supplies
gaseous nitrogen to the cryostat for camera detector cooling, to
the external window cavities for window transpiration cooling, and
to pin pullers that initiate the ejection of the advanced flexible
reusable surface insulation window protection plugs upon SILTS activation
to expose the viewing ports and camera.
obtained by the camera is recorded on the OEX tape recorder. The
data, when reduced and analyzed, will produce a thermal map of the
The SILTS experiment
is initiated by the onboard computers approximately five minutes
before entry interface, which occurs at an altitude of approximately
400,000 feet. The camera operates for approximately 18 minutes through
the forward-facing window and left-facing window, alternating evenly
between the two about every five seconds.
After the six
planned SILTS missions, an analysis of structural loads will determine
whether the SILTS pod should be removed and replaced with the original
structure or remain in position for other uses. The pod thermal
protection system is high-temperature reusable surface insulation
black tiles, whose density is 22 pounds per cubic foot.