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Space vehicles that enter a planetary atmosphere (i.e. earth) like
the Space Shuttle Orbiter require the use of a thermal protection
system (TPS) to protect them from aerodynamic heating. The aerodynamic
heating is generated at the surface of an entering object due to the
combination of compression and surface friction of the atmospheric
gas. The vehicle's configuration and entry trajectory in combination
with the type of thermal protection system used define the temperature
distribution on the vehicle. The Space Shuttle features a TPS system
based on the use of surface materials with a high temperature capability
in combination with an underlying thermal insulation to inhibit the
conduction of heat to the interior of the vehicle. The heat developed
from the aerodynamic heating process is thereby radiated back into
space by virtue of the high surface temperature. The leading edges
of wings and the nose cap are the highest temperature regions. Due
to the wide variation of these temperatures the TPS selected for Space
Shuttle was composed of many different materials. Each material's
temperature capability, durability and weight determine the extent
of its application on the vehicle. Improvements to these materials
have been the subject of much research as enhanced capability material
(i.e., more durability, higher temperature capability, greater thermal
shock resistance and lower thermal conductivity) improves thermal
protection material and vehicle performance. Future reentry vehicles
capabilities will depend upon the capabilities of TPS being developed
and available to them.
A TPS development at NASA Ames Research Center is Ultra High Temperature
Ceramics (UHTC). Ultra High Temperature Ceramics are a family of ceramic
materials with extremely high melting temperatures, good oxidation
resistance in reentry type environments, and reasonably good thermal
shock resistance for a monolithic ceramic. These materials show potential
for use as passively cooled sharp leading edges on future reentry
vehicles. Sharp leading edges offer advantages in aerodynamic performance
and crew safety over current blunt leading edges. Ames is developing
the UHTC materials for use in sharp leading edge applications. The
materials being investigated include HfB2/SiC and ZrB2/SiC composites
among other compositions. Recent work has included ground based arc
jet experiments along with the SHARP B1 and SHARP B2 flight experiments.
Future work includes improving the thermal and mechanical properties
of the materials and development of attachment designs. |
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