NASA Uncertainties Management in the TPS Design Process
Start Date
1-3-2011 9:20 AM
Description
The process of designing a thermal protection system for an entry vehicle involves many stages, including selection and sizing of the TPS material, thermal and structural design of the TPS to accommodate mission requirements, and detailed design of the resultant aeroshell to accommodate singularities such as penetrations, windows, attachments, and other engineering requirements. Unlike many spacecraft subsystems, TPS is designed primarily via computational methods, validated with component level ground tests that do not simulate all aspects of the flight environment. The employed physical models are thus used to extrapolate ground test results to the expected flight environment. This ground-to-flight traceability places great emphasis on using high-fidelity models that accurately simulate the necessary underlying physics and chemistry, so that the extrapolation to flight can be performed confidently. This talk will focus on key uncertainties primarily in the first steps of the process, namely the thermal sizing of the TPS material to meet mission requirements and reliability. The TPS sizing process requires a good understanding of the aerothermal environment encountered and the uncertainties in that environment, as well as a good TPS thermal response model that is capable of accurately determining the thickness of material required to maintain an acceptable bondline temperature in the face of the encountered environment. Uncertainties in both the environment encountered and the TPS response to that environment are captured and used to define a thermal margin policy that is tailored to the mission and overall reliability required. While a large amount of literature has been published discussing and defining the impact of aerothermal uncertainties on TPS sizing, much less work has been done to define the impact of material response uncertainties. These uncertainties tend to fall in three major areas: gas-surface interactions (the chemical interaction of the TPS material with the environment), roughness and blowing effects (the fluid dynamic interaction), and in-depth chemistry (the means by which ablating materials accommodate large heat loads efficiently). Examples will be given in each of these areas where our current lack of understanding of the underlying physics can lead to either large design margins and/or low reliability in the final design.
NASA Uncertainties Management in the TPS Design Process
The process of designing a thermal protection system for an entry vehicle involves many stages, including selection and sizing of the TPS material, thermal and structural design of the TPS to accommodate mission requirements, and detailed design of the resultant aeroshell to accommodate singularities such as penetrations, windows, attachments, and other engineering requirements. Unlike many spacecraft subsystems, TPS is designed primarily via computational methods, validated with component level ground tests that do not simulate all aspects of the flight environment. The employed physical models are thus used to extrapolate ground test results to the expected flight environment. This ground-to-flight traceability places great emphasis on using high-fidelity models that accurately simulate the necessary underlying physics and chemistry, so that the extrapolation to flight can be performed confidently. This talk will focus on key uncertainties primarily in the first steps of the process, namely the thermal sizing of the TPS material to meet mission requirements and reliability. The TPS sizing process requires a good understanding of the aerothermal environment encountered and the uncertainties in that environment, as well as a good TPS thermal response model that is capable of accurately determining the thickness of material required to maintain an acceptable bondline temperature in the face of the encountered environment. Uncertainties in both the environment encountered and the TPS response to that environment are captured and used to define a thermal margin policy that is tailored to the mission and overall reliability required. While a large amount of literature has been published discussing and defining the impact of aerothermal uncertainties on TPS sizing, much less work has been done to define the impact of material response uncertainties. These uncertainties tend to fall in three major areas: gas-surface interactions (the chemical interaction of the TPS material with the environment), roughness and blowing effects (the fluid dynamic interaction), and in-depth chemistry (the means by which ablating materials accommodate large heat loads efficiently). Examples will be given in each of these areas where our current lack of understanding of the underlying physics can lead to either large design margins and/or low reliability in the final design.