TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Integrated Modeling and Model Validation
analysis, the flight system requirements to the system testbed performance. This process is in
progress. Some requirements are understood for the near-term model validation activities, e.g.,
limits on the variability of optical material CTE, and some are clearly less defined, e.g., larger
testbeds planned for Phase A-B. The requirements for all the testbeds will be firmed up as soon
as the flight design and flight performance requirements are formally established.
By the end of the project, the primary questions asked to the analysts will be, “Why do you
believe the prediction” To help achieve this challenge, a novel modeling strategy will be
implemented on TPF-C. It is standard practice to include hardware fabrication tolerances as
margins within the error budget. For TPF-C it is proposed to treat models as “software
fabrication” by including additional margin in the error budget to account for modeling
tolerances, a.k.a. modeling uncertainties. This implies that the accuracy of the prediction will be
quantified by tracking contributions to the modeling errors during the project lifecycle.
Because the system performance objective now takes into account the predictability variances of
the analysis, the design goal is no longer to select the design which meets the best nominal
performance, but one that meets the best bounded performance including the modeling
uncertainty. This means, for instance, that from the view point of predicting performance and
meeting the error budget, a low CTE material having high variability and high uncertainty may
not be as good a design choice as a higher CTE material with low variability and low
uncertainty. Additional examples of modeling uncertainties include the nonlinear mechanics of
hinges/latches, damping, etc.
The concern that arises from this new modeling paradigm is the issue of “over-designing” the
system by imposing tighter nominal performance requirements to make up for larger margin
allocations in modeling uncertainties. This unfortunately is inevitable when analysis is the only
means to validate on-orbit system performance, as it will be for an increasing number of flight
systems in the future. The best that can be done to alleviate this concern is to address the
problem up front, and to devise means by which modeling uncertainties can be evaluated,
tracked, and, especially, reduced to minimize its overall contribution to the margin. It is
recognized that modeling uncertainties will be naturally reduced through the course of the
Project as testbeds and design mature. Nonetheless, there will still be residual uncertainties in the
prediction of those flight performances that can only be validated through analysis, and those
need to be accounted for in the V&V process.
The overall margin allocation strategy is the responsibility of the Design Team and will not be
discussed herein. At this time the design of the Baseline Mission concept is not mature and the
margin philosophy has not yet been agreed upon. Nonetheless, eventually the Design Team will
identify the required margin and levels of Modeling Uncertainty Factors (MUF) to achieve the
mission. A future version of the Technology Plan will then be able to expand on how the
required MUF levels will be validated. For now, the plan will focus on the approach and the
testbeds planned for model validation. The current version of the error budget requirements will
be used to illustrate the levels of magnitude expected from the model validation activity, but
should not be considered as the definitive model validation technology requirements.
This paradigm is fairly new to NASA missions, with JWST and SIM using engineering judgment
to define empirical uncertainty bounds through the mission lifecycle. Because the TPF-C
requirements are in a realm where there exists no past experience from which to develop
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