TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Chapter 5
engineering judgment (i.e., 10m class deployable structures, sub-mK thermal stability, pm
mechanical stability, and sub-milli-arcsec pointing stability), the Project will devise a more
rigorous approach to defining and reducing uncertainties. The plan is to:
1. Develop analytical techniques to propagate and evaluate uncertainties
2. Develop models and error budgets for each of the testbeds from which uncertainties are
evaluated by comparing predicted results to experimental data – this implies that some
testbeds will have to perform to flight levels if not better, or that scaling laws will have
been defined
3. Most importantly, develop methods of reducing uncertainties by validating physics, by
improving modeling tool accuracy and by proposing design options that minimize
uncertainties
One such means of reducing modeling uncertainty is to allow on-orbit adjustments through
control strategies, either active or passive. Current design features include the active deformable
mirror for wavefront correction, active thermal control of the secondary mirror assembly and the
aft-metering structure, active position alignment of the secondary mirror tower, and active
vibration isolation of the reaction wheel disturbances. In these instances, the control errors will
define the performance uncertainties. TPF-C will continue exploring, when necessary, other
mitigating design solutions which implement control strategies for on-orbit adjustments. Other
features that could be considered, but are not yet part of the baseline design of TPF-C, are active
or passive structural damping, active wavefront control of the primary through mechanical
actuators or distributed thermal control, or active wavefront control through a two-stage
deformable mirror.
In effect, the TPF-C modeling challenge is now turned into validation of analysis bounds,
whereby the uncertainty needs to be quantified and managed in the error budget by propagating
error contributions from the lowest level of assembly on up. Another implication of this new
modeling paradigm is that modeling margin allocations will be used to derive levels of accuracy
required from the model validation, as well as the measurement accuracy of the test facility
itself. Questions regarding what constitutes a validated model have plagued projects in the past.
Through the use of the modeling error margins, we will now be able to derive rational and
consistent acceptance criteria for the validation and delivery of models.
In summary, the most critical issues for modeling technologies on TPF-C include, in no
particular order:
1. Development of multi-disciplinary integrated analysis tools with precision numerics
2. Precision measurements of material properties (thermal, mechanical and optical)
including assessment of variability, wave length dependencies and dimensional/temporal
stability
3. Validation of the physics described in models, such as mechanism frictional stability,
scattered light behavior, contamination, sunshade thermal performance, polarization
propagation, electromagnetic masks and stops models
4. Validation of scaling laws used to extrapolate results from the ground to flight:
• Scalability to environment: 1-G to 0-G, thermal gradients, milli-G to nano-G jitter, air
to vacuum
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