TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Structural, Thermal, and Spacecraft Technology
dynamic jitters poses great challenges to the vibration isolation system. The objective of the
vibration isolation activity is to create viable passive and active isolation designs for TPF-C and
ultimately test and verify the performance of the selected isolation system.
Approach
Since TPF-C has dynamic stability requirements tighter than most current and planned missions,
it will be critical to ensure that the isolation system can reduce vibrations sufficiently. Two
fundamental approaches to vibration reduction are passive and active isolation systems. Passive
isolation uses a soft suspension to limit the transfer of disturbance energy across an interface.
Passive isolators have a significant flight heritage, and are lower risk (with relatively fewer
components and no chance of instability of the feedback loop). However, the performance is
fundamentally limited by static displacement constraints, parasitic stiffness, and high frequency
isolator modes. An active isolation system requires additional sensors and actuators to suppress
vibration based on real-time measurement feedback. A combined active isolation and spacecraft
pointing approach separates the payload pointing and vibration isolation mechanisms through a
non-contact interface between the spacecraft and payload, and non-contact sensors and actuators
at the interface. Both the pure and hybrid active approaches may potentially offer higher levels
of vibration reduction than a passive isolation design, but with additional risk and a shorter flight
heritage.
Currently there are two concepts under consideration: (1) a three-stage passive isolation system
and (2) a hybrid pointing and isolation system. The passive isolation design includes a passive
isolation stage between the spacecraft and the payload, another stage that isolates the entire
reaction wheel assembly, and a third stage that provides isolation to each of the reaction wheels.
The current hybrid design chosen as a baseline is called the disturbance free payload (DFP),
under development by Lockheed Martin. The DFP architecture involves the nearly-complete
mechanical separation between the telescope and the spacecraft, non-contact interface actuators
that allow precision inertial control of telescope pointing, and non-contact relative position
sensors for spacecraft attitude control to maintain the proximate angular separation. In this
architecture, spacecraft vibration isolation is achieved through mechanical separation, and is thus
independent of sensor characteristics, while stable telescope pointing is achieved through noncontact
actuators. 31
Integrated models, combining structural, optical, control, and isolation models, are used to
evaluate each of the isolation designs and provide end-to-end performance predictions. The
analytical work will help identify the best or most appropriate isolation design that leads to
mechanical development of the chosen design. This activity will demonstrate or verify the
isolation system performance by combining analysis efforts and hardware testing.
Progress to Date
Preliminary performance analysis of the two concepts has been completed, and the results are
discussed in this section. To compare the two isolation designs, an integrated model was created
to take reaction wheel disturbances as inputs and line-of-sight (LOS) and Zernike amplitudes as
31 Pedreiro, N., “Spacecraft Architecture for Disturbance-Free Payload”, AIAA J. Guidance, Control and
Dynamics, v.26, No.5, pp. 794-804 (September 2003).
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