TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 5
secondary mirror along the optical axis may exceed 1 nm over much of the spectrum, as shown by the
uppermost trace in Figure 5-16. Below 28 Hz, the amplitude varies, but it peaks at more than 10 nm near
17 Hz. Also between 60 and 80 Hz, amplitudes are large. They exceed the rate that can be corrected by
the fringe tracker, and they are in the section of the beamtrain that is not monitored by laser metrology,
which stops near the FOR mirror. An error budget for the beamtrain has not been developed, but it is
likely to be significantly less than 1 nm for frequencies greater than a few hertz. However, on the positive
side, the vibration amplitudes are sufficiently small that it is possible that vibration mitigation efforts
would reduce them sufficiently. Such mitigations might be (for example) improved isolation of reaction
wheels, or no reaction wheels, and spacecraft controlled by proportional thrusters or another low vibration
system. In the worst case, a laser metrology system could be added to the telescopes to measure much of
the unmonitored path. Primary-mirror vibrations were typically an order of magnitude smaller, so they are
much less likely to cause concern. Additional work to look at the major bending modes of the primary is
also desirable.
5.7 Mission Description
The current concept of the TPF-I mission begins with the launch of a single heavy-class launch vehicle
from Kennedy Space Center. The complete observatory, traveling as one integrated assembly, is flown to
the Sun–Earth L2 point. At the L2 point the observatory is inserted into a halo orbit. L2 was chosen over
an Earth drift-away orbit like that used by the Spitzer mission because L2 offers simpler
telecommunications geometry, a lower insertion energy requirement, and the option to launch groundbased
spare spacecraft to the orbit after the deployment of the original formation.
Figure 5-17 depicts a concept for the cruise stage. The cruise stage is used to transport the formation as
packaged for launch from Earth to L2. The cruise stage also protects the optics from some potential
contamination sources during launch. The stage includes a separate propulsion system, solar panels, and a
mechanical structure. The electronics on the combiner spacecraft are used to control the cruise stage. On
Cruise Stage Enclosure
Section separates to
expose Collector #2 for
deployment
Combiner
Spacecraft
Collector Spacecraft (4)
Aft Prop Module /
Collector Enclosure
Section 1 separates to
expose Collector #1 for
deployment
Cruise Stage Position and
Attitude maintained by
Combiner S/C using
Cruise Stage Forward
RCS
Cruise Stage
Collector #1 Separates
from S/C aft attach
fitting, deploys with
gentle spring push off
Cruise Stage sustains launch
loads and provides
contamination protection for
optics during launch and
cruise phases
Figure 5-17. Cruise Stage (RCS is Reaction Control System).
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