MISSION PLAN - PDS Small Bodies Node

10.0 Appendix B: Unbalanced Attitude Control Force
Modeling
The STARDUST spacecraft design imparts an unbalanced force, i.e. translational thrust
to the spacecraft, every time attitude control burns are executed. This undesirable
condition is dictated by the need to prevent contamination of the sample collector. Over
the seven year mission, the cumulative ACS activity is estimated to impart tens of meters
per second of ∆V. The sum of trajectory correction maneuvers to compensate for the
ACS burns could be intolerably large (~40 m/s) unless the ACS effects are accounted for
in advance while designing the nominal trajectories. From the navigator’s point of view,
the ACS activity is an error source that cannot be ignored in striving to achieve accurate
spacecraft delivery at comet and Earth re-entry.
This section describes the methods used to model this perturbation source and its
incorporation in the trajectory design and maintenance. The ACS perturbation is divided
into two categories: 1) almost continually acting ACS limit cycling and 2) less frequent
attitude slewing required for communications, maneuvers and special activities.
The trajectory optimization program can include the first type of perturbations, but
discontinuous second types are too cumbersome to deal with. As a result, the decision
has been made to include the latter effect in the navigation tools only in the form of a
tabular small forces predict file. This will enable accurate predictions, but will be very
slightly off-optimal in the ∆V estimates.
10.1 Limit Cycle Model
A mathematical model of the behavior of the attitude control system (ACS) is constructed
to compute ACS perturbations to the spacecraft trajectory due to cruise deadbanding or
limit cycling.
In this model, ACS thruster pulse frequencies are computed consistent with the planned
attitude history. The magnitude of the frequency depends on spacecraft mass
distribution, thruster characteristics and configuration, mission geometry, and expected
solar torques. These pulse frequencies are not incorporated into the trajectory design as
individual pulses, but instead, are converted to average perturbative accelerations.
The performance of the spacecraft’s hydrazine-blowdown system is modeled according
to the manufacturer’s specifications, and the direction of the ACS acceleration is
established based on the pre-flight plan for spacecraft attitude.
10.1.1 Spacecraft Attitude History
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