A Case Study in NASA-DoD - The Black Vault

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The AEM attitude control system does not include reaction jets as
a means of torque generation. Thus there are no limits on operational
lifetimes due to fuel considerations. However, magnetic torques are
relatively weak and as a consequence control time constants tend to be
large--on the order of an orbital period. Also, magnetic torques decrease
with increasing altitude and for the AEK design, they become
ineffective for altitude in excess of 1000 n mi.
The simplest of the STPSS designs utilizes spin stabilization.
Thus, ideally, the spin axis of the vehicle is inertially fixed. No
provisions are made for a despun platform.
A mechanical nutation
damper is provided to remove unwanted spin axis wobble and cold gas
jets are used to reorient or stabilize the direction of the spin axis.
Sun and earth sensors are used for attitude determination.
The second STPSS design is a low-cost, three-axis system with performance
specifications similar to those of the AEM spacecraft (see
Table D-l).
The attitude control system of this version of the STPSS
differs from that of the AEM in that a pitch momentum wheel is not
used to provide roll-yaw stabilization and cold gas reaction jets are
the primary means of generating control torques. Two conical scan
earth sensors provide pitch-roll attitude information, while a rate
gyro is used to detect yaw attitude errors.
Since, without a pitch momentum wheel, this version of STPSS does
not have any inherent stability, disturbances from either internal or
external torques must be countered by the reaction control system.
For low altitude orbits where aerodynamic and gravity gradient disturbance
torques can be large, control system fuel requirements for
a one-year mission might be excessive. This situation could be alleviated
by adding electromagnetic torques and a magnetometer to the
control system so that almost continuous use of the reaction jets
would not be necessary.
The third version of the STPSS is designed to attain precise
pointing accuracies and rate control.
To improve performance relative
to the low-cost three-axis design, two star trackers, two rate gyros,
In essence, the body of the AE4 spacecraft Is a despun platform
with the pitch wheel inertially stabilized.
AL