How hard was it for the Shuttle to perform attitude control for the entire ISS? Did they have to calibrate?

Question

This answer to the question How was the Space Shuttle Orbiter used for ISS Reboost? describes a procedure I never knew happened. When the shuttle was docked to the ISS it sometimes used its rear reaction control thrusters to boost the ISS' orbital altitude, something that has to be done regularly to compensate for drag.

A small part of that answer says:

Two down-firing jets were used simultaneously on this flight.

Other VRCS jets fired as needed to control undesired rotations of the stack and maintain the commanded attitude.

After the reboost was complete, control of the stack was handed back to the ISS control system.

During this time it seems the ISS shuts down its own attitude control and the shuttle takes over. I think this means that the Shuttle's ADCS has to imagine that it is no longer just the Shuttle, but the whole Shuttle + ISS superstructure, and behave accordingly.

If the whole thing could be treated as a single rigid body, then this might be as simple as updating the moment of inertia tensor. But this combination may not be rigid enough for that. If you apply a torque at one end of the ISS, it could bend or even oscillate, and depending on the time constants set in the ADCS control system, those oscillations might even become unstable.

What were the considerations necessary to implement ADCS for the Shuttle + ISS superstructure in the Shuttle's computer? Did it have to be calibrated experimentally by "pinging" one of the thrusters that were off-axis and measuring the flexure of the system, oscillation time constant and decay rate?

Or did they just model it on the ground, dock, and start boosting away?

Answer 1

I'm interpreting this question as "Did adjustments have to be made to the shuttle Digital Autopilot (DAP) while it was controlling the mated stack (shuttle + ISS) - i.e. was a maneuver tried, the results of the trial evaluated, and the DAP settings changed based on the results of the trial during the same shuttle mission?"

If that interpretation is correct, the answer is No, and

did they just model it on the ground, dock, and start boosting away?

is pretty close.

For a basic background description of the shuttle DAP and its settings, please read the explanation in this answer.

The inertias for the shuttle + payload (in this case the ISS was treated as a giant payload) were set in the CNTL ACC (voiced as "control accel") entry of the DAP settings. Referring to the table of DAP settings in the linked question you can see that only the DAP settings dealings with payloads have entries for CNTL ACC. DAPs A11 and A12 are for mated stack maneuvers, DAPs A13 and A14 are for undocked operations using the Orbiter inspection boom to scan the heat shield.

These CNTL ACC entries were determined on the ground by analysis premission. For the mated stack cases, the configuration of the station would be carefully specified: the major moving parts of the station would have been in predefined positions, the absence or presence of other visiting vehicles would be known, etc. If the on-orbit configuration of the ISS was different from the pre-planned configuration (a visiting vehicle launch failed, or a solar array joint was stuck), a different CNTL ACC could have been devised on the ground and uplinked to the Orbiter after careful testing in simulations on the ground. It would not have been done by a test / evaluate / reconfigure process in space.

Note that the two docked-ops CNTL ACCs in the table from the linked question refer to different ISS configurations expected during the mission during the installation of the Node 3 module (the mission whose DAP entries are shown, STS-130, delivered Node 3 to the ISS):

• CA6 is "Heavy Russian Segment, Prior to Node 3 at SSRMS Radiator Hold Position"
• CA7 is "Heavy Russian Segment, Beginning with Node 3 at SSRMS Radiator Hold Position"

References:

• Shuttle Crew Operations Manual discusses CNTL ACCs on page 2.13-50
• Data Processing System Dictionary page 3-18 describes the Universal Pointing page and its entries in some detail, page 3-26 describes the DAP Config page (these are the screens that the crew enters DAP parameters on)