So, TL;DR inertial is a rabbit hole. Ramble ahead.
As usual the information available is kinda vague and confused. I've been assembling an idea of where the wires connect with the different sources, and within each source, looking at how the systems interact. The sequence of commands involved in the inertial stuff is a bit clearer but I'm still missing a few pieces. Posting in case it's interesting enough.
Basically there are two systems to keep track of attitude with:
- There's the gyro system: two gyroscopes, two gimbals each, used simultaneously, which give attitude angles. Still not clear on if it can give angles for the three axes (having two of them at once equivalent to a single three-gimbal?), but it only allows manoeuvres in roll and yaw. Regardless, there is a reference to holding attitude in pitch as well. Cage them, establish the attitude to take as a reference, uncage the gyros so they get a fix on that reference for the attitude to hold.
- Then there's the BDUS unit: three one-gimbal rate gyros, which either give angular rate for all axes, or can be integrated (rate over time gives absolute angle change) to give angles in three axes and allow manoeuvres in pitch, roll and yaw. It is said switching this integration on can be done manually, problem there is I'm not quite sure which command in the signal matrix corresponds to it. The "INTEGR." one refers to the accelerometer, which measures acceleration on the longitudinal axis for main engine burns, and so is unrelated. Other than that, the documentation is vague and the commands aren't quite explicit.
It is said inertial attitude hold and manoeuvres can be accomplished with either system (gyro with BDUS rates, or just BDUS with integration), only with varying tolerances, angular rates and sequencing.
I've been trying to cross-reference the
7K-OK (first-gen) and
7K-TM documentation (second-gen, along with 7K-T), and trying to make sense of the sequence with the program timer for clues. Both sources differ, but commands seem to roughly align between the two displays in terms of order. For instance, for 7K-TM (no individual programs are documented AFAIK):
On the right column are individual commands which are activated automatically in order according to the time scale in the middle (in minutes), but supposedly can also be toggled manually on the KSU just in case. There is a direct reference to "Integrate (B)DUS" (which is missing from 7K-OK below), "Inerts. 1/2" I believe to be related to the "Prepare Inertial 1/2" commands on the KSU (gyro power on/run-up kind of stuff, caging, not sure), and also to be analogous to "ГБ-А" and "ГБ-Б" in 7K-OK (
@Urwumpe did you ever find out what "ГБ-А" and "ГБ-Б" refer to?). Then there's "Inertial Orientation" which does correspond to a command directly and should be the final activation of the attitude hold.
The 7K-OK doc provides a clearer example, in this case for the de-orbit program (Spusk 2):
First, BDUS is activated (angular rates are always needed), followed by establishing orbital orientation (prograde), then "ГБ-А" and "ГБ-Б" come in with a 10 minute separation. SDKU main engine is pressurized, accelerometer (Integr.) is turned on (so the engine can shut down at a pre-set measured Delta-V), SUS (descent control) is turned on, and at 60 minutes the SKDU is fired, simultaneously with activating inertial attitude hold so that Soyuz maintains the intended burn vector. Interesting to note here it is directly "Разарретир - Uncage", which to me suggests the gyro system is used. Not sure if that, plus the absence of an "integrate BDUS" command, indicates in the first Soyuz generation there was no BDUS integration or it was not prefered, and later in 7K-TM, there was a change in procedure.
The bottom line is, Orbiter implementation-wise it probably doesn't matter much which system is used if only attitude hold is considered, and either way I think I've pieced together enough info for manual switching between using the gyros or BDUS (there's a "Gyro Assembly (Rate - Free Gyro)" command), and I might proceed with at least the simple "on/off" functionality. But still dunno where that BDUS integration is supposed to be. My best guess is either "Select 3-Axis Orientation" or maybe "BDUS Filter".
The plus side is I also learned how the program timer display worked and getting it into Orbiter eventually seems doable, at least for main engine burns.
For reference, my current reconstructed ASTP command matrix, which I'm favouring as it should be closer to 7K-T in some places: