Control Characteristics - CMC vs SCS - Induced Yaw

Thespacer

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Hi team

Using the latest build (1678) and the default mission scn (Apollo 11 - 05 - Before SIVB Sep), I’m practising TD&E.

I’ve noticed that after CSM/LV sep, and upon commencing the pitch up move under CMC control (pitch accel cmd), the pitch-up induces a noticeable yaw rate, much more than is caused while under SCS control. It is much more noticeable pitching up than it is pitching down. I’m not sure if this is working as designed, or if not, a configuration issue. I figure it’s not a joystick issue, but then I’m not sure. Anyone else seeing it?

I appreciate some yaw is inevitable, but again it’s much more under CMC than SCS, and I cannot recall seeing this in earlier builds.
 

n72.75

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First two things to check:
1) Are your switches set correctly on panel 8 (nothing disabled that shouldn't be)?
2 When you did the DAP load with V48 was R2 11111 (or 01111, that would be okay too, the first digit is A-C vs B-D roll).

I'm 99.99999% positive that the recent builds did not affect this.
 

indy91

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Yeah what kind of attitude control configuration do you have? You load that scenario and just switch to CMC instead of SCS before starting to pitch up? CMC mode free?
 

Thespacer

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Thanks for weighing in, I appreciate I was a bit fluffy with the details. To expand:
  • Auto RCS all enabled on Panel 8, all other switches configured normally (in the default state upon scn loading)
  • DAP Load was 11103, 11111
  • I’m trying to use the procedures from the Apollo 11 Launch Operation Checklist, specifically L3-1 (Normal SC/Booster Separations). In contrast to the checklist MFD procedures, this appears to be a CMC manoeuvre. Namely, CMC Auto is enabled prior to LV sep; after sep, Pitch accel cmd is selected, pitch up of 1.5 deg per sec, V49 is then Pro’d etc.
  • My issues occur during the pitch up, I easily get up to 1 deg per sec left yaw at the same time (or up to the rate limit per V48)
  • The yaw is noticeable in any CMC mode (Auto, Hold and Free)
  • I played around with the CSM-only in a different scn (Pre PDI) and noticed similar issues

I cannot discount the fact there is noise in my joystick yaw axis, but it really seems exacerbated while in CMC mode vs SCS. I guess I do not recall this being such an issue when I last tried these scenarios some months ago.
 

rcflyinghokie13

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As a sanity check have you tried it with the keyboard to compare? Feel free to post a scn when you get these yaw moments.
 

Miriam

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...or monitor Channel 31 with a V11N10 while moving the Stick. But that's looking in the inner workings of the CMC and not everybodys taste. And you've got to be familiar with octal, so... ;)
 

Thespacer

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As a sanity check have you tried it with the keyboard to compare? Feel free to post a scn when you get these yaw moments.
Yeah, thanks for the suggestion. I did that and I’m now fairly certain the issue lies with my joystick, which has perhaps deteriorated and is exacerbating the yaw effect.

That said, I don’t know if there is an underlying issue that is contributing. What I have noticed is that, in CMC mode, the yaw axis is extraordinarily sensitive, much more than the pitch or roll. It’s as though the RHC breakout switches are instantly engaged, with 0 deadzone. In SCS, there is at least some deadzone in the yaw axis, making it less susceptible to a yaw while pitching.

For what it’s worth, I have no similar issues controlling the LM. I do not notice any undesired yaw.

I would happily post a scn but it is really just the default mission scn with the CSM-only in lunar orbit.

or monitor Channel 31 with a V11N10 while moving the Stick. But that's looking in the inner workings of the CMC and not everybodys taste. And you've got to be familiar with octal, so...
Great suggestion too, thank you. Although I can’t say I got very far with it!
 
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indy91

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It does sound like it's a problem with your joystick or at least the way its yaw axis is processed. I'm not having any issues with my joystick and the yaw axis when I use that pre CSM separation scenario.

The RHC is definitely more sensitive than the ACA though. The RHC has a maximum deflection of 11.5° and the breakout switches close at 1.5°. And that's how it is handled in NASSP as well, so about 13% of the maximum deflection of your joystick is already enough to cause the breakout switches to close. For the ACA it is 2.5° with a maximum of 13° so about 19%. And the yaw axis specifically is processed a bit differently, so potentially something with your joystick is different than with mine.

Are you building NASSP yourself or are you using the prebuild modules? If you are building it with Visual Studio yourself you could add a debug string in the RHC, just to see how it behaves. Alternatively, the tolerance for the breakout switches is 0.5° so I could make them close at 2° instead of 1.5°. That would be within specification and make it slightly less sensitive, although I would prefer to not have to do this.
 

Thespacer

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Thanks indy, I am using the prebuilt modules and doing my own builds is not within my capability unfortunately. I appreciate your other proposal but I agree it is preferable not to change code to cater for this one-off situation. I think it is pretty clear that, if nothing code-wise has changed, then it is definitely my own joystick (it definitely worked fine in the past).

What may be mentally contributing to the problem is the reversed rate indicators, I’m still struggling to get used to them!
 

Thespacer

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Just a small note for closure, got a new joystick and the noise in the yaw axis is no longer an issue. But the experience did help me continue to understand the interesting and nuanced ways in which manual spacecraft control is effected, and the differences between CMC and SCS control modes. I recall the Apollo 11 crew were not terribly happy, according to the post flight debrief, with the CMC TD&E process (noting the CMP had some minor issues with it IRL), and the CDR recommended a manual process (SCS I guess?) instead.

Anyway, thanks all for your inputs.
 
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