Apollo 11: Pre-Reentry CM RCS Check problem?

Wedge313

Well-known member
Joined
Apr 18, 2020
Messages
488
Reaction score
118
Points
58
Location
Boston
Hello, I'm getting ready for re-entry on Apollo 11. I'm following the PAMFD Checklist steps to check the CM RCS, and when I'm checking the ring 2 thrusters I seem to get a stuck thruster.

Could someone take a look at this save, and tell me what I've configured incorrectly?

Thanks
 

Attachments

  • 194+23 Test CM RCS.zip
    70.3 KB · Views: 84

rcflyinghokie

LM Junky
Addon Developer
Joined
Jun 4, 2016
Messages
619
Reaction score
331
Points
78
Location
Colorado
I am getting the same behavior, configuration looks just fine. @indy91 could this be something to do with the CMRCS changes we made?
 

Wedge313

Well-known member
Joined
Apr 18, 2020
Messages
488
Reaction score
118
Points
58
Location
Boston
FWIW I skipped over the check and pressed on with the rest of the reentry and didn't see any problem with the CM RCS.
 

indy91

Addon Developer
Addon Developer
Joined
Oct 26, 2011
Messages
1,231
Reaction score
618
Points
128
I have encountered this a couple of times. It is just SCS attitude rate damping, I think. Manual attitude switches in rate cmd, attitude deadband and rate switches in min and low. So the SCS wants to enforce a 0.2°/s rate limit. Now, you start the CM RCS check with no rates, but the way the thrusters are positioned is, pitch thruster points almost exactly through the CSM CG and causes no pitch rate. But the two roll jets you are testing both cause a small pitch rate in the same direction. And if you fire them long enough you get more than 0.2°/s and the SCS tries to get the pitch rate down. But it can't do that as the thruster used for that is ineffective for the CSM. There is also a little bit of a roll that gets induced by the pitch thruster, that is what the roll thrusters try to counter once the pitch thruster fires constantly. I think this is what is going on.

Later entry checklists (Apollo 13+, not that we have much before that...) explicitely switch to min imp mode before the test, probably to prevent this sort of issue. The only thing I could see that might be wrong in NASSP is the exact CM RCS thruster directions. They are not as perfect yet as the SM RCS thrusters. So maybe this wasn't as much of a problem in the real spacecraft. But as far as I can tell, there is nothing in the SCS behavior that changes when you switch to CM control. It tries to use the CM thrusters the same way with the full CSM as with the CM alone, same deadband and rates and all. So I think this is almost certainly correct SCS behavior. I do find it a bit strange though that there is no warning or at least a note in the AOH Volume 2 about this.
 

rcflyinghokie

LM Junky
Addon Developer
Joined
Jun 4, 2016
Messages
619
Reaction score
331
Points
78
Location
Colorado
Adding to this, on some transcript research, it appears missions as early as Apollo 8 also switched to MIN IMP before CM RCS firing. We don't have entry checklists for these missions so this got missed. I plan on adding that step to entry checklists and it should prevent this.

Apollo 8:
141:50:27 Mattingly: Very good. On page E-9, when you're getting ready to transfer the RCS to the Command Module position, if you want to avoid having the engines fire as a result of attitude correction, you might want to take the manual attitude switches to Accel Command or Minimum Impulse. And again on E-9 Alpha at step 41 Bravo, if you want to go back to attitude hold, bring the manual attitude switches back to Rate. [Pause.]


Apollo 9:
1687969101187.png
 
Top