Apollo 8 CSM-LV Separation: RCS LOGIC c/b configuration?

[Wedge313]

Good morning,

Running the Apollo 8 mission. I'm following the PAMFD checklist, I don't have many documents specific to this flight.

I just (attempted) to perform the CSM/LV separation. When I turned the THC counterclockwise, I got separation, but the SM RCS started firing continuously, and the CSM maneuvered pretty wildly.

After some troubleshooting, I think my issue is that the two RCS LOGIC c/bs on panel 8 are in? I looked at the Apollo 15 Launch Checklist and the CSM Separation Prep procedure has us open both breakers. I ran the CSM/LM with the breakers open, and everything proceeds smoothly.

I've looked through the PAMFD checklists, and I can't see where we're supposed to open these breakers. They're closed before liftoff, and I can't find any mention of them after that. It's bugging me I can't find it. (I'm using NASSP v1998)

Where am I missing this? Thanks.

EDIT: After the separation the PAMFD checklist does have us open the RCS LOGIC c/bs. But that seems too late?
Also, PAMFD checklist has no mention of latching the FC REACS VALVES.

 

[rcflyinghokie]

The Apollo 8 procedures are correct, as is the SMRCS firing when you use the THC for sep. The logic cbs should have no bearing on that function, as it only powers the RCS Transfer switch, CM RCS logic switch, CM RCS heaters switch, and the CM RCS He dump pb.

If it maneuvers wildly that tells me your in the wrong attitude control mode or your DAP or RCS isn't configured. If you have a scn where you can replicate the wild maneuvering I am happy to take a look!

Also, there is a good chunk of Apollo 8 docs here: http://www.ibiblio.org/apollo/links2.html#Apollo8

This includes the launch/TLI/Sep procedures (called the TLI checklist) as well as the G&C & partial systems checklist (CMP and LMP Checklists)

 

[Wedge313]

OK, I think I've found my error.

Here's what I expected to happen: Rotate the THC, bang, separation and the panels start to open. Then thrust forward. Then THC to neutral.

Here's what actually happens: Rotate THC, the CSM RCS fires thrusting forward (I didn't expect this, since I had not input any translation command). After brief pause bang, separation, panels begin to open.

If I don't IMMEDIATELY put the THC to neutral after the "bang", the CSM starts to veer off. As soon as the THC is neutral the CSM RCS fires to stabilize the ship.

I got locked into a "WTF is it doing?" mode when I first rotated the THC and heard the RCS firing. By the time I had thrusted forward for the 1.2 fps and returned the THC to neutral the CSM had developed a significant rate.

SO, I just needed to be quicker getting the THC to neutral.

(But I like pulling the RCS LOGIC c/bs too, I might recommend that for Apollo 9 if I survive the mission)

End result: the PAMFD is correct, user error.

 

[rcflyinghokie]

Again pulling the logic breakers should not change this outcome, and you are supposed to leave the THC CCW for 8 seconds translating +X.

If you are developing rates then something is not properly configured, can you send me a scn as I cannot seem to replicate the rates.

 

[Wedge313]

My configuration is OK, I'm just stupid. I didn't understand that this is a THC only procedure, the +X thrust occurs when you turn the THC. No Numpad 6 key required. I was leaving the THC in the CCW position WAY too long (trying to figure out why my RCS was firing) and that was causing my tumble to occur. When I follow the procedure correctly it all works fine.

I went into this expecting that when I turned the THC the only thing that would happen was the SLA panels would open, no thrusting. Then I would need to thrust forward using the Numpad 6 key to 1.2 fps. Then turn the THC to neutral. (This is what happens for me with the RCS LOGIC breakers pulled).

So the PAMFD checklist is correct, my configuration seems correct, I just need to RTFM and not incorrectly assume what's going to happen.

Sorry to bother you with this, and thanks again. I'm off to botch up some P23s.

 

[indy91]

The difference between using the THC in the CCW position and thrusting forward +X like normal is that the first option uses the Direct RCS, while the normal operation of the THC uses Auto RCS. In the control electronics using Direct RCS actually inhibits all other thrusters in that axis, so that e.g. the CMC and Direct RCS commands with the RHC don't fight each other. Side note, this is different to the LM where they would fight each other. So for the LV sep with the THC this means that the CSM isn't holding attitude while it is doing the "separation ullage".

Maybe originally this was meant for a Mode II abort. You put the THC in CCW to stop the LV engines, then after 3 seconds it separates the CSM and you keep the THC in CCW until the automatic translation got the CSM away from the LV. The problem with that is center of gravity. It is not centered enough, you just get too much of an attitude rate from using this option that overrides attitude hold. That's why the procedures have you return the THC to neutral when you know you have separated and then continue thrusting +X manually, this time with attitude hold.

 

[rcflyinghokie]

From my understanding you are also thrusting +X while leaving the THC CCW until sep time +8 sec. So shouldn't you have auto RCS helping?

003:20:52 Anders (onboard): Okay, at zero, turn Hand Controller counterclockwise, plus-X, and hold.

"Frank is occupying the left-hand seat and has access to the Translation Hand Controller. By turning the controller and pushing it forward, he fires the plus-X thrusters so that when separation occurs, the CSM will immediately begin moving away from the S-IVB. After three seconds, the vehicles separate and Frank continues firing forward for a further five seconds."