Apollo 8 SM RCS B and D Caution LTs on

[Wedge313]

In lunar orbit, preparing for LOI-2 burn, while maneuvering to the instructed ORDEAL attitude I got a SM RCS B caution lt, followed shortly by a SM RCS D caution lt. I've run the malfunction procedure on AOH Vol 2 pg. 5-87. If I'm reading the flow chart (and gauges) correctly: SM RCS He 1+2 closed, Pkg temp is normal (125), RCS SEC fuel press low (120), He press normal (3050), at this point it asks for MSFN to verify the He manifold press. What have I done to create this issue? Am I just really heavy on my RCS usage? Is there a way to MacGyver this back to normal? Thanks.

 

[indy91]

It's strange that the malfunction procedures don't mention it, but if RCS propellant is low then getting that light is normal. Maybe it's not considered a malfunction and the crew is just supposed to know this. Here is the relevant part from the SM RCS Monitoring Check which is done every so often:

When MANF PRESS <150 psia
RCS SEC FUEL PRESS A (B, C, D) - OPEN

So do that for the affected quads. You must have had really high RCS usage, normally you don't even get those lights when you have to maneuver around the LM as well, at least not until late into the mission. But you should still be able to make it through the mission. Just try more to conserve RCS from now on. If you get the lights again on the quads where you have already opened the valve to the secondary propellant tanks, then you start getting in trouble.

 

[Wedge313]

I first noticed B and D were low after TLI, I had been time accel-ing at 5x and had walked away from the computer. When I came back I could hear the RCS chattering away (I hadn't figured out yet to turn off the 16 RCS switches). Since achieving lunar orbit I have been maneuvering a lot, trying to figure out what the ORDEAL is showing me, not sure if I set it up correctly after LOI-1. Question: Scenario Editor-Propellant shows seven tanks, tanks 3 and 5 seen to correspond to RCS B and D. I wasn't worried about the low fuel quantity because I thought if all else failed I could just (cheat) add a bit. BUT, with the RCS SEC open, adding fuel on scenario editor +/- .02 causes the fuel pressure to go full scale low or high, and does nothing to the quantity. BUT if I CLOSE the RCS SEC and THEN add fuel with scenario editor, the quantity increases. In any event, I'll open the RCS SEC for now and tread lightly on the gas pedal. Thanks again for the explanation.

 

[n72.75]

With any time acceleration > 1x, it is advisable to set the attitude mode to "free".

Time acceleration works by making the simulation steps longer, so that Orbiter can fit more into the same interval of real time. This makes the simulation of things like spacecraft rotation, and rotation perturbation due to RCS less accurate and more granular, and the AGC simply can't keep up with it's goal of stabilizing rotation.

I can't advise adding fuel with the scenario editor, as we have a few other things going on in the simulation beyond the standard Orbiter handling of propellant tanks (pressurization, etc), however, if you really get into a bind, we can probably do a bit of "scenario surgery" aid fix the low level for you.

 

[MrFickles]

Adding more RCS fuel in scenario editor doesn't really work. I think it's because you're not running out of fuel, but RCS pressurant. That's why turning the SEC FUEL PRESS on allows you to fire your quads B and D again, even though you did not add fuel.

You can find an earlier scenario and copy/paste the section between SMRCSPROPELLANT_A_BEGIN and CMRCSPROPELLANT_1_BEGIN to 'refuel.' Your milage may vary, but it works for me.

 

[Wedge313]

Yea, I've been looking over the SM RCS system plumbing diagram, and if I mess with adding fuel, something has to give. Not sure how all the check valves and vents work, but I could overpressure something and just complicate the problem (envisioning panels blowing off the SM). Thanks for the tip on copy/paste, I'll go easy and hope I don't need to use it.