Apollo 17: Post-TLI S-IVB Maneuver to SEP ATT Problem

Wedge313

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Hello,

I'm having a problem with the S-IVB. When it begins it's maneuver to the SEP attitude, it develops very high rates and quickly is in an uncontrollable tumble. Viewing the maneuver from the exterior view it appears only one of the APS units is functioning.

I'm not sure how I got myself into this situation, if I've made a procedural error or if I somehow re-programmed the S-IVB while playing with the RTCC (before liftoff I was trying to see if there was a way to change the launch time. I didn't think I changed anything, ended up launching as originally scheduled without the delay, but could I have inadvertently messed up the S-IVB?). I'm attaching the .scn, hoping that someone might find where I've gone wrong and instruct me how to correct it.

Thank you
 

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indy91

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One APS module seems to have run out of propellant. That's weird, the other tank still has 63% of prop. Maybe this was caused by too much time acceleration, it can move around in the attitude deadband even during 10x. I'm usually monitoring the attitude rate and if it gets too much I go back to 1x for a minute until it has calmed down and then go back to 10x. But it's strange that one module is so much lower than the other.
 

Wedge313

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I did use 10x a bit before TLI, maybe that's what happened. I was using exterior view during the 10x so didn't see any unusual rates building up. But replaying the TLI another thing I notice is the ullage seems to be coming from only one nozzle, so I'd guess my 10x before TLI may be the culprit.

I'll try to go back to a save after I achieved orbit, and go through the pre-TLI stuff again while avoiding 10x.

Thanks for looking.
 

indy91

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If you want to find out when exactly the APS lost its propellant, you can check in the scenario editor, under propellant. There are 11 tanks, tanks 10 and 11 contain up to 143 kg, those are the APS tanks. Tank 10 is the empty one, tank 11 is still at 63% in your scenario.

This is actually one of the contingencies covered in the mission rules. If you loose one APS module before TLI they would command the IU to "S-IVB burn mode", so it will try to do attitude control with J-2 engine thrust vector control and not the APS. And the crew would then use the SM RCS for attitude maneuvers. That way you can still continue the mission and they would even do TLI like that.

This is something that can be done in NASSP as well. Use the PAMFD, IU page. Then select the option switch selector, stage is "IU" and then select the right channel. There is no list of all the channels specifically, but you can look here in the code: https://github.com/orbiternassp/NAS...samples/ProjectApollo/src_saturn/iu.cpp#L1607

What you need is:
case 31: //Flight Control Computer S-IVB Burn Mode On "A"
case 74: //Flight Control Computer S-IVB Burn Mode On "B"

So you need to select channel 31 and uplink and then channel 74 and uplink. I'm not sure right now how they would deal with roll commands still being performed by one APS module, especially for TD&E.

Or you cheat with scenario editor and give yourself a bunch of propellant in tank 10. Or go back to a good scenario. Your choice :D
 

Wedge313

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Going back is OK, and I'm not above cheating! But I might try the TLI using the APS-inop procedure (since I've mastered the normal procedures, it's time to challenge myself ?)

Thanks
 

Wedge313

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Use the PAMFD, IU page. Then select the option switch selector, stage is "IU" and then select the right channel.
I'm missing something. The bottom left says "No Target!", and if I try to uplink I get a message "Vessel has no IU". I tried using AS-512-S4BSTG as the SRC but it wouldn't accept that. What am I doing wrong? Thanks.
 

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indy91

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That screenshot is from before TLI. There is no separate S-IVB vessel at that point. So you need to use the name of the combined Saturn (with CSM), which should be "America". After CSM/LV separation, if you want to uplink anything to the IU, you need to use the name of the separate S-IVB which is then "AS-512-S4BSTG". And before CSM/LV sep, also make sure you have the IU uplink switch in accept.
 
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