# Apollo 13 Launch Scenario: What should I expect?

#### n72.75

##### Move slow and try not to break too much.
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@Wedge313 I looked through the code, and I am fairly confident that the fix for the NaNs is just to guard against the potential of divide by zero cases (basically, we calculate the resistance of all the load attached to the cell and use that to calculate voltage drop and heat and a bunch of stuff, the input of that calculation is power draw...which can be zero...).

This is a bug I introduced a few years ago with my fuel cell update, but people don't fly 13 that often so we're just finding it now. It's an easy fix and I'll have an update out soon.
This turns out not to have been the problem.... @indy91 actually added some code back in 2020 to prevent the divide by zero case that I caused.

What is actually happening here is that because everything is on FC2 and FC 2 has just the right combination of load on it, its voltage and current draw are increasing exponentially. This is obviously unrealistic only happens because the fuel cells don't [yet] care about the H2 and O2 (other than consuming it, that is).

The NaAs happen because temperature runs away and the model for FC performance breaks down at some point. Your FC-2 in the scenario you shared was about 2500°F--which is not physically possible. Temperature increase causes voltage and current to increase which in turn feed back into the rising temperature. This can actually happen--to some degree--but you need reactants for that to be possible.

I have a fix in the works that should prevent this from happening to anyone in the future.

#### Wedge313

##### Well-known member
I spent a few hours going through the first steps of the entry sequence checklist and tried to follow along looking at a schematic of the DC power system.
(Nothing good ever happens when I start looking at electrical diagrams, just ask my Fender Twin Reverb).

Anyway, what busses should be powered at the end of these steps? It seems like I should have both Main A and Main B, but based on what I'm reading on the DC meter on MDC 3 I'm only getting Main A. I get Main A when I close the Main A Bat C breaker on P275. But it seems like they're trying to get Main B through the Bat Charger c/bs and Main Bus Tie breakers? At the end of these steps I have no reading on Main B. I can power Main B by closing breakers on P275, but that's not the procedure. So something somewhere must not be configured correctly.

#### rcflyinghokie

##### LM Junky
Yeah in this case, the LM is still powering MN B and MN A is brought up because both A and B are needed to heat the CM RCS. Unfortunately we dont have the power simulation necessary yet to allow the LM to power the CSM.

#### Wedge313

##### Well-known member
I'm going to try re-configuring, un-do the LM to CM power transfer, get all the switches and breakers to a baseline config and see what that gives me when I power up. I don't think it really changes my situation though. Since the LM isn't powering Main B anyway, I'll probably have to close the Main B Bat Bus B or Main B Bat C c/b on P275, I think that will power Main B, and I think I need Main B going forward. (If the additional load drains my CM batteries too much I'll just edit them again to simulate having the LM to CM power connection).

Also I've been playing around trying to decipher the <DCBUSCONTROLLER> code. Looks like the first three numbers represent which fuel cells were tied to the bus? The fourth number looks like the Bat Bus Tie breaker. No idea what the other five numbers are, but if I start from the baseline configuration the <DCBUSCONTROLLER> MainBusAController 0 0 0 2 0 0 0 0 0 and <DCBUSCONTROLLER> MainBusBController 0 0 0 2 0 0 0 0 0 should be correct, right? That's how I think you wanted me set up.

#### rcflyinghokie

##### LM Junky
I'm going to try re-configuring, un-do the LM to CM power transfer, get all the switches and breakers to a baseline config and see what that gives me when I power up. I don't think it really changes my situation though. Since the LM isn't powering Main B anyway, I'll probably have to close the Main B Bat Bus B or Main B Bat C c/b on P275, I think that will power Main B, and I think I need Main B going forward. (If the additional load drains my CM batteries too much I'll just edit them again to simulate having the LM to CM power connection).

Also I've been playing around trying to decipher the <DCBUSCONTROLLER> code. Looks like the first three numbers represent which fuel cells were tied to the bus? The fourth number looks like the Bat Bus Tie breaker. No idea what the other five numbers are, but if I start from the baseline configuration the <DCBUSCONTROLLER> MainBusAController 0 0 0 2 0 0 0 0 0 and <DCBUSCONTROLLER> MainBusBController 0 0 0 2 0 0 0 0 0 should be correct, right? That's how I think you wanted me set up.
Yeah that has them just on the main bus tie switches.

#### Wedge313

##### Well-known member
OK making progress. As discussed I've powered Main B. SM SEP is completed, continuing through the power up. Issues that have cropped up along the way:

. As mentioned earlier, during the CM RCS preheat I need the FLT BUS powered to see anything on the P101 test meter.
. I needed Direct RCS to get the RCS hot fire check, Normal gave me nothing.
. After closing the G/N Computer Main B c/b I got a master alarm, but no indication on the DSKY that anything had turned on. So at this point you can't enter P06. It wasn't until later in the procedure when you turn on the AC power that the DSKY lights up (you also need to have the Numerics lighting up) and then you can enter P06.
. I couldn't get the RTCC to generate an Entry PAD. I re-calculated an MCC-7, thought I went through the steps correctly, but maybe there's something the RTCC doesn't like about being docked to the LM, I kept getting a CTD when I tried to get an Entry PAD. Probably operator error. I'll just use the PAD info from the actual flight.

On to the CMC power up.

#### rcflyinghokie

##### LM Junky
. As mentioned earlier, during the CM RCS preheat I need the FLT BUS powered to see anything on the P101 test meter.
This got a hotfix, with more changes on the way but the meter no longer needs power.
. I needed Direct RCS to get the RCS hot fire check, Normal gave me nothing.
Fix is in work here, there was an unnecessary condition check on the auto rcs coils.
. After closing the G/N Computer Main B c/b I got a master alarm, but no indication on the DSKY that anything had turned on. So at this point you can't enter P06. It wasn't until later in the procedure when you turn on the AC power that the DSKY lights up (you also need to have the Numerics lighting up) and then you can enter P06.
Hmm looks like another mistake in the simulation! The EL lights should have power from the DC computer cb. I will get a fix up for this as well
. I couldn't get the RTCC to generate an Entry PAD. I re-calculated an MCC-7, thought I went through the steps correctly, but maybe there's something the RTCC doesn't like about being docked to the LM, I kept getting a CTD when I tried to get an Entry PAD. Probably operator error. I'll just use the PAD info from the actual flight.
Are you doing it in the CM? Also did you save your splashdown target when you computed MCC7? That is also needed for this computation.

#### Wedge313

##### Well-known member
Before the SM SEP I was able to go through the MCC-7 calculation and then generate an Entry PAD. After the separation, I'm unable to. I've spent three mornings playing with this and can't figure out what I'm doing wrong. I've tried doing it in both the CM and LM. In both cases the simulation crashes when I get to the CLC step with MSK 363 displayed. So I can't get to the point where I'd be able to save the splashdown point and continue.

Here's a save. At this point I've got the CM O/I/G set to the values I calculated from the current LM O/I/G.

For the Actual REFSMMAT I used the REFS from Attitude option using the calculated CM O/I/G. For the Preferred REFSMMAT I want to use the Lunar Entry. But don't I need to have the MCC-7/Entry PAD process completed before I can get a Lunar Entry REFSMMAT?

Anyway, I'm curious what I'm doing to prevent myself from getting the Entry PAD generated.

Thanks

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• 140+36 Troubleshooting Alignment.zip
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#### rcflyinghokie

##### LM Junky
Looks like this is a bug, RTCC is throwing a calculation error with masses of propellant. I will let @indy91 take a look.

EDIT, I just realized you are in a CM/LM configuration, so the error is thrown because there is no propellant mass in the SM anymore. That is why its crashing. You needed to save your splashdown target before you let the CM go.

Now I think protection against this should be added in, but if you want to resume right now, you need to go back before SM sep and save the splash target when you computed MCC7

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#### indy91

There is a dedicated MFD page called Splashdown Update (in the RTE section) that you can use to get splashdown coordinates without a maneuver calculation and without having coordinates in the CMC. The Entry PAD page has a button to "downlink" the coordinates to the MFD, but you probably had the CMC powered down so the coordinates weren't updated yet. Once you have the splashdown coordinates (they are shown on the left side of the Entry PAD page) then the PAD calculation should work.

I have already put up a pull request to fix this issue that @rcflyinghokie discovered. Even if you don't need to do a RTE calculation just to get good splashdown coordinates, there might still be the case that you do a RTE targeted maneuver with the LM+CM, the SM already gone. Apollo 13 didn't do that, but the MFD shouldn't crash if you try it, so, that is getting fixed. In the same PR I did a small tweak to the Splashdown Update page so that it defaults to the mission specific reentry range, in the case of Apollo 13 that is 1250 NM.

#### Wedge313

##### Well-known member
Hey, I'm in the water! Splashdown Update worked like a charm, I was able to get the entry PAD and the REFSMMAT, the P52 opt 1 alignment went well, and the rest of the re-entry went ok, no major issues. I'll probably take it back to just prior to SM SEP, and try to run through it all without interruption. There are so many changes to the Entry Checklist it was hard for me to keep up.

I really enjoyed this flight! I would rank it as my #1 or #2 favorite mission, Apollo 15 is hard to beat, but this was right up there. Thank you very much for all the help. You guys do all the work, and I get to sit back and watch this amazing simulation (NASSP) do its thing.

Thanks again.

#### rcflyinghokie

##### LM Junky
Hey, I'm in the water! Splashdown Update worked like a charm, I was able to get the entry PAD and the REFSMMAT, the P52 opt 1 alignment went well, and the rest of the re-entry went ok, no major issues. I'll probably take it back to just prior to SM SEP, and try to run through it all without interruption. There are so many changes to the Entry Checklist it was hard for me to keep up.

I really enjoyed this flight! I would rank it as my #1 or #2 favorite mission, Apollo 15 is hard to beat, but this was right up there. Thank you very much for all the help. You guys do all the work, and I get to sit back and watch this amazing simulation (NASSP) do its thing.

Thanks again.
Glad you enjoyed it! Hopefully more changes can be added in the near future to properly simulate all the things you had to work around.

Also thanks for finding a few bugs They normally wouldnt have been noticed on a routine flight.

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