Pre-MCC4: "Set DET"?

[Wedge313]

I'm running the Apollo 8 mission, at about 60:42, getting ready for MCC4. The maneuver only requires a dV of 2.2, so we'll be using the RCS. Question: in the checklists for both P30 and P41 it directs me to "set DET". What do I set it to? Thanks.

 

[Thespacer]

In short, set the DET to a value which, when counting up, will mean that it hits 00:00 exactly at the time of ignition (Tig).

The last item of P30 is the flashing Verb 16 Noun 45 (F 16 45). Register two displays the time until Tig, counting down (assuming you have started P30 within an hour of Tig). Since you have to exit P30 (pressing PRO) to move to P40/41, you lose the countdown. By setting the DET (counting ”up”) before you exit P30, you can have the DET remind you of the time till Tig (and then the burn time itself, as it will keep counting up from 00:00).

(DET time) + (Time till Tig) = 60 mins

 

[Wedge313]

Good explanation, thank you. So it's not used as a countdown clock. If the Tig is 40 min, I set the DET to 20 min and let it count up, so that at Tig it would read 00:00 and then continue counting up so I can monitor the burn time, same as we did for the TLI burn. Thanks.

 

[Thespacer]

That’s exactly correct. Sometimes you will also see, in some checklists (mostly in the LM), a note saying “update DET.” This happens most frequently in rendezvous when the initially-calculated Tig shifts slightly, so all you will be doing is resetting the DET to accommodate the new Tig.

I also understand that, in some of the later Apollo missions, the DET had the ability to automatically count up, after counting down to 0 (contrary to the behaviour in NASSP). If that were the case, then it would obviously be easier to set the DET to align with the F 16 45 (my mental math has very occasionally exposed my DET-setting skills... the Tig will come and go while my DET is still counting up...)

 

[MrFickles]

Good explanation, thank you. So it's not used as a countdown clock. If the Tig is 40 min, I set the DET to 20 min and let it count up, so that at Tig it would read 00:00 and then continue counting up so I can monitor the burn time, same as we did for the TLI burn. Thanks.

You can use the event timer to count down to TIG, but when the event timer counts down, it won't cycle from 00:00 back to 59:59. Counting up allows you to monitor burn time as well. This becomes more important when performing DPS burns under manual throttle. Another benefit of setting the DET to count up is that it allows you to 'count past' 59:59 when your TIG is over an hour away.

But in the case of RCS burns, burn time is not really a metric that I personally find important to monitor. To be honest, I don't even think burning a few seconds before or after TIG will make much of a difference, barring rendezvous burns. I will just zero out the EMS when I reach TIG, then trim as necessary. It's easier (and less mistake prone) to set DET to count down to 00:00 than it is to count up to 60:00, so that's what I normally do if TIG is <1h away.

 

[Wedge313]

As far as the residuals, I don't think I have that figured out. If after the burn the three registries show 00000, 00002, 00000 I should tweak the pitch, correct? But when I tweaked the pitch, the roll and yaw error jumped up. After a few minutes of flailing I settled in on 00001, 00002, 00001, and wish I hadn't messed with the original. Any tips? Also, the P21 ground track shows my ALT at -0221.2. Seems to me that MCC4 should have tried to prevent me from impacting the moon. Am I going to get another chance to correct this, or at least will they name the crater after me? Thanks

 

[Thespacer]

When you say “tweak the pitch,” you’re referring to translating along the pitch ( y ) axis? If so, then technically yes, that is what you should be doing to null that residual velocity (according to your R2 numbers there). Whether you actually need to do that is a different matter, sometimes the flight plan would say something like “only null X-axis residuals to within 2fps.” In your case, the 0.2 fps probably can be corrected in the LOI burn.

As for P21, I’m not sure how accurate that number should be outside of orbit. But I’ll let the experts explain that one.

 

[Wedge313]

Yes, my error. I meant up/down (Y), not pitch. It got lost in the "translation".

 

[Wedge313]

Hey, I just terminated the PTC at 66:00, I hit F1 to look around and saw I was venting/dumping quite a lot of something. No warning lights, quantities look ok. If I hadn't happened to be looking at the CSM from the outside I would not have known. Anyone know what was going on? Thanks.

 

[indy91]

The main thing that can be venting in NASSP is waste water. Your tank is probably full and you have it set to automatic relieve. That would be the pressure relief selector valve on panel 352. 2 is the automatic setting, Dump A and B are manually. Maybe set it to Dump A for like 15 minutes and then switch it back to 2.

Also, just to make sure because you had performance issues, you are using the Orbiter_ng.exe and not the Orbiter.exe, right? Orbiter_ng is required for the DX9 client. Orbiter.exe uses the default DX7 graphics and I remember it having some problems with particle effects (like venting) being very exaggerated. Aside from it performing much worse than DX9.

Your P21 result is a bit worrying, it does use an integrated trajectory, so it should show accurate numbers. But I wonder if that isn't more of a problem with your state vector than your actual trajectory. How much DV was MCC-4? Normally that would be a smaller burn, below 5 ft/s. Maybe you have an alignment problem or something. You did the P52 option 1 before MCC-4, right?

And about the residuals, one pulse from the accelerometers to the CMC is about 0.19 ft/s. So it can't actually resolve residuals more accurately than that. So trying to get any residual of 0.1 ft/s down to 0 is quite pointless, you will just end up with -0.1. I would say 0.2 is the limit of what I would try to trim, usually I don't bother until 0.3.

 

[MrFickles]

As far as the residuals, I don't think I have that figured out. If after the burn the three registries show 00000, 00002, 00000 I should tweak the pitch, correct? But when I tweaked the pitch, the roll and yaw error jumped up. After a few minutes of flailing I settled in on 00001, 00002, 00001, and wish I hadn't messed with the original. Any tips? Also, the P21 ground track shows my ALT at -0221.2. Seems to me that MCC4 should have tried to prevent me from impacting the moon. Am I going to get another chance to correct this, or at least will they name the crater after me? Thanks

I've encountered similar numbers when running P30 for LOI and the predicted Pe comes back negative.

I'm not exactly sure what causes this, although my hypothesis is that the SV is referencing Earth SOI (and therefore Earth Pe). The LOI burn does bring me into the correct orbit.

 

[indy91]

I've encountered similar numbers when running P30 for LOI and the predicted Pe comes back negative.

I'm not exactly sure what causes this, although my hypothesis is that the SV is referencing Earth SOI (and therefore Earth Pe). The LOI burn does bring me into the correct orbit.

In P30 it is different. It actually simply adds the DV of the LOI burn to the velocity before LOI as if it was an instantaneous maneuver and uses that to calculate HA and HP. But of course LOI is a maneuver that is many minutes long. The result would be an impacting trajectory.

Here is what the Colossus 3/Luminary 1E program notes has to say about that:

N42 values of ha and hp (in P30) are preburn predictions and will vary slightly from N44 values (post-burn estimates). N42 assumes the DV will be burned impulsively. The larger the DV the greater the error in N42. Recognition: Difference in displays.

Apollo 11 got both sets of numbers (expected post LOI HA/HP that V82 would give you and the one that P30 is predicting).

072:51:24 McCandless: ... 0169.2, plus 00610 ... The values which you would see on Noun 42 prior to LOI burn are HA, plus 431.3; HP, minus 128.2. Read back, over.

P21 however should give accurate results for the trajectory. On Apollo 8 they used it quite a bit, to check the results of P23 and simply to test the capability of the AGC to predict the trajectory in the long term. So a negative alttiude in P21 is usually not good.

 

[Wedge313]

Yes, using orbiter-ng. Yes, the pressure relief was set to 2, so that's probably what I saw. And now that I'm only 2+ hours from the moon, when I open External MFD it opens to Orbit MFD, which is now referenced to the moon. It's showing a PeA of 110.5k, which is 68.66 miles. So maybe this is going to work out. I'll know in 2 hours. Thanks.

Addendum: Success. Orbit 67.6 miles by 194.3 miles. Those MCC guys are pretty good, I need to have more faith.

 

[indy91]

The Apollo program actually used nautical miles and not statute miles. And it also defined its altitude relative to the lunar landing site instead of the mean lunar radius. P21 and V82 will show altitude relative to the landing site, Orbit MFD and the Project Apollo MFD relative to the mean radius. The Apollo 8 "landing site" is at an altitude of -0.82NM. Considering all those things your pre and post LOI-1 orbit looks even better to me than your numbers in statute miles.

 

[Wedge313]

I'm constantly amazed by this simulation, truly amazing. One MAJOR screwup by me was at T-00:05 before ignition, the checklist said "Auto Ignition", so I'm (foolishly) expecting auto ignition. A few seconds later I realize it isn't totally automatic unless I hit "PRO". So learn, rewind, and second time's the charm.

A question about the waste water: Am I looking at the correct gauge? These indications seem frozen. Down on panel 352 Pressure Relief I've selected Dump A and Dump B, I can see the dump using external view, but I don't see any decrease in quantity.

Thanks

 

[indy91]

Yeah, you are looking at the right gauge. I just did a bunch of testing on this and I am very skeptical that this all works as in the real ECS. But how it works in NASSP right now is that the quantity of water in the waste water tank can be offscale high. So you have to vent for a while before it reaches the 100% mark and the needle starts going down.