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Author Topic: LV GUID Light  (Read 1586 times)
abr35
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« on: June 26, 2017, 10:40:54 PM »

Flying Apollo 12 with the latest Alpha and ran into some minor issues so far. H-dot did not become positive as early as it should have on the third stage. Shortly before SECO (GET +10:00 roughly) I got an LV GUID lt on the LV display. I had to perform SECO manually using the keypad minus. Again with TLI I needed to manually shutdown the S-IVB. TB6 and ignition were all automatic. TD&E is next up, so far I'm on a less than perfect but usable translunar trajectory.

First I'm correct in assuming Apollo 11-14 all have a pad load? And the LVDC++ also somewhat good to go on these missions? Any idea what the cause of the issues with the S-IVB might be? Scenario attached.

* LV Guid Error.scn (182.81 KB - downloaded 63 times.)
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indy91
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« Reply #1 on: June 27, 2017, 03:25:11 AM »

Flying Apollo 12 with the latest Alpha and ran into some minor issues so far. H-dot did not become positive as early as it should have on the third stage. Shortly before SECO (GET +10:00 roughly) I got an LV GUID lt on the LV display. I had to perform SECO manually using the keypad minus. Again with TLI I needed to manually shutdown the S-IVB. TB6 and ignition were all automatic. TD&E is next up, so far I'm on a less than perfect but usable translunar trajectory.

First I'm correct in assuming Apollo 11-14 all have a pad load? And the LVDC++ also somewhat good to go on these missions? Any idea what the cause of the issues with the S-IVB might be? Scenario attached.

Hmm, the guidance light is something I added only recently. Previously Orbiter would crash if the LVDC ran into trouble calculating the trajectory during the Inertial Guidance Mode and started dividing by 0. Now it doesn't crash and you get the light. No idea why that happened though, I'll have to try it myself. In any case, the LVDC found itself in a state where it can't make orbit anymore. I almost would have blamed rcflyinghokie's mass changes, but there haven't been any mass changes yet for the Apollo 12 scenario.

In terms of padload, Apollo 7 to 17 should all have good padloads for the AGCs. The LVDC++ is only configured for launch through TLI for Apollo 8, 11 and 14. Other missions need a guidance update (TLI maneuver solution) calculated with the RTCC MFD and then "uplink" that solution to the LVDC. The launch itself should always work correctly though.

EDIT: Navigation-wise the LVDC looks good. I guess just the guidance was an issue. Do you have a scenario a minute before launch or so?
« Last Edit: June 27, 2017, 05:07:27 AM by indy91 » Logged
abr35
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« Reply #2 on: June 27, 2017, 07:46:53 AM »

Only at T-1:00 I believe, I can run it all again later and see what happens, then send files at different parts of the ascent if I still have the same problem. The SII did not have an inboard engine cutoff as well if that is any clue as to what is going on. The trajectory around the time of the light suggested to me the booster was too heavy. I was well past the apoapsis and the booster had to burn an extra 45s or so to reach orbit.
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meik84
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« Reply #3 on: June 27, 2017, 08:52:16 AM »

Well, 11 doesn't work out well either. I don't get a 100 nm x 100 nm orbit, but something around 65 x 100, so that you partially re-enter during LEO. Not good.
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indy91
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« Reply #4 on: June 27, 2017, 08:56:19 AM »

Only at T-1:00 I believe, I can run it all again later and see what happens, then send files at different parts of the ascent if I still have the same problem. The SII did not have an inboard engine cutoff as well if that is any clue as to what is going on. The trajectory around the time of the light suggested to me the booster was too heavy. I was well past the apoapsis and the booster had to burn an extra 45s or so to reach orbit.

The early inboard engine cutoff of the later missions isn't simulated yet. Also, I think the issue might be a navigation problem after all. At orbital insertion the LVDC still does a cheaty IU state vector update. So in your scenario the state vector is obviously good.

Well, 11 doesn't work out well either. I don't get a 100 nm x 100 nm orbit, but something around 65 x 100, so that you partially re-enter during LEO. Not good.

Yeah, some people seem to occasionally get this issue. And then on the next attempt it works without issues. I haven't been able to reproduce this problem. Do you still have the lvlog file for that launch attempt?
« Last Edit: June 27, 2017, 09:01:46 AM by indy91 » Logged
abr35
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« Reply #5 on: June 27, 2017, 11:01:08 AM »

One other issue I had was that the RTCC uploads the TLI to the AGC as a P30 rather than to the IU when I press UPL. Is there a way to swap it to the IU?

Edit: Sorry found the answer elsewhere on the forum and that is no.
« Last Edit: June 27, 2017, 11:26:27 AM by abr35 » Logged
indy91
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« Reply #6 on: June 27, 2017, 11:31:48 AM »

One other issue I had was that the RTCC uploads the TLI to the AGC as a P30 rather than to the IU when I press UPL. Is there a way to swap it to the IU?

Edit: Sorry found the answer elsewhere on the forum and that is no.

On the Translunar page the two options for TLI (free return and nodal targeting) should both be "uplinking" to the IU, instead of the AGC. There currently is no indication to the user that the process was successful. But I don't think that was the case in the real spacecraft either. A few missions had a IU state vector update and the crew was informed by MCC-H about it.
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meik84
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« Reply #7 on: June 27, 2017, 11:35:54 AM »

Quote
Also, I think the issue might be a navigation problem after all.
Guess so, too. According to the attached lvlog, the LVDC thinks that it reaches the required terminal velocity. Interestingly, the CMC thinks that it is in a perfectly 100 x 100 nm orbit, too.

* lvlog.txt (1382.52 KB - downloaded 78 times.)
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indy91
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« Reply #8 on: June 27, 2017, 11:53:28 AM »

Quote
Also, I think the issue might be a navigation problem after all.
Guess so, too. According to the attached lvlog, the LVDC thinks that it reaches the required terminal velocity. Interestingly, the CMC thinks that it is in a perfectly 100 x 100 nm orbit, too.

Thanks for the log file! Looks like an IMU issue then, probably both alignment and accelerometers. The "SV Accuracy" value in the log file is accurate until liftoff, then it suddenly is 100 meter off and increases by 5 meters per guidance cycle. That gets worse and worse and at insertion it seems to be 200 kilometers off. That's quite a bit.

Do you get steady 60 fps in Orbiter 2016? In the past, using my crappy netbook, I sometimes had issues with the state vector accuracy at liftoff and staging. During these events the state vector suddenly started to get fairly bad. That was the case in Orbiter 2010 already. And I wonder if maybe the new touchdown points of Orbiter 2016 are screwing up the state vector now even more. Also, there currently is an Orbiter 2016 bug that prevents out accelerometers from working while landed. So we artificially induce a force, which makes the Saturn wobble a little bit and triggers the physics. With a bad frame rate it might not have settled in time for GRR and that could cause the alignment to be off, which of course during the flight can also make the state vector worse and worse.

The weird thing is that I don't really have these issues with my more powerful PC. In a bad case the state vector can be off a few hundred meters at insertion, but not more.
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meik84
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« Reply #9 on: June 29, 2017, 12:33:00 PM »

What I don't get is that Apollo 8 works smoothly. It shouldn't -or does the LM do some things we don't anticipate here?
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abr35
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« Reply #10 on: July 12, 2017, 09:58:58 AM »

Sorry I did not attach the lvlog. I wanted to fly through the whole mission and then report any issues all together so I wouldn't forget any files. I encountered a few issues on the Apollo 12. This is my completed first landing mission so some may be user error:

1) The LV GUID lt that I began this topic with. I haven't been able to reproduce it, but if I do I will attach the log and the scenario.

2) TLI required a manual shutdown using the Keypad -. The burn was up-linked to the IU with the RTCC and the EMS dV counter was set. I reproduced it without running Program 47, but that has no effect on the IU. The lvlog and scenario are attached.

3) When Orbiter's REF was the Sun, the RTCC MFD would crash when minimizing the MFD on the P30 Maneuver PAD page. I have not been able to reproduce this after updating. Perhaps it has been handled?

4) I would get a program alarm 20430 (overflow of the P00h-do loop) before lunar lift off. There was an LGC restart during Lunar Stay which caused the clock to reset. Updating the clock gave me a COMP ACTY light in P00H until the alarm. I think this is just the realistic behavior of the LGC iterating the state vectors after a massive clock update, but I have included the scenario before and after updates. I tried an upload with no program running (so V36E, Clock update, LM SV, CSM SV, V37E 00E) and the error persisted but became less frequent. Copying the pad-load from the launch scenario solved the problem and did not interfere with launch and rendezvous. This may not be a bug but more a consequence of the buggy EPS. I included the files anyway.

* lvlog (TLI overburn).txt (782.85 KB - downloaded 63 times.)
* TLI overburn.scn (191.75 KB - downloaded 61 times.)
* P00Hdoo overflow.scn (228.08 KB - downloaded 53 times.)
* P00Hdoo overflow no updates.scn (228.96 KB - downloaded 54 times.)
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indy91
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« Reply #11 on: July 12, 2017, 10:21:22 AM »

Thanks for the detailed report!

Just the first few observations for the TLI issue. The parameters for the TLI burn that were "uplinked" to the LVDC look good. The Iterative Guidance Mode is never started for the TLI burn though. That should happen a few seconds after ignition for TLI. Did that burn even result in a good trajectory after TLI? It probably would have stayed in attitude hold mode or orb rate mode throughout the whole burn. Did you manually stop the S-IVB burn for Earth Orbit Insertion in this scenario? It's possible that some flag is set incorrectly due to that, which causes problems with the IGM during the TLI burn. If that is the case, then it needs to be fixed. I will probably be able to find that out from your scenario, just have to dig deep into it. I'll get back to you on that.

The RTCC MFD issue could very well be a bug. I have some ideas what it could be, I'll investigate.

The LGC issue sounds like it was caused by power outage. I will look at the scenario though.

EDIT: Didn't have to dig all that deep. The IGM failure flag is set in your scenario. Probably due to the problem during launch. With the flag set the LVDC never uses the IGM again. I'm not entirely sure if the IGM was actually disabled forever if a problem happens during launch. But our LVDC right now will not steer the Saturn anymore after a failure has been detected. All you can do in that case is using the manual TLI procedure. I will intevstigate whether the IGM failure (and as a result the Guidance Failure light) is really permanent or if it can be used for an automatic TLI after the Saturn was manually flown into orbit.
« Last Edit: July 12, 2017, 10:30:08 AM by indy91 » Logged
abr35
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« Reply #12 on: July 12, 2017, 12:15:14 PM »

EDIT: Didn't have to dig all that deep. The IGM failure flag is set in your scenario. Probably due to the problem during launch. With the flag set the LVDC never uses the IGM again. I'm not entirely sure if the IGM was actually disabled forever if a problem happens during launch. But our LVDC right now will not steer the Saturn anymore after a failure has been detected. All you can do in that case is using the manual TLI procedure. I will intevstigate whether the IGM failure (and as a result the Guidance Failure light) is really permanent or if it can be used for an automatic TLI after the Saturn was manually flown into orbit.

To answer the burn resulted a workable but poor trajectory. I assumed it was mainly because I had to shut down manually. The IU did point the S-IVB slightly out of plane (about a 5 degree left yaw) and there was a lot of thruster activity when the uplink was complete, but I guess no IGM. I'll see if I can dig through some pdfs as well.
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indy91
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« Reply #13 on: July 12, 2017, 12:29:19 PM »

To answer the burn resulted a workable but poor trajectory. I assumed it was mainly because I had to shut down manually. The IU did point the S-IVB slightly out of plane (about a 5 degree left yaw) and there was a lot of thruster activity when the uplink was complete, but I guess no IGM. I'll see if I can dig through some pdfs as well.

Ah yes, how that works is the uplink updates a rotation matrix defining the desired plane of insertion. When the LVDC calculates the TLI with the onboard parameters, then this isn't done until ignition. With the uplink it gets updated immediately, so the S-IVB will track the desired orbital plane after TLI from then on, while still waiting for TLI to start. Usually it's very close to the actual orbital plane of the stack at the time, so it won't move more than a few degrees out-of-plane. During the TLI burn the S-IVB performed the burn in the correct plane for you, which will have helped a lot. But it didn't do any other kind of steering to get on the right trajectory.
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