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Author Topic: New issue concerning vAGC+ LTMFD  (Read 1912 times)
jalexb88
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« on: December 06, 2009, 02:55:00 PM »

Hello, I took a break using NASSP since about June, and now i'm getting back into it. I'm in the middle of the Apollo 8 vAGC mission using LTMFD. The 1st MCC went fine. On the second MCC burn after entering lunar SOI, the SC won't align to the proper burn attitude calculated by LTMFD. I've done the SV update in lunar SOI, P52 is done but it doesnt give the correct gimbal angles for the burn. This a little frustrating because I remember this working before. I just did a complete CVS checkout and built the latest modules. Back in June all my burns aligned perfectly with vAGC+LTMFD.

Here is a scenario:


* apollo8 test.scn (76.06 KB - downloaded 86 times.)
« Last Edit: December 06, 2009, 02:57:15 PM by jalexb88 » Logged
jarmonik
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« Reply #1 on: December 07, 2009, 10:42:57 AM »

There are many things changed since June like Engine gimball offsets, star tables, reference frame settings. (See the last 20 posts in Virtual AGC thread). But if the MCC1 goes well then none of these should be a reason for failure.  It is possible that LTMFD will compute the burn relative to the Earth and vAGC will assume that the data is the Moon relative or the other way around. There is also a burn data update feature added for NASSP in LTMFD. But anyway the reference frame error is my best guess at this time. I'll take a look.
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jalexb88
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« Reply #2 on: December 07, 2009, 11:28:18 AM »

Thanks for the quick reply!!  I was suspecting it may be something to do with the reference frame. Just to give you an update on my mission progress, my LOI went flawless with LTMFD+ vAGC. Its really only within the vicinity of the moon within lunar LOI that it has problems, In lunar orbit all is OK. Weird.

Alex
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jarmonik
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« Reply #3 on: December 07, 2009, 01:05:38 PM »

Yes, it looks like the change of reference frame is the problem. LTMFD and vAGC will change the reference frame origin at different points. I need to add a manual override for that in LTMFD and the current reference frame should be displayed in Delta-V display. But we could also improve the synchronization of LTMFD and vAGC. So, what's the distance to the Moon when the vAGC is changed from Earth centered space to the Moon centered space ?

The frame that is used in LTMFD is displayed in IFP-SDK_Err.html:

(Mon Dec 07 17:24:59 2009) EXE-Press
(Mon Dec 07 17:24:59 2009) ---Starting to run TLCC prog---
(Mon Dec 07 17:24:59 2009) _RIgn = [ -3.6681618e+008, 31785942, 267356.53 ]: 3.6819088e+008
(Mon Dec 07 17:24:59 2009) _VIgn = [ -839.70566, -61.307674, -39.411641 ]: 842.86269
(Mon Dec 07 17:24:59 2009) LTMFD: TLCC: Vessel state vectors are Earth relative
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jalexb88
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« Reply #4 on: December 07, 2009, 01:21:47 PM »

Nice find! About the distance from the moon that the reference frame switches, thats a good question. Maybe some of the vAGC experts can answer? In Apollo 8, the MCC#2 was done at around 22000 NM from the moon. So my guess is at least 25000 NM
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Tschachim
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« Reply #5 on: December 07, 2009, 01:25:32 PM »

So, what's the distance to the Moon when the vAGC is changed from Earth centered space to the Moon centered space ?

If I understand GSOP 5.2.2 correctly, it's the rSPH constant, which is 64 373 760 m for Colossus 249.

Cheers
Tschachim
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jarmonik
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« Reply #6 on: December 07, 2009, 01:36:12 PM »

If I understand GSOP 5.2.2 correctly, it's the rSPH constant, which is 64 373 760 m for Colossus 249.
Thank you  Happy
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jalexb88
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« Reply #7 on: December 07, 2009, 02:00:23 PM »

I just tested the MCC #2 at a closer position to the moon, about 7000 NM. The burn went perfect, it aligned perfectly to LTMFD. So I think this confirms your findings!

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jarmonik
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« Reply #8 on: December 07, 2009, 06:45:41 PM »

LTMFD was changing the reference at 88M therefore it should have failed between 64M - 88M. Can you read the statevector reference from the vAGC and verify the change between 63M and 66M. I have reconfigured the switch point from LTMFD and the manual override is added in action page (ACT). However, I haven't tested the override. I am not so familiar with vAGC.

What's the distance to the Moon when the MCC2 is executed by the checklist ?
« Last Edit: December 10, 2009, 08:04:23 AM by jarmonik » Logged
jalexb88
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« Reply #9 on: December 07, 2009, 10:02:56 PM »

Thumbs Up Thumbs Up

I just tried out the modules and so far so good! The burn aligns perfectly even at 40M which is aprx. the real distance from the moon that the MCC 2 burn was performed (By the checklist).  The test was in the override mode. Thanks!
« Last Edit: December 07, 2009, 10:07:10 PM by jalexb88 » Logged
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