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Author Topic: TVC DAP Issues  (Read 7362 times)
kneecaps
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« on: January 14, 2009, 03:30:24 PM »

Hi all,
   Can we combine any TVC DAP issues relating to SPS burns in here.

In addition to the instant gimballing of the SPS engine I'm convinced another issue is causing problems. I'm convinced that the lack of sloshing propellant is part of the problem. The DAP has code to account for the sloshing of the SPS prop during burns. We don't have this slosh!

I haven't investigated the exact workings of the mechanism yet to put some numbers on how it could be causing the attitude excursions seen during attempted SPS burns. However as soon as I have some time I will!!

This popped into my head the other day, once I searched docs I started seeing lots of refs to dealing with prop slosh.
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"Okay. As soon as we find the Earth, we will do it."
- Frank Borman, Apollo 8

Current Work: ?? What next??

Future Work:
I know the AGC pretty well so anything need doing there?

On Hold/Completed:
SPS TVC in P40 issues.
P11 FDAI Error Needles (98%) complete. Comitted. Working A7 scenario.
P06 AGC Standby. Concluded. It's done by the AGC PSU.
Got us a complete AOH Volume II
kneecaps
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« Reply #1 on: January 14, 2009, 03:46:03 PM »

I've found a few documents (circ 1966) saying that the actuators may have operated at rates as low as 0.51 rad/sec (about 8.6 degrees/sec).

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"Okay. As soon as we find the Earth, we will do it."
- Frank Borman, Apollo 8

Current Work: ?? What next??

Future Work:
I know the AGC pretty well so anything need doing there?

On Hold/Completed:
SPS TVC in P40 issues.
P11 FDAI Error Needles (98%) complete. Comitted. Working A7 scenario.
P06 AGC Standby. Concluded. It's done by the AGC PSU.
Got us a complete AOH Volume II
Tschachim
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« Reply #2 on: January 14, 2009, 03:53:44 PM »

Just a copy of irnenginer's comments from here: http://www.ibiblio.org/mscorbit/mscforum/index.php?topic=1825.0

1. TVC DAP instability : The TVC DAP is unstable, meaning that the Spacecraft can and will spin off wildly during AGC controlled SPS burns of of durations above a few seconds. This has been tracked to an issue between the interaction of the AGC and how the TVC is currently implemented. Currently agc control commands are translated immediately into thrust vector changes. There needs to be a control loop implemented so that TVC movement is damped. We cannot fly a mission without this

Cheers
Tschachim
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kneecaps
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« Reply #3 on: January 14, 2009, 04:08:01 PM »

Just a copy of irnenginer's comments from here: http://www.ibiblio.org/mscorbit/mscforum/index.php?topic=1825.0

1. TVC DAP instability : The TVC DAP is unstable, meaning that the Spacecraft can and will spin off wildly during AGC controlled SPS burns of of durations above a few seconds. This has been tracked to an issue between the interaction of the AGC and how the TVC is currently implemented. Currently agc control commands are translated immediately into thrust vector changes. There needs to be a control loop implemented so that TVC movement is damped. We cannot fly a mission without this

Cheers
Tschachim

Thanks, I'm convinced that the 'instant' rate is causing the majority of the issues. Instant thrust vector changes are not good for a system expecting a rate of 0.15 rad/sec (relatively slow).

I need to determine just how much of a problem the lack of sloshing prop may be!

Logged

"Okay. As soon as we find the Earth, we will do it."
- Frank Borman, Apollo 8

Current Work: ?? What next??

Future Work:
I know the AGC pretty well so anything need doing there?

On Hold/Completed:
SPS TVC in P40 issues.
P11 FDAI Error Needles (98%) complete. Comitted. Working A7 scenario.
P06 AGC Standby. Concluded. It's done by the AGC PSU.
Got us a complete AOH Volume II
irnenginer
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« Reply #4 on: January 14, 2009, 04:47:17 PM »

Some time before Christmas I did some more work on this, in which I did some tests but not enough to say for sure what I think I had discovered. To help understand the operation of the TVC control loops and where our stability problems where coming from I started to put together a a SCILAB model of the TVC and CSM system with a mash of computed and empirical data. This model is based on our CSM characteristics (COG location and lack of sloshing components).

After some playing around in the model I gained a much better knowledge of the TVC parammeters and how they effect spacecraft dynamics. With the model I developed set of paramenters that appear to have good TVC control during burns. I managed about 1 test of these improved parameters before the Christams rush hit and I was too busy to look at it again.

Things are still busy but I will try to get something together so you guys can try these parameters to see if we are on the right track.
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kneecaps
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« Reply #5 on: January 14, 2009, 05:33:55 PM »

Some time before Christmas I did some more work on this, in which I did some tests but not enough to say for sure what I think I had discovered. To help understand the operation of the TVC control loops and where our stability problems where coming from I started to put together a a SCILAB model of the TVC and CSM system with a mash of computed and empirical data. This model is based on our CSM characteristics (COG location and lack of sloshing components).

After some playing around in the model I gained a much better knowledge of the TVC parammeters and how they effect spacecraft dynamics. With the model I developed set of paramenters that appear to have good TVC control during burns. I managed about 1 test of these improved parameters before the Christams rush hit and I was too busy to look at it again.

Things are still busy but I will try to get something together so you guys can try these parameters to see if we are on the right track.

Wonderful stuff! Would it be possible to have your SCILAB model experiment with?

Pete
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"Okay. As soon as we find the Earth, we will do it."
- Frank Borman, Apollo 8

Current Work: ?? What next??

Future Work:
I know the AGC pretty well so anything need doing there?

On Hold/Completed:
SPS TVC in P40 issues.
P11 FDAI Error Needles (98%) complete. Comitted. Working A7 scenario.
P06 AGC Standby. Concluded. It's done by the AGC PSU.
Got us a complete AOH Volume II
irnenginer
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« Reply #6 on: January 15, 2009, 01:31:24 AM »

Here you go. There are 2 files in the zip.

TVCnoGimbal.cos :Scicos file for a simplified version of the model that is basically what the current TVC system is set up as. The two graphs show thrust angle and SC angle wrt 0 in radians. It shows only one axis because the two axis are independent and as long as there is no significant roll will not interact with each other. The upper left portion is the model of the AGC function and the lower right is the model of the spacecraft. the link between the two is the equivalent of the IMU gimbals. The simulation starts with the SC being .01 radians out of true.

TVC.cos: same as above but includes a model that is somewhat simplified of the model used in TVC simulations referenced in R-577. It mainly models the gimbal actuators and motors and the engine bell dynamics.

I am sure you will have questions so let me know.

* TVC.zip (22.95 KB - downloaded 157 times.)
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kneecaps
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« Reply #7 on: January 17, 2009, 12:03:57 PM »

Here you go. There are 2 files in the zip.

TVCnoGimbal.cos :Scicos file for a simplified version of the model that is basically what the current TVC system is set up as. The two graphs show thrust angle and SC angle wrt 0 in radians. It shows only one axis because the two axis are independent and as long as there is no significant roll will not interact with each other. The upper left portion is the model of the AGC function and the lower right is the model of the spacecraft. the link between the two is the equivalent of the IMU gimbals. The simulation starts with the SC being .01 radians out of true.

TVC.cos: same as above but includes a model that is somewhat simplified of the model used in TVC simulations referenced in R-577. It mainly models the gimbal actuators and motors and the engine bell dynamics.

I am sure you will have questions so let me know.

Thanks! I'm going to experiment with this!!
Logged

"Okay. As soon as we find the Earth, we will do it."
- Frank Borman, Apollo 8

Current Work: ?? What next??

Future Work:
I know the AGC pretty well so anything need doing there?

On Hold/Completed:
SPS TVC in P40 issues.
P11 FDAI Error Needles (98%) complete. Comitted. Working A7 scenario.
P06 AGC Standby. Concluded. It's done by the AGC PSU.
Got us a complete AOH Volume II
bluespace88
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« Reply #8 on: January 27, 2009, 04:24:39 PM »

I was working on it before the christmas rush, but I really have no idea what equation to use to simulate the gimbaling. 
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jalexb88
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« Reply #9 on: January 28, 2009, 06:27:42 PM »

I am doing the Apollo 7 mission and the SPS is working fine under CMC control, is this problem with the thrust vector only affecting the Apollo 8 vAGC?   If it works on Apollo 7, shouldnt it be simple to fix Apollo 8?

Alex
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bluespace88
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« Reply #10 on: January 28, 2009, 07:33:04 PM »

were your sps gimbal motors on?  Thats the main problem that we're trying to fix with the TVC.  Right now, the engine moves instantaneously to the commanded position with no damping, causing the csm to go out of control.
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jalexb88
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« Reply #11 on: January 28, 2009, 07:37:20 PM »

Yes, the SPS gimbal motors are all on, I followed the checklist in the Apollo 7 Flight Plan, ran the P40 SPS thrust program and the Burn is controlled by the AGC and does not go out of control at all. It gets the predicted orbit to within .5 nm. This is the Apollo 7 vAGC scenario.

Alex
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jc121081
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« Reply #12 on: January 29, 2009, 10:45:39 AM »

Ooops! I thought I had grayed out the Gimbal Motors/TVC from the Ap7 FP, but just took a look and remembered that I only appended that to the Ap8 FP. The changes have been committed.

Sorry about that!
JC
« Last Edit: January 29, 2009, 11:01:50 AM by jc121081 » Logged



Current Work:
Overhaul of Ap8 vAGC FP, Wiki FP's

Future Work:
Ap14 Q FP, Ap15 Q FP, Ap16 Q FP, Ap17 Q FP
jalexb88
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« Reply #13 on: January 29, 2009, 01:37:00 PM »

Yes, but why would you gray it out if it works fine in apollo 7? Like I said at lest for me, the TVC has no crazy behavior in apollo 7, my burns are very accurate and the AGC keeps the ship very steady.  Havent tried on apollo 8 yet though.

Alex
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jc121081
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« Reply #14 on: January 29, 2009, 01:53:10 PM »

You're right. On testing in Ap7 you probably won't have any problems, as these burns are shorter. There still are a few problems with it from what I can understand, so as a precaution, until it is rectified, I grayed it out from the FP. You'll definitely have problems with it if you tried using it on Apollo 8, on a longer burn, say, LOI or TEI.
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Current Work:
Overhaul of Ap8 vAGC FP, Wiki FP's

Future Work:
Ap14 Q FP, Ap15 Q FP, Ap16 Q FP, Ap17 Q FP
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