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Author Topic: vAGC solar and TVC issues  (Read 5379 times)
irnenginer
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« on: February 04, 2008, 10:27:04 PM »

When jc121081 was developing the flight plans for Apollo7 and Apollo 8 he came across some issues in vAGC. I have been working on them rather quietly but since people are going through the missions I quess its time to let the cat out.

Solar Emphresis and an issue with vAGC lockup during middle phase of lunar transit:
It turns out that when the spacecraft is located in the region between Earth and lunar orbit there is a phase where the orbital integrator in the vAGC takes into account the sun's gravitational component. If the solar emphresis is not defined it creates a problem similar to what happened when the lunar emphrsis was not defined. I developed the solar emprsis for both Apollo7 and Apollo8 and jc121081 and I tested it and it looks good and solves the problem. I have yet to get that totally committed because of the second and much more sticky problem below.

TVC DAP gone wild Bangs Head
The TVC DAP goes wild, particularly at LOI, which is a long burn. The behavoir is much like it was when we first had the TVC DAP going. It appears to be an issue with the DAP and the characteristics of the spacecraft. I went back over all of the earsables for the DAP and the documentation and I am reasonably sure that the values I have are match historically. I believe that the source of the issue are one or a combination of the following:

Center of Mass:
The vAGC takes into account changes in the spacecraft center of mass dependent on SPS fuel load. As SPS fuel and other consumables decrease the COM shifts from behind the RCS plane to in front. This shift in relation to the TVC changes the torque applied to the spacecraft for a given TVC pitch angle. I considered that if the COM of the spacecraft is very different then what the AGC compensates for the spacecraft could become unstable. I still need to run some tests but after some research I am not sure that this is a significant issue. the COM only shifts a few feet over the entire course of the mission.

TVC gimbal dynamic:
This is the one I am betting more on. The TVC is currently has a instant response. There is no modeling of the TVC movement. If the TVC DAP loops take into account the response time of the gimbal motors it might try to "lead" the spacecraft it could very well cause an unstable output. Luckily for us there is data on the TVC movement in section 6 of R-577.


I would propose that implementing some modeling of a more realistic TVC movement would go a long way to fix this issue. I will also continue to work to see if the pad load can be tweaked to give a a good outcome, however the TVC DAP in CSM/LM mode is hard coded and changing that would require recompiling C249 with a special build, something I really would like to avoid.
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lassombra
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« Reply #1 on: February 05, 2008, 02:00:41 AM »

On the COM issue, if the Center of mass is moving from behind the RCS thrusters to in front, or viceversa, that could cause significant change, more than would be expected from a simple few foot change in the COM.  The importance of the shift is more an angle relative to the thrusters than an actual distance, and in this case, as it is moving along the plane, the shift could be significant enough to change the behavior of the thrusters through that change, forcing the designers to compensate with the actual CMC.

I also would tend to agree more with the gimbal time being the greater problem, but perhaps we should be modeling this shift anyways, as it clearly happens, it might be of some significance to the handling of the space craft.
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My current Project Apollo work:

Quickstart to the Moon initiative (Quickstart_to_the_Moon): Done through earth orbit.  Working on new method of calculating TLI.

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  • MFD Interface 99%(Minor cleanup and future features remain)
  • Panel Interface 99%
  • Excel interface and logic 99%
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  Approximately 80% done. (Checklist Controller
Christophe
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« Reply #2 on: February 05, 2008, 03:03:01 PM »

I'm thinking about something as far as the COM variation during a burn is concerned:
The SPS uses two different propellants, the fuel and the oxydizer, and each of them has a different density. Since the tanks are not placed one behind the other, both along the X axis,  but side by side, may be it's possible that the total propellant comsuption causes a displacement of the COM not only from aft to front but also slightly from "left to right".
I means that it could be a COM variation component along the Z and Y axis.
If I'm right, it may cause to get the TVC "crazy", no?
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irnenginer
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« Reply #3 on: February 06, 2008, 03:41:41 PM »

You are indeed correct that the COM moved in all three axis during the mission. The DAP itself did not take this into account though, it mearly adjusted iteself accordinly by changing the trim values.

Here is the doc I have used to research the dynamics of the mass properties:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730060772_1973060772.pdf

The more I have looked into it I beleive more the the major contributer is how the TVC is implemented as we have it versus a more realistic model. Looking at how the TVC clutch system worked it had a control loop onto itself. Basically the farther from the commanded angle the gimbal was to faster the bell would move. It is probable that the DAP would "slam" the gimbal around to quicken the gimbal response response (ie an underdamped reponse at the DAP to result in a crtically damped reposnse at the gimbal). Remove the slower response and the gimbal then begins to go unstable.

For some background of what I am talking about see
http://en.wikipedia.org/wiki/Damping

Beyond all the jargon I would work on the TVC gimbal before playing with the COM.
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lassombra
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« Reply #4 on: February 06, 2008, 05:03:52 PM »

Here's a random question.  In a burn, does the DAP use the RCS for anything but roll?  I was under the impression that it controlled pitch and yaw strictly through the gimbal.  If that's the case then I agree with the previous ideas that we need to slow down the TVC.
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My current Project Apollo work:

Quickstart to the Moon initiative (Quickstart_to_the_Moon): Done through earth orbit.  Working on new method of calculating TLI.

Checklist Controller: 
  • MFD Interface 99%(Minor cleanup and future features remain)
  • Panel Interface 99%
  • Excel interface and logic 99%
  • LEM Event code.0%
  • DSKY Interface code. Framework complete, creating "buttons"
  Approximately 80% done. (Checklist Controller
Tschachim
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« Reply #5 on: February 06, 2008, 05:23:44 PM »

In a burn, does the DAP use the RCS for anything but roll? I was under the impression that it controlled pitch and yaw strictly through the gimbal.

Pitch and yaw is controlled by the SPS TVC only, your impression is correct.

Cheers
Tschachim
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kneecaps
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« Reply #6 on: April 21, 2008, 11:24:42 AM »

Has any progress been made with this issue? I've got some free time again now and this looks like an issue that I could look at!

My first step would be to understand what's currently being done currently in the code for the TVC system and then look at implementing the control loop along the lines of whats found in the AOHs?

EDIT: Just checked out the projectApolloSource module and grabbed VC2008. It's been a long time since I used VC (i've been on the gnu toolchain for years), any starting point to building the modules. A project file maybe?
« Last Edit: April 21, 2008, 03:38:03 PM by kneecaps » 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 #7 on: April 21, 2008, 11:30:41 PM »

I have not accomplished much lately. I have been very busy with other stuff to work on it. Gloomy What work I have been able to  work on is attempting to analyze the TVC characteristics laid out in section 6 of R-577 to give a starting point to work on an implementation. When I have got something I will pass it on
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kneecaps
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« Reply #8 on: April 22, 2008, 06:10:46 AM »

I have not accomplished much lately. I have been very busy with other stuff to work on it. Gloomy What work I have been able to  work on is attempting to analyze the TVC characteristics laid out in section 6 of R-577 to give a starting point to work on an implementation. When I have got something I will pass it on

Understood, then prehaps I can look at implementing the mechanical side of the SPS TVC, with a view to getting realistic drive rates and feedback.

That way we are working on different sides of the same system? Also what info are you using from section 6 R-577, the SPS in the Digital Simulation section? I'm going to see what the AGC TVC dap is trying to do also.

(Got to track down whats currently implemented in the code first!)
« Last Edit: April 22, 2008, 06:41:41 AM by kneecaps » 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
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