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Author Topic: LVDC++ Software  (Read 12014 times)
catnap
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« Reply #15 on: September 25, 2011, 04:40:41 AM »

http://www.ibiblio.org/apollo/Documents/SaturnVLaunchVehicleGuidanceEquations.pdf
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eddievhfan1984
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« Reply #16 on: September 25, 2011, 05:43:32 AM »

So, as I read it, the coordinate plane of the LVDC is:

  • X-axis aligned with the gravity vector, from Earth to the launch pad for +X (AKA, where the rocket's pointing before launch)
  • Y-axis aligned with the launch azimuth
  • Z-axis is rotated 90° left from launch azimuth, in coincidence with the right-hand rule

This is continually updated by the ground until 17 seconds before launch, when guidance goes internal. IGM starts at exactly T+0:03:20.

Apologies if I'm sounding pedantic, but just trying to understand. Since a previous post mentioned we may be using the wrong coordinate system, I just want to ask:

Do we use an Orbiter-based state vector system already, or something else?
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meik84
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« Reply #17 on: September 25, 2011, 06:26:01 AM »

Quote
Do we use an Orbiter-based state vector system already, or something else?
I don't know which system we use, but I think it's good. The Pre-IGM part of the guidance works perfectly, it wouldn't do that if the platform would be oriented wrong. The problem is the step from reading the accelerometer outputs to the updated state vector. The well known Boeing doc does only contain the guidance ("we are here, we want to go there, so we must fly this way") equations, but not the navigation ("where are we?") equations, those were published in a separate doc. The docs I mentioned in my previous post give a vague understanding on the LVDC's navigation, but lack any detailed information, especially on the orbital navigation when the LVDC stops reading the accelerometers and predicts athmospherical drag and venting effects on its own.
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meik84
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« Reply #18 on: January 16, 2012, 05:43:11 PM »

I have had some time the last days and smoked over the LVDC++ again. Something I could verify is, that our "non official" navigation procedure is good. I compared the logged SVs from the sim with the original ones from NASA and got only minor deviations (+/- 100 ft in position and around +/- 10ft/s in velocity) during the SIC-phase. I've recalculated some variables for the target orbit and changed some to the values specified in the Boeing doc. With those, IGM doesn't crash anymore (which is a good sign, I guess). Although, it's not working correctly yet: it "wobbles", i.e. one IGM cycle commands a "pitch up", the next cycle commands a "pitch down" and so on, finally leading to a pitch angle of 90°.  I guess that our simulated SII does not behave in the way the IGM expects it to do. One of the factors in IGM are the exhaust velocities of the engines, and something wrong in that sector could lead to this strange behaviour.
Nonetheless: now I strongly believe that the equations are correct and that it is possible to bring the LVDC++ to work. I'll go ahead next month or so. There's an skipper's exam waiting for me...
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Tschachim
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« Reply #19 on: January 17, 2012, 01:25:28 PM »

Good news, I hope you'll get it working!  Thumbs Up
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meik84
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« Reply #20 on: January 17, 2012, 08:42:10 PM »

Do you know if there's a function which returns the present acceleration? I could need that, this 'force-divided-by-mass-stuff' in the actual LVDC++-code doesn't work and causes a whole bunch of calamities. Yuck! (And is historically incorrect: the LVDC plainly used it's accelerometers for that -I'd like to do that, too...)
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Tschachim
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« Reply #21 on: January 24, 2012, 05:25:32 PM »

The "acceleration calculation", i.e. the LVIMU should be fine code-wise, but the LVDC timestep call "lvdc_timestep" needs to be moved to clbkPostStep (as the IMU time step), otherwise the acceleration calculations aren't stable. I can do that if you like, do you have a scenario (and the other changes if any) by chance, so I can test the effect? 
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dseagrav
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« Reply #22 on: January 25, 2012, 09:04:26 AM »

Do you know if there's a function which returns the present acceleration? I could need that, this 'force-divided-by-mass-stuff' in the actual LVDC++-code doesn't work and causes a whole bunch of calamities. Yuck! (And is historically incorrect: the LVDC plainly used it's accelerometers for that -I'd like to do that, too...)

By "accelerometers" do you mean the IMU? The only external accelerometers in the IU were in the Saturn 1B ONLY, the Saturn V did not have them.
See http://dl.dropbox.com/u/12368207/Screenshots/14.jpeg for reference. This accelerometer was used for attitude control only.

http://dl.dropbox.com/u/12368207/Screenshots/15.jpeg shows the flow by which navigation works. It clearly says "computation".

The IMU will give you your acceleration, but it does so in the IMU's frame of reference. I thought the whole F/M mess was trying to get forward acceleration.
« Last Edit: January 25, 2012, 09:06:13 AM by dseagrav » Logged
meik84
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« Reply #23 on: January 25, 2012, 01:37:22 PM »

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By "accelerometers" do you mean the IMU?
Jepp.
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It clearly says "computation".
...by doing something with the readouts from the platform, at least during normal flight. During staging and after orbit insertion the LVDC used equations for that and did not read the accelerometers.
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I thought the whole F/M mess was trying to get forward acceleration.
F/m seems to be the total accel. Forward accel should be obtained by rotating the IMU's readouts to this earth-centered plumbline system. Interestingly, the guidance equations don't need that, though.
LVDC's navigation is really a problem: there's no detailed documentation available. There seems to be a Boeing doc on that matter, but I can't find it on the internet. Any "LVDC equation defining document" (there seemed to be one for every launch) would be great, too.
I'm translating the whole guidance equation stuff to excel at the moment, so that I can feed them with 'real' data. There are dozens of docs on the net which contain LVDC state vector data, obviously everybody investigated if and how good this LVDC stuff was, but how it worked seemed to be common knowledge for everybody.
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meik84
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« Reply #24 on: March 15, 2012, 07:21:50 PM »

One word before I go to sleep: I think I got that ba... -well, you know. Wink
It works from IGM start to SIVB ignition -which isn't implemented yet. Sad Obviously dseagrav didn't expect to see it run...
I hope I can do this SIVB thing on my own. If not, I'll come back. Shouldn't be more complicated then rotating coordinate systems, trigonomic functions and integrals... Bangs Head

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meik84
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« Reply #25 on: March 20, 2012, 10:53:49 AM »

Some questions for the booster guys: how do I:

- fire SIVB's ullage thrusters
- jettison them
- start SIVB's main engine
- shut it down again
- fire the APS after orbit insertion?

The plain C++ instructions should be enough, I'll do the rest. Wink
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dseagrav
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« Reply #26 on: March 20, 2012, 11:08:58 AM »

I never got that far, so I didn't research it. Did you actually get IGM to work then?
How badly did I screw it up?
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meik84
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« Reply #27 on: March 20, 2012, 11:24:25 AM »

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Did you actually get IGM to work then?
Yes. Happy -till SIVB ignition, that's why I'm asking.
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How badly did I screw it up?
Oh, you did nothing wrong. NASA (or the one that wrote the Boeing doc) did. They obviously didn't know in which direction they got to rotate the range angle. Plus they sometimes declare angles in degrees, but they are clearly radiant. And their arctan-function was somewhat different then our C++ arctan-function.
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Tschachim
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« Reply #28 on: March 21, 2012, 06:35:20 PM »

Hi meik84,

do you still have problems with the acceleration calculation stability? Shall I do this change mentioned a couple of posts before:

The "acceleration calculation", i.e. the LVIMU should be fine code-wise, but the LVDC timestep call "lvdc_timestep" needs to be moved to clbkPostStep (as the IMU time step), otherwise the acceleration calculations aren't stable. I can do that if you like, do you have a scenario (and the other changes if any) by chance, so I can test the effect? 

For the S-IVB booster control functions I need to do some code mining, I'm a little bit out of the loop...

Cheers
Tschachim
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meik84
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« Reply #29 on: March 22, 2012, 05:36:07 AM »

Quote
do you still have problems with the acceleration calculation stability?
Well, I do have some minor problems towards the end of the second stage of guidance (i.e. from MRS to SII cutoff). I first thought that there's something wrong with my tau calculations and my parameters, but your tip could be the explanation for that. So:
Quote
Shall I do this change mentioned a couple of posts before:
Yes! Wink What do I have to change in the code?
edit: I can confirm that acceleration inconsistency: the LV IMU senses about 0.2 m/sec^2 less then the actual force/mass.
« Last Edit: March 24, 2012, 09:46:52 AM by meik84 » Logged
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