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Project Apollo - NASSP
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LVDC++ Software
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Topic: LVDC++ Software (Read 13904 times)
eddievhfan1984
Full Member
Posts: 231
Re: LVDC++ Software
«
Reply #75 on:
October 26, 2012, 04:24:23 PM »
1.It just started a right roll for a few minutes, even with constantly hitting KILLROT in the Scenario Editor. I'd managed to get it docked with a slight bit of cheating (scenario editor KILLROT and roll commands), at which point, I found myself in a 1.5 deg/s roll while docked. I went through the rest of the separation procedure, evaded the booster, and that was that.
I'm starting another mission as soon as I can double-check which TLI parameters are the right ones (see my other post in News & Discussion).
2. Yes, I should've said SM RCS PRIM/SEC PROP B. LOL 'S all right?
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Tschachim
Project Apollo - NASSP
Administrator
Hero Member
Posts: 3687
nassp.sf.net
Re: LVDC++ Software
«
Reply #76 on:
October 26, 2012, 04:49:16 PM »
Quote from: eddievhfan1984 on October 26, 2012, 04:24:23 PM
1.It just started a right roll for a few minutes, even with constantly hitting KILLROT in the Scenario Editor. I'd managed to get it docked with a slight bit of cheating (scenario editor KILLROT and roll commands), at which point, I found myself in a 1.5 deg/s roll while docked.
That sounds strange, do you have scenarios saved before and/or after separation?
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eddievhfan1984
Full Member
Posts: 231
Re: LVDC++ Software
«
Reply #77 on:
October 26, 2012, 05:06:04 PM »
Sadly, no. But do you think you could answer my TLI question?
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meik84
Project Team Member
Sr. Member
Posts: 299
Re: LVDC++ Software
«
Reply #78 on:
October 27, 2012, 07:16:48 PM »
Short 'comp acty light on' from me:
- the short SIVB burn problem is solved; when there's no LM (LTA does not count as LM), the values for LM ascent & descent fuelin the .scn are ignored. Solution: add LTA's mass to SIVB dry-mass.
- orbital guidance works now. According to CMC's V83, LVDC++ points me on the local horizon +/- 1 deg, which is in the FCC attitude deadbands. Orbital nav is yet nothing else as powered flight nav minus accelerometer processing. I don't know in how far we are simulating any venting and/or atmospheric influences; if not, this should be enough.
- the annoying eccentric orbit problem remains. I've biased out our 'tail-off thrust' now, so that I hit terminal velocity +/- 0.5 m/s. Has improved the situation a little (difference between H-Apo and H-Per now around 15 nm), but that's still not good. Gonna play around with ROV a little (this is IGMs gravity compensation), maybe this helps. If not, the only remaining thing would be my navigation -which I can't validate, as I have no idea how to convert orbiters SV to earth-centered plumb-line.
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eddievhfan1984
Full Member
Posts: 231
Re: LVDC++ Software
«
Reply #79 on:
October 27, 2012, 08:07:55 PM »
Quote from: meik84 on October 27, 2012, 07:16:48 PM
Short 'comp acty light on' from me:
- the short SIVB burn problem is solved; when there's no LM (LTA does not count as LM), the values for LM ascent & descent fuelin the .scn are ignored. Solution: add LTA's mass to SIVB dry-mass.
- orbital guidance works now. According to CMC's V83, LVDC++ points me on the local horizon +/- 1 deg, which is in the FCC attitude deadbands. Orbital nav is yet nothing else as powered flight nav minus accelerometer processing. I don't know in how far we are simulating any venting and/or atmospheric influences; if not, this should be enough.
- the annoying eccentric orbit problem remains. I've biased out our 'tail-off thrust' now, so that I hit terminal velocity +/- 0.5 m/s. Has improved the situation a little (difference between H-Apo and H-Per now around 15 nm), but that's still not good. Gonna play around with ROV a little (this is IGMs gravity compensation), maybe this helps. If not, the only remaining thing would be my navigation -which I can't validate, as I have no idea how to convert orbiters SV to earth-centered plumb-line.
I thought that Orbiter's "ref. equator (fixed)" readout mode in the Scenario editor did do that (that is, Z-axis aimed at the center of the major sphere of influence).
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meik84
Project Team Member
Sr. Member
Posts: 299
Re: LVDC++ Software
«
Reply #80 on:
October 28, 2012, 08:20:10 AM »
No. The axes are completely different and orbiter uses a left-handed system, whereas PACSS 13 is a right-handed system.
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dseagrav
Project Admin
Hero Member
Posts: 831
Re: LVDC++ Software
«
Reply #81 on:
October 28, 2012, 12:30:19 PM »
Can you request the Orbiter coordinates of the Earth and then compare them to your own? The difference would be your earth-centric position.
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meik84
Project Team Member
Sr. Member
Posts: 299
Re: LVDC++ Software
«
Reply #82 on:
October 28, 2012, 02:39:08 PM »
When I read the docs right, the subject is not that simple. I guess we've got to rotate orbiters values around 3 axes to get the desired data. Looks like another excel sheet...
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eddievhfan1984
Full Member
Posts: 231
Re: LVDC++ Software
«
Reply #83 on:
October 28, 2012, 02:52:17 PM »
Well, what IS the difference between Orbiter's fixed equatorial system and the LVDC's coordinate system?
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meik84
Project Team Member
Sr. Member
Posts: 299
Re: LVDC++ Software
«
Reply #84 on:
October 28, 2012, 03:29:37 PM »
Ok, here we go...
Orbiter: left-handed, origin: earth's center, y through north pole, x and z in the equatorial plane (which means they point at what? Scenario Editor doc remains silent on that)
PACSS 13: right-handed, origin: earth's center, x parallel to the reference ellipsoid normal through the launch site, positive upward, z is parallel to the aiming azimuth, positive downrange, y completes the system
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eddievhfan1984
Full Member
Posts: 231
Re: LVDC++ Software
«
Reply #85 on:
October 28, 2012, 03:58:41 PM »
From reading the Scenario Editor doc, it looks like both the ecliptic and fixed equatorial systems use the vernal equinox in the J2000.0 epoch as +X. When I plugged a positive value (Orbiter Earth radius, 6371012.6 m) into Rx and Ry, it gave my position as (time dependent longitude) W by 45.00 N, radius 9.010Mm.
Assuming the following names:
X_o: Orbiter x-axis in fixed equatorial, +X towards the vernal equinox for J2000 epoch (1/1/2000 at 00:00 UTC)
Y_o: Same, but in the y-axis, +Y aimed towards the geographic north pole
Z_o: Same, but in the z-axis, +Z aimed so as to create a left-handed system
X_lv: LVDC x-axis, +X from the center of the Earth through the launch site
Y_lv: LVDC y-axis, +Y aimed so as to create a right-handed system
Z_lv: LVDC z-axis, +Z aimed so as to point in the direction of the launch azimuth
(Is it me, or does the LVDC coordinate system sound like the pad REFSMMAT?)
So, let's assume it's launch day for Apollo 7.
X_o should be rotated in the Y_o*Z_o plane so as to line up with the pad longitude. For the launch date and time, the vernal equinox is 114.25 deg E (again, in the J2000 system), as determined by the same method as above.
Y_o is then rotated in the new X_o*Z_o plane to center X_o on the pad in longitude and latitude, forming X_lv.
Z_o should then be rotated in the new X_o*Y_o plane to point parallel to the launch azimuth. This will simultaneously form the Y_lv and Z_lv axis.
Have I screwed anything up?
«
Last Edit: October 28, 2012, 06:04:29 PM by eddievhfan1984
»
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meik84
Project Team Member
Sr. Member
Posts: 299
Re: LVDC++ Software
«
Reply #86 on:
October 29, 2012, 06:51:15 AM »
Quote
From reading the Scenario Editor doc, it looks like both the ecliptic and fixed equatorial systems use the vernal equinox in the J2000.0 epoch as +X.
When you read it with that intention, you are right. But it's not clearly stated.
Quote
Is it me, or does the LVDC coordinate system sound like the pad REFSMMAT?
Pad REFSMMAT relates the IMU orientation at launch to PACSS4. IMU orientation at launch is x-axis parallel to launch azimuth, z-axis to center of earth, y-axis completes the right handed system. PACSS4 is x- axis to vernal equinox (in case of AGC the one from Jan, 0.525 (sic!) 1969), z-axis through the north pole, y- completes the right-hand system; origin: center of earth. So they may sound alike, but they aren't.
Quote
X_o should be rotated in the Y_o*Z_o plane so as to line up with the pad longitude. For the launch date and time, the vernal equinox is 114.25 deg E (again, in the J2000 system), as determined by the same method as above.
So we need a way to determine the angle between pad longitude and vernal eqinox of 1/1/2000...
Quote
Y_o is then rotated in the new X_o*Z_o plane to center X_o on the pad in longitude and latitude, forming X_lv.
Only latitude, IMHO? We've aligned x in longitude the step before.
Quote
Z_o should then be rotated in the new X_o*Y_o plane to point parallel to the launch azimuth. This will simultaneously form the Y_lv and Z_lv axis
This should be 90° - Azimuth, right? Plus we've got to invert the signs on the y-axis, otherwise we won't get a right-handed system.
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eddievhfan1984
Full Member
Posts: 231
Re: LVDC++ Software
«
Reply #87 on:
October 29, 2012, 07:49:44 AM »
I've basically been compiling a short text file of experimentally determined equinoxes. I just plugged in a +X and +Y in the equatorial state vector in the scenario editor (with enough length to clear the Earth radius), zero the velocity vector, and pause once Orbiter updated. My ground position was the vernal equinox and 45.00 degrees N each time. Since the Earth is still a moving frame, the equinox slowly shifts across the sky (though in ecliptic terms, it only precesses .014 deg/year), so it HAS to be locked to the launch time position, if you want to convert from Orbiter equatorial SV to LVDC. I'm sure I can work out the Euler angles from transferring each one if I stare at Excel long enough. LOL And the LVDC only needs to maintain accuracy for ~3 hours if we take into account TLI, right?
Here's the file with Apollo 7, 8, and 11 equinox positions recorded. Hope it helps.
Apollo vernal equinoxes.txt
(0.12 KB - downloaded 8 times.)
«
Last Edit: October 29, 2012, 07:51:34 AM by eddievhfan1984
»
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meik84
Project Team Member
Sr. Member
Posts: 299
Re: LVDC++ Software
«
Reply #88 on:
October 29, 2012, 08:39:45 AM »
When I understand right, this is nothing else as determining LMST i.e. GMST + gamma at GRR-Time in degrees. That's easy, I guess.
Edit: Oh, we're stupid. It's easily available -as longitude of ref. equator (fixed) in polar coordinates!
«
Last Edit: October 29, 2012, 10:24:59 AM by meik84
»
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eddievhfan1984
Full Member
Posts: 231
Re: LVDC++ Software
«
Reply #89 on:
October 29, 2012, 10:33:21 AM »
I see your wall bang and raise you a few fistfuls of hair.
I'm annoyed it didn't come to me to use polar/spherical as well. When did you say the epoch of equinox was the PASSC4 system (i.e., translate the mistyped instruction for me)?
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