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Old 03-22-2019, 09:08 AM   #211
indy91
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Originally Posted by GLS View Post
 Plus, the pad being in the wrong place, means that even launching at the correct time will very likely not work perfectly.
Quickly ran the numbers for geodetic vs. geocentric latitude for LC-39, targeting 51.6 inclination. I might have simplified the calculation too much, but I am getting a LAN difference of 0.183. So definitely significant. You would need to adjust the launch time by 44 seconds to account for that.

NASSP of course has to account for that as well and we are using the historically flown targeting polynomials (inclination/LAN as a function of launch azimuth) for the lunar missions with the Saturn V. And the Iterative Guidance Mode really just does a little bit of yaw steering to correct for the wrong launchpad latitude, not very noticable, so it's not really a performance issue.

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Plus, for 28.45 orbits, launching on time almost certainly will not work perfectly as the pad isn't where it should, so the vehicle does a dog leg to correct the inclination, and doing this with no control over LAN... how knows exactly what happens.
I've tried launching STS-82 to the HST. The first attempt was at the actual launch time, which happened at the beginning of the launch window, so without yaw steering that gave me a very high relative inclination, 4 or so. Then I tried the middle of the launch window which resulted in about 1. Then I got tired of it, because I always had to adjust the HST state vector in the launch scenario etc. So I simply moved the Shuttle in the right orbit. I'm sure by manually tweaking the launch time I could have gotten better results, at least good enough so that the HST can be reached without cheating. Just as an example.

And I'll look into the time definition issue, maybe I can figure out how much of a problem it really is. It's also relevant for NASSP of course, but it hasn't caused any noticable problems so far.
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Old 03-22-2019, 09:18 AM   #212
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Orbiter uses MJD, which means, leap seconds are not correctly included.

There is the \Delta T the difference between UT and TT, those could explain it (UT is based on earths real rotation, TT assumes a fixed day length like Orbiter). Its about 60 seconds between the origin of the Julian Date (1858) and today.
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Old 03-22-2019, 09:27 AM   #213
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Originally Posted by indy91 View Post
 Quickly ran the numbers for geodetic vs. geocentric latitude for LC-39, targeting 51.6 inclination. I might have simplified the calculation too much, but I am getting a LAN difference of 0.183. So definitely significant. You would need to adjust the launch time by 44 seconds to account for that.
If we had an ascent guidance that targeted an actual plane, that would be almost a no-issue, as 44s is pennies in a 10m launch window. I think working ascent is what I'll do first for SSU 6.0... just hope it doesn't take 1 year like the landing did...


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Originally Posted by indy91 View Post
 I've tried launching STS-82 to the HST. The first attempt was at the actual launch time, which happened at the beginning of the launch window, so without yaw steering that gave me a very high relative inclination, 4 or so. Then I tried the middle of the launch window which resulted in about 1. Then I got tired of it, because I always had to adjust the HST state vector in the launch scenario etc. So I simply moved the Shuttle in the right orbit. I'm sure by manually tweaking the launch time I could have gotten better results, at least good enough so that the HST can be reached without cheating. Just as an example.
The way SSU gets to a 28.45 orbit is by heading off to 90 and waiting for vehicle the latitude to drop below the target orbit inclination, then the "normal" orbital inclination logic kicks in... pretty much an open-loop solution, which is not very flexible or good in terms of performance, but it allows the vehicle to get to the correct orbit of a target.


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Originally Posted by indy91 View Post
 And I'll look into the time definition issue, maybe I can figure out how much of a problem it really is. It's also relevant for NASSP of course, but it hasn't caused any noticable problems so far.
I'm sure the difference is just a few seconds, but at 7km/s, the distances accumulate fast. Plus there could be 3 time systems at play here: Orbiter, TLEs and "wrist clock".
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Old 03-22-2019, 10:07 AM   #214
indy91
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 If we had an ascent guidance that targeted an actual plane, that would be almost a no-issue, as 44s is pennies in a 10m launch window. I think working ascent is what I'll do first for SSU 6.0... just hope it doesn't take 1 year like the landing did...
Many years ago I played around with the Shuttle ascent targeting, I even got the logic working for using throttling to intercept a target in orbit (for Design Reference Mission 3B). That code is not really in a very usable state, but if you are lacking documentation, I can at least point you to the useful documents.

And once SSU has that stuff implemented I can implement some launch window targeting in the FDO MFD.
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Old 03-22-2019, 10:27 AM   #215
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Originally Posted by indy91 View Post
 Many years ago I played around with the Shuttle ascent targeting, I even got the logic working for using throttling to intercept a target in orbit (for Design Reference Mission 3B). That code is not really in a very usable state, but if you are lacking documentation, I can at least point you to the useful documents.
DRM-3B, a.k.a., total insanity.
I've some pdfs with some general info on ascent guidance, and the coordinate systems, but I haven't sat down with them to know if they are enough or not. But, even if just the general scheme of things is presented, we can at least start to walk in the right direction.

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 And once SSU has that stuff implemented I can implement some launch window targeting in the FDO MFD.
Yeah, that way you could estimate a nominal SV for post MECO (that SSU would have to meet), and the flight plan could be calculated on the ground pre-launch.
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Old 03-22-2019, 10:32 AM   #216
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About adjusting for the errors and glitches above: with Non Spherical disabled it is just a matter of changing the launch time. Problem is how do we do that when Non Spherical is enabled. I wonder if there is a way to predict Nodal Regression at a future time (hence the magnitude of my desired RInc at MECO) if my chasing orbit keeps changing from launch to Rendezvous time.
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Old 03-22-2019, 10:48 AM   #217
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 Yeah, that way you could estimate a nominal SV for post MECO (that SSU would have to meet), and the flight plan could be calculated on the ground pre-launch.
Yes. By the way, is the MPS dump after ET sep propulsive in SSU? That would be important to know for that. The rendezvous launch window targeting that was used generated a post-MPS dump state vector to then be used in all the orbital calculations.

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Originally Posted by Wolf View Post
 About adjusting for the errors and glitches above: with Non Spherical disabled it is just a matter of changing the launch time. Problem is how do we do that when Non Spherical is enabled. I wonder if there is a way to predict Nodal Regression at a future time (hence the magnitude of my desired RInc at MECO) if my chasing orbit keeps changing from launch to Rendezvous time.
It's difficult to do that in any manual way, because that difference in LAN that you want after the ascent is not constant, but will depend on the phase angle as well. Large phase angle needs a low orbit for the Shuttle to catch up and the lower orbit has a stronger effect on the differential nodal regression. Small phase angle will need a higher Shuttle orbit to avoid catching up too quickly and then the difference in nodal regression isn't very large. So far these numbers have (roughly) varied from 0.1 to 0.5 for me.

And inclination is also not constant, but varies over one orbit (with a mean inclination that always stays constant). So, I think, the insertion inclination you would want is the osculating inclination that the target has at the same point in orbit. Luckily that effect is smaller than than the LAN difference, but if you are simply using the mean inclination of the target then that might still cause NPC maneuvers that are 0-20 ft/s larger than they should be.

All of this can only really be solved by some prelaunch targeting that would give you the right launch time and inclination (and later the LAN).

Last edited by indy91; 03-22-2019 at 10:51 AM.
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Old 03-22-2019, 10:59 AM   #218
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Originally Posted by indy91 View Post
 Yes. By the way, is the MPS dump after ET sep propulsive in SSU? That would be important to know for that. The rendezvous launch window targeting that was used generated a post-MPS dump state vector to then be used in all the orbital calculations.
Yes it is. From memory I think it should be +/- 11fps... but then they started using it to pitch the vehicle up to put the ET in the overhead windows, so if that is made the dV is not all in one direction...
Plus, some missions had the +X RCS burn, so there's another variable to add.
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Old 03-22-2019, 03:44 PM   #219
Tim13
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And once SSU has that stuff implemented I can implement some launch window targeting in the FDO MFD.
That statement almost made me wet myself.

Your MFD has made the SSU go from an A+ to an A+++

It's greatly appreciated!

Tim
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Old 03-22-2019, 04:52 PM   #220
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 That statement almost made me wet myself.

Your MFD has made the SSU go from an A+ to an A+++

It's greatly appreciated!

Tim
Not yet.... but let's hope a rewrite of the ascent guidance/FCS doesn't take as much as the entry did...
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