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Space Shuttle - Development

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Re: Space Shuttle - Development

Postby GinGin » Fri Jan 25, 2019 7:50 pm

I tested the last push, thanks for the update on Inertial value and RR tracking in Spec33.It works very well

Image


I find a nice thing to calculate the time of rendez vous with a good precision.
I entered in item 2 the expected time of rendez vous I calculated based on catch up rate, item 28, and we have in Downrange the position of the Shuttle at that time.
With some iterations, it gives us a quite solid Time of Rendez vous

Image



Last screen to show your great work on Lambert targeting.
Numbers are almost spot on at designated time :)

Image
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Re: Space Shuttle - Development

Postby GinGin » Wed Jan 30, 2019 8:58 am

I made a quick sum up of my "most" succesfull rendez vous so far
https://forum.flightgear.org/viewtopic.php?f=87&t=35078&p=342077#p342077

Just a more detailled feed back after hours played with rendez vous.


Lambert targeting works really well, I am looking forward to export the ISS state vector to use LEO tools

Save/resume works well, ie. it replaced me where I left and ISS also, But it also reload the ISS position from the mission file and Spec 34 is taking data from this ISS and not the saved one ( and the rendez vous canvas takes information for the saved ISS)
But it is really useful once we merged with ISS and started the prox ops


Apogee and perigee are fluctuating quite a lot ( couple of miles in one orbit) , is the J3 and non spherical gravity effects are that strong ? It is impressive :shock:
Regarding that, is there a way to have ISS perigee and apogee to affinate our rendez vous targeting ?

Last point, ISS is usually showing well below 15 kft, sometimes it flickers a bit and stabilizes when we are closer

It is lot of fun to use lambert equations, and it works already well globaly, awesome job : )

I almost managed a full rendez vous from ground, entertaining xD
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Re: Space Shuttle - Development

Postby Thorsten » Wed Jan 30, 2019 11:27 am

Apogee and perigee are fluctuating quite a lot ( couple of miles in one orbit) , is the J3 and non spherical gravity effects are that strong ?


Compare black and red line... the red is what goes for a circular orbit in J3. Gives some idea of what is going on.

Image

Regarding that, is there a way to have ISS perigee and apogee to affinate our rendez vous targeting ?


Not any time soon.

But it also reload the ISS position from the mission file and Spec 34 is taking data from this ISS and not the saved one ( and the rendez vous canvas takes information for the saved ISS)


I think I explained that (and that I have a tentative solution which is not yet pushed).
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Re: Space Shuttle - Development

Postby eatdirt » Wed Jan 30, 2019 3:09 pm

Compare black and red line... the red is what goes for a circular orbit in J3. Gives some idea of what is going on.


I have been silently following the thread, so if I understand, that's because we're using analytical spherical gravity equations to have both ISS and the Shuttle around at the same time, while the Shuttle is following J3 gravity. That sounds that there is no way to remove the fluctuations indeed.

I am posting because I've noticed a similar effect on elliptical orbits. At apogee, the DPS in OPS 3xx displays a HP which over the course of the orbit usually goes down by a few miles, till the perigee is reached, and then goes up again by a few miles. I would understand if the DPS was using spherical gravity equations, but if I remember well, Thorsten, a while ago you've made the DPS displaying the exact same thing as the J3 gravity of the sim, then I do not understand what could be the source of these fluctuations then?

Cheers,
Chris.
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Re: Space Shuttle - Development

Postby Thorsten » Wed Jan 30, 2019 3:17 pm

At apogee, the DPS in OPS 3xx displays a HP which over the course of the orbit usually goes down by a few miles, till the perigee is reached, and then goes up again by a few miles. I would understand if the DPS was using spherical gravity equations, but if I remember well, Thorsten, a while ago you've made the DPS displaying the exact same thing as the J3 gravity of the sim, then I do not understand what could be the source of these fluctuations then?


Since orbital elements are not constants of motion in J3 gravity, what meaning would you assign to them (short of inserting the current state vector into the point mass formula)?
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Re: Space Shuttle - Development

Postby eatdirt » Wed Jan 30, 2019 4:39 pm

Since orbital elements are not constants of motion in J3 gravity


Arf!.. of course... Sorry, I got confused, I was wrongly interpreting these HP and HA numbers as the actual smallest and highest altitudes of the trajectory. Makes sense now, thanks!
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Re: Space Shuttle - Development

Postby Thorsten » Wed Jan 30, 2019 5:50 pm

Interpreting them as actual altitudes could be quite misleading, as the periapsis (measured by radius) might not coincide with the lowest altitude above the IERS ellipsoid at all.

All these things turn crazy when you wonder for too long about what they *really* should be... :shock:
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Re: Space Shuttle - Development

Postby GinGin » Wed Jan 30, 2019 6:09 pm

. Gives some idea of what is going on.


Wow indeed, what a difference.
What is Theta T?



I think I explained that (and that I have a tentative solution which is not yet pushed).


Alright, I missed that then.


All these things turn crazy when you wonder for too long about what they *really* should be.


Indeed :) It gives quickly headache
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Re: Space Shuttle - Development

Postby Thorsten » Wed Jan 30, 2019 6:11 pm

What is Theta T?


A PEG-4 parameter.
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Re: Space Shuttle - Development

Postby GinGin » Wed Jan 30, 2019 6:26 pm

Ah ok theta from Peg4, thanks :)
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Re: Space Shuttle - Development

Postby Thorsten » Sun Feb 03, 2019 8:13 pm

I've been trying to get 'something like' CW targeting in the proximity of ISS to work. One issue of course is that you want to move some few hundred meters relative to the station - yet in actual reality you move a few thousand kilometers, so even if you have an accuracy of less than 1/10.000 error, that'll give you a few hundred meters of error relative to ISS. So we are struggling against numerics here.

The other issue (as I discovered as well) is that simply rotating with DAP-A into burn attitude can give you some 20 cm/s translational motion - which for the next 500 seconds transfer also gives about a 100 meter error.

Which basically explains why I can transfer from 1500 m distance vbar to 300 m rbar in one go only with about a hundred meters error margin.

I guess one can beat that down a little still, but partially the issues are really intrinsic, so the close-by work needs to be manually.
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Re: Space Shuttle - Development

Postby Thorsten » Wed Feb 13, 2019 3:23 pm

I've changed the prediction for the ascending node longitude at launch time to a numerical table - that seems to be working rather well (assuming you launch from KSC or at least the same latitude...not sure how this works for any launch site...) , I've managed relative inclinations of 0.05 degrees with ISS in my tests.

Now I have to get a feeling for the phasing part of the launch window... Some 13 degrees behind ISS doesn't seem too bad for a first try...
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Re: Space Shuttle - Development

Postby GinGin » Sun Feb 17, 2019 12:53 am

Very nice.
That is a totally viable value :)
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Re: Space Shuttle - Development

Postby Thorsten » Tue Feb 19, 2019 6:47 pm

Added some tools today to have MCC compute a plane align burn. Since the problem is miserably ill-conditioned for small wedge angles, I used a numerical procedure to find the next zero crossing of the y-coordinate and determine the burn from ydot at that point.

Seems to be working reasonably well - but the routine is moderately expensive, so I yet have to figure out when to run it to display a sensible node crossing time on SPEC 33 :?:

Part of the trouble is that node precession etc. makes it hard to see what is *really* displayed anywhere - it's not even clear to me that a node is defined by y=0 in J3, that's just one possible definition I can see (trouble with it is that if you're not at the same z-coordinate, you can bring y to zero, but it won't stay there, so in fact your orbital plane after alignment is manifestly not 'the same'...)
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Re: Space Shuttle - Development

Postby Thorsten » Thu Feb 21, 2019 4:03 pm

Tinkering with the rendezvous state vector filters on SPEC 33 - anyone has an idea what kind of accuracy to expect from the radar in ranging/angle or from teh star tracker?
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