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Re: Space Shuttle - Flight

Postby eatdirt » Wed Jan 08, 2020 12:44 pm

@Gingin, very nice explanation thanks!
The T2 burn for the Hohmann transfer was surprisingly good, it started outside the proximity zone, ended inside, and the remaining velocities to cancel were about a few m/s.

A quite long while ago, soon after Thorsten added ISS, I tested a similar Hohmann transfer, but by then, I had to perform two T2 burns to come at rest. So I don't know if something has changed in the frame matching, but the thing is currently working perfectly! Although, as Thorsten said, the windows to get a small DeltaVz is very small, not more than 1 minute.
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Re: Space Shuttle - Flight

Postby GinGin » Wed Jan 08, 2020 6:16 pm

Thorsten wrote in Wed Jan 08, 2020 12:35 pm:I'm not sure how OMS/RCS heaters are implemented, need to check - in reality they're thermostats, not 'always on' devices, so they wouldn't draw anything if the temperature were currently okay.


Thanks.
If it there is no heating at the time of powerdown, no changes are expected in Amps, good point.

And actually, I saw a difference in AMPS after trying several configurations :)

2 fuel cells out, powerdown include: GPC, 2 Avionics fan bay ( cycle on and off), FES heaters, some cryo heaters, all the APU and boiler heaters, Ku antenna, 2 IMU, All the house keeping heaters ( APU, RCS/OMS, hyd heaters), CRT

Endless scenarios

I tried to unload enough with 2 FC out , ended up there

Image

Still too much for bus tie with one fc, I lost the screen
Missing one or two volts ahah
I think it should be possible in real, they speak of bus tie on one remaining fc here and there

Image



Ok, I think I am ready for the big one in Orbit and contigency deorbit :mrgreen:




The T2 burn for the Hohmann transfer was surprisingly good, it started outside the proximity zone, ended inside, and the remaining velocities to cancel were about a few m/s.

A quite long while ago, soon after Thorsten added ISS, I tested a similar Hohmann transfer, but by then, I had to perform two T2 burns to come at rest. So I don't know if something has changed in the frame matching, but the thing is currently working perfectly! Although, as Thorsten said, the windows to get a small DeltaVz is very small, not more than 1 minute.


Truly impressive, I always like to read some precise rdv like that
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Re: Space Shuttle - Flight

Postby eatdirt » Fri Jan 17, 2020 11:06 pm

For the week-end, I decided to visit ISS. Took the space bus today, and after some serious fight with the He valves to reach orbit, and a segfault, here we go with an monster Hohmann transfer:

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Quite nice on the RDV window:

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The clouds, and their fantastic shadow:

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MET 6h9min, docked on ISS

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Image

I am spending the week-end there to shot a few pictures and will go back early next week. See you!

;)
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Re: Space Shuttle - Flight

Postby GinGin » Sat Jan 18, 2020 4:04 pm

Another very nice Rendez Vous burn.
When the lambert solver comes up with almost zero radial velocity, that is satisfying, isn't it ? :)
I need to test those kind of steeper approach

I am always into the nearly co elliptic and minimum fuel comsumption thing, but those seem really nice to plan

How do you forecast those burns before using Lambert targeting?

You should do a more exhaustive flight sum up sometimes, always interesting to read how others proceed and enjoy the sim.
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Re: Space Shuttle - Flight

Postby eatdirt » Sat Jan 18, 2020 5:44 pm

Hi Gingin,
this time, had to do two T2 burns, seems I was lucky last time. For the forecast, taking my initial orbit, something ha=175 hp=100, the one of ISS, I am using the rdv dialog to know the remaining angle. Then, I do a little calculation with std spherical gravity to guess the MET at which the angle will be 0, let's call it MEET. Then, again in spherical gravity, I calculate the time of a Hohmann transfer from my current orbit to ISS, something close to 45mn then, and I am using T1 = MEET-45ish on SPEC 34. Usually, I am getting a deltaVz non-vanishing, but non stupidly big neither. Then, I am changing T1 by something like +10 min if DeltaVz < 0, or -10 min otherwise, with ITEM +17 always set to the same transfer time, 45mn-ish, and I iterate a few times to get close to Delta Vz=0. Done! Prediction uncertainties are around 10000ft, so 20mn after the T1 burn I do mid-course corrections with Lambert again, and then the prediction accuracy goes down to a few 100ft. Then comes the T2 burn, or burns.

I'll do a story-like at some point, when bugs will be stabilized, and I am pretty sure there are many ways to optimize the T2 burn by playing with the DeltaX/Z on arrival.

Cheers.
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Re: Space Shuttle - Flight

Postby GinGin » Sun Jan 19, 2020 12:26 pm

Nice thanks.
I got also one or two stories to tell when I will be back behind the computer :)
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Re: Space Shuttle - Flight

Postby Thorsten » Tue Jan 21, 2020 8:00 am

How do you forecast those burns before using Lambert targeting?


Actually, one of my plans for LEO targeting is to add a TIG searcher for a Hohmann-like targeted transfer (since that's the energetically cheapest option) - that would basically just spit out that kind of burn.
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Re: Space Shuttle - Flight

Postby GinGin » Tue Jan 21, 2020 1:35 pm

Thorsten wrote in Tue Jan 21, 2020 8:00 am:
How do you forecast those burns before using Lambert targeting?


Actually, one of my plans for LEO targeting is to add a TIG searcher for a Hohmann-like targeted transfer (since that's the energetically cheapest option) - that would basically just spit out that kind of burn.


Very nice.
That would be really useful for planning in many different cases .
That would take into account the target in game SV ?
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Re: Space Shuttle - Flight

Postby Thorsten » Tue Jan 21, 2020 2:16 pm

That would take into account the target in game SV ?


No, it's still LEO targeting and an external tool, so you have to specify a target just for the Lambert server, it doesn't know anything about FG.
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Re: Space Shuttle - Flight

Postby GinGin » Tue Jan 21, 2020 8:05 pm

Thorsten wrote in Tue Jan 21, 2020 2:16 pm:No, it's still LEO targeting and an external tool, so you have to specify a target just for the Lambert server, it doesn't know anything about FG.


Nice, thanks.
So we need to feed the target with : lat lon hdg alt vup vtot ?
I was wondering if if would be possible to feed it with the delta position ( or other parameters from rendez vous canvas)



Standard Sv for shuttle from Fg
Delta x,y,z Rinc and alt vup vtot for target

Don’t know if it define fully a proper orbit or not like with standard parameters
That would be super handy then to have a good target approximation to use with Leo tool
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Re: Space Shuttle - Flight

Postby Thorsten » Wed Jan 22, 2020 6:49 am

Delta x,y,z Rinc and alt vup vtot for target - Don’t know if it define fully a proper orbit or not like with standard parameters


Okay, let's take some time to explain this issue, as it's going to haunt us otherwise...

What, exactly, is Rinc for instance?

You certainly know that the orbital plane of an object can be specified by inclination and the longitude of the ascending node, so if we have two orbiting objects, each has their plane, and the angle between the normals of these planes defines the relative inclination. Right?

Unfortunately, in J3 gravity, inclination isn't constant. It changes with latitude - as you go to higher latitudes, the equatorial bulge of Earth tends to pull you further inward than you'd be in spherical gravity. That means, if you have two objects in the same orbit with one just crossing the equator and one reaching the highest latitude (so they're a quarter of an orbit separated), they will have different inclination in spite of the fact that they are in the same orbit. But if you wait for 1/8 orbit, you'll find that they have the same inclination - waiting further will reverse the sign of Rinc - there will be an oscillation around a mean value of 0.

So if you accept a definition of Rinc as the angle between the local plane normals, you'll have the unfortunate result that Rinc = 0 does not necessarily mean the objects orbit in the same plane, and Rinc > 0 may mean that they do. [1].

Which means, unless you're fine with being ~50 km off at rendezvous, the above definition of Rinc isn't very useful. A better one can be constructed on the notion of parallel transport - you follow the chaser orbit to the next crossing with the target orbit and compute the angle under which the tracks cross at that point. Note that this happens to be a fairly useful definition in practice, because it specifically investigates the nodes at which you'd do any correction burns. It also means that if you are in the same orbit, it will actually be zero.

But it also means that Rinc isn't a continuum of values at all times, it's a discrete value valid from one crossing to the next. In practice more annoying, while the angle between normals is cheap to compute, the computational cost of parallel-transporting and finding an intersection point numerically is easily a million times higher.

Well, unfortunately if you're not quite in the same orbit but have different eccentricity, you get to enjoy this effect as well:

Image

The longitude of the ascending node (and with it the orbital plane) drifts over time - and it drifts at a rate that depends on your orbital altitude. What does that mean for our notion of parallel transport?

It means that if you parallel-transport based on the current orbital plane, you can have a Rinc of zero, but when you actually fly there time passes and by the time you arrive Rinc is not zero any more.

So in fact you have to make a decision on whether you want to geometrically parallel-transport (basically using the deformed orbital planes, i.e. a 2-dim problem) or whether you want to fully-dynamical parallel-transport (solving the full 3-dim problem in which the solution depends on every orbital parameter - in particular the variation of orbital altitude) [2].

To summarize that - the cheap and intuitive definition of Rinc is not precise, any precise definition of Rinc is not intuitive and the situation that you specify some value of Rinc but the simulation will not give back to you what you might think at the crossing point is fairly normal.

Basically in order to use a precise definition of Rinc without bugging me endlessly with reports about odd observations, you would need to fully understand the problem, but if you full understand the problem you'd come to the conclusion that this is a bad parameter choice in the first place, so you would not use it [3]. :D



[1] Incidentially, that's why the Shuttle rendezvous dialog refuses to give a value of Rinc (obtained with that definition) when it's getting smaller than the oscillation amplitude - it ceases to make any sense

[2] That may not be a big thing for any orbits the Shuttle can do - but LEO targeting can do way more than Shuttle orbits

[3] Pretty much the same is true for specifying 'standard orbital parameters' for a target - it's not clear what you mean when you specify e.g. inclination - at the equator? At highest longitude? Average value? At local position?
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Re: Space Shuttle - Flight

Postby GinGin » Wed Jan 22, 2020 7:17 am

Ok I see, nice explanation.
Fascinating how non spherical gravity can be tricky and tough to forecast.



And what about the idea to just fill the target block with thedelta position plus alt vtot /up ( easily accessible parameters )and assume that chaser /target plan is the same ( after good use of a launch window etc)
Back to 2 dim problem like you said

Idea is just to easily feed target infos to have a good approximation for planning the burn during first orbit

I don’t know how you manage to feed lat lon hdg from fg target into Leo ?
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Re: Space Shuttle - Flight

Postby GinGin » Sat Jan 25, 2020 7:04 pm

Sunset Launch For the Pole

Image

Image

Image


Hello Groenland

Image
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Re: Space Shuttle - Flight

Postby GinGin » Wed Jan 29, 2020 2:17 pm

A bit of cockpit pano during dynamic phase

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Preaparing for the OMS 2

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Space dusk peacefulness

Image
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Re: Space Shuttle - Flight

Postby GinGin » Mon Feb 03, 2020 12:26 pm

First post edited with latest Adventures

Hubble
https://forum.flightgear.org/viewtopic.php?f=87&t=36311

Contigency Deorbit
https://forum.flightgear.org/viewtopic.php?f=87&t=36862



Polar sighting

Image
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