Advanced Question ISS Sync with Non-Spherical gravity and gradient torque enabled?

[Usquanigo]

After learning how to orbit, then use the MFDs to reach the moon, then dock with the ISS, and of course land without turning into an expanding ball of incandescent gas..... I decided to try turning on Gravity Gradient Torque and Non-Spherical Gravity sources.

As Clarkson would say "how hard can it be?"

And I found exactly the same result as he does every time he says it. lol

The Apogee and Perigee on the ISS are constantly spinning around. I can't use the tutorial's instructions of finding out the target's alt at my PeA, then make my PeA match it's alt (at that point), then push the ApA as needed to adjust the speeds and meet up at the reference point - if the target's alt is constantly varying at whatever point I choose to use as a reference.

So.... are those options realistic? (I assume they are and that's why they are there)
If so, then there clearly is a way to do it, as it is done in real life - can anyone shed some light on the tricks to do it?

I don't want to be stuck in "easy mode" forever.

 

[Andy44]

Yes, the options are realistic. In fact, in real life there are other, less powerful perturbations which aren't even modeled in Orbiter.

The way they deal with this in real life is to use numerical propagation methods, something you can't do on a bar napkin.

They was I deal with it is to just get as close as I can and try to get into an orbit in front of or behind the target, and then move in closer over several revolutions. The closer you get to the target, the more the perturbations cancel out, since they are acting on both vehicles at the same time. Don't worry about aligning planes perfectly until you are getting very close.

Someone once said that if you set your intercept up near the ascending or descending nodes it works better, but I'm not sure if there's any truth to that.

Also, remember that gravity gradient don't affect your orbit, only your attitude and the ISS's attitude. It's still a pain in the butt, but it's realistic.The real ISS used big heavy gyroscopes to deal with it.

 

[Tommy]

With non-spherical gravity sources, it's far easier to use one of the intersect modes on SyncOrbitMFD. I usually set my ApA in about the middle of my targets Alt range, sometimes a little higher (assuming I'm coming from a lower orbit). There's a little more relative velocity, but most ships have enough Delta-V to handle it.

The ISS's Pe will tend to be in about the same place time the ISS actually gets there, but be prepared for a couple KM more alt than the PeA.

As for Gravity Gradient Torque, it makes docking more difficult because your ship will tend to rotate on its own, even after you killrot. Means a lot of switching RCS mode, but it can be done.

And yes those effects are realistic, that's one of the reasons the shuttle takes so long to dock.

 

[Usquanigo]

Thanks for the advice, I'll have to give it a try. Of course, more advice is always welcome. lol

And here I was thinking it was a pain to do this *without* those things turned on. lol (new challenges are good though)

 

[zerofay32]

Having Gravity Gradient Torque on will show you why the shuttle docks with its tail to the Earth. In this attitude the shuttle, with its heavier end being pulled down, will stay in this attitude with minimal RCS.

Zerofay32

 

[Usquanigo]

I just read the tech-note on non-spherical gravity in the install directory, it said that not only would the periapsis rotate (which I witnessed in Orbit MFD), but so would the LAN. If the ISS was moving it's LAN, wouldn't that take it off a trjectory that would be reachable in the manner they intended?

Does the ISS have orbit correction IRL, or is just left to fly as it seems to be in Orbiter?

I'm still nervous about trying to sync up like this, since I don't have the ground team working out mile long calculations in advance for me. lol

 

[Zatnikitelman]

I've been synching to stations for about 1.5 years now successfully with non-spherical gravity turned on and have had no problems. I just line up the marker line with whatever reference point (sh periapsis, intersect points, etc.) burn till Dtmin~0 and just make small corrections when I'm back at the line and don't worry about the rotating nodes and LAN.

 

[Usquanigo]

Well, I took your word and tried it, and 'tis a christmas miracle! lol I actually did it, but it was touch and go, and I think as much luck as anything else. Seemed more of an art than a science. (Docking took forever as a result too, at first I thought it was because of the odd sync'ing method, but later I realized that I made things worse with my impatience and not using the docking hud like I should have been lol)

Still with the stock DG, but I guess it's still progress. :clap:


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w00t! I actually did it! lol :speakcool:

I wanted to make sure it wasn't a fluke, so I took the DG-XR1 up (little bit more of a challenge than the stock DG) and did it again. Unfortunately the scenario was really timed poorly, so it took me 25 hours to catch and dock with it (it was almost half-way around the other side, but just close enough that I had to chase it). It really does seem more art than science, but I am finding as long as I'm relatively close to it, above, below, ahead, or behind, as long as it's 250Km or less I can just manually and more-or-less directly chase it down.

I'm guessing it's not the "correct" way to do it though. lol