Unstable ISS orbit?

[foundpra]

Hi,

ISS orbit was totally regular and stable.

I added a bunch of addons.

Now in Orbit MFD graphic, yellow solid and open circles move randomly.
And ApA and PeA change.

Makes it tough to sync up...

What happened?

Thanks!

--PDN

 

[Urwumpe]

Hi,

ISS orbit was totally regular and stable.

I added a bunch of addons.

Now in Orbit MFD graphic, yellow solid and open circles move randomly.
And ApA and PeA change.

Makes it tough to sync up...

What happened?

Thanks!

--PDN

I would say, physics happened.

http://www.heavens-above.com/IssHeight.aspx

Which add-ons changed it and does it matter if you simply disable the add-ons?

 

[foundpra]

Interesting link! Does Orbiter download current ISS data?

If problem persists, I'll try disabling addons.

Thanks!

---------- Post added at 07:11 PM ---------- Previous post was at 07:10 PM ----------

I added I-MFD, GPC MFD, TransX, Attitude, LunarTfrMFD, Radio/mp3 Panel (sound pkg),
AMSO, and the other Apollo (NASSP?).

 

[Urwumpe]

Interesting link! Does Orbiter download current ISS data?

Luckily not!

But the orbit of the ISS does also vary in Orbiter. The Orbit MFD only displays osculating elements, which are orbit elements valid only for a short segment of the trajectory of the spacecraft. Perturbations on the orbit by non-spherical gravity or the gravity of sun and moon are showing up more extreme in Orbit MFD than they really are. For example, if you would only read the numbers during perigee passage.

I added I-MFD, GPC MFD, TransX, Attitude, LunarTfrMFD, Radio/mp3 Panel (sound pkg),
AMSO, and the other Apollo (NASSP?).

• IMFD
• GPC MFD
• TransX
• Attitude MFD
• Lunar Transfer MFD
• OrbiterSound

Can be disabled in Orbiters configuration. Aside of Lunar Transfer MFD and Attitude MFD, I have all those on my machine and never had any negative issues.

 

[indy91]

Do you have nonspherical gravity enabled and didn't before? NASSP would kind of force you to enable it.

 

[N_Molson]

Hi,

ISS orbit was totally regular and stable.

That's the most unusual fact. I always experienced "unstable" orbital elements, and that's how things are in real life.

I agree that the "non-spherical gravity sources" and other advanced parameters like this one are probably what you are looking for (to un-enable them). I always checked all realism options on the first day I got Orbiter, roughly 15 years ago

Yes, from a navigation point of view, KSP looks a bit boring in comparison :lol:

 

[Urwumpe]

That's the most unusual fact. I always experienced "unstable" orbital elements, and that's how things are in real life.

I agree that the "non-spherical gravity sources" and other advanced parameters like this one are probably what you are looking for (to un-enable them). I always checked all realism options on the first day I got Orbiter, roughly 15 years ago

Yes, from a navigation point of view, KSP looks a bit boring in comparison :lol:

I remember that one solar system for one add-on consisted only of sun and Earth, maybe he experienced such a setup. This has only very little disturbances to the orbit because of a lack of other gravity sources.

 

[foundpra]

Ah -- I'm sensing its the non-spherical gravity switch that did it!

I'll attach a 20 second vid I made of Orbit MFD -- open and solid green and yellow circles moving back and forth (at accel speed). They used to (perhaps before I selected non-sph grav) stay stable.

Also, ApA and PeA for both ISS and me are varying as I orbit -- never used to.

Makes it much harder to meet up with ISS (in shuttle) -- any tricks?

Thanks! And see attached if you have a moment.

--PDN

https://www.youtube.com/watch?v=gXxzmppzJDU&feature=youtu.be

 

[Urwumpe]

Makes it much harder to meet up with ISS (in shuttle) -- any tricks?

Like hedgehogs in love. Very very veeeery carefully.

Like in reality, you have to approach the station in small steps.

 

[foundpra]

The 3 factors which seem to matter are: RInc=0, the two yellow numbers in SyncOrbit must be exactly equal, and the ApA (if meeting at apo) of the ship must equal the altitude of the ISS when it crosses the ship's apo.

At a certain point I get close enough such that I can't adjust these anymore. And it seems things change enough in that last orbit such that I can only get to within around 10km. The old way (with non-sph-grav off?) when nothing moved around, I could get within 1km.

When 10km away, need several correction burns to get there...

 

[Urwumpe]

Again, just take your time. Don't make them equal too fast, because if your measurements are not accurate, your manoeuvres won't be so as well. RInc = 0 is always good, but the yellow numbers must not be instantly equal.

 

[N_Molson]

With a bit of experience, you'll anticipate those orbit variations. There is definitively a feeling about this. The idea is that the pertubations are (for LEO) predictible. The major perturbation source depends of the Earth-Moon-Sun configuration, and this isn't going to change a lot for a rendez-vous manoeuver, that typically lasts 1 or 2 days at worst (in real life, they currently they do it in less than 6 hours with the Soyuz-TMA-M).

So it takes some practice, definitively. In comparison, intercepting the orbit of Mars takes much more planning, but the trip is rather smooth after the injection (provided you don't miss the planned correction burn, and even then a few hours of delay are not going to waste it).

 

[boogabooga]

The video you posted is definitely the effect of non-spherical gravity.

Since you have IMFD, you can use the IMFD map in conjunction with Orbit MFD. It takes into account the non-spherical gravity and will show information for your actual apogee and perigee.

Also, with non-spherical gravity I find that it helps to have an intentionally elliptical orbit. That way, apogee and perigee locations are a bit more stable.

 

[N_Molson]

Also, with non-spherical gravity I find that it helps to have an intentionally elliptical orbit. That way, apogee and perigee locations are a bit more stable.

It definitively does. I've read somewhere that this property is used by some satellites.

 

[kuddel]

I've read somewhere that this property is used by some satellites.

https://en.wikipedia.org/wiki/Sun-synchronous_orbit