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Old 01-29-2019, 09:59 PM   #1
statisticsnerd
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Default Changes in orbital trajectory over time

I performed the TEI from the Moon and I've noticed that PeD of Earth gradually increases under time acceleration, and thus a few course corrections are needed.

What causes that?
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Old 01-30-2019, 12:22 AM   #2
Urwumpe
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Originally Posted by statisticsnerd View Post
 I performed the TEI from the Moon and I've noticed that PeD of Earth gradually increases under time acceleration, and thus a few course corrections are needed.

What causes that?

Remember, that Orbit MFD displays so-called osculating elements. That is a two-body orbit solution that fits best to your current position and velocity vector. In Orbiters simulation though, you are in a multi-body trajectory, influenced for example by Earth, Sun and Moon. Thus your osculating elements will change, trying to fit to the real trajectory you are flying.
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Old 01-30-2019, 07:26 AM   #3
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Generally the orbital elements are constants of motion only for a two-body pointmass problem - in a realistic gravity field of an extended non-spherical body and multiple bodies around, some of them will oscillate around a mean value, others will in addition drift as precession alters the orbit.

For a few numerical examples I may point to my numerical orbital mechanics tutorial covering J3 gravity and multibody perturbations.
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Old 01-30-2019, 02:39 PM   #4
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I'm a little confused (so what else is new). When I'm coming up on perapsis (365.3k), my apoapsis reads (412.9). Which is what I want. But when I get there, my periapsis is the same, but my apoapsis and my alt, read (403.6). What am I missing ? I'm flying a docked ship btw.
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Old 01-30-2019, 02:45 PM   #5
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Originally Posted by Donamy View Post
 I'm a little confused (so what else is new). When I'm coming up on perapsis (365.3k), my apoapsis reads (412.9). Which is what I want. But when I get there, my periapsis is the same, but my apoapsis and my alt, read (403.6). What am I missing ? I'm flying a docked ship btw.
Aside of what was already told?

Not sure how the MFD measures the COG - if it is using the COG of your focus vessel and not the superstructure COG, you could really have a few km difference at apogee between prediction and true trajectory because of the common superstructure.

But more likely is, that something simply slowed you down on the way to Apoapsis... is the difference periodic, so, when you get back to periapsis, its about 365 again?
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Old 01-30-2019, 02:53 PM   #6
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Originally Posted by Donamy View Post
 I'm a little confused (so what else is new). When I'm coming up on perapsis (365.3k), my apoapsis reads (412.9). Which is what I want. But when I get there, my periapsis is the same, but my apoapsis and my alt, read (403.6). What am I missing ? I'm flying a docked ship btw.
It means the Moon and Sun (and other objects) are changing your trajectory as you move around in your orbit.
Simple scenario: the Moon is in a place in relation to your orbit, such that in one half of your orbit (e.g., going up from the perigee to the apogee) it pulls you forward, so it will accelerate you a bit, thus you will get to an higher apogee that you thought you would half an orbit back. Then the opposite happens, the Moon now pulls you back, so you will loose a bit of speed, thus when at the perigee the apogee will seem lower. Add more bodies in different places, and the effect gets more complex.

Like it was said above, OrbitMFD only looks at the current orbital data, so as the vessels move around, and get pulled in different directions by different bodies, things change. Predicting the future is possible, but one would have to take into account the gravitational perturbations of (at least) the large masses.
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Old 01-31-2019, 03:33 PM   #7
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 Aside of what was already told?

Not sure how the MFD measures the COG - if it is using the COG of your focus vessel and not the superstructure COG, you could really have a few km difference at apogee between prediction and true trajectory because of the common superstructure.

But more likely is, that something simply slowed you down on the way to Apoapsis... is the difference periodic, so, when you get back to periapsis, its about 365 again?

Yes, when I get back to periapsis, it is 365 and apoapsis is once again 412. Makes HST rendezvous very difficult.
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Old 01-31-2019, 06:43 PM   #8
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Yes, when I get back to periapsis, it is 365 and apoapsis is once again 412.
At that altitude, the main effect is rather j3 gravity than the moon...

Quote:
Makes HST rendezvous very difficult.
It just means you're using the wrong tool (or need to switch to pointmass gravity). In a realistic environment, orbital elements change over time. Which in a nutshell means your orbit isn't an ellipse, it wobbles quite a bit around that dependent on where you are, longitude of the ascending node and argument of the periapsis drift over time in addition to the wobble,...

Trying to pretend what you target is an elliptical orbit in real gravity means you'll be off by a few kilometers easily.

(In realty, there's also no such thing as a circular orbit - you always get altitude variations of a few km as you go around the planet).

So if you want to catch an object moving in real gravity, either you need to bring propellant to iteratively improve your targeting as you get closer, or you need a tool computing a rendezvous solution based on numerical fast-forward of the state vectors (they remain meaningful). But eye-balling orbital elements won't cut it any more.
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Old 01-31-2019, 06:52 PM   #9
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10 km error over a distance of 22000 km isn't that bad anyway.
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Old 01-31-2019, 06:54 PM   #10
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Not at all, all things considered.

Except when you code a targeting routine with that accuracy, I guarantee you users won't see it that way and endlessly complain how they missed again by a huge margin
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Old 01-31-2019, 07:43 PM   #11
Donamy
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I keep aligning the plane at the nodes, and also adjust the Dtmin at the (what I thought I established as 412.9) apoapsis which is what the HST is at that point. The rendezvousMFD tool, which is great, can't be used with docked vessels. I will try to use it manually.

Last edited by Donamy; 01-31-2019 at 07:48 PM.
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Old 01-31-2019, 10:30 PM   #12
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 I keep aligning the plane at the nodes, and also adjust the Dtmin at the (what I thought I established as 412.9) apoapsis which is what the HST is at that point. The rendezvousMFD tool, which is great, can't be used with docked vessels. I will try to use it manually.

Can't you use it "manually", just using it for providing the guidance solutions for you?
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Old 01-31-2019, 10:35 PM   #13
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That's what I will try.
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