How to stop a Rotating Orbit

[Venus]

Hi,

I'm new here, and new to Orbiter. I can get into Orbit, and have seen the tutorials/playbacks of Syncing an orbit with the ISS. Once my planes are aligned (which they are), I'm supposed to set my PeA to the Altitude of the ISS when it crosses my PeA. My problem is after the plane change, and correction of the planes; my orbit is rotating, and my PeA is constantly changing. Is there any way to stop this, or prevent it from happening in the future?

I'm ahead of the ISS by about 76 seconds. My T is about 570 higher than the ISS T, so it should catch me up, but I want to remove problems in docking by getting the numbers pretty well spot on. The ISS Alt at my Pea is 348k, but after setting my PeA to that, it's wandering above & below by as much as 10k.

Venus

 

[Urwumpe]

You can't prevent this. That is why the elements in orbit MFD are called osculating elements - they are only an approximation of the orbit at the current position and change over time.

 

[Linguofreak]

This happens because certain parts of the Earth are more or less dense than others, so there are slight variations in Earth's gravity at different points. If you have nonspherical gravity turned on in the options, Orbiter will simulate this, and it will cause your orbit to drift. You should still be able to get close enough to the ISS to rendezvous and dock. (Also, even with nonspherical gravity off, you will get gravitational influences from other bodies, primarily the moon if you're in Earth orbit, that will cause your orbit to shift).

 

[Venus]

Ok, thanks guys. I did eventually manage to dock to the ISS, though it was very hard to control the relative vel, the cross was never anywhere near where it needed to be, and it didn't get onto where it was supposed to be, until I was 20m from docking. Not easy, and took me all night. lol

Still, alot of fun. Can't wait when I can use this properly.

Venus

 

[Tommy]

Congratulations! Docking is one of the most difficult operations to perform. The main thing is patience, it's easy to mess up if you try to hurry. Keep the relational velocity low, and use time acceleration to avoid the temptation to lay on to much RVel.

 

[Linguofreak]

The only problem with high relative velocity is that a) it wastes fuel, and b) if you aren't careful you may end up overshooting if you don't give yourself space to slow down. But other than that, it actually helps to some degree, because orbital effects have less time to act.

I myself tend to use TransX to set up a course that passes within a few hundred meters of the station, and Burn-time to bring myself to a stop at just the right moment. Oftentimes my rendezvous velocity will be a kps or more.

 

[Tommy]

Realistically, you wouldn't want to be burning full throttle that close to the ISS. The last couple kilometers you would want to keep your RVel under 10 m/s or so. This is even more important during dock alignment and approach. Too much RVel in any direction and you'll have a hard time getting it nulled before you miss the dock or ram it at excessive speed.

Once you get some experience, higher RVels aren't as big a problem, but for beginners it's best to keep it slow.

 

[Venus]

Thankyou for the extra info. One thing I failed to mention in the first post, was the main problem with the rotating orbit, was I couldn't catch my Apoapsis. For about 5-10 orbits, the closest I got to it was about 70-90 deg. As it looked like it was going to come around (as it always has before), it would accelerate away. I found this very strange and couldn't really make sense of why it was doing it. I can only put it down to the speed of the orbital rotation was higher than mine, but if that were the case, then I wouldn't even catch my PeA, which I did.

Can anyone explain why this occurred?

I've been having another problem which is annoying me. On the DGIV, I loaded the Autopilot scenario for docking. This is what prompted me to try to dock myself. A few times since I managed to dock it myself, I've reloaded the scenario before docking (within 50km - even within 2km of ISS) and tried to run the Auto Docking from the flight computer. Each time it complains that Nav 1 is not set, even though I have set it for 134.40 (for docking port 1 on ISS). Can someone please explain the correct way to setup the Nav so the autopilot will work? I want to become familiar with all aspects of the DGIV.

 

[Tommy]

The correct frequency is 137.40, not 134.40.. Use Object Info (available from the <F4> menu) to find the correct IDS frequency for a stations docking port.

 

[Hielor]

The correct frequency is 137.40, not 134.40.. Use Object Info (available from the <F4> menu) to find the correct IDS frequency for a stations docking port.

.....

Brilliant. I've always just had to look at the config file, and then convert the "channel ID" to the in-game channel.

 

[Tommy]

I make it a point to re-read the Orbiter manual every couple of months. Although I've been through it dozens of times, on each re-read I pick up on at least one useful tidbit I had overlooked before! Just another aspect of the steepness of the learning curve. There's just too much information to absorb, and it's too easy to miss a "little detail" such as that.

It took me over a year to figure out that you can make an external MFD "sticky" to a particular ship even if you change focus to another, and that comes in pretty handy if you're flying multiple vessels, or trying to use a UMMU to dock a DGIV cargo pod to the ISS!

 

[Andy44]

Actually, CTL-I is the easiest way to look up radio freqs, (Orbiter manual pg. 39).

 

[Hielor]

Actually, CTL-I is the easiest way to look up radio freqs, (Orbiter manual pg. 39).

.....
Also brilliant. Thanks.

 

[Tommy]

Like I said, there's always something I missed. Have to give the [ctrl] i a try!

 

[Omhra]

Also; if your Pe is drifting away from you, it means you have one hell of a round orbit (well done) and if you prograde burn... you are AT Pe..