Flight Question Newbie question -- always losing orbit stability?

[Caleb9849]

Hi, I just started playing with Orbiter a few days ago, and I'm having a problem in that whenever I change my orbital plane or, worse, try to sync up my position with another object (i.e. Mir or ISS), I lose a *lot* of orbit stability (eccentricity goes to ~0.05 or even 0.1 or higher). I know that it shouldn't stay perfectly stable through these transitions and that I'm supposed to correct it afterward, but should it really be this bad? I've read and re-read the manual and can't tell that I'm doing anything wrong. A few minutes ago I was practicing by moving from ISS to Mir and things somehow got so hosed that my "time until perogee" and "time until apogee" on my orbit MFD were *increasing* instead of *decreasing*, and my vertical speed was rapidly spiraling into the negatives, sending me on a crash course towards Earth. I started to panic a little bit (funny how real the simulator feels..... ) but then calmed down and started trying some experiments to get back into a decent orbit. No luck. Had to quit completely and start over. In this case I *may* have hit something else inadvertently, something really bad, and never figured out what it was I did, but I dunno.

Can anyone tell me if these extreme losses of orbit stability are normal when making these transitions, and if not, does anyone have any immediate ideas what I may be doing wrong? Thanks very much in advance!

 

[kuddel]

Hi Caleb9849,

to the forum.

Eccentricity is nothing about the 'stability' of the orbit. It's just a number that tells how 'round' your orbit is.
If your apogee and your perigee are (almost) equal in height, you have a eccentricity of 0. Once one of these two bekome higer or lower, it is not a perfect circle anymore and you get numbers like 0.9 for example.
When your are in a perfect circular orbit (ecc=0) every point on your orbit is apogee and perigee, 'cause non of them is 'highest' or 'lowest' point in your orbit.
When you burn prograde at any point of such an orbit you will raise the point opposit of your current position and you have created an appogee there and you'll find yourself at the (new) perigee. This orbit is naturaly not a perfect circle anymore and has therfore a non zero eccentricity.
That's still a 'stable' orbit. As long as your perigee does not hit the atmosphere of corse

So don't be worried, most of your travels will be in orbits that are not perfect circles. Any hohmann-transfer only works when you have a clear perigee and apogee...and therfore a non-zero eccentricity.

When you do a pure plane-change manouvre (perpendicular to your current orbitel plane) you might theoretically be able to keep the circular orbit and just alter the inclination, but as said: that's more theoretical. I reality (and in Orbiter) your eccentricity might slightly alter. But normally that's nothing to worry about.

Happy orbiting,
Kuddel

 

[Jarvitä]

I don't think you have a very accurate understanding of what "eccentricity" means.

http://en.wikipedia.org/wiki/Eccentricity_(mathematics)

 

[Urwumpe]

Also... a perfect circular orbit does not exist in reality. Sun and Moon will for example still accelerate you and change your eccentricity. As long as you stay within limits for your mission, all is fine.

Sun and Moon can raise a satellite by 50000 km over multiple years and back, that happens in reality even more likely than in Orbiter.

 

[n72.75]

Are you getting this from the OrbitMFD. If so, know that it doesn't report the true eccentricity, only eccentricity of a true keplarian orbit. If you use the flight data recorder and something like MATLAB for visualization, you will see that the orbit is actually much more stable than you would think.

 

[PeriapsisPrograde]

If you have nonspherical gravity sources turned on, (under parameters tab of launchpad), that can really mess with your orbit. Orbit MFD doesn't account for this. It will make your orbit look very erratic while it is actually stable.

 

[ADSWNJ]

Hi Caleb - welcome to the forum. I'm assuming you are in some kind of orbit around Earth. First job - you want to get your Periapsis (the lowest part of your orbit) safely up to 200km or higher. (Below 80km or so you would be dragging in the atmosphere, and below 30-40km or lower, you would crash!) 200km is a decent starting height where you can get a smooth orbit. So ... bring up the Orbit MFD, wait to you get close to your Apoapsis (ApT counts down to say 180), hit Prograde autopilot to orient "forwards", wait to ApT say 10 secs, and burn the engine to get PeA (Periapsis Altitude) where you want it. (If you have a PeD instead, then hit DST to flip from Distance form the center of the Earth to Altitude about sea level). OK - so PeA is now good.

Wait a half orbit (zooming time with T to say 10x or 100x), and watch PeT count down to close to zero. You will now adjust ApA at the low point of your orbit. Burn prograde again, trying to match your PeA. A quick tip here ... have your retro doors open, so you can do a few puffs of NUMPAD-MINUS if you overshoot. For finer tuning, you can use translation mode forwards and backwards (NUMPAD 6 and 9) to get it very close. You'll see Ecc come down to 0.000 as you do this. Congratulations - you now have a very near circular orbit.

Now - if you want to adjust the inclination, then you need to use Align Planes MFD. You will be burning Normal+ or Normal- (i.e. right angles to prograde, "up" or "down"). The critical thing to remember here is that you must burn on the 'nodes'. Look at Align Plane MFD, and give it a target (e.g. ISS or Mir), or hand-enter the target. Remember AN on the AN (ANTI-normal on the ASCENDING Node), but if you get it back to front, just use your retros.

Following these steps should get you circular and into the plane you want. (Note - if you are launching from the ground, getting the plane close - say within a few degrees - is much easier as you take off than once you are in orbit. You can burn a lot of fuel adjusting plane by 10's of degrees!).

Hope this gives you some quick pointers. Let me or the other guys here know if we can help more...

 

[Caleb9849]

Thank you for all the replies they're very much appreciated.

Most of what you've told me is stuff that I already know, however. I guess I was wrong to use the word "stability," I know what the eccentricity means. Whatever I did to get me crashing down to the surface is something that I haven't done since, so I guess I don't have that problem anymore. I also knew that having a bit of non-zero eccentricity wasn't really a "bad" thing but wanted to make sure I wasn't doing something wrong to induce it, and that's been confirmed for me. Thanks again!

 

[Aviator]

If you use a joystick maby check the deadzone setting on your joystick throttle?

That caught me out a few times, engine was thrusting slightly without me knowing.