Flight Question Orbit Question

[astroavion]

If I put myself into a near circular Earth orbit around 200k and I get into the orbit normal position to do a plane change at the DN, why does my apoapsis/periapsis change? I thought this was supposed to remain relatively fixed. What gives? All peturbations are off, with complex flight model on.

Thanks,
Andrew

 

[Thorsten]

Is gravity set to non-spherical (Earth is not a sphere...) or spherical?

If the perturbations you see are sinusoidal (i.e. you're back to the same apses after a complete orbit) it's most likely that.

 

[Urwumpe]

You always also have the influences by sun and moon.

That is why you read OSC in Orbit MFD. osculating elements, which means, its the Kepler orbit that fits best to the velocity and radius at the current moment.

 

[dgatsoulis]

The reason that the OP sees the periapsis and apoapsis change during a normal+ or normal- burn, even though perturbations have been switched off, is because the built in autopilot that keeps the orientation of the spacecraft can't keep up with the rate of the inclination change.

@astroavion: You will notice that as you burn your engines your spacecraft isn't exactly pointed at +90 or -90. This leads to the change you see at the periapsis and apoapsis altitudes.
To correct for this either throttle down your engines or use a different AP to orient the spacecraft (for example BrianJ's IEAT MFD).

 

[astroavion]

Thanks guys. I'll try that autopilot, dgatsoulis.

 

[Thorsten]

So this is really during the burn (this isn't obvious from the initial question)?

 

[astroavion]

Yes, it is during the burn. I have always notice some wobbling around when doing plane change burns. I figured that was why the discrepancy.

Also, I tried the MFD which Dmitri (I think) suggested, above. I'm using 2016, so that autopilot is way outdated and I couldn't get it to work. Is there another presently, for 2016? I haven't been able to find one.

I haven't tried yet, but I think the suggestion of using less thrust is the answer. Never even thought of that. I have been using Burn Time Calc, and that operates with the assumption of a full thrust burn, as you probably know.

 

[dgatsoulis]

IEAT MFD works just fine in Orbiter 2016.
First orient the spacecraft using the default orbiter autopilot, then open IEAT MFD, press REF.
Enter the name of the body you are orbiting and then press ATT to hold the orientation.
It's rock solid.

 

[astroavion]

Strange. You're right. It works fine now. Musta did something wrong with the extraction, even though I now see it's simply one module file. I'll try it next up.

One more question - This wobbling does not seem to occur when doing standard pro/retrograde burns. Why is this? Or does it, and I am not noticing?

Thanks again,
Andrew

 

[dgatsoulis]

During a pro/retrograde burn the orientation of your orbit doesn't change much. To put it better, the plane upon which your orbit "sits" remains the same.
(If you press Z to zoom you will notice that the AP isn't exactly at the prograde or retrograde orientation but slightly off, but that is a different matter).

On the other hand, during a plane change burn, the orientation of your orbit changes a lot, depending on your spacecraft's acceleration capabilities of course. As you are burning at a normal+ or - direction, the AP is trying to keep up with a moving target. (the +/-90° orientation relative the prograde direction changes during each second you burn your engines).

Hope this helps


Dimitris

 

[astroavion]

Yes, I think I understand. It's not a 'path of least resistance', so to speak.

Thanks Dimitris,

Andrew

 

[martins]

If I put myself into a near circular Earth orbit around 200k and I get into the orbit normal position to do a plane change at the DN, why does my apoapsis/periapsis change? I thought this was supposed to remain relatively fixed. What gives? All peturbations are off, with complex flight model on.

Thanks,
Andrew

When you say the apo/periapsis changes, do you mean the apo/periapsis longitude or the altitude? If it is the longitude, this is mainly due to the fact that in a nearly circular obit the apoapsis and periapsis longitudes are not well defined (for a perfectly circular orbit they are completely undefined). So even small perturbations will have a large effect on the longitudes. This is a classic ill-posed problem.

 

[astroavion]

Altitude. I'll try it a few more times.

---------- Post added at 01:46 AM ---------- Previous post was at 12:53 AM ----------

Yes, Dimitris, the IEAT MFD worked well. I had a 200km circular orbit, tried a 10 degree plane change, and was only off by +/- .3km in altitude.

And even in the Orbiter manual, it says:

"If the angle between the initial and target OP is large it may be necessary to adjust the orientation of the spacecraft during the maneuver to keep it normal to the OP." Or in this case, simply use an attitude hold MFD. I guess it was just a matter of not being exactly on center. Zooming in helped. (duh)

Thanks for your help.

Andrew

Awesome sim, Martin (THE Martin?), I can't thank you enough.