Magically circularising orbit

[Esme]

I'm playing a Science game in 1.04, and have just got as far as getting a manned capsule into orbit for the first time. Well, kind of. Valentina's capsule found itself out of fuel and in an orbit about 125km by 67.5km, so I've been patiently waiting for gradual aerobraking to bring her back down to Kerbin. On this current orbit, I noticed something whilst watching the apoapsis and periapsis figures on MechJeb - whilst out of the atnmosphere, the periapsis was rising and the apoapsis dropping!

The peripasis rose from 67.127km to 67.285m, and the apoapsis dropped from just over 111km to just under. Then, when the atmosphere was entered again, the orbit started decaying as normal.

At first I wondered whether the fact that I have MechJeb keeping the ship pointed retrograde might have had something to do with it, but even with that turned off, the same thing was happening. Odd.

Edited July 15, 2015 by Esme

[SpenSpaceCorp]

Were you by chance rotating the ship or anything? I believe that as the vessel rotates about the center of mass, the command part (capsule/probe core) is some meters away, thus reading a change in Ap/Pe height, while the true orbit (in regards to the center of mass) remains unchanged. Thus, this may just be a "data reading oversight" by the mechanics of the game.

[Aladran]

RCS on? Mechjeb can use your thrusters to propel the ship when out of fuel. Maybe you didn't noticed that the throttle wasn't completely cutoff?

[sal_vager]

Moved to Gameplay Q's, though I think SpenSpaceCorp is correct, also floating point rounding errors will change your orbit slightly over time.

[Esme]

No thrusters - haven't got the tech for RCS yet; absolutely nothing to throttle aboard! :-} Ship was just a one-Kerbal capsule with an equipment bay, materials science unit and heat-shield below it. Nothing like large enough for changing centre of mass to be causing over a hundred metres of lift of periapsis. I did think of that - which is why I tested with Mechjeb turned off (so that the ship remained pointing teh same direction) as well as with it on (so that the ship remained pointing retrograde).

A rounding error is possible, but it seems odd that it's consistently raising the periapsis whilst the ship is above the atmosphere. Periapsis is at about the 9am position (erm, ok, about midway between dawn and midday, anyway) - did wonder whether it might be due to Kerbin orbiting Kerbol, but that'd seem more likely if the periapsis wsa on the evening side of Kerbin. Or whether the fact that part of the orbit is in the upper atmosphere might be confusing things a tad. It's ciontinuing to occur in subsequent orbits, incidentally - just seen it creep up from 67.0km.. currently at 67.028km (and close to the 9pm position) by 108.934km

Anyway, it's not large enough to bother me, and the ship IS still, overall, coming down, just thought I'd mention it in case it's of interest. When I manage to get a ship into an orbit fully outside Kerbin's atmosphere, I'll look to see if it still happens.

Edited July 15, 2015 by Esme

[Padishar]

This has been reported before: http://bugs.kerbalspaceprogram.com/issues/3939

[Esme]

This has been reported before: http://bugs.kerbalspaceprogram.com/issues/3939

Ah, I wasn;t aware of that section of things. Yes, I agree that does fit what I'm seeing. I'll mark this thread as 'answered'.

Esme

[juanml82]

Pff, Val can push the ship into a lower periapsis.

It's a bit tricky, so save before. I'll also recomend Navhud (here) so you have some sort of navball while on eva. Send Val out of the ship while near periapsis and have her use the thrusters in her suit to push the module into a retrograde direction. You should be able to lower your Pe that way. Otherwise, it will take forever for that orbit to decay.

[Sharpy]

Well, in Science she can. If that was a Career game, he'd be quite a bit from EVA, so no "pff".

[SalehRam]

Before 1.0.2 I used to not seeing this situation too much before, and in before 1.0 I never experience this, so I think it is something that was done to the engine or the game stuff (what exactly I have not idea) that made it more common than before... all I'm hoping is a fix whether from KSP side or Unity side...

Love for both though, Unity for being an engine for such an awesome game, and KSP for being this awesome game

[Sharpy]

I believe it may be a result of different physics calculations between atmospheric and vacuum. Get a vessel of a rather high part count and observe what happens to your framerate the moment you cross 70000m. A different physics engine (or its module) kicks in, and I guess some values may get affected.

[mrclucks]

It may be due to the angle your craft was hitting the atmosphere causing a lift effect Which would be a radial/antiradial (I'm not quite sure which) effect. Are you activly using the craft? Are you using physics warp?

[Sharpy]

It may be due to the angle your craft was hitting the atmosphere causing a lift effect Which would be a radial/antiradial (I'm not quite sure which) effect. Are you activly using the craft? Are you using physics warp?

That would not return the prior periapsis upon leaving the atmosphere.