Unstable orbits

[Dungas Kerman]

So I launched a Jool misison with the goal of landing on Laythe for the first time. I park in a 105km orbit, dock the mothership and lander, so far so good. I'm not in an ideal spot to launch so I timewarp to the Jool transfer window, but halfway through the warp it stops because I have entered Kerbin's atmosphere. I'm confused but ok, maybe I made a little mistake somewhere.

 

I quickload to the start of the warp and raise my orbit to 130km just to be safe. I double check that my RCS is off and all my engines are manually shut down. OK good to go for timewarp. For extra safety I stop the warp halfway to check my orbital height and what do you know, my orbit has changed from a stable 130 to 85km periapsis and 160km or so apo. I did nothing, and there is no atmospheric drag or anything that could have influenced my orbit. 

 

Is this a bug or am I missing something?

[DeadJohn]

35 minutes ago, Dungas Kerman said:

I timewarp to the Jool transfer window

How are you starting timewarp? Are you clicking the ">>" button to warp to your next maneuver?

There are roundoff errors in the stock game that can change orbits, but those changes are usually subtle, rather than the big 30km apoapsis change you've reported. It would help us if we knew what mods you had installed.

One trick to lessen roundoff errors during timewarp is to ease into timewarp. Manually go to 10x warp before clicking the ">>" button.

 

[Dungas Kerman]

38 minutes ago, DeadJohn said:

How are you starting timewarp? Are you clicking the ">>" button to warp to your next maneuver?

There are roundoff errors in the stock game that can change orbits, but those changes are usually subtle, rather than the big 30km apoapsis change you've reported. It would help us if we knew what mods you had installed.

One trick to lessen roundoff errors during timewarp is to ease into timewarp. Manually go to 10x warp before clicking the ">>" button.

 

Hey thanks for your reply!

 

I just clicked the green arrow thing on the top right to go at max timewarp. The phase angle for Jool was quite a ways off and those planets move damn slow so I maxed it out.

 

I have no mods installed (wouldn't even know where to find them tbh), but I do have the breaking ground and making history dlcs.

[jimmymcgoochie]

Every time you start timewarp the physics system has to switch to “on rails” and this can introduce small imprecisions; the greater the jump in speed, the greater the imprecision will be and the more your orbit can change. Always start timewarp yourself by bumping it to the first speed, then click the “warp to node” button or speed it up yourself. It’s also a good idea to slow down timewarp before SOI changes or your trajectory can shift too- KSP does this automatically, but at high timewarp speeds it might not slow down fast enough.

[surge]

There is a setting called "Max Physics Delta" you may want to change if your craft is complicated and enormous. Later fixes in the game (>= 1.8 from memory) mean that "on-rails" and part loading switching no longer screws up orbits as drastically as it once did, but if you don't give the game enough time to calculate the physics properly, wierd excrements like this can happen. Traditionally, it has been blamed on "The Kraken".

 

I'm about to launch a similar orbitally constructed ship to Laythe and Bop (to see the dead Kraken :), but my setting is 0.04s. I seem to have no problems with timewarp in a 100km, near 0 eccentricity orbit.

 

Note that there are now additional settings: "Ease in Gravity" and "Orbital Drift Compensation". The latter I suspect you may wish to ensure is on (for some reason they made the previously mentioned on-rails bugfix an option).

 

[Dungas Kerman]

I can't find that setting in the settings menu or the f12 menu, but what Jimmymcgoochie suggested worked out perfectly. If I start gradually and increase to max timewarp in something like 10 seconds my orbit remains stable. It's a bit annoying to have to think about but if it works it works, I aint complaining

 

Surge seems to be correct that this problems occurs with complicated ships. I've started a second Jool mission and the same problem occurred. The ship's size is simply ludicrous so I suspect that's the culprit.