Jumping orbits

[Warzouz]

I'm in the middle of a Duna mission.

I've launched 2 ships (one station+fuel and one lander/probes/return vehicle), they should meet around Duna. Then I had a lately mission for a base on Duna, which I packed quicly and launched a bit late (that should be OK)

All mission are now in Kerbol SOI 30 days from the mid course correction burn ; while I'm planning for the upcomming Eeloo window.

My problem is that 2 of the 3 flights have jumping orbits (the space station and the "late" base). They aren't stable, I'm worried which one is the real orbit and which one is a glitch. Strangely, the thrid one is stable even its nearly the same as the space station (I even had trouble to do the ejection burn as they where so close...)

Planning nodes is very hard, even with "node editors", tajectory jumps a lot. I didn't experience that in my 14 fights to Jool on beta 0.9. My Duna transfert orbits aren't exotic. I don't cross Ike SOI and I don't have any other encounter before or after Duna.

- What is the reason of those jumps ?

- How can I know what trajectory I'll get ?

- How can I fix them ? (I'm not sure small correction burn will do anything in that matter)

[DeMatt]

I'm in the middle of a Duna mission.

I've launched 2 ships (one station+fuel and one lander/probes/return vehicle), they should meet around Duna. Then I had a lately mission for a base on Duna, which I packed quicly and launched a bit late (that should be OK)

All mission are now in Kerbol SOI 30 days from the mid course correction burn ; while I'm planning for the upcomming Eeloo window.

My problem is that 2 of the 3 flights have jumping orbits (the space station and the "late" base). They aren't stable, I'm worried which one is the real orbit and which one is a glitch. Strangely, the thrid one is stable even its nearly the same as the space station (I even had trouble to do the ejection burn as they where so close...)

Planning nodes is very hard, even with "node editors", tajectory jumps a lot. I didn't experience that in my 14 fights to Jool on beta 0.9. My Duna transfert orbits aren't exotic. I don't cross Ike SOI and I don't have any other encounter before or after Duna.

- What is the reason of those jumps ?

Floating-point precision errors. Since the numbers KSP uses to calculate orbits have a limited precision (= number of digits behind the decimal place), the further out you ask KSP to calculate a trajectory, the more sensitive the calculation becomes to even minor changes - like turning your ship.

- How can I know what trajectory I'll get ?

Accept that you'll need to make corrections all the way up to your final orbit and rendezvous.

- How can I fix them ? (I'm not sure small correction burn will do anything in that matter)

Instead of doing your orbit planning while at 1x time acceleration, do it at 5x time acceleration. This "turns off" the physics calculations and sets your ship on a fixed course.

[Warzouz]

Well I would agree if I passed trough several SOI, but that's a simple Duna transfert ! And one of my ships don't have jumping orbits.

I don't mean those AP/PE jumping around on a near circular orbit, I mean trajectory lines juming from one orbit to another, not even passing through Duna SOI.

[ElWanderer]

I often get this "encounter, no encounter, encounter, no encounter..." effect when transferring to Minmus (the smaller the target's sphere of influence, the smaller the change or error in trajectory calculation needed to miss it - Jool's sphere of influence is massive). It's quite distracting as Kerbal Alarm Clock keeps creating then deleting a sphere of influence change alarm. I find that going into time warp makes the 'wrong', no encounter orbit show, but if I do nothing, I end up where I was aiming rather than where the game showed me I was going.

I'm not sure if this is a problem with floating point calculations or a display issue. Or both. The one thing I find that helps is to plot a dummy manoeuvre node just outside the sphere of influence change. That often kicks it into showing the right trajectory. If not, Kerbal Alarm Clock will let me know when I'm close so I don't warp through the encounter.

Edited June 15, 2015 by ElWanderer

[tutike2000]

Yeah, conics are pretty glitchy. I haven't been able to use the Advanced Transfer feature in MechJeb for months now. I create a node that takes around 925 m/s, then the 'jumping' happens, and my node gets recalculated and recalculated, and it keeps increasing 2000 m/s, 6000 m/s, 20,000 m/s until I finally give up and do it by hand.

[Warzouz]

Can it depend on the patch conics "mode" ? I've changed from 3 to 0 in the settings/parameters/somewhere I don't recall...

[Red Iron Crown]

- What is the reason of those jumps ?

Two things: The aforementioned floating point errors and KSP calculating orbital path from the root part rather than the center of mass. The latter means ship orientation or wobble will change the trajectory slightly.

- How can I know what trajectory I'll get ?

- How can I fix them ? (I'm not sure small correction burn will do anything in that matter)

Go into low on-rails timewarp, say 5x. The path will stabilize and you can tweak the node more accurately. This is because on-rails physics assumes a point mass for the vessel and uses 64-bit floats for its calculations, eliminating the root part issue and reducing FP errors.