Wobble causing trajectories to jitter

[michelcolman]

When you're flying a tall, wobbly rocket, the trajectory lines jitter about and it's very hard to plan precise course corrections for distant intercepts and flybys.

I initially thought the jittering was just due to rounding errors causing uncertainty, but today I found out it's caused by wobbling of the spacecraft, which really ought to have no effect on its trajectory since the wobble does not affect the center of gravity. 

Apparently, the projected trajectory lines (orbits) are based on the current speed of the cockpit (or root part) while they ought to be based on the speed of the CG.

Apart from the obious annoyance from this jittering, this error can also be exploited to gain free energy because when you activate time warp, the currently displayed trajectory line is fixated and the craft ends up actually flying that trajectory. 

I used that trick today to get an escape trajectory out of Minmus using only the reaction wheels. I had a contract to activate an engine on an escape trajectory out of Minmus, but the previous stage ran out of fuel when I was still in orbit with a 1400 km apoapsis. So I started to wobble the rocket using "A"/"D", the apoapsis went up and down between 1450 and 1350, I did my best to press the period key at the right time and got a 1420 km apoapsis. Back out of time warp, wobble again, time warp: 1435 km. I got it all the way up to escape trajectory step by step (above 2000 km!), so I could activate the next stage in the required conditions and finished the contract.

The time warp trick does not work if the wobble is too intense (cannot activate time warp during acceleration), but once the intensity of the wobble has decreased somewhat, it does work.

I filed bug report 9131.