MJ2 and the Case of the Shuddering Ship

[maddog59]

I just built a small 'skiff' - not much more than a MK2 cockpit and long adapter with engine to get around the K/M/M system. It flew just fine, but after refueling at Minmus, when I activated MJ2 to execute a node, once it lined up on the target marker, it started to 'shudder' ... not enough to leave the target, but there's constant input to the roll/yaw axis' back and forth. Visually, it looks like a chihuahua that's not been let out in too long. There's just the one tank (the long adapter), so the CoM, and CoT shouldn't have changed. I have autostruts enabled (heaviest part) but like I said, that was how I built it and it launched and travelled out to Minmus, docked and rendezvoused with a couple of different ships, before re-fueling, and it flew perfectly normally. And when  I line it up manually on the target and execute, there's no shuddering involved. So it definitely seems to have something to do with MJ2.

Anyone else see this, or have an idea of what might be the cause/fix?

[jimmymcgoochie]

MechJeb can be twitchy sometimes, it seems to depend on how fast the game is running as it can end up overcorrecting itself from one extreme to the other. Try switching to fine control mode with caps lock, disabling RCS for a while to let the reactions wheels stabilise the vessel or pointing in a different direction entirely then returning to the desired heading.

[maddog59]

When you say 'how fast the game is running,' can you explain? Not sure I understand what you mean.

In my particular instance, I came back the next day after shutting everything down, the ship was still exhibiting the same behavior, then in the middle of the return to Kerbin when I tried again for a course correction, everything was smooth as silk and running normally - no shudders, etc. 

I've never tried the fine control mode, I'll need to look into that. Thanks for all of the suggestions, I'll try them if/when it happens again.

[Curveball Anders]

6 hours ago, jimmymcgoochie said:

MechJeb can be twitchy sometimes,

Especially on small crafts with OP controls.

Try to dial down the power of RCS and/or reaction wheels.

[jimmymcgoochie]

2 minutes ago, maddog59 said:

When you say 'how fast the game is running,' can you explain?

The game physics doesn’t run in a continuous manner, instead it gets chopped up into ‘ticks’. When the game runs slower e.g. due to a large number of parts, the ticks are slower and so a greater amount of time will pass in each tick- MechJeb applies a small correction in one tick, but by the next one the craft has rotated too far so it corrects heavily in the other direction, and so on in a constant loop of over correction from one extreme to the other. Changing the physics delta in the main menu will affect this as it affects how much time can be in each tick- a bigger number will tend to make the game run slightly better with large or high part count vessels, but will also increase the likelihood of getting this behaviour from MechJeb.

Generally speaking the physics ticks will correspond to your game’s frame rate so if the game seems to be stuttering it’s probably because the physics engine is struggling to keep up. The time displayed in the top left will also change from green to yellow and then red if the frame rate (and by extension the physics processing speed) drops. Increasing physics warp makes this worse as you’re trying to cram 2-4 times as much game time into the same number of physics ticks.

Although I can’t say any of that for certain, so take everything I just said with a pinch of salt.

2 minutes ago, Curveball Anders said:

Especially on small crafts with OP controls.

Try to dial down the power of RCS and/or reaction wheels.

I’ve seen it on even moderate sized craft with tiny RCS (45N thrusters in RO/RP-1), so I think that the physics system is more likely to be to blame; overpowered controls will naturally make it worse as they can overcorrect more violently.

[Serenity]

On 3/5/2021 at 6:15 PM, maddog59 said:

I have autostruts enabled (heaviest part) 

Anyone else see this, or have an idea of what might be the cause/fix?

Not saying its that but its never a good idea to have heaviest for the long run, the krakens like them a lot, especially after multiple docks/burns/rendervouz etc, try to stick with grandparent.

Edited March 6, 2021 by Boyster