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Conservation of momentum physics issue

Wilhelm shared this bug ago

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I swear I submitted this bug, but I can't find it with an extensive search. Reposting.

There is an issue with the physics engine, specifically with unbalanced thrust. I can actually reproduce this with SE1 and SE2, but the former only recently (with the prosperity update or perhaps the one prior, which may narrow it down if there was any code merging). It is possible that it was there all along and only once I figured out how to duplicate the issue in SE2 that I started to also see it in SE1, but I don't think so.

This issue is that your momentum seems to 'attach' to a facing of the ship and once that happens you can turn the ship and the ship's momentum continues on that facing. Flying, I rely heavily on turning or rolling the ship to improve turn rates by turning the heavier thrust facings of the ship into the turn. Example, flying in atmosphere with 2 large atmos thrusters in the down face but one small thruster to the left and right, if I'm turning, I'll rotate the ship 45% into the turn to allow the down facing large thrusters to assist in the turn.

Often my ships will have unbalanced (lighter) thrust to the sides than forward/back and up (for atmospheric performance). Okay, with that in mind, the actual bug is:

Fly straight ahead and get to top speed.
Disable inertial dampeners
Rotate the ship 90 degrees so that the right side of the ship is facing the direction of movement
Enable inertial dampeners (the underpowered side thrust starts to try and slow the ship down)
this should trigger the bug (you may have to try it a few times)
At this point, rotate another 90 degrees so that the rear of the ship (heavier thrust) is facing the original direction of travel. Some % (usually significant) of the momentum rotates with the ship so that the same side thruster that is struggling to slow the ship is still firing 100% even though it should have been rotated out of needing to provide any thrust at all. The actual direction of travel of the ship actually rotated 90%. I have used this to literally steer around obstacles.
It doesn't happen every time, but once that thrust is assigned to that underpowered side thruster, any rotation simply shifts the momentum of the ship until that thruster fully stops the ship (then the issue goes away until next time).

I have the same bug

Replies (5)

Wilhelm ●

If anyone else can repro this please share, I have gotten (in space) to a point where I could make it happen pretty reliably. The ship I was using was testing with the most was the excellent mission runner ship by Diggrok: https://steamcommunity.com/sharedfiles/filedetails/?id=3621168964 but I could also make it happen with the old starting hydrogen ship and my own designs.

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SE2 Community Support ●

Hello Wilhelm,

This may be caused by Super Dampeners. Have you tried reproducing this issue with Super Dampeners OFF?

You can turn the Super Dampeners On / OFF in Advanced Settings of a World in Load Menu.

Kind Regards,

Bartosz

Keen Software House

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Wilhelm ●

I'll try it tomorrow, thanks Bartosz.

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Wilhelm ●

I just verified that I already had the super dampeners turned off

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Noruzenchi86 ●

It was indeed there all along, and valid physics (for what the thrusters are trying to do). It's especially noticeable with ships that have more thrusters in one direction than another.

As a ship rotates, the dampening thrust to counter the slip reacts immediately and at full force.

Since you're almost never at a perfect 90, the tiny amount of offset you're askew forces your ship forward into your facing direction.

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Now, what about the back thrusters pushing against that facing direction? They're pushing against the net force aren't they?

Not by much. The dampers power proportional to your vector, so while your side thrusters may be full power, the back thrusters might only be full power for a split second before smoothing to zero.

They just happen to be really good (albeit quirky) dampers.

Now if you wanted to reduce/eliminate weathervaning, the dampers would have to reduce power slightly on the side thrusters to let the back thrusters continue full force.

Or you could simply adjust the smoothing such that less smoothing is applied to a vector that's more transverse, making its return to zero much slimmer/faster than the more axial side thrust.

Try setting up a test experiment where you have two of the same ship attached to eachother and rotated, delete the connection block, throw on a thrust override on a timer, and watch. Also try a hinge for the 60 degree test (half as much thrust override on the forward thrusts over the side thrusters, or however much needed to straighten the test).

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Wilhelm ●

Good analysis, just to make sure, it doesn't do it every time with the same process so it's likely not intentional physics. When I start going forward at 100%, I then turn off the dampeners and turn 90%, no thrusters are firing. You can see the speed lines going horizontal across the screen, definitely not perfectly 90% as you say. Turn dampeners on, side thrusters fire and the other thrusters correct all the side momentum caused by being not at 90%. After a split second all the other thrusters have stopped except the ones that are trying to stop the ship. Now turn 90%, or rotate the ship 90% or whatever. Many times, not always, the speed remains constant and the direction of travel shifts instantly in the direction of the rotation or turn. No speed is lost and none of the other thrusters fire. That's not how momentum works.

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Marcus Parriott ●

are you guys also taking into account that thrusters assist at different angles... not quite thrust vectoring, but if you rotate so that you have multiple thrusters half way both sets of thrusters push in that reversing motion at what ever power they can add from that angle?

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Wilhelm ●

Yep, in this case when the issue happens (it's not difficult to reproduce) none of the thrusters are firing except the side thrusters that are underpowered (i.e. it can take a minute or two for it to slow down). I described where you rotate on a plane 90 and then another 90 degrees, the rear (main) thruster should be firing to stop the majority of the momentum since it's a rapid turn. However, I can also get going sideways as above and then rotate the ship in a roll where you shift the leading wing up, again the speed remains constant and only the underpowered side thrusters fire. The speed lines you get in space likewise shift completely with the turn (instantly with the turn or rotation). As I mentioned, I've used this to steer completely around an asteroid as the deceleration curve remains constant while the side thruster grinds away trying to stop.

Then other times it doesn't happen with the same steps and you crash into said object.

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Wilhelm ●

This is a checkback for Bartosz, do you still need more information?

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LadyHawk ●

Not sure if relevant but I figured I'd post it anyway. I've noticed that even without using any tricks, a ship's speed will "turn" along with the ship itself if you turn the nose.

Turn off backwards thrust, leave dampeners enabled, and at say 200kph, try to make a circle around an asteroid. In SE1 once you got halfway round the asteroid, you'd be stopped dead in space which makes sense. In SE2 though, make a complete circle around an asteroid and you'll still have 180kph left somehow and it happens each and every single time. I do have super dampeners enabled, but I have no clue if this is one of the super dampener "features".

Does this affect your investigation at all?

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Wilhelm ●

it sounds similar to what I'm seeing if I'm picturing what you're describing correctly. The main thing I see is that when it happens I can rotate the nose in a roll and the momentum will turn as the leading wing goes up or down, or if I rotate the nose in the plane of travel the motion lines turn with the ship's nose (and the momentum).

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LadyHawk ●

Sounds like exactly the same thing to me. The only difference is that you're rolling your ship before you turn, while I just turn

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Wilhelm ●

I can do either, it's easiest seen when you have unbalanced thrust in one direction (sides) so that the unbalanced side thrust struggles to slow the ship down in that direction. Once the thrust is active trying to slow the ship in that side direction, any rotation of the nose seems to shance the ship's direction of movement in relation to that side thrust (while drifting, not actively trying to thrust.

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