Bug #26898

Extraneous part mass

Added by tehmattguy 5 months ago. Updated 3 months ago.

Ready to Test
Target version:
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Breaking Ground, Core Game
English (US)
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Arrow u r green
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Certain parts or part combinations have gained mass which isn't accounted for in the game.
Seems to affect low-mass, physics-enabled parts like grip pads, small robotic parts, and empty dumpling fuel tanks.
Also affects low-mass physics-less parts if they're attached to a robotic part.
Crafts using these parts have gained a significant amount of mass which isn't shown in the editor, nor accounted for in the delta-v calculations.

test1.png (1.18 MB) test1.png Rocket with two grip pads attached. It should have 1029m/s delta-v but comes up 34.6m/s short due to extraneous part mass. tehmattguy, 12/20/2020 02:17 AM
screenshot13.png (2.43 MB) screenshot13.png Screenshot from 1.10. Scale weighing 5 grip pads vs 2 communotrons. Both sides weigh 0.01t so the scale is balanced. tehmattguy, 12/20/2020 02:35 AM
screenshot18.png (2.44 MB) screenshot18.png Screenshot from 1.11. Editor shows part mass is unchanged but the grip pads are evidently much heavier than before. tehmattguy, 12/20/2020 02:35 AM
seesaw2.craft (12.3 KB) seesaw2.craft core-game craft made in version 1.7.3 [email protected], 12/20/2020 03:38 PM
Scale.craft (76.9 KB) Scale.craft Breaking Ground balance built in 1.10.1. Throttle up to loosen the main pivot. tehmattguy, 12/23/2020 07:10 PM
screenshot21.png (2.48 MB) screenshot21.png Screenshot from 1.11 showing that a single grip pad balances against 6 communotrons, altogether weighing 0.03t tehmattguy, 12/23/2020 08:45 PM
screenshot22.png (2.05 MB) screenshot22.png 1.11 fuel tanks will not fall below 0.03t while draining. An empty dumpling tank weighs as much as a partially-fueled one. tehmattguy, 12/23/2020 08:45 PM
screenshot23.png (2.35 MB) screenshot23.png 1.11 physics-less parts also weigh 0.03t, but only if they're attached directly to a robotic part. tehmattguy, 12/23/2020 08:45 PM
test6.png (1.08 MB) test6.png 1.11 In this situation each individual communotron, being attached directly to a robotic part, will add 0.03t to the rocket. The game estimates 868m/s but external delta-v calculations put us at 828.1m/s, a difference of 39.9m/s as shown in-game. tehmattguy, 12/23/2020 08:46 PM
screenshot0.png (2.19 MB) screenshot0.png 1.11.1 A Tiny Nosecone (0.001t) balances against two Cubic Octagonal Struts (0.001t each). tehmattguy, 01/30/2021 03:46 AM
screenshot1.png (2.27 MB) screenshot1.png 1.11.1 A Communotron 16, Cubic Octagonal Strut, and an RFP-0 Flag (0.0065t total) balances against a single Cubic Octagonal Strut when it's attached directly to a robotic part. tehmattguy, 01/30/2021 03:46 AM


#1 Updated by tehmattguy 5 months ago


I did more testing and it seems like the game raises the mass of low-mass parts to be at least 0.03t.
Empty dumpling tanks, empty oscar-B tanks, small grip pads, and small robotic parts apparently weigh 0.03t despite editor info.

#2 Updated by [email protected] 5 months ago

  • File seesaw2.craft seesaw2.craft added
  • Status changed from New to Confirmed
  • % Done changed from 0 to 10
  • Expansion Core Game added

Confirming with just core-game parts. Two (2) '10-kg' Z-200 batteries outweigh one (1) '50-kg' Z-1000 battery, when loaded in KSP version 1.11.0

Not all parts behave as if they have this 30-kg minimum mass, but many do.

#3 Updated by tehmattguy 5 months ago

#4 Updated by tehmattguy 5 months ago


Added more screenshots/ info to help clarify the 0.03t issue

#5 Updated by tehmattguy 5 months ago

  • File deleted (test4.png)

#6 Updated by [email protected] 4 months ago

Reported in this thread
where Stamp 20 points out the new line in Physics.cfg:

partRBMassMin = 0.03 // Minimum mass that a parts RigidBody can have - If this is too small then PhysX will not behave when it is dropped as a vessel, this is the default if minimumRBMass is not defined in the part cfg.

This observation inspires the obvious edit to 'Physics.cfg'
partRBMassMin = 0.005
which solves the problem for me. (I tried a Module-Manager patch but it did not take effect on KSP startup.) The adjusted minimum mass was 5kg, to match the minimum mass of all parts that have their mass attached to their body (as opposed to the smallest parts, flagged PhysicsSignificance=1, for which KSP adds their mass to the parent part).

The obvious potential side-effect, based on the comment in physics.cfg, would be if parts with mass in the range 5kg to 30kg start behaving badly when dropped on the ground during EVA construction.

However, some mods have lighter parts that can be independent rigid bodies, specifically the GP-004 grip pad from Breaking Ground used in some examples posted above. So for games including these parts, a lower minimum

  partRBMassMin = 0.002

New bug-report #26939 might have the same cause, but shows a different symptom in that the trajectory plotter sees a different center-of-mass than the physics engine.

#8 Updated by Well 4 months ago


However, some mods have lighter parts that can be independent rigid bodies, specifically the GP-004 grip pad from Breaking Ground used in some examples posted above. So for games including these parts, a lower minimum

My mod add 0.625 rocket parts to the game, so all my parts have a really small mass. In 1.11 with this mass added to all small parts, my mods is just broken, Rocket can't reach space anymore with a huge amount of False DeltaV showed. A lot of work broken with this update. Could be great to think about modders when you update things like that, Playing with KSP physics is never a good idea. I hope this could have a fix, i suggest the lower min mass possible to avoid any more problem, even if i think adding "virtual" mass to a parts wich is not defined to have this mass is not a good idea.

#9 Updated by victorr 3 months ago

  • Status changed from Confirmed to Ready to Test
  • Target version set to 1.11.1
  • % Done changed from 10 to 80

We've made some changes in the latest version of the game and would like some feedback on this issue. Thanks.

#10 Updated by tehmattguy 3 months ago


victorr wrote:

We've made some changes in the latest version of the game and would like some feedback on this issue. Thanks.

Thanks for the update, from what I can tell reducing partRBMassMin to 0.002 fixed the mass issues for most physics-enabled parts. However I believe any physics-enabled part lighter than 0.002t will still be affected by the 0.002t minimum. For example the Tiny Nosecone, a physics-enabled part which should weigh 0.001t, still has an apparent weight of 0.002t.

Also, while testing for physics-less parts I found that attaching them directly to a robotic part seems to give them a minimum mass of 0.0065t. This mass isn't shown in the editor but is accounted for in-flight through the delta-v readouts. This effect is also present in version 1.10.1 so I don't know whether it's intentional or not.

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