Bug #20683

engines with variants have shape for drag different from the visible shape

Added by 2 months ago. Updated 10 days ago.

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Core Game
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Arrow u r green
Arrow d r red


Many of the new engines in Making History expansion have variants models, often with and without tank butts.

1) The PartDatabase.cfg is populated with drag-cubes that correspond to the areas and shapes of these engines with shrouds attached. This produces drag in-game that can be surprising if you chose an engine that would fit the stack without its shroud. (Usually the drag-cube has 'Shielded' and 'Clean' entries but this is turned off with useMultipleDragCubes = false in the new engines, probably to let the drag-cube entries line up simply with the variants.)

2) The models with shrouds have holes passing thru so the drag areas do not fill the expected {1.5, 2.5, 3.75}-meter-diameter circles. This leaves an area mismatch in the joints of a stack, and KSP applies flat-plate drag that users would not expect, similarly to the behaviour of tubes in #19376.

The drag a player feels is very sensitive to mismatches in area where two parts join along the stack. Desired behavior is to have at least the ends with nodes have areas in the drag model matching the visible area of the model, with any holes filled in, which often results in an area very near (nominal diamter)² × pi/4

Something like the Kodiak, with three variants and shroud/no-shroud configurations, would seem to need 6 drag-cube entries for the 6 possible visible shapes.

skiff.jpg View - bare engine feels drag as if it had the shroud (95.9 KB), 12/16/2018 06:41 AM

kodiak.jpg View - shrouded engines counted as filling less than their enclosed area, leaving flat-plate drag at joints (111 KB), 12/16/2018 06:41 AM

TerrierRace.craft - (67.9 KB), 12/25/2018 03:04 AM

TerrierRace.jpg View - (44.3 KB), 12/25/2018 03:05 AM

unvariantDrag.cfg - restore the 'Fairing' vs 'Clean' distinction in drag cubes (2.8 KB), 02/10/2019 03:07 AM



#1 Updated by 2 months ago

  • Description updated (diff)

#3 Updated by 2 months ago

  • File drag race.craft added
  • File dragrace.craft added

#4 Updated by about 2 months ago

  • Version changed from 1.5.1 to 1.6.0
  • Expansion Core Game added
  • Expansion deleted (Making History)

Now that the Terrier and Spark have variants, with version 1.6.0, this affects the core game.
If you put the default-variant Terrier between two 1.25-m parts, those neighboring parts get the drag of flat-plates of nearly half their cross-sectional area.

The difference in drag between variants are tiny compared to the difference with versus without the shroud.  It is straightforward to add a custom drag specification (attached unvariantDrag.cfg) to restore the Shielded/Clean options for the Terrier and Spark.

The new engines are trickier because there is no configuration mechanism for each combination of variant and shroud. With a bit of a hack (also in unvariantDrag.cfg) I get drag that feels right if I
1) specify each engine's areas A along the stack to match a disk that fills the cross-section with shroud attached, and
2) specify the Cd along the stack so that A×Cd is reasonable for the engine without shroud.

#5 Updated by about 2 months ago

  • File unvariantDrag.cfg added

#6 Updated by about 2 months ago

  • File deleted (unvariantDrag.cfg)

#7 Updated by about 2 months ago

  • File deleted (dragrace.craft)

#8 Updated by about 2 months ago

  • File deleted (drag race.craft)

#11 Updated by about 1 month ago

  • File unvariantDrag.cfg added

#12 Updated by 10 days ago

  • File deleted (unvariantDrag.cfg)

#13 Updated by 10 days ago

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