Discrepancy in decoupling behaviour between Service Module and Avionics Ring

[sumghai]

As reported by forum user Bomoo and confirmed by my own testing:

The decoupler in the Avionics Ring appears to separate from the stock Mk1-2 Command Pod with greater force than the Service Module, even though both have ejectionForce = 200.

Suspect minute differences in colliders.

[sumghai]

No differences in collider geometry or Unity configuration.

Possible test cases for determining the cause of discrepancy

• Side-by-side unaltered comparison
• Replace Avionics Ring model with Service Module model
• Alter Avionics Ring model mass to match that of Service Module model

[sumghai]

Test payload: Mk1-2 Pod + SDHI Heatshield

Side-by-side unaltered comparison (SM/AR sitting on pad)

• SM decoupler: 74->85m (11m altitude gain)
• AR decoupler: 73->85m (12m altitude gain)

Side-by-side unaltered comparison (SM/AR suspended by launch clamp)

• SM decoupler: 76->80m (4m altitude gain)
• AR decoupler: 76->80m (4m altitude gain)

Side-by-side unaltered comparison (SM sitting on pad, AR sitting on Rockomax Brand Adapter 02)

• SM decoupler: 74->85m (11m altitude gain)
• AR decoupler: 73->79m (6m altitude gain)

Side-by-side unaltered comparison (SM sitting on pad, AR sitting on Rockomax X200-16 Fuel Tank)

• SM decoupler: 74->85m (11m altitude gain)
• AR decoupler: 75->86m (11m altitude gain)

Side-by-side unaltered comparison (SM sitting on pad, AR sitting on Rockomax Jumbo-64 Fuel Tank)

• SM decoupler: 74->85m (11m altitude gain)
• AR decoupler: 80->92m (12m altitude gain)

[sumghai]

Increasing the mass of the AR from 0.48 to 1.1 had no effect on decoupling distance.

[sumghai]

Replacing the AR model and stack node definitions with the SM model while keeping the mass at 0.45 resulted in greater decoupling distance (74-88m, 14m altitude gain)

[sumghai]

In conclusion:

When tested alone, there may sometimes be differences in decoupling distance, but since the Avionics Ring is typically used with a whole bunch of other stock parts to become a user-built Service Module (and becomes heavier in the process), the difference tends to be negated.

[sumghai]

Additional feedback / info from Bomoo

• According to him, the inconsistent behaviour is not just limited to decoupling distance, but also decoupling direction when used with the KSPX LES - the command pod may jerk left or right depending on whether an SM or AR is used.
• His most recent post says he may have found the cause: Kerbal Joint Reinforcement. If this is a mod conflict edge-case, then it's probably outside my area.