Stabilizing Physics Joints During Massive Explosions

Applying massive explosive impulses to interconnected rigid bodies often leads to simulation divergence, manifesting as catastrophic jitter, flying disconnected limbs, or game-crashing NaN values. This article covers practical physics-engine strategies to mitigate divergence when detonating high-energy explosives near jointed structures like ragdolls, vehicles, and destructible contraptions. Key techniques include temporal impulse spreading, velocity clamping, dynamic constraint relaxation, sub-stepping, and hierarchy-aware force distribution.

Distribute Impulses Over Multiple Sub-Steps

Iterative constraint solvers (such as Projected Gauss-Seidel) calculate forces sequentially under the assumption that positions change smoothly over time. A single-frame, high-magnitude impulse injects massive kinetic energy instantly, moving bodies beyond the solver's convergence radius before constraints can react.

Rather than applying the full velocity change (\(\Delta v\)) in a single simulation step, distribute the explosion's force over a brief window (typically 3 to 10 physics ticks). Spreading the impulse gives the constraint solver the intermediate frames necessary to resolve tension and angular limits without accumulating extreme positional error.

Implement Velocity and Acceleration Clamping

Enforce strict upper bounds on both the linear and angular velocity that any single body can inherit from an explosion. Unchecked impulses can launch rigid bodies faster than the physics world's broadphase culling and continuous collision detection (CCD) can process.

  • Linear Velocity Clamping: Cap the maximum exit velocity to a threshold that the solver's integration timestep can safely resolve within a single frame.
  • Angular Velocity Clamping: Explosions striking peripheral colliders can impart extreme torque. Clamping angular velocity prevents severe rotational shearing across hinge and socket joints.

Soften Constraints Dynamically During Detonation

Interconnected bodies fail most frequently when rigid constraints (such as fixed or locked spherical joints) fight opposing explosive vectors applied to adjacent parts.

Temporarily reduce constraint stiffness upon detonation by adjusting spring and damping properties:

  • Increase compliance (soft constraints) for a fraction of a second during the blast wave pass-through, allowing joints to stretch elastically instead of snapping.
  • Use Baumgarte stabilization or post-solve projection to gradually pull stretched bodies back to their target offsets once the initial impulse dissipates.

Apply Breakage Thresholds

In reality, extreme explosive forces tear physical connections apart. Forcing unbreakable joints to endure infinite stress is the primary cause of mathematical divergence.

Configure joints with realistic break thresholds for both linear force and angular torque:

  • Monitor joint reaction forces every physics tick.
  • Sever constraints instantly if the applied explosive force exceeds a predetermined tolerance.
  • Once the constraint breaks, the affected bodies become independent rigid bodies, eliminating the internal system stress that causes divergence.

Apply Impulses to the Root or Center of Mass

Applying distinct point impulses to every sub-collider in a ragdoll or articulated assembly creates high internal shear forces. If the blast wave strikes the hand of a ragdoll with extreme velocity while the torso is shielded, the solver must stretch the entire arm chain to maintain equilibrium.

To prevent this:

  1. Treat the interconnected structure as a unified system during the initial blast calculation.
  2. Apply the primary translational impulse to the assembly's overall center of mass or the root bone.
  3. Apply lower-magnitude, localized secondary impulses to individual limbs to produce visual variation without over-stressing the connecting joints.

Increase Solver Iterations Dynamically

Global physics settings often favor performance over stability by using low solver iteration counts (e.g., 4 to 8 position/velocity iterations). While sufficient for ambient movement, this is inadequate for explosive shockwaves.

Implement a localized solver-boost zone:

  • Detect interconnected assemblies situated within the explosion's blast radius.
  • Temporarily elevate position and velocity solver iterations (e.g., to 16 or 32) for the affected bodies for several frames following the blast.
  • Enable temporary time sub-stepping (\(dt\) reduction) globally or regionally to ensure the simulation converges stably through the high-energy event.