Good progress. Ditch the damping and whatever was offsetting the dire…

…ction, but this is on the Unity side.

ModelMultiParticlePersistFX.cs

@@ -249,9 +249,14 @@ public void FixedUpdate()
                       double volumeChange = currentVolume - estimatedInitialVolume;
                       double density = (estimatedInitialVolume * initialDensity + volumeChange) / currentVolume;
                       double atmosphericDensity = FlightGlobals.getAtmDensity(FlightGlobals.getStaticPressure(pPos));
+                      double dragCoefficient = .5; // Computed by Pifomètre (R). Make it a setting?
+                      double mass = density * currentVolume;
+                      // Weight and buoyancy.
                       Vector3d acceleration = (1 - (atmosphericDensity / density)) * FlightGlobals.getGeeForceAtPosition(pPos);
+                      // Drag. TODO(robin): simplify.
+                      acceleration += - 0.5 * atmosphericDensity * pVel * pVel.magnitude * dragCoefficient * Math.PI * r * r / mass;
                       // Euler is good enough for graphics.
-                      // pVel = pVel + acceleration * TimeWarp.fixedDeltaTime;
+                      pVel = pVel + acceleration * TimeWarp.fixedDeltaTime;
                       particles[j].velocity = (peristantEmitters[i].pe.useWorldSpace ? (Vector3)pVel : peristantEmitters[i].pe.transform.InverseTransformDirection(pVel));
                     }
 
@@ -264,14 +269,14 @@ public void FixedUpdate()
                             if (hit.collider.name != "Launch Pad Grate")
                             {
                                 Vector3 hVel = Vector3.Exclude(hit.normal, pVel);
-                                Vector3 reflectedNormalVelocity = collideRatio * (hVel - pVel);
+                                Vector3 reflectedNormalVelocity = hVel - pVel;
                                 float residualFlow = reflectedNormalVelocity.magnitude * (1 - collideRatio);
                                 // An attempt at a better velocity change; the blob collides with some
                                 // restitution coefficient collideRatio << 1 and we add a random horizonal term
                                 // for flow conservation---randomness handwaved in through fluid dynamics:
                                 float randomAngle = UnityEngine.Random.value * 360.0f;
                                 Vector3d flowConservationTerm = Quaternion.AngleAxis(randomAngle, hit.normal) * hVel.normalized * residualFlow;
-                                pVel = hVel + reflectedNormalVelocity + flowConservationTerm;
+                                pVel = hVel + collideRatio * reflectedNormalVelocity + flowConservationTerm;
                                 particles[j].velocity = (peristantEmitters[i].pe.useWorldSpace ? pVel : peristantEmitters[i].pe.transform.InverseTransformDirection(pVel));
                             }
                             else