diff --git a/src/CounterDrone.Core/Algorithms/CloudExpansionModel.cs b/src/CounterDrone.Core/Algorithms/CloudExpansionModel.cs
index b35c34e..86aa15a 100644
--- a/src/CounterDrone.Core/Algorithms/CloudExpansionModel.cs
+++ b/src/CounterDrone.Core/Algorithms/CloudExpansionModel.cs
@@ -49,6 +49,26 @@ namespace CounterDrone.Core.Algorithms
             return volume > 0.001f ? (float)_ammo.SourceStrength / volume : (float)_ammo.CoreDensity;
         }
 
+        /// <summary>达到指定浓度所需时间（密度随膨胀下降，较晚时间密度更低）</summary>
+        /// <returns>浓度首次低于threshold的时间(s)，如果不会低于则返回MaxDuration</returns>
+        public float TimeToDensity(float threshold)
+        {
+            // 密度 ≈ SourceStrength / (4/3 π R³)，随R增大而下降
+            // 求R使得密度=threshold: R³ = SourceStrength / (threshold * 4π/3)
+            float src = (float)_ammo.SourceStrength;
+            float volume = src / Math.Max(threshold, 0.000001f);
+            float r = (float)Math.Pow(volume * 3f / (4f * (float)Math.PI), 1f / 3f);
+            return TimeToReach(r);
+        }
+
+        /// <summary>覆盖指定距离需要的云团数量（间距=2R）</summary>
+        public int RoundsNeeded(float requiredCoverage)
+        {
+            float R = RadiusAt(30f);
+            float spacing = 2f * R;
+            return Math.Max(1, (int)Math.Ceiling(requiredCoverage / spacing));
+        }
+
         /// <summary>达到指定半径所需时间 (s)</summary>
         public float TimeToReach(float radius)
         {
diff --git a/src/CounterDrone.Core/Algorithms/DefaultDefensePlanner.cs b/src/CounterDrone.Core/Algorithms/DefaultDefensePlanner.cs
index 41d3861..cfcdcff 100644
--- a/src/CounterDrone.Core/Algorithms/DefaultDefensePlanner.cs
+++ b/src/CounterDrone.Core/Algorithms/DefaultDefensePlanner.cs
@@ -361,16 +361,12 @@ namespace CounterDrone.Core.Algorithms
         private int CalcRoundsNeeded(DroneGroup threat, AmmunitionSpec ammo,
             CombatScene env, bool isAirBased, float turbulentRadius)
         {
+            var cloudModel = new CloudExpansionModel(ammo, env);
             float avgSpeed = (float)threat.Target.TypicalSpeed / 3.6f;
-            float spacing = 2f * turbulentRadius;
-
             var aerosolType = (AerosolType)ammo.AerosolType;
             float neededExposure = aerosolType == AerosolType.ActiveMaterial ? 2f : 6f;
-            float singleCoverage = neededExposure * avgSpeed;
-            float requiredCoverage = singleCoverage;
-
-            return Math.Max(1,
-                (int)Math.Ceiling(requiredCoverage / spacing));
+            float requiredCoverage = neededExposure * avgSpeed;
+            return cloudModel.RoundsNeeded(requiredCoverage);
         }
 
         private float ComputeInterceptProbability(DroneGroup threat,
@@ -399,7 +395,8 @@ namespace CounterDrone.Core.Algorithms
             var events = new List<FireEvent>();
 
             var mid = ThreatMidpoint(threat);
-            var (effectiveR, expansionTime, _) = ComputeEffectiveRadius(threat, ammo, env);
+            var cloudModel = new CloudExpansionModel(ammo, env);
+            float expansionTime = cloudModel.TimeToReach(cloudModel.RadiusAt(30f));
 
             // 编队 Y 偏移：按车道分布
             float laneSpacingZ = yLanes > 1 ? formationWidth / (yLanes - 1) : 0f;
@@ -407,7 +404,7 @@ namespace CounterDrone.Core.Algorithms
 
             float tx = mid.X + targetOffset;
 
-            // 按目标点位置计算无人机到达时间，非中点
+            // 无人机到达目标点时间
             float droneSpeed = (float)threat.Target.TypicalSpeed / 3.6f;
             if (droneSpeed <= 0) return events;
             float startX = (float)threat.Waypoints[0].PosX;
@@ -420,7 +417,6 @@ namespace CounterDrone.Core.Algorithms
             distToTx = Math.Max(0, Math.Min(totalDist, distToTx));
             float txArrival = distToTx / droneSpeed;
             float recommendedTiming = txArrival - expansionTime;
-            // 来不及：无人机到达时云团未形成 → 跳过此目标点
             if (recommendedTiming <= 0f) return events;
 
             float fireTime;