- fixed scale bug (balloons were displayed twice two small)

- added spring rendering code

Plugin/ModuleAerostat.cs

@@ -84,11 +84,17 @@ namespace Aerostats
         [KSPField(isPersistant = false, guiActive = false)]
         public float BalloonDragCoeff = 0.3f;
 
+        /// <summary>
+        /// Length of spring that can not be extended (no force applied under this length)
+        /// </summary>
+        [KSPField(isPersistant = false, guiActive = false)]
+        public float SpringRestLength = 10.0f;
+
         /// <summary>
         /// Force applied by the spring between the balloon and the vessel, proportional to spring extension, in newton per meter
         /// </summary>
         [KSPField(isPersistant = false, guiActive = false)]
-        public float SpringHardness = 20000.0f;
+        public float SpringHardness = 5000.0f;
 
         private bool Staged;
 
@@ -104,6 +110,7 @@ namespace Aerostats
         private float Inflation = 0;
 
         private GameObject Balloon;
+        private LineRenderer Spring;
 
         private void PlayAnimation(string animationName, float animationSpeed)
         {
@@ -157,6 +164,23 @@ namespace Aerostats
 
             LiftingGasQuantity = Math.Min(MinimumFillQuantity, RemainingCompressedGas);
             RemainingCompressedGas -= LiftingGasQuantity;
+
+            Spring = Balloon.AddComponent<LineRenderer>();
+            Spring.useWorldSpace = true;
+            Spring.material = new Material(Shader.Find("VertexLit"));
+            Spring.SetColors(Color.black, Color.black);
+            Spring.SetWidth(0.1f, 0.1f);
+            Spring.SetVertexCount(2);
+
+            part.OnJustAboutToBeDestroyed += OnPartDestroyed;
+        }
+
+        private void OnPartDestroyed()
+        {
+            Destroy(Balloon); // another option could be to let it float freely, but in this case the buoyancy code should be implemented in a separate MonoBehavior
+            Balloon = null;
+            Spring = null;
+            part.OnJustAboutToBeDestroyed -= OnPartDestroyed;
         }
 
         private void FixedUpdate()
@@ -226,7 +250,7 @@ namespace Aerostats
                     
                     // V = 4/3*pi*r^3
                     float radius = Mathf.Pow(currentGasVolume * 0.75f / Mathf.PI, 0.333f);
-                    float scale = radius + 0.1f;
+                    float scale = radius * 2.0f + 0.1f;
                     Balloon.transform.localScale = new Vector3(scale,scale,scale);
                     Balloon.rigidbody.mass = (BalloonEmptyMass + balloonGasMass) * 0.001f;
 
@@ -241,28 +265,24 @@ namespace Aerostats
                     Balloon.rigidbody.AddForce(-airVelocity.normalized * dragForce / 1000.0f, ForceMode.Force);
 
                     // spring between balloon and base
-                    float restLength = 2.0f + radius;
+                    float restLength = SpringRestLength + radius;
                     var springVec = Balloon.rigidbody.position - part.rigidbody.position;
                     float springLength = springVec.magnitude;
                     float springForceMag = 0;
+                    var balloonAttachPoint = Balloon.rigidbody.position - Balloon.transform.up.normalized * radius;
                     if(springLength > restLength)
                     {
                         float tensingLength = springLength - restLength;
                         springForceMag = tensingLength * SpringHardness;
                         var springForce = springVec * (springForceMag / springLength * 0.001f);
                         part.rigidbody.AddForce(springForce, ForceMode.Force);
-                        Balloon.rigidbody.AddForceAtPosition(-springForce, Balloon.rigidbody.position - Balloon.transform.up * radius, ForceMode.Force);
+                        Balloon.rigidbody.AddForceAtPosition(-springForce, balloonAttachPoint, ForceMode.Force);
                     }
                     Util.PostSingleScreenMessage("spring force", "Spring force = " + springForceMag + "N");
-                }
-            }
-        }
 
-        public void Update()
-        {
-            if (Balloon != null)
-            {
-                Gizmos.DrawLine(part.transform.position, Balloon.transform.position - Balloon.transform.up * Balloon.transform.localScale.x);
+                    Spring.SetPosition(0, part.transform.position);
+                    Spring.SetPosition(1, balloonAttachPoint);
+                }
             }
         }
     }