refactored the balloon module so that there is now a balloon case that can deploy an independent balloon (which starts attached by a spring, but can be detached later)

Parts/HeliumBalloon/heliumBalloon.cfg

@@ -29,6 +29,6 @@ PART
 	bulkheadProfiles = size1, srf
 	MODULE
 	{
-		name = ModuleAerostat
+		name = ModuleBalloonCase
 	}
 }

Plugin/Aerostats.csproj

@@ -47,7 +47,9 @@
     </Reference>
   </ItemGroup>
   <ItemGroup>
-    <Compile Include="ModuleAerostat.cs" />
+    <Compile Include="GasUtilities.cs" />
+    <Compile Include="ModuleBalloonCase.cs" />
+    <Compile Include="ModuleBalloon.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Util.cs" />
   </ItemGroup>

Plugin/GasUtilities.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace Aerostats
+{
+    public static class GasUtilities
+    {
+        // amount of gas Q is specified in m^3 at stp (standard pressure of 100kPa and temperature of 0°C = 273K).
+        // Ideal gas equation P.V = n.R.T gives us the equivalent number of moles: n = P.V/R/T = 100000.Q/8.314/273 = 44.Q mol
+
+        public static readonly float R = 8.314f;
+        public static readonly float ZeroCelsius = 273.15f;
+        public static readonly float StandardPressure = 100000.0f;
+        public static readonly float StpVolumeToMoles = 100000.0f / R / 273.15f;
+    }
+}

Plugin/ModuleBalloon.cs

@@ -0,0 +1,139 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using UnityEngine;
+
+namespace Aerostats
+{
+    public class ModuleBalloon : PartModule
+    {
+        public float MaxBalloonVolume { get; private set; }
+        public float GasDensity { get; private set; }
+        public float BalloonDragCoeff { get; private set; }
+        public float BalloonEmptyMass { get; private set; }
+
+        public float LiftingGasQuantity { get; private set; }
+        public float CurrentVolume { get; private set; }
+        public float BalloonLift { get; private set; }
+        public bool IsVenting { get; private set; }
+
+        /// <summary>
+        /// Inflation ratio of the balloon (0=empty, 1=maximum)
+        /// </summary>
+        private float Inflation = 0;
+
+        public float Radius { get; private set; }
+
+        /// <summary>
+        /// Must be called right after creating a balloon
+        /// </summary>
+        public void Initialize(float emptyMass, Vector3 initialVelocity, float maxVolume, float gasDensity, float dragCoeff)
+        {
+            MaxBalloonVolume = maxVolume;
+            GasDensity = gasDensity;
+            BalloonDragCoeff = dragCoeff;
+            BalloonEmptyMass = emptyMass;
+
+            gameObject.AddComponent<Rigidbody>();
+            rigidbody.mass = BalloonEmptyMass;
+            rigidbody.velocity = initialVelocity; // start with the same velocity or everything explodes when deploying from a moving vessel
+            rigidbody.angularDrag = 10.0f;
+
+            InjectGas(0.0001f);
+        }
+
+        /// <summary>
+        /// Injects (positive quantity) or removes (negative quantity) gas, and returns the contained quantity after that operation
+        /// </summary>
+        public float InjectGas(float quantity)
+        {
+            LiftingGasQuantity += quantity;
+
+            float currentMaxQuantity = GetCurrentMaxQuantity();
+
+            // balloon security valve
+            if (LiftingGasQuantity > currentMaxQuantity)
+            {
+                IsVenting = true;
+                Util.PostSingleScreenMessage("security valve", "Some gas has been vented by the balloon security valve");
+                LiftingGasQuantity = currentMaxQuantity;
+            }
+            else
+            {
+                IsVenting = false;
+            }
+
+            Vector3d worldPos = vessel.GetWorldPos3D();
+            CelestialBody body = vessel.mainBody;
+
+            float externalTemperature = (float)FlightGlobals.getExternalTemperature(worldPos, body);
+            float balloonInternalTemperature = externalTemperature;
+            float externalPressure = Math.Max((float)FlightGlobals.getStaticPressure(worldPos, body) * 1000.0f, 0.00001f);
+
+            float currentGasMoles = GasUtilities.StpVolumeToMoles * LiftingGasQuantity;
+            CurrentVolume = currentGasMoles * GasUtilities.R * balloonInternalTemperature / externalPressure;
+            Inflation = CurrentVolume / MaxBalloonVolume;
+
+            Util.PostSingleScreenMessage("inflation", "Inflation = " + (Inflation * 100.0f).ToString("0.00") + "%");
+
+            float airDensity = (float)FlightGlobals.getAtmDensity(externalPressure / 1000.0f, externalTemperature, body);
+            float currentGasDensity = GasDensity * balloonInternalTemperature / GasUtilities.ZeroCelsius / externalPressure * GasUtilities.StandardPressure;
+            BalloonLift = CurrentVolume * airDensity;
+            float balloonGasMass = currentGasDensity * CurrentVolume;
+            Util.PostSingleScreenMessage("lift", "Air density = " + airDensity + "kg/m^3, Lift = " + (BalloonLift - balloonGasMass) + "kg");
+
+            // V = 4/3*pi*r^3
+            Radius = Mathf.Pow(CurrentVolume * 0.75f / Mathf.PI, 0.333f);
+            float scale = Radius * 2.0f + 0.1f;
+            transform.localScale = new Vector3(scale, scale, scale);
+            rigidbody.mass = (BalloonEmptyMass + balloonGasMass) * 0.001f;
+
+            return LiftingGasQuantity;
+        }
+
+        /// <summary>
+        /// Computes the maximum gas quantity that the balloon can current hold, given the external temperature and pressure conditions
+        /// </summary>
+        public float GetCurrentMaxQuantity()
+        {
+            Vector3d worldPos = vessel.GetWorldPos3D();
+            CelestialBody body = vessel.mainBody;
+
+            float externalTemperature = (float)FlightGlobals.getExternalTemperature(worldPos, body);
+            float balloonInternalTemperature = externalTemperature;
+            float externalPressure = Math.Max((float)FlightGlobals.getStaticPressure(worldPos, body) * 1000.0f, 0.00001f);
+            Util.PostSingleScreenMessage("external atmo", "Temperature = " + externalTemperature + "K, Pressure=" + externalPressure + "Pa");
+
+            float currentMaxQuantity = MaxBalloonVolume / GasUtilities.R / balloonInternalTemperature * externalPressure / GasUtilities.StpVolumeToMoles;
+
+            return currentMaxQuantity;
+        }
+
+        private void FixedUpdate()
+        {
+            Vector3d worldPos = vessel.GetWorldPos3D();
+            CelestialBody body = vessel.mainBody;
+
+            InjectGas(0); // update security valve venting
+
+            var gravityAccel = FlightGlobals.getGeeForceAtPosition(worldPos);
+            //Util.PostSingleScreenMessage("gravity", "Gravity accel = (" + gravityAccel.x + ", " + gravityAccel.y + ", " + gravityAccel.z + ")");
+
+            float externalTemperature = (float)FlightGlobals.getExternalTemperature(worldPos, body);
+            float balloonInternalTemperature = externalTemperature;
+            float externalPressure = Math.Max((float)FlightGlobals.getStaticPressure(worldPos, body) * 1000.0f, 0.00001f);
+            float airDensity = (float)FlightGlobals.getAtmDensity(externalPressure / 1000.0f, externalTemperature, body);
+
+            rigidbody.AddForce(-gravityAccel * BalloonLift / 1000.0f, ForceMode.Force);
+
+            // balloon drag
+            var airVelocity = rigidbody.velocity + Krakensbane.GetFrameVelocity() /*- vessel.mainBody.getRFrmVel(vessel.GetWorldPos3D())*/;
+            float sqVel = (float)airVelocity.magnitude;
+            sqVel *= sqVel;
+            float dragForce = 0.5f * airDensity * sqVel * BalloonDragCoeff * (Mathf.PI * Radius * Radius);
+            Util.PostSingleScreenMessage("balloon drag", "Drag = " + dragForce + "N");
+            rigidbody.AddForce(-airVelocity.normalized * dragForce / 1000.0f, ForceMode.Force);
+        }
+    }
+}

Plugin/ModuleBalloonCase.cs

@@ -6,14 +6,8 @@ using UnityEngine;
 
 namespace Aerostats
 {
-    public class ModuleAerostat : PartModule
+    public class ModuleBalloonCase : PartModule
     {
-        // In this class, amount of gas Q is specified in m^3 at stp (standard pressure of 100kPa and temperature of 0°C = 273K). Ideal gas equation P.V = n.R.T gives us the equivalent number of moles: n = P.V/R/T = 100000.Q/8.314/273 = 44.Q mol
-        private static readonly float R = 8.314f;
-        private static readonly float ZeroCelsius = 273.15f;
-        private static readonly float StandardPressure = 100000.0f;
-        private static readonly float StpVolumeToMoles = 100000.0f / R / 273.15f;
-        
         /// <summary>
         /// Maximum rate at which gas can be injected in the balloon to fill it, in m^3/s at stp
         /// </summary>
@@ -95,14 +89,7 @@ namespace Aerostats
         [KSPField(guiName = "Volume", isPersistant = false, guiActive = true)]
         public string VolumeStatus;
 
-        /// <summary>
+        private ModuleBalloon Balloon;
-        /// Inflation ratio of the balloon (0=empty, 1=maximum)
-        /// </summary>
-        private float Inflation = 0;
-
-        private float Radius = 0.01f;
-
-        private GameObject Balloon;
         private LineRenderer Spring;
 
         private Vector3 EstimatedNextFramePosition;
@@ -118,21 +105,19 @@ namespace Aerostats
             UnityEngine.Debug.Log("Aerostats: staged");
             Util.PostScreenMessage("staged");
 
-            Balloon = GameObject.CreatePrimitive(PrimitiveType.Sphere);
+            var balloonObject = GameObject.CreatePrimitive(PrimitiveType.Sphere);
-            Balloon.transform.position = part.Rigidbody.position + part.Rigidbody.transform.up;
+            balloonObject.transform.position = part.Rigidbody.position + part.Rigidbody.transform.up;
-            Balloon.AddComponent<Rigidbody>();
+            Balloon = balloonObject.AddComponent<ModuleBalloon>();
-            Balloon.rigidbody.mass = BalloonEmptyMass;
+            Balloon.part = part; // TODO: remove this hack and properly instantiate a new vessel?
-            Balloon.rigidbody.velocity = part.rigidbody.velocity; // start with the same velocity or everything explodes when deploying from a moving vessel
+            Balloon.Initialize(BalloonEmptyMass, part.rigidbody.velocity, MaxBalloonVolume, GasDensity, BalloonDragCoeff);
 
-            Balloon.rigidbody.angularDrag = 10.0f;
+            LiftingGasQuantity = Balloon.InjectGas(part.RequestResource("Helium", MinimumFillQuantity));
 
-            LiftingGasQuantity = part.RequestResource("Helium", MinimumFillQuantity);
+            Spring = part.gameObject.AddComponent<LineRenderer>();
-
-            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.SetWidth(0.2f, 0.2f);
             Spring.SetVertexCount(2);
 
             part.OnJustAboutToBeDestroyed += OnPartDestroyed;
@@ -146,8 +131,8 @@ namespace Aerostats
             if (Destroyed || !Deployed)
                 return;
 
-            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;
+            Destroy(Spring);
             Spring = null;
             part.OnJustAboutToBeDestroyed -= OnPartDestroyed;
             Destroyed = true;
@@ -192,21 +177,16 @@ namespace Aerostats
                 }
                 EstimatedNextFramePosition = part.rigidbody.position + part.rigidbody.velocity * Time.fixedDeltaTime;
 
-                float externalTemperature = (float)FlightGlobals.getExternalTemperature();
+                float currentMaxQuantity = Balloon.GetCurrentMaxQuantity();
-                float balloonInternalTemperature = externalTemperature;
-                float externalPressure = Math.Max((float)FlightGlobals.getStaticPressure() * 1000.0f, 0.00001f);
-                Util.PostSingleScreenMessage("external atmo", "Temperature = " + externalTemperature + "K, Pressure=" + externalPressure + "Pa");
-
-                float currentMaxQuantity = MaxBalloonVolume / R / balloonInternalTemperature * externalPressure / StpVolumeToMoles;
 
                 LiftingGasTargetQuantity = Math.Max(MinimumFillQuantity, LiftingGasTargetQuantity);
-                if(LiftingGasTargetQuantity > LiftingGasQuantity)
+                if(LiftingGasTargetQuantity > Balloon.LiftingGasQuantity)
                 {
-                    // infalting balloon
+                    // inflating balloon
-                    float stepFinalQuantity = Math.Min(LiftingGasQuantity + MaxGasFillRate * Time.fixedDeltaTime, Math.Min(LiftingGasTargetQuantity, currentMaxQuantity));
+                    float stepFinalQuantity = Math.Min(Balloon.LiftingGasQuantity + MaxGasFillRate * Time.fixedDeltaTime, Math.Min(LiftingGasTargetQuantity, currentMaxQuantity));
-                    float step = Math.Max(stepFinalQuantity - LiftingGasQuantity, 0.0f);
+                    float step = Math.Max(stepFinalQuantity - Balloon.LiftingGasQuantity, 0.0f);
                     float stepResource = part.RequestResource("Helium", step);
-                    LiftingGasQuantity += stepResource;
+                    LiftingGasQuantity = Balloon.InjectGas(stepResource);
                     if(step > 0.0f)
                     {
                         Status = stepResource > 0.0f ? "inflating" : "out-of-gas";
@@ -219,56 +199,24 @@ namespace Aerostats
                 else
                 {
                     // deflating balloon
-                    Status = LiftingGasTargetQuantity == LiftingGasQuantity ? "nominal" : "deflating";
+                    Status = LiftingGasTargetQuantity == Balloon.LiftingGasQuantity ? "nominal" : "deflating";
-                    LiftingGasQuantity = Math.Max(LiftingGasQuantity - MaxGasVentRate * Time.fixedDeltaTime, LiftingGasTargetQuantity);
+                    float quantityAfterVenting = Math.Max(Balloon.LiftingGasQuantity - MaxGasVentRate * Time.fixedDeltaTime, LiftingGasTargetQuantity);
+                    LiftingGasQuantity = Balloon.InjectGas(quantityAfterVenting - Balloon.LiftingGasQuantity);
                 }
 
-
+                if(Balloon.IsVenting)
-                // balloon security valve
-                if (LiftingGasQuantity > currentMaxQuantity)
                 {
                     Status = "full (venting)";
-                    Util.PostSingleScreenMessage("security valve", "Some gas has been vented by the balloon security valve");
-                    LiftingGasQuantity = currentMaxQuantity;
                 }
-
+                
-                float currentGasMoles = StpVolumeToMoles * LiftingGasQuantity;
+                VolumeStatus = Mathf.Round(Balloon.CurrentVolume) + " / " + Mathf.Round(MaxBalloonVolume);
-                float currentGasVolume = currentGasMoles * R * balloonInternalTemperature / externalPressure;
-                Inflation = currentGasVolume / MaxBalloonVolume;
-                VolumeStatus = Mathf.Round(currentGasVolume) + " / " + Mathf.Round(MaxBalloonVolume);
-
-                Util.PostSingleScreenMessage("inflation", "Inflation = " + (Inflation * 100.0f).ToString("0.00") + "%");
-
-                float airDensity = (float)FlightGlobals.getAtmDensity(externalPressure / 1000.0f, externalTemperature, vessel.mainBody);
-                float currentGasDensity = GasDensity * balloonInternalTemperature / ZeroCelsius / externalPressure * StandardPressure;
-                float balloonLift = currentGasVolume * airDensity;
-                float balloonGasMass = currentGasDensity * currentGasVolume;
-                Util.PostSingleScreenMessage("lift", "Air density = " + airDensity + "kg/m^3, Lift = " + (balloonLift - balloonGasMass) + "kg");
-                var gravityAccel = FlightGlobals.getGeeForceAtPosition(vessel.GetWorldPos3D());
-                //Util.PostSingleScreenMessage("gravity", "Gravity accel = (" + gravityAccel.x + ", " + gravityAccel.y + ", " + gravityAccel.z + ")");
-                    
-                // V = 4/3*pi*r^3
-                Radius = Mathf.Pow(currentGasVolume * 0.75f / Mathf.PI, 0.333f);
-                float scale = Radius * 2.0f + 0.1f;
-                Balloon.transform.localScale = new Vector3(scale,scale,scale);
-                Balloon.rigidbody.mass = (BalloonEmptyMass + balloonGasMass) * 0.001f;
-
-                Balloon.rigidbody.AddForce(-gravityAccel * balloonLift / 1000.0f, ForceMode.Force);
-
-                // balloon drag
-                var airVelocity = Balloon.rigidbody.velocity + Krakensbane.GetFrameVelocity() /*- vessel.mainBody.getRFrmVel(vessel.GetWorldPos3D())*/;
-                float sqVel = (float)airVelocity.magnitude;
-                sqVel *= sqVel;
-                float dragForce = 0.5f * airDensity * sqVel * BalloonDragCoeff * (Mathf.PI * Radius * Radius);
-                Util.PostSingleScreenMessage("balloon drag", "Drag = " + dragForce + "N");
-                Balloon.rigidbody.AddForce(-airVelocity.normalized * dragForce / 1000.0f, ForceMode.Force);
 
                 // spring between balloon and base
-                float restLength = SpringRestLength + Radius;
+                float restLength = SpringRestLength + Balloon.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;
+                var balloonAttachPoint = Balloon.rigidbody.position - Balloon.transform.up.normalized * Balloon.Radius;
                 if(springLength > restLength)
                 {
                     float tensingLength = springLength - restLength;
@@ -293,7 +241,7 @@ namespace Aerostats
 
             if (Deployed)
             {
-                var balloonAttachPoint = Balloon.rigidbody.position - Balloon.transform.up.normalized * Radius;
+                var balloonAttachPoint = Balloon.rigidbody.position - Balloon.transform.up.normalized * Balloon.Radius;
 
                 Spring.SetPosition(0, part.transform.position);
                 Spring.SetPosition(1, balloonAttachPoint);