KSPAerostats/Plugin/ModuleBalloon.cs

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);
        }
    }
}