Merge pull request #4946 from starling13/atmospheric-flight

Improve Atmospheric Flight, Fix Atmosphere Temperature

src/DynamicBody.cpp

@@ -97,6 +97,18 @@ void DynamicBody::SaveToJson(Json &jsonObj, Space *space)
 	jsonObj["dynamic_body"] = dynamicBodyObj; // Add dynamic body object to supplied object.
 }
 
+void DynamicBody::GetCurrentAtmosphericState(double &pressure, double &density) const
+{
+	Frame *f = Frame::GetFrame(GetFrame());
+	Body *body = f->GetBody();
+	if (!body || !f->IsRotFrame() || !body->IsType(Object::PLANET)) {
+		pressure = density = 0;
+		return;
+	}
+	Planet *planet = static_cast<Planet *>(body);
+	planet->GetAtmosphericState(GetPosition().Length(), &pressure, &density);
+}
+
 void DynamicBody::PostLoadFixup(Space *space)
 {
 	Body::PostLoadFixup(space);
@@ -190,16 +202,11 @@ void DynamicBody::SetFrame(FrameId fId)
 
 double DynamicBody::CalcAtmosphericDrag(double velSqr, double area, double coeff) const
 {
-	Frame *f = Frame::GetFrame(GetFrame());
-	Body *body = f->GetBody();
-	if (!body || !f->IsRotFrame() || !body->IsType(Object::PLANET))
-		return 0.0;
-	Planet *planet = static_cast<Planet *>(body);
 	double pressure, density;
-	planet->GetAtmosphericState(GetPosition().Length(), &pressure, &density);
+	GetCurrentAtmosphericState(pressure, density);
 
 	// Simplified calculation of atmospheric drag/lift force.
-	return 0.5 * density * velSqr * area * coeff;
+	return density > 0 ? 0.5 * density * velSqr * area * coeff : 0;
 }
 
 vector3d DynamicBody::CalcAtmosphericForce() const
@@ -248,7 +255,7 @@ void DynamicBody::CalcExternalForce()
 	if (f->IsRotFrame()) {
 		vector3d angRot(0, f->GetAngSpeed(), 0);
 		m_externalForce -= m_mass * angRot.Cross(angRot.Cross(GetPosition())); // centrifugal
-		m_externalForce -= 2 * m_mass * angRot.Cross(GetVelocity()); // coriolis
+		m_externalForce -= 2 * m_mass * angRot.Cross(GetVelocity());		   // coriolis
 	}
 }
 

src/DynamicBody.h

@@ -93,6 +93,8 @@ class DynamicBody : public ModelBody {
 protected:
 	virtual void SaveToJson(Json &jsonObj, Space *space) override;
 
+	void GetCurrentAtmosphericState(double &pressure, double &density) const;
+
 	virtual vector3d CalcAtmosphericForce() const;
 
 	static const double DEFAULT_DRAG_COEFF;

src/Planet.cpp

@@ -54,15 +54,15 @@ void Planet::InitParams(const SystemBody *sbody)
 	const double GAS_CONSTANT = 8.3144621;
 	const double PA_2_ATMOS = 1.0 / 101325.0;
 
-	// surface gravity = -G*M/planet radius^2
-	m_surfaceGravity_g = -G * sbody->GetMass() / (sbody->GetRadius() * sbody->GetRadius());
-	const double lapseRate_L = -m_surfaceGravity_g / specificHeatCp; // negative deg/m
-	const double surfaceTemperature_T0 = sbody->GetAverageTemp(); //K
+	// surface gravity = G*M/planet radius^2
+	m_surfaceGravity_g = G * sbody->GetMass() / (sbody->GetRadius() * sbody->GetRadius());
+	const double lapseRate_L = m_surfaceGravity_g / specificHeatCp; // deg/m
+	const double surfaceTemperature_T0 = sbody->GetAverageTemp();	//K
 
 	double surfaceDensity, h;
 	Color c;
 	sbody->GetAtmosphereFlavor(&c, &surfaceDensity); // kg / m^3
-	surfaceDensity /= gasMolarMass; // convert to moles/m^3
+	surfaceDensity /= gasMolarMass;					 // convert to moles/m^3
 
 	//P = density*R*T=(n/V)*R*T
 	const double surfaceP_p0 = PA_2_ATMOS * ((surfaceDensity)*GAS_CONSTANT * surfaceTemperature_T0); // in atmospheres
@@ -72,7 +72,7 @@ void Planet::InitParams(const SystemBody *sbody)
 		//*outPressure = p0*(1-l*h/T0)^(g*M/(R*L);
 		// want height for pressure 0.001 atm:
 		// h = (1 - exp(RL/gM * log(P/p0))) * T0 / l
-		double RLdivgM = (GAS_CONSTANT * lapseRate_L) / (-m_surfaceGravity_g * gasMolarMass);
+		double RLdivgM = (GAS_CONSTANT * lapseRate_L) / (m_surfaceGravity_g * gasMolarMass);
 		h = (1.0 - exp(RLdivgM * log(0.001 / surfaceP_p0))) * surfaceTemperature_T0 / lapseRate_L;
 		//		double h2 = (1.0 - pow(0.001/surfaceP_p0, RLdivgM)) * surfaceTemperature_T0 / lapseRate_L;
 		//		double P = surfaceP_p0*pow((1.0-lapseRate_L*h/surfaceTemperature_T0),1/RLdivgM);
@@ -136,9 +136,9 @@ void Planet::GetAtmosphericState(double dist, double *outPressure, double *outDe
 	// lapse rate http://en.wikipedia.org/wiki/Adiabatic_lapse_rate#Dry_adiabatic_lapse_rate
 	// the wet adiabatic rate can be used when cloud layers are incorporated
 	// fairly accurate in the troposphere
-	const double lapseRate_L = -m_surfaceGravity_g / specificHeatCp; // negative deg/m
+	const double lapseRate_L = m_surfaceGravity_g / specificHeatCp; // deg/m
 
-	const double height_h = (dist - sbody->GetRadius()); // height in m
+	const double height_h = (dist - sbody->GetRadius());		  // height in m
 	const double surfaceTemperature_T0 = sbody->GetAverageTemp(); //K
 
 	Color c;
@@ -157,10 +157,10 @@ void Planet::GetAtmosphericState(double dist, double *outPressure, double *outDe
 	}
 
 	//*outPressure = p0*(1-l*h/T0)^(g*M/(R*L);
-	*outPressure = surfaceP_p0 * pow((1 - lapseRate_L * height_h / surfaceTemperature_T0), (-m_surfaceGravity_g * gasMolarMass / (GAS_CONSTANT * lapseRate_L))); // in ATM since p0 was in ATM
+	*outPressure = surfaceP_p0 * pow((1 - lapseRate_L * height_h / surfaceTemperature_T0), (m_surfaceGravity_g * gasMolarMass / (GAS_CONSTANT * lapseRate_L))); // in ATM since p0 was in ATM
 	//                                                                               ^^g used is abs(g)
 	// temperature at height
-	double temp = surfaceTemperature_T0 + lapseRate_L * height_h;
+	double temp = surfaceTemperature_T0 - lapseRate_L * height_h;
 
 	*outDensity = (*outPressure / (PA_2_ATMOS * GAS_CONSTANT * temp)) * gasMolarMass;
 }
@@ -222,9 +222,9 @@ void Planet::GenerateRings(Graphics::Renderer *renderer)
 	{
 		Color *row;
 		row = buf.get();
-                std::fill_n(row, RING_TEXTURE_WIDTH, Color::BLACK);
+		std::fill_n(row, RING_TEXTURE_WIDTH, Color::BLACK);
 		row = buf.get() + (RING_TEXTURE_LENGTH - 1) * RING_TEXTURE_WIDTH;
-                std::fill_n(row, RING_TEXTURE_WIDTH, Color::BLACK);
+		std::fill_n(row, RING_TEXTURE_WIDTH, Color::BLACK);
 	}
 
 	const vector3f texSize(RING_TEXTURE_WIDTH, RING_TEXTURE_LENGTH, 0.0f);

src/Ship.cpp

@@ -379,50 +379,57 @@ inline int sign(T num)
 vector3d Ship::CalcAtmosphericForce() const
 {
 	// Data from ship.
-	double m_topCrossSec = GetShipType()->topCrossSection;
-	double m_sideCrossSec = GetShipType()->sideCrossSection;
-	double m_frontCrossSec = GetShipType()->frontCrossSection;
+	const double topCrossSec = GetShipType()->topCrossSection;
+	const double sideCrossSec = GetShipType()->sideCrossSection;
+	const double frontCrossSec = GetShipType()->frontCrossSection;
 
 	// TODO: vary drag coefficient based on Reynolds number, specifically by
 	// atmospheric composition (viscosity) and airspeed (mach number).
-	double m_topDragCoeff = GetShipType()->topDragCoeff;
-	double m_sideDragCoeff = GetShipType()->sideDragCoeff;
-	double m_frontDragCoeff = GetShipType()->frontDragCoeff;
+	const double topDragCoeff = GetShipType()->topDragCoeff;
+	const double sideDragCoeff = GetShipType()->sideDragCoeff;
+	const double frontDragCoeff = GetShipType()->frontDragCoeff;
 
-	double m_shipLiftCoeff = GetShipType()->shipLiftCoefficient;
+	const double shipLiftCoeff = GetShipType()->shipLiftCoefficient;
 
 	// By converting the velocity into local space, we can apply the drag individually to each component.
-	auto m_localVel = GetVelocity() * GetOrient();
-	auto m_lVSqr = m_localVel.LengthSqr();
+	const vector3d localVel = GetVelocity() * GetOrient();
 
 	// The drag forces applied to the craft, in local space.
 	// TODO: verify dimensional accuracy and that we're not generating more drag than physically possible.
 	// TODO: use a different drag constant for each side (front, back, etc).
 	// This also handles (most of) the lift due to wing deflection.
-	vector3d fAtmosDrag = vector3d(
-		CalcAtmosphericDrag(m_lVSqr, m_sideCrossSec, m_sideDragCoeff),
-		CalcAtmosphericDrag(m_lVSqr, m_topCrossSec, m_topDragCoeff),
-		CalcAtmosphericDrag(m_lVSqr, m_frontCrossSec, m_frontDragCoeff));
 
-	// The direction vector of the velocity also serves to scale and sign the generated drag.
-	fAtmosDrag = fAtmosDrag * -m_localVel.NormalizedSafe();
+	// Get current atmosphere parameters
+	double pressure, density;
+	GetCurrentAtmosphericState(pressure, density);
+	if (is_zero_exact(density))
+		return vector3d(0.0);
+
+	// Precalculate common part of drag components calculation (rho / 2)
+	const auto rho_2 = density / 2.;
+	// Vector, which components are square of local airspeed vector components
+	const vector3d lv2(localVel * localVel);
+	const vector3d fAtmosDrag(
+		-sign(localVel.x) * rho_2 * lv2.x * sideCrossSec * sideDragCoeff,
+		-sign(localVel.y) * rho_2 * lv2.y * topCrossSec * topDragCoeff,
+		-sign(localVel.z) * rho_2 * lv2.z * frontCrossSec * frontDragCoeff);
 
 	// The amount of lift produced by air pressure differential across the top and bottom of the lifting surfaces.
-	vector3d fAtmosLift = vector3d(0.0);
+	vector3d fAtmosLift(0.0);
 
-	double m_AoAMultiplier = m_localVel.NormalizedSafe().y;
+	double AoAMultiplier = localVel.NormalizedSafe().y;
 
 	// There's no lift produced once the wing hits the stall angle.
-	if (std::abs(m_AoAMultiplier) < 0.61) {
+	if (std::abs(AoAMultiplier) < 0.61) {
 		// Pioneer simulates non-cambered wings, with equal air displacement on either side of AoA.
 
 		// Generated lift peaks at around 20 degrees here, and falls off fully at 35-ish.
 		// TODO: handle AoA better / more gracefully with an actual angle- and curve-based implementation.
-		m_AoAMultiplier = cos((std::abs(m_AoAMultiplier) - 0.31) * 5.0) * sign(m_AoAMultiplier);
+		AoAMultiplier = cos((std::abs(AoAMultiplier) - 0.31) * 5.0) * sign(AoAMultiplier);
 
 		// TODO: verify dimensional accuracy and that we're not generating more lift than physically possible.
 		// We scale down the lift contribution because fAtmosDrag handles deflection-based lift.
-		fAtmosLift.y = CalcAtmosphericDrag(pow(m_localVel.z, 2), m_topCrossSec, m_shipLiftCoeff) * -m_AoAMultiplier * 0.2;
+		fAtmosLift.y = CalcAtmosphericDrag(pow(localVel.z, 2), topCrossSec, shipLiftCoeff) * -AoAMultiplier * 0.2;
 	}
 
 	return GetOrient() * (fAtmosDrag + fAtmosLift);