/*
	Planet.java
*/

import Vec3D;


class Planet
{
	/*	A Planet is a body from which a Rocket is launched.  It is assumed
		that the planet is the only relevant gravitational attractor in
		the Rocket's environment.  A Planet can calculate the gravitational
		force and air drag vectors, given a position and velocity.
	*/
	
	public double radius;		// mean surface radius (m)
	public double G;			// accel. due to gravity at surface (m/s^2)
	public double airDensity;	// air density (kg/m^3) at surface
	public double airFalloff;	// height to 1/e air density (m)
	
	// constructors
	public Planet()		// by default, construct Earth
	{ radius = 6378150; G = 9.8; airDensity = 1.29; airFalloff = 8000; }
	public Planet(double pRadius, double pG, double pAir, double pFalloff)
	{ radius = pRadius; G = pG; airDensity = pAir; airFalloff = pFalloff; }
	
	public Vec3D GetGravity( Vec3D position, double mass )
	{
		// gravity falls with the square of the distance from the center
		// but we'll assume it doesn't increase if you're below ground!
		double r = position.Length();	// radius from center of planet
		double a;						// acceleration
		if (r <= radius) a = G;
		else a = G * radius*radius / (r*r);
		
		// Now we have the acceleration; we need to compute the force
		// vector.  Since position is relative to our center, the
		// direction is simply the inverse of this position.
		// We normalize (divide by its length), then multiply by
		// the magnitude of the force (F=ma).
		
		return position.Times(-mass*a/radius);
	}
	
	public double GetAirDensity( Vec3D position )
	{	return GetAirDensity( position.Length() - radius ); }
	
	public double GetAirDensity( double altitude )
	{
		if (altitude < 10) return airDensity;
		
		// I assume that air density falls off exponentially:
		return airDensity / Math.exp( altitude / airFalloff );
	}
	
	public double GetAirDrag( Vec3D position, Vec3D velocity,
			double cd, double area )
	{ return GetAirDrag( position.Length() - radius,
				velocity.Length(), cd, area ); }
	
	public double GetAirDrag( double altitude, double speed,
			double cd, double area )
	{
		// I'll assume basic Newtonian drag for now:
		return speed*speed * area * cd * GetAirDensity(altitude) / 2.0;
	}

}