// Functions for calculating thrust values.
@lazyglobal off.

local G is 9.81.
lock G to SHIP:BODY:MU / ((SHIP:BODY:RADIUS+SHIP:ALTITUDE)^2).

// Returns the throttle value you should use to achieve the
// target TWR. If TWR can't be achieved, returns 1.0. (full throttle)
function ThrottleToTWR {
  parameter targetTWR is 1.5.

  local m is SHIP:MASS.
  return min((targetTWR*m*G)/SHIP:AVAILABLETHRUST, 1.0).
}

// Calculates the ship's current TWR.
function TWR {
  local m is ship:mass.
  local t is THROTTLE * SHIP:AVAILABLETHRUST.
  return t/(m*G).
}

function BurnTime {
  parameter dV.

  local en is list().
  list ENGINES in en.

  local f is en[0]:MAXTHRUST * 1000.  // Engine Thrust (kg * m/s²)
  local m is SHIP:MASS * 1000.        // Starting mass (kg)
  local e is CONSTANT():E.            // Base of natural log
  local p is en[0]:ISP.               // Engine ISP (s)

  return G * m * p * (1 - e^(-dV/(G*p))) / f.
}