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

// 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.
  local g is 9.81.
  if HAS_GRAV_SENSOR = true {
    set g to SHIP:SENSORS:GRAV:MAG.
  }
  return min((targetTWR*m*g)/SHIP:AVAILABLETHRUST, 1.0).
}

// Calculates the ship's current TWR.
function TWR {
  local m is ship:mass.
  local g is 9.81. // fallback to this approximation that doesn't deal with
                   // altitude change.
  if HAS_GRAV_SENSOR = true {
    set g to SHIP:SENSORS:GRAV:MAG.
  }
  local t is THROTTLE * SHIP:AVAILABLETHRUST.
  return t/(m*g).
}

// Check for various sensors and set appropriate global constants.
// Currently only checks for grav sensor, as that's the only one used by this library.
global HAS_GRAV_SENSOR is false.
function SensorCheck {
  local SensorList is 0.
  list SENSORS in SensorList.
  for s in SensorList {
    if s:type = "grav" {
      print "Gravometric sensor detected".
      set HAS_GRAV_SENSOR to true.
      return.
    }
  }
  set HAS_GRAV_SENSOR to false.
}

function BurnTime {
  parameter dV.

  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)
  local g is 9.80665.                 // Gravitational acceleration constant (m/s²)

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