kOS-scripts/lib/orbital_equations.ks

@LAZYGLOBAL OFF.


function semi_major_axis {
    //
    // Semi-major axis
    // Based on:
    //  https://en.wikipedia.org/wiki/Semi-major_and_semi-minor_axes
    //  http://www.orbiterwiki.org/wiki/Front_Cover_Equations
    //
    parameter ap is APOAPSIS.
    parameter pe is PERIAPSIS.
    
    return (ap + pe + 2*BODY:radius) / 2.  // Add 2x body radius because KSP measures apoapsis/periapsis from the surface
}


function orbital_period {
    //
    // The orbital period.
    // Note that for all ellipses with a given semi-major axis, the orbital period is the same, disregarding their eccentricity. 
    // Based on:
    //  https://en.wikipedia.org/wiki/Semi-major_and_semi-minor_axes#Astronomy
    //  http://www.orbiterwiki.org/wiki/Front_Cover_Equations
    //
    parameter sme is semi_major_axis().
    
    return 2 * CONSTANT:PI * sqrt(sme^3 / BODY:mu).
}

function orbital_period_from_ap_pe {
    parameter ap, pe.
    return orbital_period(semi_major_axis(ap, pe)).
}


function orbital_velocity {
    //
    // The required velocity to maintain orbit at provided altitude.
    // Based on:
    //  https://en.wikipedia.org/wiki/Orbital_speed#Precise_orbital_speed
    //  http://www.orbiterwiki.org/wiki/Front_Cover_Equations
    //  https://en.wikipedia.org/wiki/Vis-viva_equation
    parameter altitude.
    parameter sma is semi_major_axis().
    
    return sqrt(BODY:mu * ((2 / (altitude+BODY:radius)) - (1 / sma))).  // add body radius because KSP measures altitude from the surface
}

function orbital_velocity_from_ap_pe {
    parameter altitude, ap, pe.
    return orbital_velocity(altitude, semi_major_axis(ap, pe)).
}

function orbital_velocity_circular {
    parameter altitude.
    return orbital_velocity_from_ap_pe(altitude, altitude, altitude).
}


function orbital_eccentricity {
    //
    // The orbital eccentricity, i.e. the ratio by which the orbit deviates from a perfect circle, where e=0 is circular.
    // Based on:
    //  https://en.wikipedia.org/wiki/Orbital_eccentricity#Calculation
    //  http://www.orbiterwiki.org/wiki/Front_Cover_Equations
    parameter ap is APOAPSIS.
    parameter pe is PERIAPSIS.
    
    return (ap - pe) / (ap + pe).
}