def buckle_thickness(D_o, P_p, sig_y):
    """
    Number [m], Number [Pa], Number [Pa] -> Number [m]

    Returns the nominal buckle thickness based on the propagation pressure.

    Considers the worst case maximum external pressure and ignores internal
    pressure.

    Equation (G.21)
    """
    return D_o * (P_p / (10.7 * sig_y)) ** (4 / 9)
