/* 1. setup thrust, isp (or mdot) vehicle stage parameters, and mu for the central force */
            public double thrust { get { return this.thrust; } set { this.thrust = value; _update_mdot(); } }
            public double isp { get { return this.isp; } set { this.isp = value; _update_mdot(); } }
            public const double g0 = 9.80665;
            public double mdot;
            public double mu;
            private void _update_mdot() { if (isp != 0) { mdot = thrust / (g0 * isp ); } }