when sailing downwind or decelerating when sailing into a headwind. Under certain conditions and in some designs, some of the plasma wind force may be lost as indicated by on the right side. All magnetic sail designs assume a standoff between plasma wind pressure and magnetic pressure of the same form with parameters specific to a plasma environment, differing only in a constant coefficient as follows: where (m/s) is the apparent wind velocity and (kg/m3) is the plasma wind density for a specific plasma environment, (T) the magnetic field strength at magnetopause, μ0 (H/m) is the vacuum permeability and is a constant that differs by reference as follows for corresponding to modeled as dynamic pressure with no magnetic field compression, for modeled as ram pressure with no magnetic field compression and for modeled as ram pressure with magnetic field compression by a factor of 2 Equation can be solved to yield the required magnetic field (T) that satisfies the pressure balance at magnetopause standoff as: The solar wind plasma density decreases in inverse proportion to the square of the distance from the Sun and hence from the above, decreases in inverse proportion to . Since magnetic field strength at radius is this means that the magnetic sail magnetopause radius will increase with distance from the Sun, where the increased effective size of a sail compensates for the reduced dynamic pressure of the solar wind. The force derived by a magnetic sail for a plasma environment is determined from MHD equations as reported by many researchers is: where is a coefficient of drag determined by numerical analysis and/or simulation, (Pa) is the dynamic wind pressure, and (m2) is the effective blocking area of the magnetic sail with magnetopause radius (m). Note that this equation has the same form as the drag equation in fluid dynamics. is a function of coil attack angle on thrust and steering angle. The power (W) of the plasma wind is the product of velocity and a constant force where equation was used to derive the right-side yielding the same result as equation (9). MHD applicability test Through analysis, numerical calculation, simulation and experimentation an important condition for a magnetic sail to generate significant force is the MHD applicability test, that states that the standoff distance must be significantly greater than the ion gyroradius, also called the Larmor radius or cyclotron radius: where (kg) is the ion