![]() ![]() Solar sails are limited in their performance however, by their dependence upon sunlight, which decreases in strength with the square of the distance, and the laws of reflection, which dictate that the direction of thrust can only lie within 90 degrees of the vector of sunlight. The best known propellantless system is the solar sail, which derives its thrust by reflecting light emitted by the Sun. For this reason, engineers have long been interested in propulsion systems that require no propellant. ![]() The performance of rockets as propulsion systems is greatly limited by their need to carry onboard propellant, which adds to the mass which must be propelled exponentially as the extent of propulsive maneuvers is increased. It therefore offers potential as a means of achieving ultra-high velocities necessary for interstellar flight. In contrast to the electric sail, the ultimate velocity of the dipole drive is not limited by the speed of the solar wind. The dipole drive can achieve more than 6 mN/kWe in interplanetary space and better than 20 mN/kWe in Earth, Venus, Mars, or Jupiter orbit. To deorbit, the negative screen is positioned forward, turning the screen into an ion reflector. There are thus two exhausts, but because the protons are much more massive than the electrons, the thrust of the ion current is more than 42 times greater than the opposing electron thrust, providing net thrust. Ions entering are then propelled from the positive to the negative screen and then out beyond, while electrons are reflected. ![]() To accelerate within a magnetosphere, the positive screen is positioned forward in the direction of orbital motion. If the screen is perpendicular to the solar wind, only drag is generated but the amount is double that of electric sail of the same area. Ambient solar wind protons entering the dipole drive field from the negative screen side are reflected out, with the angle of incidence equaling the angle of reflection, thereby providing lift if the screen is placed at an angle to the plasma wind. In contrast to the single positively charged screen employed by the electric sail, the dipole drive is constructed from two parallel screens, one charged positive, the other negative, creating an electric field between them with no significant field outside. The dipole drive remedies two shortcomings of the classic electric sail in that it can generate thrust within planetary magnetospheres and it can generate thrust in any direction in interplanetary space. The dipole drive is a new propulsion system which uses ambient space plasma as propellant, thereby avoiding the need to carry any of its own. Read on for a look at a form of enhanced electric sail the author has christened the Dipole Drive. Zubrin presented the work at last April’s Breakthrough Discuss meeting, and today he fills us in on its principles and advantages. Now Zubrin looks at moving beyond both this and solar wind-deflecting electric sails toward an ingenious propellantless solution. Robert Zubrin’s work with Dana Andrews has helped us see how so-called magnetic sails (magsails) could be used to decelerate a craft as it moved into a destination system. ![]() But even as we enter the early days of solar sail experimentation in space, we look toward ways of improving them by somehow getting around their need for solar photons. One reason we look so often at sail technologies in these pages is that they offer us ways of leaving the propellant behind. ![]()
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