Robert Zubrin has provided more details and analysis of his new dipole drive to Centauri Dreams. The main new information is Zubrin describes the relative simplicity of building simple double sail systems.
* dipole drives are buildable to improve cubesat propulsion
* dipole drives could be used to move around the earth and moon and reduce fuel usage
* dipole could reduce trip times for space telescopes to gravitational lens points 500 AU away to less than ten years. This would be 2 to 5 times less than other gravitational lens missions.
* power beaming could further enhance dipole drive performance
The dipole drive is made from two parallel screens, one charged positive, the other negative, creating an electric field between them with no significant field outside.
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. This provides lift if the screen is placed at an angle to the plasma wind.
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.
To accelerate within a magnetosphere, the positive screen is positioned forward in the direction of orbital motion. Ions entering are then propelled from the positive to the negative screen and then out beyond, while electrons are reflected. 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.
Assuming near-term space power systems with about one kilogram per kilowatt, 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.
Power beaming could enable a more powerful dipole drive.
Aluminized Spectra or aluminized carbon fiber for dipole sails and building dipole sails
The deployment of large scoops composed of two parallel, oppositely charged meshes poses operational and design issues.
If there were two sails of 500 meters radius separated by 500 m with a 2 kV potential difference. Then the electric field between them will be 4 volts/meter.
The total electrostatic force of each sail will be 0.1 mN. This is about a tenth the thrust force exerted by the screens themselves. Nevertheless, as small as they are, both of these forces will need to be negated. This can be done either with structural supports or by rotating the spacecraft and using artificial gravity to hold the sails out perpendicular to the axis of rotation. An alternative is to use the self-repulsion of the charge of each sail to help hold it out flat. In such a configuration two sails held separate from each other by a boom
A critical issue is the material to be used to create the dipole drive. In his original paper on the classic electric sail, Pekka Janhunen suggested using copper wires with diameters between 2.5 and 10 microns. This is not an optimal choice, as copper has a much lower strength to mass ratio than aluminum, and such thin strands would be quite delicate. For this reason, in the above examples we specified aluminum wire with 100-micron diameters. A potentially much better option, however, might be to use aluminized Spectra, as spectra has about 10 times the yield strength of aluminum, and roughly 1/3 the density (Aluminum 40,000 psi, 2700 kg/m3, compared to Spectra 400,000 psi, 970 kg/m3.). Spectra strands with 100-micron diameters and a coating of 1 micron of aluminum could thus be a far superior material for dipole drive system, and classic electric sails as well. An issue however is Spectra’s low melting point of 147 C. Kevlar, however, with a yield strength of 200,000 psi, a density of 1230 kg/m3, and a melting point of 500 C could provide a good compromise. Still another promising option might be aluminized strands made of high strength carbon fiber, such as the T1000G (924,000 psi, 1800 kg/m3) produced by Toray Carbon Fibers America.
Unlike magnetic sails and electric sails, it can generate both lift and drag, and its maximum velocity is not limited by the speed of the solar wind. Near-term dipole drives could be used to provide a reliable, low cost, low mass technology to enable propellantless movement of spacecraft from one orbit to another, to provide station keeping propulsion, or to deorbit satellites, as required.
A dipole drive could potentially reduce the mass of a manned lunar mission to within the launch capacity of a single Falcon Heavy. Because it needs no propellant, the dipole drive offers the unique advantage of being able to provide its propulsion service to any spacecraft indefinitely.
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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