One reason we look so often at sail technologies in these pages is that they offer us ways of leaving the propellant behind. 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. 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. Now Zubrin looks at moving beyond both this and solar wind-deflecting electric sails toward an ingenious propellantless solution. Zubrin presented the work at last April’s Breakthrough Discuss meeting, and today he fills us in on its principles and advantages. Read on for a look at a form of enhanced electric sail the author has christened the Dipole Drive.
by Robert Zubrin
Abstract
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. 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. 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. 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. 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. 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. To deorbit, the negative screen is positioned forward, turning the screen into an ion reflector. 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. In contrast to the electric sail, the ultimate velocity of the dipole drive is not limited by the speed of the solar wind. It therefore offers potential as a means of achieving ultra-high velocities necessary for interstellar flight.
Background
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. For this reason, engineers have long been interested in propulsion systems that require no propellant.
The best known propellantless system is the solar sail, which derives its thrust by reflecting light emitted by the Sun. 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. Moreover, because photons move so swiftly, the amount of thrust that can be derived by reflecting light is at best 0.0067 mN/kW (at 100% reflectance, full normal incidence), which means that very large sails, which necessarily must have significant mass and be difficult to deploy, must be used to generate appreciable thrust. As a result, while solar sails have been studied since the time of Tsiolokovsky [1], we are only now beginning to experiment with them in space.
An alternative to the solar sail is the magnetic sail, or magsail, which was first proposed by Zubrin and Andrews in 1988, and subsequently analyzed extensively by them in a variety of further papers [2,3] in the 1990s. The magnetic sail uses a loop of superconducting wire to generate a magnetosphere to deflect the solar wind. Assuming the development of high temperature superconducting wire with the same current density as existing low temperature superconductors, a magsail should be able to generate significantly higher thrust to weight than is possible with solar sails. However such wire has yet to be developed.
Another propellantless propulsion system of interest is the electric sail [4], which like the magsail operates by deflecting the solar wind, in its case by using an electrostatic charge. As a result, like the magsail, the classic electric sail (electric sail) cannot operate inside of a planetary magnetosphere other than as a drag device, has its thrust decrease with distance from the Sun, and is limited in the potential direction of its thrust. Because of the low momentum density of the solar wind, electric sails must be even bigger than solar sails. However, because only sparsely spaced thin wires are needed to create sail area, higher thrust to mass ratios can be achieved than are possible using solar sails which require solid sheets of aluminized plastic.
Electrodynamic tethers [5] have also been proposed, which use the interaction of a current in a tether with the Earth’s geomagnetic field to produce thrust. In addition to facing a variety of engineering and operational issues, however, such systems can only operate in a planetary magnetic field and can only thrust in a direction normal to the field lines, a consideration which limits their applicability.
Finally, we note recent claims for a system called the EM Drive [6], which according to its proponents can generate about 1 mN/kWe, in any direction, without the use of propellant, an external light source or plasma wind, or magnetic field. Such performance would be of considerable interest. However, as it appears to contradict the laws of physics, there is reason to suspect that the measurements supporting it may be erroneous.
As a result, there clearly remains a need for a new type of propellantless propulsion system, which can operate both inside and outside of a planetary magnetosphere, can thrust in a multitude of directions, and which is not dependent upon sunlight or the solar wind as a momentum source. The dipole drive is such a system.
The Dipole Drive
The principle of operation of the dipole drive while accelerating a spacecraft within a planetary magnetosphere is illustrated in Fig. 1 below.
Fig. 1. The Dipole Drive Accelerating within a Magnetosphere.
In Fig. 1 we see two parallel screens, with the one on the left charged positive and the one on the right charged negative. There is thus an electric field between them, and effectively no field outside of them, as on the outside the field of each screen negates the other. There is also a voltage drop between the two, which for purposes of this example we will take to be 64 volts.
Protons entering the field region from the left are accelerated towards the right and then outward through the right-hand screen, after which they escape the field and experience no further force. Protons entering from the right are reflected towards the right, adding their momentum to that generated by the protons accelerated from left to right. There is thus a net proton current from left to right, and a net proton thrust towards the left.
In the case of electrons, the situation is exactly the opposite, with a net electron current from right to left, and a net electron thrust towards the right. Note that while electrons entering from the right will be greatly accelerated by the field, reflected electrons will only be reflected with their initial velocity. There will also be an electron current through the outside plasma to neutralize the net proton flow to the right.
Because space plasmas are electrically neutral, the number density of both electrons and ions (which for the moment we will consider to be protons, but may which – advantageously – be heavier species, as we shall discuss later) will be the same, so the proton and electron electrical currents will be equal, as will the power associated with each of them. However because the mass of a proton is about 1842 times as great as the mass of an electron, the thrust of the proton current will be about 43 times greater than the opposing electron current thrust (because the momentum of particles of equal energy will scale as the square root of their mass, sqrt(1842)=43) and the system will generate a net thrust. The acceleration of the electrons is a form of drag, which is provided for by loss of spacecraft kinetic energy. It therefore could, in principle be used to generate electric power, partially compensating for the power consumed to accelerate the protons. In the following examples, however, we will assume that there is no provision for doing this, i.e. that the efficiency of any such energy recovery is zero.
To see what the performance of a dipole drive might be, let us work an example, assuming a 500 W power source to drive the system. The electron current negates about 2% of the thrust (1/43rd) produced by the proton current. The maximum possible jet power is thus about 490 Wj. Assuming additional inefficiencies, we will round this down to 400 Wj, for a total system electrical to jet power efficiency of 0.8.
A Coulomb of protons has a mass of 0.011 milligrams. If the jet power is 400 W, and the potential difference is 64 V, so the proton current will be 6.25 A, and have a mass flow of 0.0652 mg/s.
The relationship of jet power (P) to mass flow (m) and exhaust velocity (c) is given by:
P = mc2/2 (1)
Taking P = 400 W and m = 0.0652 mg/s, we find that c= 110,780 m/s. Since thrust (T) is given by T=mc, we find:
T = mc = 7.2 mN (2)
This is a rather striking result. It will be recalled that the electrical power driving this system is 500 W. So what we are seeing here is thrust to power ratio of 14.4 mN/kWe, more than ten times better than that claimed for the EM Drive, but done entirely within the known laws of physics!
If it is desired to deorbit (decelerate) a spacecraft, the direction of the screens would be reversed, with the negative screen leading in the direction of orbital motion. In this case, the screens would become a proton reflector. An electric sail could also be used as a drag device to serve the same purpose. However, because the dipole drive doesn’t merely create drag against passing protons, but reflects them, it would create twice the drag of an electric sail of the same area. If the dipole drive is positioned obliquely to the wind angle, it can reflect protons, with the angle of incidence equaling the angle of reflection. For example, if it is tilted 45 degrees to the wind, a force will be generated perpendicular to the wind, that is “lift” will be created. Such maneuvers could also be done with the dipole drive in acceleration mode, deflecting protons to combine lift with thrust. Using this capability, a dipole drive propelled spacecraft in orbit around a planet could execute inclination changes.
To summarize, in contrast to the electric sail which can only create drag against the wind to lower its orbit, the dipole drive can thrust in any direction, raising or lowering its orbit or changing its orbital inclination. In addition, when used as a drag device, the dipole drive can create twice the drag per unit area as the electric sail.
The Dipole Drive in Planetary Orbit
Let us therefore analyze the system further. The dipole drive exerts no field outside of its screens, so the only plasma it collects is the result of its own motion through the surrounding medium. So how big does its screen need to be?
We consider first the case of the above described dipole drive system operating in LEO at an altitude of 400 km, being used to thrust in the direction of orbital motion. It is moving forward at an orbital velocity of 7760 m/s. The average density of ions at this altitude is about 1,000,000 per cc. Assuming (conservatively) that all the ions are protons, the required ion mass flow of 0.0652 mg/s would be swept up by a screen with a radius of 127 m.
It may be noted however, that at 400 km altitude there are also O+ ions, each with a mass 16 times that of a proton, with a numerical density of about 100,000/cc. These therefore more than double the ion mass density provided by the protons alone. If these are taken into account, the required scoop radius would drop to about 80 m.
Another way to reduce the scoop size would be by going to higher voltage, so that more power can be delivered to a smaller number of ions. If, for example, we quadrupled the voltage to 256 volts, the exhaust velocity would double, to 222 km/s, allowing us to cut the mass flow by a factor of four, and the scoop radius by a factor of two, to just 40 m. The thrust, however, would be cut in half, giving us 3.6 mN/kWe.
As we go up in altitude, the plasma density decreases, as does the orbital velocity, requiring us to go to larger scoops. Examples of 500 W dipole drive systems operating at a variety of altitudes are provided in Table 1. In Table 1, Vo and C are orbital velocity and exhaust velocity, in km/s.
Table 1. Dipole Drive Systems Operating in Earth Orbit (Power=500 W)
It can be seen that the dipole drive is a very attractive system for maneuvering around from LEO to MEO orbits, as the high ion density makes the required scoop size quite modest. It should be emphasized that the above numbers are for a 500 W system. If a 5 W dipole drive thruster were employed by a microsatellite, the required scoop areas would be reduced by a factor of 100, and the radius by a factor of 10.
It may be noted that Mars, Venus and Jupiter all have ion densities in low orbit comparable to those above. For example, Mars has 500,000/cc at 300 km, Venus has 300,000/cc at 150 km, and Jupiter has 100,000/cc at 200 km, making the dipole drive attractive for use around such planets as well. Many of the moons of the outer planets also have ionospheres, and the dipole drive should work very well in such environments.
As one ascends to higher orbits, the density of ions decreases dramatically, while the orbital speed decreases as well. For example, in GEO, the ion density is only about 20/cc, while the orbital velocity is 3 km/s. These two factors combine to make much larger scoops necessary. So, for example, in GEO, a 500 W dipole drive operating at 1024 volts would need a scoop 3.6 km in radius.
Because the effectiveness of the dipole drive decreases at higher altitudes while operating within the magnetosphere, the best way for a dipole drive propelled spacecraft to escape the Earth is not to continually thrust, as this would cause it to spiral out to trans GEO regions where it would become ineffective. Rather, what should be done is to only employ it on thrust arcs of perhaps 30 degrees around its perigee, delivering a series of perigee kicks that would raise its apogee on the other side of its orbit higher and higher until it escaped the magnetosphere and became able to access the solar wind.
The Dipole Drive in Interplanetary Space
The dipole drive can also operate in interplanetary space. Compared to planetary orbit, the ion densities are lower, but this is partially compensated for by much higher spacecraft velocities relative to the plasma wind. As a result, the required scoop sizes are increased compared to planetary orbital applications, but not by as much as considerations of ion density alone might imply.
Let us consider the case of a dipole drive traveling in heliocentric space at 1 AU, positioned at an angle of 45 degrees to the wind, with its negative screen on the sunward side. It would thus reflect solar wind protons 90 degrees, thereby accelerating itself forward in the direction of orbital motion. A diagram showing the dipole drive operating as a sail in interplanetary space is shown in Fig. 2.
Fig. 2 The Dipole Drive Operating as a Sail in Interplanetary Space.
The solar wind has a velocity of 500 km/s, so to insure reflection, we employ a voltage of 2028 volts, sufficient to reverse the motion of a proton moving as fast as 630 km/s. With a density of 6 million protons per cubic meter, the wind has a dynamic pressure of 1.25 nN/m2. As the sail is positioned 45 degrees obliquely to the wind, its effective area will be reduced by a factor of 0.707, with the thrust reduced to 0.9 nN/m2. In this case, virtually all of the protons hitting the sail will be coming from the sunward side, and since they are reflected without adding any kinetic energy, no power is required to drive them. However, we still have an electron current coming from the sunward side being accelerated outward. This requires power. With 500 W, total radial thrust would be 1.27 mN, with 1.27 mN also delivered in the direction of orbital motion, for a L/D ratio of 1. The total effective screen area would therefore need to be 1,414,000 m2, with an actual area of 2,000,000 m2, requiring a radius of 798 m. Total thrust to power would be 3.6 mN/kWe.
If instead we had not concerned ourselves with obtaining complete deflection of each particle, we could have used a lower voltage. This would increase the thrust per unit power, but increase the required sail area for a given amount of thrust. So, for example, if we chose 512 volts, we would have a total thrust of 3.6 mN, for a thrust/power ratio of 7.2mN/kWe, but need a sail radius of 1127 m.
It may be noted that all of these results are for a 500 W dipole drive. A microsatellite might employ a 5 W dipole drive, in which case the required scoop radii would drop by a factor of 10.
The thrust and diameter of a 1 kWe dipole drive system operating as a solar wind sail in interplanetary space at 1 AU is shown in fig. 3.
Fig. 3. Thrust and Diameter of a 1 kWe dipole drive system operating as a solar wind sail in interplanetary space.
Use of the Dipole Drive for Interstellar Flight
In contrast to the electric sail, the dipole drive can be used to accelerate a spacecraft at velocities greater than that of the solar wind. For example, consider a spacecraft moving away from the Sun at a velocity of 1000 km/s. The solar wind is following it at a velocity of 500 km/s, so relative to the spacecraft there is a wind moving inward towards the sun at a velocity of 500 km/s. In this case, to accelerate the spacecraft would direct its positive screen away from the sun. This would cause it to accelerate protons sunward, while reflecting electrons outward, for a net outward thrust. At 500 km/s the protons are approaching the spacecraft with a kinetic energy equal to 1300 volts. It can be shown that employing a screen voltage difference that is about triple the kinetic voltage produces an optimal design for an accelerating system, while one using a voltage difference equal to the kinetic voltage is optimal for deceleration. This is illustrated in figs 4 and 5 which respectively show the kinetic voltage as a function of velocity, and the relative power/ thrust and area/thrust ratios of the spacecraft as a function of the dimensionless parameter Z, where Z=(engine voltage)/(kinetic voltage.)
Fig 4. Kinetic Voltage as a function of spacecraft velocity.
Fig 5. Relative Power/Thrust and Area/Thrust as a function of Z=(engine voltage)/(kinetic voltage.) There is a step factor of 2 increase in thrust during deceleration when Z reaches 1, because protons are reflected. For acceleration, Power/Thrust ~ 1 + sqrt(1+Z), while Area/Thrust ~ 1/(-1 + sqrt(1+Z)).
If we add 3900 volts to the incoming protons, quadrupling their energy, we will double their velocity relative to the spacecraft, thereby providing an effective exhaust velocity of 500 km/s. The solar wind has a density of 6 million protons/m3 at 1 AU, with ambient density decreasing to 1 million/m3 in interstellar space. If we take the former value, we get a thrust of (1.67e-27 kg/proton)(500,000m/s)2(6,000,000/m3) = 2.5 nN/m2. If we take the latter value, it would be 0.42 nN/m2. The proton current at the smaller value would be 80 nA/m2, which at 3900 volts works out to 0.312 mW/m2. The thrust to power ratio would therefore be 1.35 mN/kW. (This ratio would also hold true at the 1 AU value, but the magnitudes of both the thrust and power per unit area would be six times greater.)
If a dipole drive powered spacecraft were receding 500 km/s directly away from the Sun, it would see no relative wind and thus produce no thrust. However, like a modern sailboat that can sail faster crosswind than downwind, because it can generate lift, the dipole drive can get to speeds above 500 km/s by sailing across the wind. As the spacecraft’s crosswind speed increases, it becomes advisable to turn the sail to ever greater angles to the solar wind and increasingly normal to the crosswind. As this occurs, the L/D resulting from solar wind reflection increases while the total solar wind thrust decreases. At the same time, however, thrust resulting from the acceleration through the screens of crosswind protons increases, maintaining total thrust constant at ever higher L/D (relative to the solar wind) levels. Once the crosswind velocity exceeds the solar wind velocity the solar wind becomes increasingly irrelevant and the dipole drive becomes a pure acceleration system, driving the incoming crosswind plasma behind it to produce thrust,
As the speed of the spacecraft increases relative to the wind, it is necessary to increase the voltage in order maintain thrust/power ratio efficiency. For example, let’s say we want to achieve 3000 km/s, or 0.01c. Then the kinetic energy equivalent voltage of the approaching protons would be 47 kV. So, to double this velocity we need to quadruple the total voltage, or add a sail voltage drop of 141 kV. The proton current would have a value of 480 nA/m2, with a power of 68 mW/m2. The thrust would be 15.1 nN/m2, for a thrust to power ratio of 0.22 mN/kW.
It may be observed that since the necessary voltage increases as the square of the velocity, with power increasing with voltage but thrust increasing with velocity, the thrust to power ratio of the dipole drive decreases linearly with velocity. This puts limitations on the ultimate velocity achievable. For example, the most optimistic projections for advanced large space nuclear power systems project a mass to power ratio of 1 kg/kW. If we accept this number, then, neglecting the mass of any payload or the dipole drive system itself, then the system described in the previous paragraph performing with a thrust to power ratio of 0.22mN/kilowatt at 3000 km/s would have an acceleration of 0.00022m/s2, or 7 km/s per year. The average acceleration getting up to 3000 km/s would be twice this, so the spacecraft would take 214 years to reach this speed. During this time it would travel 1.07 light years. To reach 6000 km/s (0.02 c) starting from negligible velocity would require 857 years, during which time the spacecraft would travel 8.57 light years. The performance of such a system is shown in Table 2. Note 63,000 AU = 1 light year. The performance shown assumes an advanced 1 kg/kWe power supply. If a more near-term power system with a higher mass/power is assumed, the time to reach any given distance increases as the square root of the mass/power ratio. So for example, if we assume a conservative near-term space nuclear power reactor with a mass/power ratio of 25 kg/kW, the time required to reach any given distance would increase by a factor of 5.
Table 2. Advanced Dipole Drive Performance for Ultra High-Speed Missions (1 kg/kW power)
It can be seen that advanced dipole drive spacecraft could be quite promising as a method of propulsion for missions to near interstellar space, for example voyages to the Sun’s gravitational focus at 550 AU. Unless much lighter power systems can be devised than currently anticipated however, they would still require centuries to reach the nearest stars. Power beaming may provide an answer. However such technologies are outside the scope of this paper.
If a spacecraft has been accelerated to interstellar class velocities, whether by means of the dipole drive or any alternative technology, the dipole drive provides a means of deceleration without power (it could actually generate power) by creating drag against the relative plasma wind. This feat can also be done by a magnetic sail or an electric sail. However because it can also create lift as well as drag, the dipole drive offers much greater maneuverability during deceleration as well as a means to freely maneuver within the destination solar system after arrival.
Dipole Drive Design Issues
Let us consider the case of a 2 kg microsatellite operating in LEO, with 5 W of available power to drive a dipole drive. (Note, a typical CubeSat has a mass of 1.3 kg. At 20 kg/kWe, a 5 W solar array should have a mass of about 0.1 kg.) If we operate it with a voltage of 16 Volts, it will produce 28.8 mN/kWe, or 0.144 mN thrust over all. It would have an acceleration of 0.000072 m/s2. This would allow it to generate a ΔV of 2288 m/s in a year, sufficient to provide extensive station keeping propulsion, substantially change its inclination, or to raise it from a 400 km altitude orbit to a 700 km orbit in 1.6 months. To generate this much thrust at 400 km would require a scoop with a radius of 16 m, while doing so at 700 km would require a scoop with a radius of 58 m. Let us assume that the scoop is made of aluminum wire mesh, using wires 0.1 mm in diameter separated by distances of 2 m. Each square meter of mesh would thus have about 1 m length of wire. This needs to be doubled as there are two meshes, one positive and one negative. Therefore, a scoop with a radius of 16 m would have a mass of 32 grams. If the propulsion system were used simply for station keeping, inclination change, or deorbit functions at the 400 km altitude, that’s all that would be needed. To operate at 700 km, a 116 gram scoop would be required. From these examples we can see that the use of the dipole drive to provide propulsion for microsatellites in LEO could potentially be quite attractive, as the modest scoop sizes required do not pose major deployment challenges.
Now let us consider a 100 kg interplanetary spacecraft in interplanetary space, operating with 500 W at a voltage of 2028 volts. From the discussion above it can be seen that this would generate about 2.54 mN of thrust in the direction of orbital motion. The scoop would need to have a radius of about 800 m. In interplanetary space, the Debye shielding length is ~60 m, and so a screen with a 20 m mesh would suffice. Such a screen would have a mass of about 8.5 kg, which would be well within the spacecraft mass budget. The 2.54 mN thrust would accelerate the spacecraft at 0.000025 m/s2. It could thus impart a V to the spacecraft of about 804 m/s per year. Higher accelerations could be provided by increasing the spacecraft power to mass ratio.
The deployment of large scoops composed of two parallel, oppositely charged meshes poses operational and design issues. Prominent among these is the fact that the two opposite charged screens will attract each other. However the total force involved is not that large. For example, let us consider a configuration consisting to two sails of 500 m radius separated by 500 m with a 2 kV potential difference. Then the electric field between them will be 4 volts/m. The area of each screen will be 785,400 m2. From basic electrostatics we have EA = Q/ε, so Q, the charge of each screen will be given by Q=(4)(785,400)(8.85 e-12) = 0.000028 coulombs. The electrostatic force on each sail is given by F=QE, so 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 attached to their centers could be expected to curve towards each other at their edges until the stiffening self-repulsive force on each sail from its own charge balanced the bending forces exerted by the spacecraft’s acceleration, the push of the wind, and the attractive force of the opposite sail.
One way to avoid such issues would be to design the system as a literal dipole, with a rod holding a positive charge at its end to the front of the spacecraft, and a rod holding the negative charge pointing to the rear of the spacecraft. Seen from a distance, such a configuration is electrically neutral and would exert negligible field. However, in the zone between the charges, there is a strong field from one pole to the other. Particles entering this field along the rod center lines would experience the full voltage drop. Particles entering the field at some distance from this central axis would experience a lower voltage drop. The overall functional voltage of such a system, from the point of view of power consumption and exhaust velocity, would be an average over many particles entering the dipole field at all distances from its axis. This is obviously a more complex configuration to analyze than that of the two parallel screens discussed so far, but it may be much simpler to implement in practice on an actual spacecraft.
A critical issue is the material to be used to create the dipole drive. In his original paper on the classic electric sail [4], 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.
Some options for dipole drive spacecraft configurations are show in in Fig. 6. As can be seen, small dipole drive systems can be used for spacecraft control, for example as an empennage. Such small dipole drive units could also be used for attitude control on non-dipole drive spacecraft, such as solar sails.
Fig. 6. Options for dipole drive spacecraft configuration. Small dipole drive systems can be used for attitude control.
As with the electric sail, the dipole drive must deal with the issue of sail charge neutralization caused by the attraction of ambient electrons to the sail’s positive screen. In reference 4, P. Janhunen showed that the total such current that an electric sail would need to dispose of would be modest, entailing small power requirements if ejected from the spacecraft by a high voltage electron gun. In the case of the dipole drive, the current would be still smaller because the spacecraft has no net charge. In addition electrons acquired by the positive screen could be disposed of by using the power source to transport them to the negative screen. Alternatively, if an electron gun were used, its required voltage would be less than that needed by an electric sail because external to the screens, the dipole drive’s field is much weaker and falls off much more quickly. For these reasons, the issue of sail charge neutralization on the dipole drive should be quite manageable.
Because the dipole drive does not interact with plasma outside of the zone between its screens, the issue of Debye shielding of its screen system to outside charges is not a concern. Debye shielding of its individual wires within screens can be dealt with by means of adequately tight wire spacing. As shown by Janhunen [4], such spacing may be quite liberal (~60 m in near Earth interplanetary space), enabling sails with very low mass to area ratios. [7]
Conclusion
The dipole drive is a promising new technological concept that offers unique advantages for space propulsion. Requiring no propellant, it can be used to thrust in any direction, and both accelerate and decelerate spacecraft operating within planetary magnetospheres, in interplanetary space, and interstellar space. 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. Then dipole drive could also be used as a method of capturing interplanetary spacecraft into orbit around destination planets, or of lowering the orbits of spacecraft captured into initial elliptical orbits using high thrust propulsion. The latter application is particularly interesting, because it could enable a small lightweight lunar ascent vehicle to carry astronauts home from the Moon by launching directly from the lunar surface to trans-Earth injection and then subsequently lower itself to LEO to rendezvous with a space station or reentry capsule spacecraft without further use of propellant. Such an approach 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. While the dipole drive is most attractive in orbital space whether ambient plasma is thickest, it can be used in interplanetary space and even enable interstellar missions as well, becoming more attractive for such applications as ancillary technologies, such as power generation evolve.
There are many technical issues that need to be resolved before practical dipole drive spacecraft can become a reality. However both the theory of dipole drive operation and it potential benefits are clear. Work should therefore begin to advance it to flight status. The stars are worth the effort.
References
1. Jerome Wright (1992), Space Sailing, Gordon and Breach Science Publishers
2. D. G. Andrews and R. Zubrin, “Magnetic Sails and Interstellar Travel”, IAF-88-553, 1988
3. R. Zubrin and D.G Andrews, “Magnetic Sails and Interplanetary Travel,” AIAA-89-2441, AIAA/ASME Joint Propulsion Conference, Monterey, CA July 1989. Published in Journal of Spacecraft and Rockets, April 1991.
4. Pekka Janhunen, “Electric Sail for Spacecraft Propulsion,” J. Propulsion, Vol. 20, No. 4: Technical Notes, pp763-764. 2004.
5. Cosmo, M.L., and Lorenzini, E.C., Tethers in Space Handbook, NASA Marshall Space Flight Center, 1997
6. D. Hambling, “The Impossible EM Drive is Heading to Space,” Popular Mechanics, September 2, 2016.
7. “Debye Length,” Plasma Universe.com, https://www.plasma-universe.com/Debye_length accessed Feb 18, 2018.
Dr. Zubrin,
It’s a fascinating concept but I’d like to see a comparison toDr. Young Bae’s laser thruster concept for solar system travel.
https://www.nextbigfuture.com/2017/02/yk-bae-can-now-amplify-photonic-laser.html
Also, regarding your statement that the EMDrive, there are a range of reported results up hundreds and even around 1000 mN/KW for the Cannea results. The 1 mN/KW number is not the complete data set so far and further, is not from optimal engineered drives but low power test systems.
There is big difference between EMdrive and Dipole drive – EMdrive seams to be based on some new principles, and have to be tested and researched in the labs. When Dipole drive claimed (by author) to be based on the known physic , but in reality has nothing common with physical laws. The basic concept of Dipole drive supposes , that protons in electrical field (in described by author situation and system) will create greater thrust than electron – is absolute false.
Probably , there will be some electrical current between electrodes in this dipole, but no any thrust is possible in neutral media (plasma) the forces from positive and negative charged particles will be equal, and the particle mass difference plays no role in this fact. After charged particles acceleration in dipole electrical field summary forces and energy balance will stay ZERO…
I might be mistaken, but isn’t this basically the same kind of propulsion as with an ion drive. But the dipole drive use no Xenon fuel, instead propels with plasma from the surrounding space.
So it could work, but like one Ion drive, it will need to insert electrons together with the Ion so the craft don’t get electrical charge. And the ‘exhaust’ dont perhaps even fold back on the craft and cancel the trust.
Real life Ion drive takes care about electrons :-)
Dipole drive concept supposes (falsely) that :
if you will apply electrical potential to system composed by two electrodes (anode and cathode) that located in ionized media , the external drug force (thrust) will be applied to this system and this force has negative to positive electrodes direction.
Other words the pressure on this system from the negative electrode side will be higher than from positive electrode…
“The basic concept of Dipole drive supposes , that protons in electrical field (in described by author situation and system) will create greater thrust than electron – is absolute false.”
You’re assuming the plasma between the grids is neutral, because the plasma entering the front is neutral. But that’s not true: Protons being much more massive, they have a much longer residence time inside the grid than the electrons.
So you have a net positive plasma exposed to the inter-grid voltage. Thus a net force.
Another way of looking at it is that both protons and electrons are given the same increment of *energy* if they pass through the grid.
But energy is 1/2mv2, while momentum is mv. So a given amount of energy produces more momentum for heavier particles. Again, net production of momentum.
***
I can see other potential problems, though. If you’re reflecting protons at the front of the grid, and accelerating electrons out the back, the local plasma in front of the grid will be net positive charge, and behind net negative. Wouldn’t this tend to negate the dipole field of the grid itself?
This concept claims to be “real physic”, but in basic source , it have no connection to wishful thinking…
There is lot of fault I have found in this idea, but basic one is enough to eliminate whole idea…
Despite proton mass is more than 3 order higher than electron’s mass, proton and electron will always get exactly same energy amount (and as seauence same impulse) from the same electric field – it is the basic law of electrostatic… So in the neitral media that contans the same amount of negative and positive charged particles it will not produce any thrust…
There is additional discrepancy in the article, but this one fact is more than enough…
Sorry , but EMdrive or Mach drive are much more scientific based than this “idea”.
XDDDDD So a drive that contradicts well stablished physical principles is more real than one based on well stablished physical principles XDDDDDD
Sorry, but proposed Dipole drive contradict all modern physical principles, take this article and to any good high school physics teacher he can explain why this idea has no scientific ground.
There’s no specific example worked out. To assess whether such a spacecraft makes sense we need a clear description of the spacecraft’s embodiment: what is its mass, size, energy source for maintaining the potential, etc.?
I also wonder whether the ambient medium that the spacecraft is to be in, such as a planetary magnetosphere, would allow electrons coming in from the side to short out the potential. To assess that we would have to know what the local electric field is but, although the voltages are specified, the electric field is not because the gap has not been given. I’d like to know whether or not there’s any possibility that electrons coming in from the sides could short out this potential.
Energy that will get (give) charget particle in electric field is calculated by very simple equation:
W = C*U,
where :
C – is electrical charge
U – electrical potential difference (in this case voltage) between electrodes
So both proton and electron will get the exactly same energy from the proposed Dipole thruster, as sequence in electrically neutral media – there will be same thrust from electrons and protons (or ions), i.e. no thrust from this device, useless.
Now before I comment to what you have just written above, I have to stress that I only made a quick scan of what was being said in the article above, and so this is more reflexive and off-the-cuff, then a detailed rebuttal to what you have just stated.
But haven’t you made the assumption within your argument above that there exist an EQUAL number of positive and negative ions entering into the gap that exists between the two potential surfaces? Unless I’m mistaken, the article did not make a statement that at any particular instance, there existed an equal number of negative and positive charges existing that would diffuse into the gap. Admittedly, the author did state that OVERALL the ion species that exist in outer space is a type of neutral soup, but he did not specifically state that ions (positive and negative) diffuse into the electrified gap in equal numbers.
Or am I missing something?
Charley,
You make simple things more complicated that it is in reality :-)
Author wtires about neutral medai (plasma), this means that summary volume charge is ZERO, i.e. summary charge of positive and negative charged particles is equal.
Equal charges (positive and negative) will create the equal force i.e. equal energy collected (spreaded) in both directions, the difference in charged particles masses plays no role in this situation.
Authors makes huges mistake supposing that more massive positive charged particles will give higher force and as sequence thrust – modern physic deny this, author should invent the new electristatic laws the get this things work…
Yes, but you’re not quite getting it right. The energy a proton receives from the dipole is equal to the energy an electron receives, but the momentum is not. For example, if we take the proton as weighing 1600 times the electron (a crude approximation), then the speed added to the proton is 1/40 of the speed added to the electron. But since the proton weighs 1600 times as much, it will acquire 40 times as much momentum.
@Thomas Goodey
isnt both energy AND momentum conserved ?
Sure, but they’re not equal. They’re separately conserved.
For instance, energy is conserved within a rocket, so is momentum. But it still moves.
That is correct. If same amount of energy is given to two objects of differrnt mass, the heavier will receive more momentum, in proportion to the square root of the ratio of the two masses.
@Robert Zubrin,
“That is correct. If same amount of energy is given to two objects of differrnt mass, the heavier will receive more momentum, in proportion to the square root of the ratio of the two masses.” ?????????
From Wikipedia
The electric field, E → in units of newtons per coulomb or volts per meter, is a vector field that can be defined everywhere, except at the location of point charges (where it diverges to infinity).[2] It is defined as the electrostatic force F → in newtons on a hypothetical small test charge at the point due to Coulomb’s Law, divided by the magnitude of the charge q , in coulombs.
means force F = Eq ; means momentum = force F x
delta t;
means momentums are EQUAL , RIGHT ?
Yes agreed, if the charge is equal, the force shold be equal on both particles. F =qE where E is the field intensity.
https://books.google.co.uk/books?id=qzNdDtZUPXMC&pg=PA75
If the mass ratio is 1831 the ratio of accelerations is 1831. Then delta v = at gives final delta v in the ratio of 1831 to 1, so the momentum change is too, so it seems there should be a net momentum change equal and opposite for protons and electrons. As for the amount of energy, the electron is accelerated more so gets more energy as it depends on the square of the velocity.
However, the electron, traveling faster, would spend less time in the field so would accelerate less, if they both start at rest. S = 0.5 * a * t^2 so t^2= 2s/a so if the ratio is 1831 to 1, then the time is the square root of that so the total change of momentum changes as the square root of the mass ratio.
So – it does seem possible…
They aren’t at rest of course, so the analysis would be more complex. But my first impression is that yes – it does seem possible…
It would depend on the orientation and seems particles would spend more time in the field if it is orientated to slow them down rather than accelerate them. So if it was orientated to slow down the protons aned accelerate the electrons, the advantage would be even greater.
What do others think, is that analysis correct, or am I missing something?
In more detail: the electron, traveling faster, would spend less time in the field so would accelerate over a shorter period of time, if they both start at rest. S = 0.5 * a * t^2 so t^2= 2s/a so if the ratio is 1831 to 1, then the time varies inversely as the square root of the acceleration, so the total change of velocity changes as the square root of the mass ratio. While the energy change is the same for both. For proton m v^2, for electron (1/1831) m (v*sqrt(1831))^2 where m is the mass of a proton. For momentum, for proton mv, for electron, (1/1831) m (v*sqrt(1831))
So energy change is same for both, rather than momentum, which is 23 times greater for the proton.
It would depend on the orientation and seems particles would spend more time in the field if it is orientated to slow them down rather than accelerate them. So if it was oriented to slow down the protons and accelerate the electrons, the advantage would be even greater. If oriented to slow down the electrons then some of the advantage is lost.
^1831 should be 1842 and 23 should be 43 in the above
I suppose that you (and author) cannot apply momentum conservation law to this particular case, i.e. when dipole is moving through the some media composed by charged particles (plasma). In this case expected movement can be caused only by electrical forces and stochastic plasma’s (Brownian ) motion , no other (propultion) mass/momentum exchange, It is the basic author’s mistake and it seams to me now, that most commenters in this topic do not understand this fact…
By the way there is lot of other mistakes in this concept, some of it was posted here by other commenters.
My notes about momentum conservation law are related only to the particular case – i.e. proposed dipole drive and fallwing speculations/calculation of expected momentum, i.e. non correct use of this physic law.
To me this looks a lot like the Bussard ramjet, without the nuclear fusion.
I really appreciate Dr. Zubrin’s post here, as it expands on his talk and addresses some questions I had with the device.
In my ignorance of the technology, I am missing the knowledge on what conditions allow the protons to pass the +ve shield and be accelerated by the -ve shield in one condition (Fig 1), but be reflected by the +ve shield in another (Fig 2).
As the acceleration of the drive is quite low, especially in interplanetary space, I wonder if there might be a role for “plasma scoops” to focus more particles onto the shield.
As particle density is important, this might well be a candidate for a drive using particle beams to increase the acceleration potential.
Given the scaling possibilities, it seems that a CubeSat with this drive could be easily manufactured, deployed and tested with very low cost.
As regards deployment, as the wires are so delicate, perhaps it is worth the mass penalty of putting the wires on an inflatable structure to deploy them from a compact state for testing purposes. For large structures, rotation seems like a good way to proceed. The mesh would look like a spiderweb with small masses at the end of the radial treads.
Dear Bob
Very interesting concept. I see this as a possible second stage for an interstellar solar sail. Of course, the proton density of the local interstellar medium is a lot less than the value in your illustration. If power-beaming technology of Project Breakthrough pans out, this might be how the necessary s/c power is provided. So I hope that the Dipole Drive proves feasible. All the best with it!
Regards, Greg
I like this concept! I really do!
It seems to be a new and relatively refreshing idea that I don’t believe it. I’ve ever heard about in any other venue. Who is the original originator of this particular idea? Does anybody know?
The distinct advantage that I see behind this is that you take advantage of ubiquitous space ions which (I presume) exist virtually, and freely appear to be a fairly common feature of the outer space environment. It only necessitates that one carry some type of electrical power source that you can readily tap into to provide the electric field necessary to create your propulsion.
The only disadvantage is that I see here within the concept is the idea that:
1. The craft obtains a sufficient velocity and begins to encounter other external drag factors which can limit (which I presume) are present in the environment.
2. The very electrical charge that you used to obtain your field effect would (seemingly almost by necessity) extend beyond the boundaries of your sail and would interact perhaps in a non-predictive manner and would result in both drag and perhaps torquing effects. But that is totally speculative. Otherwise, the compactness and simplicities speak well for the concept in and of themselves.
This is only one disadvantage of dipole drive (as it is dexcribed by author) – modern physic dies not allow it to create any thrust.
Good point – this drive probably can be used as vacuum tube to amplify ETI radio signals …
” Who is the original originator of this particular idea? ”
No idea. I thought it up back in the early 70’s as a bright teenager brainstorming potential interstellar propulsion technologies. but then immediately rejected it because the thrust was so very low. (Had a whole stack of 3×5 card with goofy ideas. Haven’t seen it in decades.) Admittedly, I was analyzing it at lower voltages with both ion species assumed to pass through the grid. It didn’t occur to me to analyze it in a reflection mode.
I suspect it’s one of those ideas that occur to a lot of people, and get dropped immediately without any substantial analysis.
“Unless much lighter power systems can be devised than currently anticipated however, they would still require centuries to reach the nearest stars. ”
What’s wrong with centuries? It’s millennia that start becoming impractical!
“(Aluminum 40,000 psi, 2700 kg/m3, compared to Spectra 400,000 psi, 970 kg/m3.)”
You slipped there. Don’t mix unit systems!
The idea of this system is very clever.
Although it make sense to me, but I think the accumulation of electron on the one side of the payload will quickly neutralize the potential. The dipole is described as two electrostatic grid with no current flowing. I do not see where the power requirement is calculated from. What mechanism is used to feed the power into the system?
Maybe the power requirement is to compensate for the electron accumulating on the positive side? I think something is missing in this concept (or the description of it).
I will try again…
Quote from autor’s article:
“because the mass of a proton is about 1842 times as great as the mass of an electron, the thrust of the proton current will be about 43 times greater than the opposing electron current thrust ”
Sorry, but it is the false conclusion, electristatic forces for positive (protons, ions) and/or negative charged particles (electrons) and also forces applied to Dipole electrodes are equal, so no any thrust can be expected from this concept.
Agreed. In the case of a stationary (with respect to plasma current) scenario the reaction forces due to accelerated cations and electrons would be balanced. However when the + screen is moving into a plasma wind, acting as a proton scoop, one would expect no electrons to be entering the – screen (from the opposite direction) to generate a balancing force. An unbalanced force will cause an acceleration. So perhaps this concept still has some application after all?
I suppose that this particular Dipole (as it is described by author) will not move even if it will be placed into tha plasma composed by charged particles that all have the same charge (i.e. protons/cations only, or electrons/anions only) :-)
The charged particles contained in the Earth’s magnetic field, e.g., the Van Allen belts are within Earth’s gravity well. Consequently, Zubrin’s dipole drive can’t reach escape velocity without the addition of chemical propellants. Also VASIMR is much smaller, and even with only 500 KM power source it uses today, the VASIMR is faster than Zubrin’s dipole drive. VASIMR already works, is proven and built and ready to go to send astronauts into deep space.
Excuse 500 KW power source.
The VASIMR engine has taken some 20 years to reach TRL 5 next year. Several of the commentators here are not sure if the theoretical basis of this engine is sound. I do hope to see this in my lifetime
Wall of text. Submit to a legitimate peer reviewed Journal please.
If Robert Zubrin didn’t exist, NASA would need to engineer an replacement at high cost! Always a pleasure to read his work!
Though I think I found a very very minor typo: I guess the voltage of 2028 should be 2048 volt instead (as the author goes stepwise from 1 volt by doubling the voltage multiple times).
The interesting question is if accelerating electrons and protons would accelerate an spacecraft (or if the forces would cancel out), as some have pointed out here.
First of all, I need to look up why ion thrusters tend to use heavy ions (as initially I thought: “Duh, ion thruster use heavy ions for a reason”):
It is the ratio between ion mass to ionization energy. As the ionization energy (as far as I know) depends on electron orbital energies, and should stay (within less than an order of magnitude I guess) roughly the same, ion thrusters favor heavy ions (e.g. Xenon).
For the dipole drive, this is not the case: the protons (and electrons) are already “ionized”.
So would accelerated protons and electrons impart a different force onto the spacecraft?
Both electrons and protons see the same force, giving different accelerations:
a = f / m
f = a * m
The electron will get accelerated 42 times as much, imparting on both the same energy *PER TIME*.
ae = f / me
ap = f / mp
ap * mp = ae * me
mp = me * 42
ap * (me * 42) = ae * me
ae = 42 * ap
But the electron will leave the field much faster. How much faster?
The time is given by:
t = d / ( 0.5 * SQRT( 2 * a * d ) )
If we set the distance d to 1 meter, and the acceleration of the proton ap to 1 meter per second squared we get
tp = 1.41 seconds
With ap of 42 meters per second squared
te = 0.21 seconds
tp / te = 6.48
So as a rough estimate (neglecting the real acceleration in a real electric field) the energy imparted onto the proton should be 6.5 times the energy imparted onto the proton (and conversely the energy imparted into the spacecraft).
Of course, this was just a quit “back of an envelope” calculation, I could be wrong, I could be missing things, …
Should read:
With ae of 42 meters per second squared we get
te = 0.21 seconds
Damn, used the wrong ratio of 42 instead of 1842.
Egg, meet face.
My point still stands, while the numbers are obviously wrong.
So for d=1 meter and ap=1 meter / second squared we get
te = 0.03295 seconds
and
tp / te = 42.9185
Sorry to disturb you, but energy imparted to proton will be always same as energy imparted to electron… it is physical law, if during your caclulation you got to opposite conclusion, you should return to the beginning and make it again , till you will get equality.
Meanwhile I see that many commenters confused by comparation between two unrelated things, it is same as comare equality between “round” and “green” :-)
By the way, why do you compute acceleration of proton and electron?
Why don’t you compute dipole drive acceleration from the same forces?
According your (and Zubrin’s) approach and conclusion dipole drive will get much higher acceleration than electron – because Dipole drive has mass multiple orders higher than electron or proton :-)
Simple open your mind and forget physic.
AlexT:
You need to brush up on kinematics!
Please be so kind to take time to understand these topics before posting again.
1. An electric field will impart the same force on an electron as on the proton. So it will accelerate the (light) electron much much stronger than the much heavier proton.
Acceleration = Force / Mass
In layman terms:
The electron will fly off (out of the field) very very quickly, while the proton lingers much longer in the electric field.
So the electron will experience the force only for a very very short period of time, while the proton will experience the force much longer.
2. Force is equal to opposing force.
In layman’s terms:
There is the force acting on the spacecraft you inquired about.
My sincere apologies but your phrase (quote following): “the energy imparted onto the proton should be 6.5 times the energy imparted onto the proton”
Has nothing common not with kintecis, nor with physic in our Universe.
Some notes on you calculations:
The kinematic energy that charged particle will collect (loose) moving in electric field (let suppose for simplicity it is moving in direction of electric field vector) can be calculated by two “alternative” ways (equations):
1. W = Q * U / 2, where:
Q – is charge of particle (Coulomb)
U – potential difference (voltage) along measured way (Volts)
2. Purely kinematics equation:
W = F * S, where
F – force applied to particle (Newton)
S – distance that particle “travels” in electrical field (meter)
So wandering how can you get to conclusion that energy collected by proton will be higher than energy collected by electron , when both moving in the same electrical field along the same distance…
Your analysis is I believe quite correct, even if you have made a calculational mistake.
However, in thinking this over just a bit more I began to wonder whether or not this particular concept may suffer from one small flaw.
If you are accelerating charges with the intention of creating force on the spacecraft you have a situation in which you have spatial charge separation, which inevitably do to charge buildup begins to create an external field. How such a charge separational field should interact with the grid and perhaps begin to neutralize its accelerational properties remains to be seen. It may be expensive enough, this charge separated field to in fact, nullify the ability to accelerate the space plasma, as well as possibly create facts with the space plasma that is to enter the grid for acceleration.
If you think that next Tony’s calculation is correct:
“the energy imparted onto the proton should be 6.5 times the energy imparted onto the proton”, you think you should not be bithered by any “charge separationsl field” , everything is possible in the Universe , that is not limited by first law of thermodynamics.
I have been thinking about the opposing force created by the external field, and my very very rough back-of-an-envelope-guesstimate is that the opposing force of the external field will be about one third (1/3) of the desired force created by the internal field.
So for every Newton the internal field creates, the external field will give an opposing force of about 0.33 Newton – give or take…
My reasoning goes as follows:
First we take a 2D view of the two poles (the two plates of the “capacitor”)
Then divide the area by four lines:
– Two lines through the plates
– Two line at the ends of the plates, at 90 degrees to the plates
(This should work in 3D as with planes as well, but 2D is simpler to explain and visualize)
Now we have 9 sections: The internal field in the middle, surrounded by 8 sections of the external field.
The external field consists of the following sections:
– One section “in front”, and one section “behind”, both roughly with field lines going in the wrong direction. (Boooo!)
– One section “on the left”, and one section “on the right”, both roughly with field lines going in the right direction. (Hurray!)
– Finally there are the four corner sections, where the contribution of the field lines cancel each other out. (We can live with that.)
If we are pessimistic, we say that the sections “on the side” don’t matter because they are so small, and for the other 6 external sections we end up with about 2 out of 6 where the field that contributes to an opposing force – so this is a very rough estimate that yields an opposing force of one third of the force created by the internal field. But we would need to go and put different spacecraft geometries into a simulation to see what effect different plate size to plate distance ratios have.
AlexT:
If you can not contribute anything meaningful besides scathing cynicism, why do you not stay quiet?
And why are you so impolite anyways? If you are right, you can calmly explain why you are right and why we are wrong.
After all, if we are as wrong as you say, we should be able to embarrass ourselves without your help.
And until you start to contribute something meaningful, I will ignore you.
Charley: I have been skeptically wondering along similar lines of thought – my gut told me reality could be be harsher.
With regards to field neutralization, I think this will be a no problem in reality. Only on very long distance flight (think interstellar), one might get problems with the deposition of protons on one dipole, or erosion of the other pole … that is not a question the back of an envelope can answer. But other than that, on “short distances” I think maintaining charge separation should be no big obstacle.
With regards to the external field: That is a much more interesting question, and I fear this could bring down the actual efficiency of the dipole drive by quite a bit. This is a question simulation should be able to answer.
Charley, after some more thinking about this, I think I was wrong about the “1/3 opposing force” of the external field.
The external field will cancel itself out (or even create a little additional desired force) – as far as I see the external field should not create any problems with regards to thrust.
The field lines go in one arc from one plate around the spacecraft to the other plate. On each field line you can mark two “reversal points” where the field line is parallel to the poles/plates. With this we can divide each arc into three sections:
– First one arc-section from the pole/plate to the first reversal point (with the field lines pointing in the wrong direction, giving force in the wrong direction)
– Then an arc-section from the first reversal point to the second (with the field lines pointing in the right direction, giving force in the right direction)
– And finally the arc-section from the second point to the other plate (and again wrong direction of field lines, giving force in the wrong direction)
Now, we can ignore the “sideways” component, because for every field line on one side of the craft, there is a field line on the other side of the craft.
That leaves us with the component parallel to the acceleration axis. And if we take only that component, then the (undesired) component of the first arc-section, plus the (undesired) component of the last arc-section (plus the pole/plate distance!) is equal to (desired) component of the second arc-section.
So roughly the two forces created by the external field (one against the thrust of the spacecraft, and one in direction of the thrust) should cancel each other out.
Precise numbers would need a simulation, but I expect that the residual force by the external field is much much smaller than the force created by the internal field (and the force might even possibly be in the desired direction).
There is additional problem in the Dipole drive concept , it is described in cytate below:
“In Fig. 1 we see two parallel screens, with the one on the left charged positive and the one on the right charged negative. There is thus an electric field between them, and effectively no field outside of them”
The physic teach us that outside electric field of described dipole (in reality – it is classic parallel capacitor) will be close to zero for distances from outside surface of electrode that much bigger than distance between elctrodes , when distance from charged particle to outside surface of electrode is comparable with space between anode/cathode electrical field will have huge inluence on charged particle, so proton will be repelled by external surface of positive plate and after proton will pass through “dipole” it will be attracted by negative plate… (this force will be in opposite direction to supposed “thrust”)
Further speculation you can do by you self…
Yes, the net zero charge is a far field approximation only. There is no magical boundary established by the first screen, just a local maximum of the net charge along a path normal to the plane of the screens. With the large (multiple meter) spacing of grid wires being specified the local maximum should not even be large.
So, how about you have a three grid system?
The grid facing the incoming plasma flow would be negatively charged. The center grid positively charged. And then the third grid, at the back side, would again be negatively charged.
Protons would still be reflected, but electrons would exit the back at their original velocity, thus reducing the power consumption.
So now we have tetrode rather than a triode (the external ion source is the cathode) in a CRT arrangement where the ions pass through rather than hit the anode. At least we don’t need a glass envelope in space!
While I have not even tried to work through the details of Zubrin’s proposal I suspect AlexT is on the right track.
They may be no need for a voltage generator if the material from which the sail is made has a high electron emission due to UV from the Sun.
As I stated above, the mere fact that you are performing charge separation to obtain net acceleration of the craft may be enough to nullify the accelerating potential between the two grids (due to the accumulation of external separated charges outside the grids).
That’s why I believe whenever you have an ion drive, in which the latest manifestation of this uses the noble gas xenon, you ionize and accelerate the ions through a charge grid just before the exhaust is exhausted into space. You do so because then the positive ions are neutralized by the negative grid and you have neutral exhaust. I’m not seeing this type of neutralization in this particular scheme; and that may create the charge separation problems that I’ve just spoken of.
A fascinating concept, but I am with Jocelyn Boily above: how does energy (power) get transferred from the power supply to the protons?
A small amount of energy is used initially to charge up the two grids and create the electric field. Thereafter protons passing through increase their energy. How does this energy get from the power supply to the protons? Electrons are ok – they are reflected with a change of direction but no change in energy.
VASIMR is at TRL 9 this year: https://www.nextbigfuture.com/2018/03/advanced-vasimr-plasma-drives-and-spacex-bfr-trip-times.html
This article states that vasimr is at trl 5 or above. Since it hasn’t been flown in space yet there’s no way it’s close to trl 9
https://thecostaricanews.com/canadian-space-agency-signs-us-1-5-million-rd-agreement-boosting-ad-astra-rocket-companys-vasimr-development/
At what MW level?
AFAIK, the VASIMIR test at the ISS was canceled because the ISS can’t supply that power level.
When I read about VASIMIR, I keep keep reading about power levels of 20 MW or even 200 MW. How do you intend to power such a VASIMIR engine? How much mass do you have to allocate to a power source (solar panels I presume)?
A good material for the mesh would be capillary filled and joined nano-tubes, very light weight.
https://www.nanowerk.com/spotlight/spotid=6371.php
The 39 day mission to Mars using VASIMR propulsion takes 200 Megawatts total input power driving 4-to-8 yet to be designed VASIMR engines that will sink 25-to-50 MW each.
This is sheer and highly purified fantasy. Whey do they keep on talking about a 39 day mission to Mars when such power levels are barely in the realm of speculation? The VASIMIR seems to be more of a hype drive than hyper drive.
My apologies, this response was meant for the VASIMIR post above.
I believe I can answer my own question above. When a proton both enters and exits the electric field it ‘sees’ a large changing electric field which creates a magnetic field transverse to the electric field (Maxwell’s equations). This magnetic field acts via v x B forces to change the direction, again transverse to the original direction of motion, and so there are two impulses opposite to the electric field thrust. Without doing any math, my guess is that the overall effect is that the proton just changes direction and there is no net increase in speed. This means the thrust will be less than that calculated using the electric field alone.
Would the nearby medium become charged by the drive in a way that’s detrimental to the setup? I don’t know how much would be absorbed vs flying on past. I imagine you would be kicking off a bunch of charged particles into the direction you want to travel and behind, that would act in a negative way. Basically would that sort of close the system?
I may be missing something, but I think the thrust from this all comes from the approximation of a parallel plate capacitor’s field as uniform between the plates and 0 elsewhere. The actual field looks like this: http://farhek.com/a/a/em/em-beauty-the-parallel-plate-capacitor-electric-field-distribution-of-two-oppositely-charged-metal-plates-colors-represent-magnitude_electric-field-of-a-capacitor_10nf-capacitor-hvac-start-.jpg (If that image link doesn’t work, just do a google image search for “parallel plate capacitor electric field” and look at the one that is a blue-to-orange gradient.) The non-approximated electric field has a strong, roughly uniform electric field going one way between the plates but also a weak but large electric field going the other way outside the plates. I am not entirely confident, but I think these would cancel each other out for a charged particle making the journey from far away, across the plate, and far away again.
I would love to be wrong about this! Possibly the plasma somehow interacts with the large, weak field to make my reasoning wrong.
@Peter Reid,
You are absolutely right.
Sadly, but this mistake is not single one in the discussed article.
This conversation is noisy and bordering on the impolite. I do wonder about a massive influx of solar protons from a CME. Can such a drive in interplanetary space experience a (helpful) surge in acceleration? Can this drive provide shielding to the space craft?
Regarding the relative thrust between two particles with the same kinetic energy but with a mass ratio of 1600+, the heavier particle would have a velocity about 1/40 of the lighter particle but 40 times the momentum. That is high school physics. I suppose the basic question is whether the drive would actually create similar kinetic energies in protons and electron. If so, it will produce the predicted thrust withstanding all of the other factors discussed above.
I share your concern about the tone of the conversation. The best thing to do when things get heated is to remember the posting rules on the front page of the site, and in particular the injunction ‘civility counts.’ This is by way of a general admonition to keep the tone constructive, and is not aimed at any particular individual. Overall, the short statement of the comment policy is:
“Centauri Dreams publishes selected comments on the articles under discussion here. The primary criterion is that comments contribute meaningfully to the debate. Among other criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations. A long form of the policy can be viewed on the Administrative page. The short form is this: If your comment is not on topic and respectful to your fellow readers, I’m probably not going to run it.”
I am rather surprised at the intensity of the criticism that this concept has attracted from some quarters. At the very worst (which I don’t necessarily think is the case), it would seem that modifying the arrangement of the dipole drive to be like that of a gridded ion thruster (including an exhaust beam neutralizer [an electron gun or a hot cathode filament], placing a ring-shaped electromagnet coil between the screens, etc.) would ensure that it would work, and:
Making the screens spin-rigidized would help prevent the oppositely-charged screens from being electrostatically drawn toward each other (dielectric spacer rods at the edges, and at other points between the screens, would also help to keep them separated). While all of these features would, of course, add mass, making the dipole drive very large would make those added parts a smaller percentage of the total mass; a spin-deployed-and-rigidized dipole drive could be miles across–also:
Given even Dr. Zubrin’s original dipole drive’s similarity to a gridded ion thruster (multiple-grid ones have been tested–one 1960s-vintage Martin-built ion engine had five grids), I wonder if a Hall Effect thruster analog of the dipole drive might produce more thrust than the original dipole drive, all else being equal? (Hall Effect thrusters usually produce more thrust than comparable gridded ion thrusters, at lower specific impulse [lower exhaust velocity].)
The third problem with this concept :
Direct current calculation, the current between electrodes can be caused by two things:
1. Charged particles that “fall” directily to electrodes
So if we accept that plasma dencity used in authors calculation are currect, expected constant currecnt will be much lower than used in this calculations this current is function of voltage between electrodes, relation between cunducting area vs. electrode’s free space area and particle motion vector…
2. Alternating current, that is caused by electromagnetic disturbance from charged particles that are pasing through the Dipole (particles that are not thouching electrodes). Calculation of this current is complicated electrodynamic task.