Laser-driven sails are limited to about 6.67 Newtons per gigawatt. The Three Gorges Dam has a capacity of about 22.5 GW. If this was transmitted with 100% efficiency to a light sail it would provide thrust equivalent to the force required to lift a 15 kg mass on Earth.
A new propulsion system wants to use electric fields to push against the charged particles of the interstellar medium.
Recently Robert Zubrin published his independent work on a ‘dipole drive’ concept which bears a striking resemblance to this concept (called SWIMMER) which they describe.
The Magsail and electric sail concepts are based on the fact that we can interact with significant mass in the ISM (or the heliosphere) using relatively low mass structures consisting of charged or current carrying wires. How then can we interact with the medium to accelerate rather than decelerate?
Above – A schematic representation of a paddy-wheel SWIMMER in operation. The top frames show the paddy wheel with sail A positively charged. The inset shows sail A in its frame of reference pushing against the positive ions in the ISM which appear to be streaming towards it. Bottom frame shows a later time in the rotation cycle during which neither sail is moving with negative velocity, and the electric potential difference is removed with both sails neutrally charged.
Rotating version with cycling of the positive and negative charges
Two electric sails are mounted opposite each other at the ends of two long tethers which are electrically connected, and which we can apply a potential difference across. The tethers are mounted to a reaction wheel in the center, and the whole system is set spinning with the spin axis perpendicular to the direction of travel (which we define as the positive direction). As one of the electric sails (sail ‘A’) approaches the portion of the cycle when its velocity is negative with respect to the ISM, we apply a potential difference, charging sail A positively and sail B negatively. At this point in the cycle in the frame of sail A, ions in the ISM are streaming towards it and pushing it in the desired direction of travel. We note that simultaneously the negatively charged sail B will be reflecting electrons in the positive direction causing some drag. Fortunately, as ions out-mass electrons by at least a factor of mp/me=1836, the electrons contribute negligible drag, and in general we will ignore them here and throughout the analysis.
Two layer sail with positive and negative charges but pulsing power
Another configuration described is a large pair of wire grids with opposite charges to push on the ambient ISM, is very similar to that recently described by Robert Zubrin as the dipole drive. In the case of the dipole drive, however, the electric field is apparently static rather than pulsed, the wire grids are separated by some distance, and they push on the charged particles as they pass between the plates. At first look, this seems like a reasonable and simpler approach. Introductory E&M tells us that two oppositely charged infinite plates produce a strong electric field between them and no electric field outside, so if we can simply push the heavy ions between the plates in the correct direction this static electric field should give us thrust. Unfortunately, the approximation of infinite plates leads us astray here. In fact, a finite system of parallel plates will produce an electric field outside the plates pushing in the opposite direction. Although these fields will be weaker than the field between the plates, they will also extend over a larger region, canceling out the thrust gained from particles between the plates. Indeed, any system of static charges over a finite area must leave the electric potential zero at infinity.
Space probe rendezvous at α Cen
A relatively early stage SWIMMER mission might have the goal of transporting a modest space probe, mpay = 1000 kg to α Cen A and then decelerating to allow gravitational capture for a permanent orbital space telescope. We will assume a modest electrical power delivered to the SWIMMER of 10 MW. The pusher plate will be made up of several long tethers. In practice these tethers will consist of very fine braided filaments to prevent failure due to micrometeoroid and interstellar dust collision, as described for the electric sail (Janhunen 2004), but we will consider them to be single wires with an effective diameter of 30 µm. This is equivalent in material to eight filaments with diameters of about 10 µm. We will also include strategically weak breakpoints in our tethers which can be activated by simply increasing the spin rate such that the centripetal force exceeds the breakpoint capacity. As we reach higher velocities then, we may leave behind mass from the pusher plate. Given the pulsed nature of the SWIMMER electric field, the wire tethers should be made out of superconducting materials.
The total mass of the SWIMMER ship is comprised of mpay=1000 kg, mpower=2500 kg (given by our 10 MW electric power supply and its assumed specific power), and mpusher. At the moment it is unclear how much mass to devote to mpusher, however we will show that a mass of 7400 kg is useful. The mass for the tethers could be mined in situ from asteroids. This mass provides for a total summed tether length of 4.1×109 meters.
While this is seemingly a very long tether, it does not in any way represent the spatial scale of the SWIMMER as the pusher plate will be made up of several thousand tethers, possibly splitting off from each other at greater radial distances. The summed length is merely a useful value for determining the total cross-sectional area in plasmas of different temperatures and densities.
After 1.5 years the SWIMMER enters the ISM at 100 AU with a velocity of 4.0×105 meters per second.
Upon entering interstellar space, the SWIMMER begins normal mode operations. Simultaneously the ion density drops and the cross sectional area of our tethers increases by a factor of λD(ISM)/λD(helio) = 2.3. At this distance from the sun our tethers will be superconducting, and we can begin applying our 10 MW of power.
We will also begin discarding mass from the pusher plate as it accelerates. The optimal rate to discard mass will change based on the specific details of any given mass distribution, power, and journey length.
A minimum 1 pc travel time of 263 years for our SWIMMER with χ = 0.13, and ψ = 0.53. The ship arrives with a velocity of 6.02×106 meters per second (0.02 c, 2% of lightspeed). Without allowing the pusher plate mass to be discarded en route, the journey would take slightly longer at 340 years. For comparison, an ideal light sail dominated by mpay = 1000 kg (IE ignoring the light sail mass and assuming perfect reflectivity) pushed with the same delivered power, would take 793 years to complete the same journey.
As the SWIMMER approaches α Cen A it begins destination braking. This would begin in nearby interstellar space at a distance of ∼12500 AU from α Cen A. By this point the SWIMMER has significantly reduced the mass of its pusher plate to 518 kg, with a corresponding interaction area of 1.25×1010 m2. After 23 years of braking in the ISM, the SWIMMER enters the α Cen A heliosphere at a distance of 100 AU and a velocity of 1.13×106 meters per second relative to α Cen A
Researchers propose a new mode of transport which relies on electric-field moderated momentum exchange with the ionized particles in the interstellar medium. While the application of this mechanism faces significant challenges requiring industrial-scale exploitation of space, the technological roadblocks are minimal, and are perhaps more easily addressed than the issues presented by light sails or particle beam powered craft. This mode of space travel is particularly well suited to energy efficient space travel at velocities less than 5% of light speed, and compares exceptionally well to light sails on an energy expenditure basis. It therefore represents an extremely attractive mode of transport for slow (~multi-century long) voyages carrying heavy payloads to nearby stellar neighbors. This could be very useful in missions that would otherwise be too energy intensive to carry out, such as transporting bulk materials for a future colony around Alpha Cen A, or perhaps a generation ship.
Due to their extremely favorable performance at lower power and velocities, SWIMMERs would make excellent transporters for large masses that can take long timescales. This could be used as the basis of a generation ship, or perhaps a transporter for bulk colony materials sent out ahead of time before a fast moving low mass people transporter arrived. For this example we will assume a payload mass, mpay=8×109 kg, equivalent to the Super Orion ship discussed by Dyson (2002). Since such a mission would likely only be attempted after significant technological advances, we will slightly improve our material properties by assuming a specific power in our power conversion systems of 10 kW kg−1 , and superconducting materials which are able to passively operate beyond 3 AU. We will take our delivered SWIMMER power to be 10000 GW, thus mpower=1×109 kg. We will use a pusher plate of mass mpusher=3.7×1010 kg, with a summed tether length of 2.0×10M16 m. As before, this pusher plate mass is based on our optimization of the travel time during the normal SWIMMER operation as a function of velocity, ψ, and χ.
By disposing of onboard reaction mass they circumvent the rocket equation, and by exchanging momentum
with ions in the ISM they improve by orders of magnitude over the energy efficiency of traditional
light sails. The key to this momentum exchange is the changing electric field which allows us to create inhomogeneities in the surrounding plasma, and then push on these inhomogeneities to create thrust. SWIMMERS perform exceptionally well at lower velocities, with their advantage over light sails diminishing quickly at velocity over 0.05 c. Furthermore, by relying on the ambient ISM as a momentum exchange medium, they are quite versatile, able to accelerate either away or towards a beamed energy
source, opening up myriad opportunities to serve as oneway transport, roundtrips, or even statites in stationary positions.
The examples discussed here only scratch the surface of the possible roles for SWIMMERs in our spacefaring future. Their characteristics make them ideal for any mission with large masses in which relatively low velocities (v less then 0.05 c) are acceptable. They are unlikely to be the sole mode of space transport due to their diminishing advantages at high velocities and their structural complexity which requires onboard power conversion systems with significant mass. Nonetheless, SWIMMERS will play an important role in future space exploration and augment other modes of transport. They might, for
instance, also be well suited to aiding the construction of a fast interstellar highway by transporting massive particle beam stations along with their fuel supply out to stationary positions between us and our target destinations. These particle stations could be used to swiftly carry light weight Magsails along the path, and simultaneously augment the power of future SWIMMERs by replacing the stationary ISM with a corridor of fast moving beamed particles.