Of Sails and Supernovae

by Paul Gilster | Feb 4, 2020 | Sail Concepts | 37 comments

When we consider pushing a sail to 20 percent of lightspeed, which is the target velocity for Breakthrough Starshot, it’s interesting to think about how laser propulsion differs from sunlight. After all, while constructing a huge laser array presents numerous challenges on Earth, we already have a star to work with, and sail technology that is beginning to be tested in space. Consider, too, that we have operational spacecraft like the Parker Solar Probe that are exploring regions close to the Sun, helping us learn more about heat shields, even as we plan missions like Solar Cruiser, whose sail would enable interesting non-Keplerian orbits near the Sun.

Wouldn’t it be easier to find ways to use our Sun’s own energies to drive our starship by getting a boost from gravitational effects? A closer look reveals the power of solar sails in nearby space (i.e., within the system), while illuminating the problems at interstellar distances.

For getting a solar sail up to the highest possible speeds, we will probably need an Oberth maneuver, which means falling deep into a gravitational well and applying propulsion at the time when velocity in that well peaks, which squeezes maximum effect out of our boost. With a sail, that would mean a so-called ‘Sundiver’ mission, bringing a furled sail shielded by an occulter (perhaps a small asteroid) breathtakingly close to the Sun and then opening the sail at perihelion for the best possible kick.

Depending on shielding and thus how close we can get to our star, we do get a substantial boost over the velocities of our two interstellar Voyagers, one of which moves at just over 17 kilometers per second. We might squeeze 100 kilometers per second out of the Sundiver maneuver, and perhaps a bit more, with future craft using sails made out of new metamaterials achieving even better. But we’re nowhere near 20 percent of the speed of light.

Solar sails certainly have broad applications as we build a Solar System infrastructure, and we can use them with Sundiver maneuvers to get into nearby interstellar space. But if we find long mission times unacceptable, we need to be thinking of alternatives when we mount a true star mission.

Other civilizations won’t necessarily have to work with the same constraints, depending on the kind of star they orbit. I’m always interested in how intelligence might exploit natural objects to achieve interstellar goals, so I took note when I saw Avi Loeb’s latest piece in Scientific American. Loeb (Harvard University), who chairs Breakthrough Starshot, is not one to avoid speculation, without a healthy dose of which we can hardly work with concepts involving SETI and highly evolved civilizations.

So let’s leave the realm of what humans can do with our small G-class star far behind as we consider what sufficiently advanced technologies might attempt. If a culture were to be a billion or more years old, how would we know where to look for it? One way into the problem is to consider the need for energy useful to Kardashev Type II and III civilizations, energy which is available in abundance around certain natural phenomena.

And given the recent attention to Betelguese and the question of when it might become a supernova, Loeb has been moved to consider the power such an event would unleash. The results make working with the flux from a G-class star seem trivial indeed. Even the best Sundiver maneuver at Sol yields velocities that would take hundreds of years to reach the nearest stars. A similar maneuver around the most luminous stars we know might reach 10 percent of lightspeed (not too shabby!). But for maximum kick, a lightsail riding the shockwave of a supernova from a star like Betelguese or Eta Carinae could be pushed to a high percentage of c.

Image: Hubble Space Telescope-Image of Supernova 1994D (SN1994D) in galaxy NGC 4526 (SN 1994D is the bright spot on the lower left). Credit: NASA/ESA.

Could a civilization time a supernova explosion accurately enough to ride the shockwave? That’s a question we can’t answer, nor can we say what kind of timeframes such a culture might operate within. Loeb imagines numerous lightsails parked around a star nearing the end of its life, perhaps placed by a civilization nearby, and perhaps left there indefinitely. If this civilization wanted to use the supernova for propulsion, it would still face numerous problems:

First, as in Starshot, the sails must be highly reflective so as not to absorb too much heat and burn up. Second,once the sails are placed in orbit around the massive star, they will be pushed away by the bright starlight or mass loss prior to the explosion. To avoid this danger, one could deploy the sails in a folded configuration and equip them with a switch that would open them up like umbrellas as soon as the explosion flash begins to rise. Third, even though the launch can start from a distance that is a hundred times larger than the size of the exploding star, care must be taken in selecting particularly empty acceleration paths – clear of any stellar debris.

Indeed, given dust particles with a relative speed close to the speed of light, such a sail would have to be folded to reduce the area it presents in the direction of flight as soon as it reached peak velocity. An even wilder prospect: A massive enough star (Loeb mentions Eta Carinae) collapsing into a black hole could produce gamma-ray bursts (GRBs) that would drive the Lorentz factor to extreme levels. Now we’re in range of Poul Anderson’s ‘Leonora Christine,’ the runaway starship in the novel Tau Zero that crosses galaxies in far less than a human lifetime as measured by the crew, while aeons pass outside their rest frame.

We’re in Dyson sphere country here, a reference Loeb himself makes, looking into ways advanced civilizations could harvest high energy sources around them. A Dyson sphere or array, gathering the maximum amount of stellar light, might be found by its infrared signature, so Dysonian SETI, which looks for evidence of ETI in our astronomical records, has a target.

Clément Vidal has also worked this notion in his book The Beginning and the End (Springer, 2014), looking into questions like extracting energy from the accretion disk around a rotating black hole or tapping the power of X-ray binaries. We are also in Olaf Stapledon territory, asking about extraterrestrial engineering that is inconceivable to ourselves, but possibly visible as a SETI signature in the ‘watering holes’ for energy the universe makes available.

It’s hard for me to imagine a civilization with the patience to wait out a supernova explosion to drive a lightsail, but when we’re dealing with technologies that may be several billion years beyond our own, we have no business imposing our own limitations on the cosmos. Anomalies in our observations of supernova remnants would be worth investigating, although Loeb admits to the difficulty of isolating artificial components within them. Even so, dying stars conceivably have reason to be pondered by civilizations advanced enough to make use of their energy.

The Electric Sail and Its Uses

by Paul Gilster | Nov 18, 2019 | Sail Concepts | 36 comments

The electric sail is an intriguing propulsion concept that Pekka Janhunen at the Finnish Meteorological Institute has been championing for some years. It’s currently the subject of a NASA Phase II study and continues to draw attention despite the fact that we’re in the early stages of turning what looks like sound physical theory into engineering. What captures the imagination here is the same thing that is so attractive about solar sails — in both cases, we are talking about carrying no propellant, but instead relying on natural sources to do the work.

Here we have to be careful about terminology, because it’s all too easy to refer to solar photons as a kind of ‘wind,’ especially since the predominant metaphor is sailing. So let’s draw the lines sharply. There is indeed a ‘solar wind’ in today’s parlance, but it refers not to light but to the stream of particles, plasma and magnetic fields flowing out from the Sun into the heliosphere. An electric sail will ride this solar wind to achieve interplanetary velocities. A solar sail, on the other hand, will use solar photons, which carry no mass but do convey momentum.

Two entirely different concepts, even if both have resemblance to traditional nautical sails. Then we have the other terminological complication: A sail designed to be pushed not just by sunlight but rather by a laser or microwave beam is sometimes called a ‘light sail,’ which is how I have always referred to it, but it still uses photons for propulsion, even if they don’t come from the Sun. Maybe Manasvi Lingam and Avi Loeb have it right in their new paper to refer to photon-pushed sails of any kind as ‘light sails,’ distinguishing these from both electric and magnetic sails (magsails) that use the ‘solar wind’ as their driver. Thus:

Light sails — solar sails and those driven by beamed arrays — use electromagnetic radiation and the momentum transfer of photons. Electric sails use the particle stream of the solar wind.

The electric sail that Janhunen continues to study is the subject of Lingam and Loeb’s new paper, which has been submitted to Acta Astronautica. At the Florida Institute of Technology and Harvard University respectively, the two scientists have calculated performance possibilities for a spinning spacecraft that deploys a number of long wires to which an electrostatic charge has been induced. Solar wind protons (not photons!) reflect off these wires to produce thrust. The wires are kilometers long, and with that slight positive bias, the spacecraft carries an electron gun to manage the charge, retaining the bias against ambient solar wind electrons.

Image: The electric sail is a space propulsion concept that uses the momentum of the solar wind to produce thrust. Credit: Alexandre Szames.

Light sails, to use the Lingam and Loeb terminology, have been considered for interstellar missions for decades now (hats off to the early work of Robert Forward, Gregory Matloff and Geoff Landis, among others), but electric sails are new enough that we need information on how well an electric sail might do for this purpose. Could this technology get us to another star?

For a species like ours, anxious to see missions completed within a few human lifetimes, the answer is no. While a huge laser array like the one contemplated by Breakthrough Starshot could send a small light sail at relativistic speeds to another star, the electric sail cannot achieve the needed velocities.

A species with a different attitude toward time might fare better. The paper explains, for example, how electric sails could leverage the stellar winds of red dwarf stars, which are by far the most common kind of star in the galaxy. Because the interstellar medium itself can decelerate the sail, turning off the electron gun in deep space is essential. Careful maneuvering from star to star over millennia then allows relativistic speeds. From the paper:

…a series of repeated encounters with low-mass stars, and taking advantage of their winds, will enable the electric sail to achieve progressively higher speeds. We showed that sampling ? 10⁴ stars could enable electric sails to achieve relativistic speeds of ? 0.2 c and that this mechanism would require ? 1 Myr. While this constitutes a long timescale by human standards, it is not particularly long in comparison to many astronomical and geological timescales. The ensuing relativistic spacecraft would be well-suited for tackling interstellar and even intergalactic exploration.

This is an eye-opener. We can’t rule out the possibility that species capable of operating in this time frame might deploy electric sails, but the time involved precludes their use as the primary propulsion method for interstellar missions by us. The authors note as well that because an electric sail will have a low cross-sectional area, its presence would be all but undetectable, whereas a light sail driven by a laser would demand huge amounts of energy and would be theoretically detectable at interstellar distances. So for a civilization hoping to explore in ‘stealth’ mode, an electric sail would have its advantages. These are not good SETI targets.

Returning to M-dwarf stars, the authors show that if stars are small enough (less than about 0.2 solar masses), the pressure of the stellar wind dominates over photon pressure, Speeds in the range of 500 kilometers per second seem feasible for electric sails near late-type M-dwarfs. Indeed, for F-, G- and K-class stars, electric sails fare better as propulsion systems in the vicinity of the home star than light sails.

So we are looking at a technology that, if it can be properly engineered, could play a role in shaping an interplanetary infrastructure, while yielding to faster methods for missions to other stars, unless we humans somehow attain an all but geological patience.

The paper is Lingam and Loeb, “Electric sails are potentially more effective than light sails near most stars,” in process at Acta Astronautica (preprint). For Pekka Janhunen’s concept of the electric sail as a fast interplanetary probe, see Electric Sails: Fast Probe to Uranus.

Sail Deployment: Reflections on LightSail 2

by Paul Gilster | Jul 24, 2019 | Sail Concepts | 19 comments

One thing that James E. Webb insisted on during his tenure as NASA administrator was that the space program was larger than an attempt to get humans to the Moon. The man who did so much to ensure that Apollo would succeed, and who will be rightfully honored in the form of the James Webb Space Telescope, was a proponent of exploration throughout the Solar System through robotic craft, and weather and communications satellites that would become part of a permanent reliance on a growing space infrastructure. Marc Millis noted some of the results in his recent essay.

For while the frustration of abandoning the Moon in the 1970s lingers, we do have over 4600 spacecraft in Earth orbit, many of them doing the kind of work Webb envisioned. We’ve completed the initial reconnaissance of the Solar System and made our first tentative ventures into the Kuiper Belt and out past the heliopause. We’re charting exoplanets and looking to explore Saturn’s largest moon. So these are things to keep in mind when frustration begins to build.

The anniversary of Apollo’s first landing was a time for looking back, but The Planetary Society has just reminded us that we have to keep looking ahead as well. With the successful deployment of the solar sail aboard LightSail 2, we are seeing the kind of change Millis talked about in action. Much of the space business has turned commercial, while we continue to sort out how to handle the change (SLS or Falcon Heavy?), and in the midst of this, private organizations using off-the-shelf hardware can produce crowd-funded missions of real value.

SAIL DEPLOYMENT COMPLETE! We're sailing on SUNLIGHT!!!!! pic.twitter.com/PA74NMa7Ry

— Planetary Society (@exploreplanets) July 23, 2019

We’re in that confusing time — one that future historians will be able to sort out more readily than we can define it today — when the pace of technological change drives new models for the accomplishment of grand goals. How we interrelate big government projects with corporate space activities and private contributions will define an era that will one day have its own name, just as the 1960s could be partially defined by the term ‘Space Race.’ And it’s understandable that Planetary Society CEO Bill Nye should be proud of what his organization has accomplished: “We are advancing space science and exploration,” says Nye. “We are democratizing space. We are innovating.”

Apollo 8 gave us gorgeous photographs of our planet seen entire, a blue and brown crystal filling the frame. Now we have LightSail 2’s view of Earth, showing vast portions of the Pacific Ocean and part of the North American landmass. CubeSats in their various configurations make it possible for organizations like The Planetary Society as well as universities and other private groups to contemplate missions that move the ball forward. In the case of LightSail 2, we will have learned more about sail deployment, and orbit raising by the pressure of sunlight alone.

Image: This image of Earth, which shows the Pacific Ocean with Baja California and Mexico on the right, was captured by LightSail 2 on 18 July 2019 at 21:45 UTC while the spacecraft was in range of its ground station at Cal Poly San Luis Obispo in California. Though LightSail 2’s altitude is only 720 kilometers, its 185-degree, wide-angle camera lenses allow it to capture horizon-to-horizon Earth imagery. Credit: The Planetary Society.

The deployment command to LightSail 2 went out at about 1445 EDT (1845 UTC) on the 23rd, with the momentum wheel, responsible for orienting the sail in relation to the Sun, spinning up successfully. Now we wait for images of the deployed sail, which should be downloaded today.

Here’s a photo of the LightSail 2 team set up for sail deployment. It’s impossible to ignore the contrast between the Mission Control operations we see at NASA (think of the operation Chris Kraft ran!) and the ad hoc effort at Cal Poly. What makes this possible is vision, public financial support and technology trends working in favor of small, light spacecraft, not to mention gritty persistence. I’ll feel better when I’ve seen actual sail images, but for right now, things look good, and the entire Planetary Society team deserves our congratulations.

Image: This image shows The Planetary Society’s LightSail 2 team on console prior to sail deployment on 23 July 2019 at the Cal Poly CubeSat lab in San Luis Obispo, California. From left: Barbara Plante, Founder and President, Boreal Space; Alex Diaz, Avionics Engineer, Ecliptic Enterprises Corporation; John Bellardo, Associate Professor, Cal Poly San Luis Obispo; Dave Spencer, LightSail Project Manager, Associate Professor at Purdue University; Bruce Betts, LightSail Program Manager, Planetary Society Chief Scientist. Credit: The Planetary Society.

You can track LightSail 2’s condition on its own Mission Control page, which offers data on temperature, degree of rotation, control mode and location over the Earth. 40,000 private donations (totalling in the region of $7 million) went into LightSail 2 over 10 years. The results of the effort will also feed a NASA project called Near Earth Asteroid Scout, which will likewise employ CubeSat technology to visit an asteroid early in the 2020s. Launched by a Falcon Heavy, LightSail 2 emphasizes today’s mix of commercial, corporate and private effort.

I fall back on what Marc Millis said in these pages on Monday:

It is my hope that progress will continue along all these fronts and improve the human condition. The next steps toward the Moon, Mars and recreational spaceflight will usher in a new era, a suitable name for which will probably be conceived years later. It’s certainly no longer a “space race” with only one finish line. It is the beginning of a new stage of humanity.

LightSail 2 Inspires Thoughts on Fictional Sails

by Paul Gilster | Jun 10, 2019 | Sail Concepts | 23 comments

Solar sails are a case of science fiction anticipating the scientific journals, though in an odd way. Engineer Carl Wiley (writing as Russell Saunders) described the physics of solar sailing and some early engineering concepts in the pages of John Campbell’s Astounding back in 1951, but he did it in a nonfiction article of the kind the magazine routinely ran. Richard Garwin would discuss sails in the scientific literature in “Solar Sailing: A Practical Method of Propulsion within the Solar System,” which ran in 1958 in the journal Jet Propulsion.

Then we waited for fictional treatments, which began with Cordwainer Smith’s wonderful “The Lady Who Sailed the Soul” (Galaxy, April 1960) and a string of stories from top authors of the time in just a few quick years — Jack Vance’s “Gateway to Strangeness” (Amazing Stories, 1962), Poul Anderson’s “Sunjammer” (Analog 1964), Arthur C. Clarke’s story of the same name, later renamed “The Wind from the Sun” (Boy’s Life, 1964). Sails of the solar kind had definitely arrived.

Because he is a personal favorite, let me run a clip from the Jack Vance story, which later became known as “Sail 25” (available in various places, but most easily in the 1976 collection The Best of Jack Vance). This is what solar sailing looked like in the days when the nautical metaphor was just beginning to be explored, and a fictional crew is learning, with the help of space veteran Henry Belt, to handle the unusual demands of sail deployment.

“Around the hull swung the gleaming hoop, and now the carrier brought up the sail, a great roll of darkly shining stuff. When unfolded and unrolled, and unfolded many times more it became a tough gleaming film, flimsy as gold leaf. Unfolded to its fullest extent it was a shimmering disk, already rippling and bulging to the light of the sun. The cadets fitted the film to the hoop, stretched it taut as a drum-head, cemented it in place. Now the sail must carefully be held edge on to the sun, or it would quickly move away, under a thrust of about a hundred pounds.

“From the rim braided-iron threads were led to a ring at the back of the parabolic reflector, dwarfing this as the reflector dwarfed the hull, and now the sail was ready to move.

“The carrier brought up a final cargo: water, food, spare parts, a new magazine for the microfilm viewer, mail. Then Henry Belt said, ‘Make sail.'”

It sounded complicated in 1962, but in the era of the IKAROS sail, we’ve learned how tricky actual deployment is, and also how incredibly thin high-performance sails will need to be, particularly as we look toward future missions with cutting-edge materials. Geoff Landis, for example, has examined sails of niobium, beryllium and transparent films of dielectric (non-conducting) materials like silicon carbide, zirconia and diamond-like carbon — a material much like diamond — that could be assembled in space with a plastic substrate.

James Davis Nicoll recently wrote up some examples of sails in science fiction that include the early ones mentioned above, but also more recent work like the novels in Vonda McIntyre’s ‘Starfarers’ sequence and the 1974 tale “The Mountains of Sunset, the Mountains of Dawn.” Also catching Nicoll’s attention is Joan D. Vinge’s “View from a Height,” which should interest anyone exploring the human motivations for immense journeys, and Alastair Reynolds’ Revenger (2016). I hope readers will supply some of their own favorites in science fictional sails. Laser sails play a prominent role in Larry Niven and Jerry Pournelle’s The Mote in God’s Eye (1974), for example, the ‘mote’ being evidence of a sail heading in our direction. And then there’s Forward’s Rocheworld… The list is extensive!

Image: Poul Anderson’s “Sunjammer” appeared in April, 1964, about a month after Arthur C. Clarke’s story of that name, and to make matters even more confusing, ran under the pseudonym ‘Winston P. Sanders.’ Both stories were milestones in early sail depictions.

Like Nicoll, I’m looking back at sail technologies partly because we’re coming up on the launch of The Planetary Society’s LightSail 2, which is now scheduled for no earlier than June 24 aboard a SpaceX Falcon Heavy. The CubeSat spacecraft about the size of a loaf of bread and weighing 5 kilograms will deploy a small sail and attempt to raise its orbit by the momentum imparted by solar photons. I’m a great partisan of CubeSats, particularly those with sail capabilities, for fleets of the inexpensive spacecraft, further miniaturized, networked and tapping solar radiation, can become a great way to deploy sensors all through the inner system.

LightSail is no 5,000-mile behemoth, like the staggering sail of Cordwainer Smith’s story — its four booms will unfurl four triangular panels with a combined area of 32 square meters, and as The Planetary Society’s Jason Davis tells us, the craft will receive a push ‘no stronger than the weight of a paperclip.’ But continual thrust is just the ticket as the effects mount up, and LightSail 2 will be in an orbit high enough (720 kilometers) that the effects of atmospheric drag can be overcome. Expect LightSail 2 to be deployed from the Prox-1 spacecraft that encloses it about seven days after launch. All good wishes on the attempt!

Image: Prox-1 deploys the LightSail 2 spacecraft in Earth orbit. Credit: The Planetary Society (CC BY-NC 3.0).

Ultrahigh Acceleration Neutral Particle Beam-Driven Sails

by Paul Gilster | Jan 3, 2019 | Sail Concepts | 52 comments

Beamed propulsion has clear advantages when it comes to pushing a payload up to interstellar flight speeds, which is why Breakthrough Starshot is looking at laser strategies. But what about a neutral particle beam in conjunction with a magnetic sail? We’ve discussed the possibilities before (see Interstellar Probe: The 1 KG Mission), where I wrote about Alan Mole’s paper in JBIS, followed by a critique from Jim Benford. Mole, a retired aerospace engineer, is now collaborating with plasma physicist Benford (CEO of Microwave Sciences) to examine a solution to the seemingly intractable problem of beam divergence. Getting around that issue could be a game-changer. Read on for the duo’s thoughts on sending a 1 kg probe to a nearby star system with a flight time in the range of 70 years. Part 2 of this study, outlining engineering issues and the practical realities of cost, will follow.

by James Benford and Alan Mole

We advance the concept for a 1 kg probe that can be sent to a nearby star in about seventy years using neutral beam propulsion and a magnetic sail. The concept has been challenged because the beam diameter was too large, due to inherent divergence, so that most of the beam would miss the sail. Increasing the acceleration from 1000 g’s to 100,000 g’s along with reducing the final speed from 0.1 c to 0.06 c redeems the idea. Such changes greatly reduce the acceleration distance so that the mission can be done with realistic beam spread. Magsail-beam interaction remains an aspect of this concept that needs further study, probably by simulations.

Central features of Neutral Particle Beam Propulsion

Use of a neutral particle beam to drive a Magsail was proposed by Geoffrey Landis as an alternative to photon beam-driven sails [1]. Compared to beam-driven propulsion, such as Starshot, particle beam propelled magnetic sails, Magsails, substitute a neutral particle beam for the laser and a Magsail for the ‘lightsail’, or ‘sailship’. The particle beam intercepts the spacecraft: payload and structure encircled by a magnetic loop. The loop magnetic field deflects the particle beam around it, imparting momentum to the sail. The general ‘mass beam’ approach has been reviewed by Nordley and Crowl [2].

Particle beam propelled Magsails require far less power for acceleration of a given mass. There’s also ~ 10³ increase in force on the sail for a given beam power. Deceleration at the target star is possible with the Magsail but not with a laser driven sail.

The neutral particle beam approach is conceptually similar to photon beams such as the laser-driven Starshot project. A disadvantage of reflecting photons from the sail will be that they carry away much of the energy because they exchange only momentum with the sail. Neutral particle beams transfer energy, which is much more efficient. The reflecting particles may in principle be left unmoving in space after reflection and thus the efficient energy efficiency can approach 100%.

The thrust per watt beam power is maximized when the particle velocity is twice the spacecraft velocity. The Magsail, with a hoop force from the magnetic field, is an ideal structure because it is under tension. High-strength low-density fibers make this lightweight system capable of handling large forces from high accelerations. The rapidly moving magnetic field of the Magsail, seen in the frame of the beam as an electric field, ionizes the incoming neutral beam particles. Nordley and Crowl discuss on-board lasers to ionize the incoming beam, although this adds additional on-board mass and power [2]. When the dipole field of the Magsail is inclined to the beam vector the Magsail experiences a force perpendicular to the beam vector, which centers it on the particle beam, perhaps providing beam-riding stability.

Ultrahigh Acceleration

Alan Mole proposed using it to propel a lightweight probe of 1 kg [3]. The probe was accelerated to 0.1 c at 1,000 g by a neutral particle beam of power 300 GW, with 16 kA current, 18.8 MeV per particle. The particle beam intercepts a spacecraft that is a Magsail: payload and structure encircled by a magnetic loop. The loop magnetic field deflects the particle beam around it, imparting momentum to the sail, and it accelerates.

Benford showed that the beam divergence is fundamentally limited by the requirement, at the end of the acceleration process, to strip electrons from a beam of negative hydrogen ions to produce a neutral beam [4,5]. Therefore neutral beam divergence is typically a few microradians. Mole’s beam had an inherent beam divergence of 4.5 µradians.

In Mole’s work, the neutral hydrogen beam at 18.8 MeV per particle and inherent beam divergence of 4.5 µradians accelerated to two-tenths of the speed of light (0.2 c) had acceleration of 10³ g’s for 50 minutes [3]. This resulted in a 411 km diameter beam spot, far larger than the Magsail diameter, which was 0.27 km. So most of the beam missed the sail.

But if we use much higher acceleration, the sail will stay within the beam until it reaches the desired final velocity, even with microradian divergence. We choose 10⁵ g’s, 10⁶ m/s² to accelerate to 0.06 c, 1.8 x 10⁷ m/s.

Numerical experiments with the model developed by Nordley [6], and later replicated by Crowl, showed that the greatest momentum delivery efficiency is when the velocity of the neutral beam is twice the sail velocity. The physics of this is straightforward: Maximum energy efficiency comes when all of the energy goes to the sail and none of it remains in the beam. For a sail that is perfectly reflective, the beam bounces off the sail at the same velocity it impinges the sail. If after reflection it is moving at zero velocity (so none of the energy is left in the beam), the initial beam velocity must be twice the sail velocity, so that it impinges on the sail at a relative velocity equal to the sail velocity.

We take the beam velocity at the end of acceleration to be the twice the final sail velocity, 0.06c The energy of a hydrogen atom is imparted by accelerating through a voltage of 6.6 MeV. The mission parameters for a hydrogen beam then become those shown in Table 1.

The lighter the particle to be accelerated, the shorter the beam driver can be at a fixed field gradient. However, lighter-particle shorter beam drivers, while they may cost less, would require a larger sail due to the higher divergence of the beam.

For a second example, a mercury beam has a minimum divergence of 0.8 µradians, but must use far higher voltage because of the larger mass [4]. Mercury beam parameters are also given in Table 1.

Table 1 Parameters of neutral particle beam-driven sail probes

We will see that when the beam divergence is in reality roughly 3 orders of magnitude higher than previous studies have assumed, from a nanoradian to microradian, rapidly moves the beam generator regime toward being very large and expensive.

Because in Table 1 the hydrogen beam sail diameter is so large, we will focus the rest of this discussion on the mercury beam. Even so, the mercury beam Magsail has a 260 m diameter and 1 kg mass, if the superconducting hoop has a density of steel, the thickness must be no larger than 0.44 cm, if the density of carbon, 0.8 cm.

Magsail-Beam Interaction

Note that the sail diameter given in Table 1 is taken to be simply the diameter of the divergent beam encountering the Magsail. The diameter of the reflection region produced by the magnetic field of the sail could well be somewhat larger than the superconducting hoop diameter. (Of course, early in the acceleration, the beam will hit it at the axis where the magnetic field is greatest.)

When a Magsail driven by a neutral particle beam is at the early stages of the acceleration, the beam will have a considerably smaller spot size on the Magsail than it will later and will hit it at the axis where the magnetic field is greatest. Later on, as the Magsail flies away, the beam will reach a size dictated by its divergence. A question is: does the initial beam high intensity of the beam on the magnetic field tend to push the sails magnetosphere outward radially and make the effective diameter of the Magsail larger? If it does, then the beam divergence can be a bit larger and still strike the Magsail. Or, conversely one could accelerate the Magsail for a longer time because some of the beam would still be captured.

Simulations show the field being compressed; but they are of solar wind, which is taken to be uniform across a magnetic dipole. There are no simulations of the beam smaller than the sail. One would expect the loop generated field to be compressed in the direction of motion, but it seems reasonable for it to be inflated radially, especially if charged particles are trapped in it for significant periods of time.

Andrews and Zubrin have done single particle numerical calculations that do not include modeling dynamic effects (such as field distortions from magnetic pressure) and do not include any such “inflation” of the mirror due to trapped beam ions [7].

Figure 1 is taken from the late Jordan Kare’s NIAC report [8]. (From his figure, he considered using a nuclear detonation to accelerate a Magsail, which is not relevant to our discussion.) From the left a uniform solar wind strikes the Magsail, which in our case would be a non-uniform neutral particle beam. The beam encounters the peak of magnetic field along the axis of the sail. On the right of the figure, the field is distorted, producing a plasma interface shock against the magnetic field of the Magsail. Inflation of the magnetic field due to a particle beam pressure could occur. However, the effect would be to allow the beam divergence to be only a bit larger.

Note also that in this diagram the sail is shown as dragging the payload behind it as it accelerates. If part of the particle beam reaches the payload it could create substantial damage. Consequently, it might it be better to distribute the payload around the superconducting hoop where it would have the most protection against incoming charged particles. Note also the stability of the superconducting loop on a beam of finite width has not been investigated to date. However, the Starshot program is looking at this issue extensively.

Figure 1: Interaction of streaming plasma flow with a Magsail. From Jordan Kare NIAC report [8].

The assumption that the moving magnetic field of the Magsail, seen in the frame of the beam as an electric field, ionizes the incoming neutral beam particles must be quantified.

Conclusions

Since beam divergence is fundamentally limited, high accelerations can be used to insure the sail will stay within the beam until it reaches the desired final velocity, even with microradian divergence. This leads to ultrahigh, 10⁵ g’s, 10⁶ m/s² to accelerate to 0.06 c. The Starshot system, a laser beam-driven 1 gram sail with the goal of reaching 0.2c, has been quantified in a detailed system model by Kevin Parkin [9]. It too uses 10⁵-10⁶ g’s. Magsail-beam interaction remains an aspect of this concept that needs further study, probably by simulations. This promising method for interstellar travel should receive further attention.

References

1. G.A. Landis, “Optics and Materials Considerations for Laser-Propelled Lightsail,” IAA-89-664, 1989.

2. G. Nordley and A. J. Crowl, “Mass Beam Propulsion, An Overview”, JBIS 68, pp. 153-166, 2015.

3. Alan Mole, “One Kilogram Interstellar Colony Mission”, JBIS, 66, pp.381-387, 2013.

4. J, Benford, “Beam-Driven Sails and Divergence of Neutral Particle Beams” JBIS 70, pg. 449-452, 2017.

5. Report to the APS of the study on science and technology of directed energy weapons, Rev. Mod. Phys 59, number 3, part II, pg. 80,1987.

6. G. D. Nordley, “Relativistic Particle Beams for Interstellar Propulsion,” JBIS 46, pp 145-150,1993

7. Andrews, D. G. and R. M. Zubrin, “Magnetic Sails and Interstellar Travel”, JBIS 43, pp. 265-272, 1990

8. J. T. Kare, “High-acceleration Micro-scale Laser Sails for Interstellar Propulsion,” Final Report NIAC RG#07600-070, 2002.
www.niac.usra.edu/files/studies/final_report/597Kare.pdf. Accessed 03 Dec 2018.

9. K. L. G. Parkin, “The Breakthrough Starshot System Model”, Acta Astronautica 152, 370-384, 2018.

The Dipole Drive: A New Concept for Space Propulsion

by Paul Gilster | Jun 29, 2018 | Sail Concepts | 185 comments

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.