Magnetic sails — ‘magsails’ — are a relative newcomer on the interstellar propulsion scene, having been first analyzed by Dana Andrews and Robert Zubrin in 1988. We saw that the particle beam concept advanced by Alan Mole and discussed this week by Jim Benford would use a magsail in which the payload and spacecraft were encircled by a superconducting loop 270 meters in diameter. The idea is to use the magnetic field to interact with the particle beam fired from an installation in the Solar System toward the departing interstellar craft.
Within our own system, we can also take advantage of the solar wind, the plasma stream flowing outward from the Sun at velocities as high as 600 kilometers per second. A spacecraft attempting to catch this wind runs into the problem that sunlight contains far more momentum, which means a magnetic sail has to deflect a lot more of the solar wind than a solar sail needs to deflect sunlight. A physical sail, though, is more massive than a spacecraft whose ‘sail’ is actually a magnetic field, so the magsail spacecraft can be the less massive of the two.
Science fiction began exploring basic solar sails in the 1960s through stories like Clarke’s “Sunjammer” and Cordwainer Smith’s “The Lady Who Sailed the Soul.” In fact, SF writers have done an excellent job in acquainting the public with how solar sails would operate and what their capabilities might be. But magsails are hard to find in science fiction, and the only novel that springs readily to mind is Michael Flynn’s The Wreck of the River of Stars, whose haunting title refers to a magsail passenger liner at the end of its lifetime.
Here’s Flynn in ‘Golden Age’ Heinlein style introducing the tale:
They called her The River of Stars and she spread her superconducting sails to the solar wind in 2051. She must have made a glorious sight then: her fuselage new and gleaming, her sails shimmering in a rainbow aurora, her white-gloved crew sharply creased in black-and-silver uniforms, her passengers rich and deliciously decadent. There were morphy stars and jeweled matriarchs, sports heroes and prostitutes, gangsters and geeks and soi-disant royalty. Those were the glamour years, when magsails ruled the skies, and The River of Stars was the grandest and most glorious of that beautiful fleet.
Image: There are few science fiction stories involving magsails, and even fewer visual depictions. The cover art for Michael Flynn’s book, by the artist Stephan Martiniere, is a striking exception.
The novel takes place, though, many years later, when the grand passenger liner has become no more than an obsolete freighter whose superconducting sail structure has been decommissioned in favor of newly developed fusion drives. What happens when she needs to power up the sail again because of a fusion emergency makes up the bulk of the tale. The Wreck of the River of Stars is not about an interstellar journey but a highly developed infrastructure within the Solar System that, for a time, used the solar wind. It will be interesting to see what science fiction tales grow out of the current interstellar thinking.
For magsails emerged in an interstellar context, and if it was Robert Zubrin and Dana Andrews who worked through the equations of what we conceive today as a magsail, it was Robert Bussard who first brought life to the idea through his notion of an interstellar ramjet that would use magnetic fields to scoop up fuel between the stars. Both Zubrin and Andrews saw the potential uses of a magsail for deceleration against a stellar wind. If beam dispersal cannot be prevented to allow an interstellar magsail to be accelerated by particle beam, we might still consider equipping a beamed laser sailcraft with magsail capabilities for use upon arrival.
And when it comes to magsails closer to home, one cautionary note is provided by a 1994 paper from the Italian physicist Giovanni Vulpetti, who describes the problems we may have operating superconductors within the orbit of Mars. The paper notes that superconductivity can be lost this close to the Sun unless massive thermal shielding is applied and that, of course, ramps up the spacecraft mass. This evidently does not preclude outer system work, but it could serve as a brake on using magsails near the Earth, at least until we make considerable advances in superconductor technology.
The Vulpetti paper is “A Critical Review on the Viability of Space Propulsion Based on the Solar Wind Momentum Flux,” Acta Astronautica 37 (1994), 641-642.
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How do we envisage keeping the superconducting wire cold? Does it need to be kept shaded and a radiator provided? Is a passive system sufficient, or do we meed active cooling? What impact do either have on the mass and performance issue? Are we better off with the non-superconducting electro-sail concept, especially as regards navigation and maneuvering?
The Martiniere picture is lovely, but the orientation of the mag-sail wires looks completely wrong to me. They look more like electro-sails.
As Jim Benford noted, particle beams will require an more complete infrastructure, like the railroad system. What might that mean? Power is best created as near to the sun as possible. But the beamers need to be much more dispersed to reduce beam divergence and hence losses. How should the power for operation be generated – e.g. from microwave beams in the inner solar system? Should these facilities be anchored on airless celestial bodies, rather than be in free space?
Today our space exploration is more like that of the C18th sea exploration. We are starting to think about the need for ports and fuel bunkers to facilitate more powerful vessels and be able to do repairs locally. Beamed power, especially particle beams, seems to fit this latter transport model. I suspect that like shipping ports and railroads, locations will be selected to maximize profits, and as yet we don’t know where they will come from.
“We are starting to think about the need for ports and fuel bunkers to facilitate more powerful vessels and be able to do repairs locally.”
I wonder if any of that thinking will materialize. I would put my money on the Chinese as the first culture to take that step in harboring manned space facilities. Their President (and military/government) seems to be rather enthusiastic about the idea, as noted here:
By Megan Gannon April 15, 2014
“In a report to Congress last year, the Pentagon noted that China was continuing to ramp up its military capabilities in space, with 18 space launches in 2012 and an expansion of its space-based intelligence, surveillance, navigation, meteorological and communications satellite networks. That report from the U.S. Department of Defense also claimed that China had improved capabilities “to limit or prevent the use of space-based assets by adversaries during times of crisis or conflict.” http://www.space.com/25517-china-military-space-technology.html
‘戚发韧：神六后中国航天面临极大挑战’ Qi Faren: China Shenzhou VI spacecraft after facing great challenges By Liang Caiheng on January 15, 2006
Carrying out space programs is not aimed at sending humans into space per se,
but instead at enabling humans to work in space normally,
also preparing for the future exploration of Mars, Saturn and beyond.”(Qi Faren, 2006) http://scitech.people.com.cn/GB/25892/4028044.html
A nice website of further interest, delving into the issue of U.S. space endeavors via documentary medium: http://www.fightforspace.com/
A funky Russian space colonization journal: http://spacecolonization.info/publications/
For those interested in Stephan Martiniere’s artwork: http://www.martiniere.com/
Intriguing Enterprise episode of the Automated Repair Station; would be cool to have one of these!: http://www.cbs.com/shows/enterprise/video/1475165420/enterprise-dead-stop/
This is as close as the U.S. has gotten to automated systems in a real space envirnoment: http://www.youtube.com/watch?v=9cZTGsDtUWo&list=UUaigdIdLyOlAEBYj8U3camg
In media, during his opening monologue for a recent ‘Late Show with Jimmy Fallon’ episode (an episode featuring Halle Berry and Fallon demonstrating the “human hamster wheel”, I believe), Jimmy used the NASA robotics concept as a punchline to a joke. Probably not what NASA intends, but it can’t be helped–at least the message got out there to those who may otherwise not be interested in automated/artificial intelligent systems and space exploration! ^^;
@Alex Tolley August 28, 2014 at 16:16
‘How do we envisage keeping the superconducting wire cold? Does it need to be kept shaded and a radiator provided?’
A reflective component and an insulator such as aerogel will be sufficient to keep a superconductor cold within much of the solar system. The poles of Mercury are very cold, well within the realm of superconductors as are the poles of the Moon.
‘As Jim Benford noted, particle beams will require an more complete infrastructure, like the railroad system. What might that mean? Power is best created as near to the sun as possible.’
In space heavy ion colliders for fusion would operate quite efficiently due to the emptyness of space and the 3D geometry allowing a more spherical compression of fuel. These fusion base stations would allow the use of the other solar system where sunlight is less powerful.
Hmmm… if i recall it correctly the SDI initiative also included neutral particle beam stations initially. Their solution was nuclear power sources and remarks this being a very cost-intensive endeavor. Also, considering the public resistance to the Cassini launch, it may be problematic within public perception.
If its a feasible solution to counter beam momentum by ion drives, then i guess free floating beamers, able to reposition single units within the grid by outfitting the beamers with magsails would provide a huge advantage over mass-anchored systems in deployment flexibility including other tasks besides propelling sail-craft. Intuitively i guess there may be an accuracy problem balancing beam drag vs ion thrust. I have no idea if that is even an issue within interplanetary distances or if the mass of a beamer is feasible for a self-positioning grid, i am guessing wildly here.
I noticed NASA is advocating public participation in their asteroid initiative forum currently. Maybe some suggestions discussed here regarding the concept should be voiced in that context. I realize Icarus Interstellar is already affiliated with NASA through other means, but still… it could open one or more doors.
Also, excuse me again for bringing up the divergence issue again, would it be possible to dispatch… lets say… small portions of the outer ring of the magsail as electromagnetic focus stations for the particle beam?
@swage August 29, 2014 at 6:00
‘Also, excuse me again for bringing up the divergence issue again, would it be possible to dispatch… lets say… small portions of the outer ring of the magsail as electromagnetic focus stations for the particle beam?’
The problem here is that a huge voltage would be needed in the focusing rings for the effect to be noticeable. Perhaps the spent charges after interacting with the magnetic field could deflected in such a way as to create a tunnel that steers the other charged particle towards the magsail.
Yes, i though maybe you could power it directly from the particle beam itself somehow. There will be losses, of course. Maybe too severe. Interestingly detaching parts of the sail also means a decreasing of mass by the… vehicle… itself. Hmmm….
@Alex: Keeping a superconductor cold in space is simply a matter of shading it from the sun. A very thin metal foil of the same area as the cross section of the coil can do this at minimal cost, although there is a need to keep it precisely aligned at all times. If truly superconducting, the loop should generate negligible energy internally, and cool by radiation. It will asymptotically, slowly, approach the temperature of empty space, which I think is around 4 K.
Such foil, however, will not protect the loop from the beam. The beam, as envisioned, has many times the power density of sunlight and will likely cause the loop to become, in quick succession, non-conducting and non-solid.
The other issue that has not been sufficiently appreciated is the effect of the beam on the magnetic field of the sail. The beam constitutes a moving plasma, and as such will sweep up field lines and carry them with it. This will reduce the effective sail cross-section of the loop, in my estimation substantially.
A good model to consider would be that of a conducting wire in an orthogonal plasma stream. With no plasma, the magnetic field of the wire extends to infinity and its sail cross section is large. As you increase the density and/or velocity of the plasma stream, the field will be blown into a long tail the frontal cross section of which becomes finite and potentially quite small.
As the radius of the wire field becomes less than the radius of the loop, the sail will become donut-shaped with a hole punched out in the middle. Much of the beam will blow right through without significant propulsive effect.
You can of course try to counteract the effect by increasing the current, but that would make the wire thicker, the loop heavier, requiring a stronger beam, and so on ad infinitum.
No magnetic sail proposal should be taken seriously until this effect has been considered and dealt with, thoroughly and quantitatively.
Previously commented in an earlier article, the Johns Hopkins University study referenced below (link provided) performed “Monte Carlo” simulations of the interaction of the current loop’s magnetic field and interaction with incident ion stream.
This study might be a good starting point for further studies and modeling for how magnetic sails scale up.
This study does not account for changes in the magnetic field due to plasma interactions. It uses a simple model of individual particles in a static field. It is completely irrelevant to the problem of field deformation.
Field deformation is very real, and well understood. All you have to do is to look at one of the many beautiful depictions of planetary magnetospheres blown about by the solar wind to see it.
Apparently I am not the first to note the field deformation objection. Essentially the same objection is raised in somewhat less friendly terms by Alexander Bolonkin, who probably did a lot more research than I:
Showing little apparent respect for existing research he has this to say:
Does he have a point? I have to say that it looks like he does. In which case the whole subject could be a wild goose chase and should be laid to rest. Perhaps we could get a practitioner or two to comment on the validity of either Bolonkin’s or my own earlier objections?
There is, of course, also this work, which we have discussed in these pages before, I believe:
A little bit different in topic from beamed propulsion, but another highly qualified piece of work that is shedding much needed light (and, unfortunately, cold water) on magnetic sail technologies.
Rare in the field for having a real plasma physicist involved…
Paul, surely you have seen this book by Vulpetti: http://books.google.com/books?id=wnsjP6M6MxUC&lpg=PR1&dq=%22Fast%20Solar%20Sailing%22&pg=PR1#v=onepage&q=%22Fast%20Solar%20Sailing%22&f=false
From reading the free samples it looks very well researched and argued to me. Not primarily about magnetic sails or particle beams, but I came across it just now and thought I’d mention it just in case.
Eniac, yes — Giovanni Vulpetti has plenty of experience with sail technologies dating back to the Aurora days in Italy, SETIsail and other offshoots. I pay attention to anything he says on the subject. At the moment, I’m going through the 2nd edition of the solar sail book he wrote with Les Johnson and Greg Matloff, to be published late this year. More on this one soon.
Ah, that’s great news, Paul. Thanks!
I know I chimed in late and most have moved on, but if there is anything you know or can find out that would reconcile the apparently diverging opinions about the potential of magsails discussed above, I would love to hear about it. The magnetic field of a conducting wire in a plasma stream sounds like a textbook problem, and by myself I am not able to reject Bolonkin’s assertion that much of the work that has been published simply ignores the issue.
I would also love to be set straight if I misinterpret the situation.
It seems there is a NASA tech note from 1963 by Albert G Munson on this “textbook problem”. I wish I could locate a copy…
Yes — Jim Benford has been going over all the comments and is preparing a response, so we should get some further ideas on this soon. I’ll publish Jim’s work as soon as I receive it.
‘Such foil, however, will not protect the loop from the beam. The beam, as envisioned, has many times the power density of sunlight and will likely cause the loop to become, in quick succession, non-conducting and non-solid.’
A combination of multi layered reflectors like that used on the James Webb telescope and because the moving charged particles in a magnetic field will tend to gyrate perpendicular to the field direction and dump energy ‘radiation’ in the direction of travel the amount of radiation falling on the coils would be reduced significantly. A direct hit by a particle is unlikely due to the shape and field strength, think about the LHC, the walls don’t get hit by 3500 gamma protons.
‘The other issue that has not been sufficiently appreciated is the effect of the beam on the magnetic field of the sail. The beam constitutes a moving plasma, and as such will sweep up field lines and carry them with it. This will reduce the effective sail cross-section of the loop, in my estimation substantially.’
Yes the magnetic field will be compressed but remember a bow shock of plasma is created, it to has a field associated with it and so the size of the effective field will not be reduced as much. Further a ‘polo mint’ type field might be aiding the removable of interstellar material ahead of the craft as it moves through the centre, so all is not lost.
‘In the incorrect works, the particle magnetic field is hundreds times stronger than a MagSail magnetic field.’
Yes the individual particles have a higher magnetic field density but the field strength ‘moment’ of the craft is much larger and therefore able to bend the particles.
‘No magnetic sail proposal should be taken seriously until this effect has been considered and dealt with, thoroughly and quantitatively.’
Without taking the concept seriously in the beginning we can’t carry out a ‘thoroughly and quantitatively’ investigation of the concept, sort of self defeating. I and others and those that follow us will continue to look at the concept, there are a few hurdles to over come but I am quietly confident they will be.
I think Bolonkin’s objection here is that if the induced fields from the particles is 100 times stronger than the original field, it means the calculations are absurdly off. Not just inaccurate, but in the wrong ballpark. After all, the induced field only needs to be of the same magnitude to completely cancel the inducing field. Bolonkin’s 100 times just shows that the model used, with plasma field ignored, is very far from applicable, and the results meaningless.
Not really. It just means that once this (I am assuming valid) objection has been raised, it would be inappropriate and unscientific to put blinders on and continue to forge ahead with models that have been so seriously challenged. Such work should absolutely not be taken seriously. Practitioners need to redo their calculation with the plasma field included, or else explain why they think Bolonkin is wrong.
So glad I came back for another look and found the new comments and links to various considerations of the effectiveness of mag sail.
There are different ideas in the discussion about the way a sail would work, and in some scenarios people have talked about the sail ejecting a plasma cloud behind it to enhance the effectiveness of the sail. But if we remove the consideration of a plasma cloud around the sail, the resulting magnetic field is just a matter of superposing magnetic effects of the various currents at play. Not a distortion, so much as changes in magnitude owing to re-enforcements and cancellations.
Maybe more to discuss after reading the links provided, thanks!
Theory of Space Magnetic Sail Some Common Mistakes
and Electrostatic MagSail:
“Within the ring magnetic field they rotate under Lawrence force and
produce their own magnetic field that is OPPOSED to the ring magnetic field”
I’m having a hard time drawing any ion paths where this can happen. E (X) B is strongest where the ion path and magnetic field are perpendicular, and this would initially be at the bottom of the loop’s magnetic torus and only after the ion has been deflected 90 degrees (and therefor has already imparted its momentum to the sail). However even at that angle the ion’s magnetic field and that of the loop would seem to be orthogonal and have no effect on each other as far as cancellation/opposition.
Of course what I imagine is only one of many paths of incidence and deflection possible, that’s why I thought the Monte Carlo analysis would be helpful.
Are there any diagrams that show how the incident ion’s magnetic field can act to cancel the current loops magnetic field?
ProjectStudio: I am not sure you are thinking about this correctly. A particle entering a magnetic field will be bent by Lorentz force. As such it will form a circular current, and generate a little field of its own, exactly opposed to the field that causes the Lorentz deflection in the first place. If there are enough such particles, as is the case in a plasma, the particle fields will add up and attenuate, or shield, the original field. The end result is a deformed field, the precise shape of which is determined by the rather complex, but tractable, equations of plasma physics (magnetohydrodynamics or some such). We see the end results of such calculations and numerous observations in the pictures of magnetospheres, such as this: http://en.wikipedia.org/wiki/Magnetosphere#mediaviewer/File:Magnetosphere_Levels.svg, where the field looks quite different than the dipole that would result if there was no plasma. In particular, while the dipole field extends to infinity, the plasma-shielded field is finite and outlined by a bow-shock, magneto-pause, and what-not (I have not studied plasma physics that much…)
If you do not take this into account, you risk that your model is useless, which is what Bolonkin is claiming.
All I am asking is that this argument be made and backed up with something a little bit more quantitative than “not as much” accompanied by hand-waving.
It appears to me Bolonkin has done the math and disagrees with you, but it would sure be good to hear another expert’s opinion.
It’s been a while since I took undergraduate E & M. My girlfriend has been puzzled watching me make strange “right-hand rule” gestures in the air, as I tend to think more qualitatively these days. I never studied plasma physics at a graduate level. The physical realities are usually more complex than our qualitative or even our quantitative models. Thinking things through in this way might help someone design a practical working model in the lab where feasibility of the mag sail could be more thoroughly assessed.
A qualitative approach won’t sort out the feasibility, but it does show me that the cancellation does not occur for the entire trajectory of the deflected ion, but only for a very specific circumstances. The cancellation (of any resultant force) occurs most strongly for “crossed field” scenarios (see link and quote below.)
Like a sailboat sailing on the water, it is possibly to contend that it cannot sail upwind depending on how you imagine the sail shape and how they are trimmed. In reality sailboats can often sail fairly far into the wind as well as they sail away from it.
See: “Lorentz Force” section off
“The most interesting situation is that where the electric and magnetic forces cancel each other out and leave a charged particle undeflected. This requires
… the electric and magnetic fields must be orthogonal to one another (often referred to as crossed fields). In addition the balance between the fields depends on the velocity of the ion.”
It would be great to see graphical modeling where all these interactions and cancellations are accounted for, and they may very well show whether a plasma cloud is helpful or a hindrance, whether a plasma cloud can even be prevented if it is a hindrance (by suitably shaping the dispersion of deflected particles), or perhaps determining whether a toroidal magnetic field is the most effective shape – or if we need to trim it to achieve a more efficient field shape?
The plasma in this case is the beam, or the solar wind in case of a solar sail, or the ISM in case of a “parachute” sail. It cannot be prevented, it is an intrinsic component of the thing we are trying to make work.
Quite separate to the ion beam, some of the designs suggest intentionally creating a cloud of plasma behind the mag sail to make the magnetic field act bigger (suggesting the intensity could fall off as slowly as 1/r).
Discounting that approach out for the moment, the essence of the mag sail’s effectiveness will be the effective transfer of momentum from the incident ions (beam) to the mag sail. So we are looking for the factors that will influence the deflection of that ion.
The ion will not affect itself. Other ions preceding it may affect it if they accumulate behind the mag sail, but this would all depend on the deflection paths and related factors (beam speed, density, etc.) as well as the shape of the magnetic field.
So a design is needed, and then a statistical model can be created. And quite valuable to that model will be to consider the effect of any accumulation of deflected ions that can influence the deflection of succeeding ions.
My guess is that an effective design can minimize this influence. It is possible that, like the Bussard Ramjet/Parachute, the mag sail would become more efficient at higher speeds?
re: ‘My guess is that an effective design can minimize this influence.’
Or, ‘an effective design can minimize – or take advantage – of this influence’ due to any accumulation of a plasma cloud from the incident beam.
Thanks to everyone who has commented.
I wrote the original paper that was discussed here April 3. Comments seemed to have petered out and I moved on. Recently discussion has started again, so I’ll try to catch up.
Many comments show confusion over what I proposed or discuss issues already addressed in the paper, so I will be happy to send you a copy if you write: RAMOLE@AOL.COM.
Geoffrey Landis has suggested that using a plasma of mercury would solve the dispersion problem because millions of atoms would condense into tiny droplets. The atoms have random lateral speeds and directions but these would cancel each other when they were grouped together so beam divergence would be very low. Jim Benford objects:
“Geoff Landis’ proposal to reduce beam divergence, by having neutral atoms in the particle beam condense, is unlikely to succeed. Just because the transverse energy in the relativistic beam is only one millionth of the axial energy does not mean that it is cool. Doing the numbers, one finds that the characteristic temperature is very high, so that condensation won’t occur. The concepts described are far from cool beams. “
I am not a beam analyst but my figures show a low equivalent temperature that should allow condensation.
First, the axial velocity is high but all the atoms go at the same axial velocity so this does not count. Only their velocity relative to each other matters as to whether they adhere and form droplets. This is indeed related to their transverse velocity.
The beam goes .2 C and diverges at 3.5x10E-6 rad, so the transverse velocity is just .2 X 3 X 10E 8 m/second X 3.5 X 10 E-6 = 210m/s . (This is for hydrogen and heavier elements diverge less, so this is conservative as to temperature. That is, mercury will have a lower transverse velocity and equivalent temperature than I calculate.)
Temperature is related to speed through the ideal gas law, or
T = V^2 * M/(3 * R)
where T is degrees K, V is average velocity,m/s, in all three directions, M is the kilogram mass of a mole of the gas (201 for hg), and R is the universal gas constant or 8.31* 10^3 for Kg moles.
Then T= 210^2 * 201/(3*8.31*10^3) = 355K = 82C
Mercury should form droplets at this temperature. Thus Landis’ idea of using mercury seems promising.
This is overconservative, because V should be the average speed in X, Y, and Z directions, and 210 m/s is only in one one direction, radial out of the beam. The two perpendicular velocities are zero. I believe the actual correct number should be 210 / sqr 3 = 121 m/s, and even less because a heavier beam diverges at a smaller angle.
Again, all the droplets are going .2 c axially relative to the ground and this would be a very high temperature (and a huge energy if they impacted a still particle, and only vapor or plasma would result.) But then again, relative to matter falling into a black hole at nearly the speed of light, we are all going .99c, but that doesn’t cause us to vaporize! Relative to each other, we’re cool. (At least those of us on this list…)
Regarding the idea of a beam of protons and a parallel beam of electrons (or one intersecting at a small angle) Jim Benford also writes:
“One method of getting a neutral particle beam might be to generate separate ion and electron beams and combine them. But two nearby charged beams would need to be propagated on magnetic field lines or they would simply explode due to the electrostatic force. If they are propagating parallel to each other along magnetic field lines, they will interact through their currents as well as their charges. The two beams will experience a JxB force, which causes them to spiral about each other. This produces substantial transverse motion before they merge. This example shows why the intense fields of particle beams create beam divergence no matter how carefully one can design them.”
I have asked him if both particle streams have the same axial velocity, i.e zero axial velocity relative to each other, whether they then feel a magnetic field from the other? Or is B just zero? Again, their velocity relative to the ground seems irrelevant; relative to each other they are still. I have received no answer and I’m not a beam expert so I simply do not know.
Galacsi writes: “The launch system as envisioned by Dana Andrews and Alan Mole would be affixed to an asteroid that would provide sufficient mass to prevent the reaction from the launch of the beam from altering the orbit of the Beamer and changing the direction of the beam itself. No quantitative valuation of this has been provided to date.’ Others question what it would take to stabilize the generator in earth orbit.
Actually I (Mole) wanted the beam generator on the ground or in earth orbit. If it is in orbit we must indeed counteract its thrust but that is not hard.
If one does not want to go at lightspeed one is better off using as much ejecta mass as possible, to get the most momentum for a given energy. That’s why jets use ever-higher bypass ratios.
The beam genertor accelerates a 1 kg (2.2 lb) mass at 1000 g for 50 minutes. That’s 2200 lbs force x 50 minutes = 110,000 lb minutes. A typical rocket – say, Falcon 1 – weighs 85,000 lbs and produces 102,000 lbs force (thrust) for 2.8 minutes, or 285,000 lb minutes. This is about three times what we need, so we could get by with 28,000 lb of rocket pushing the generator and counteracting its thrust. At $200/ lb to LEO (Musk’s goal for reuseable rockets) that comes to $5.6 million, which is trivial compared to the $17 Billion I estimated for this project. This part is easy.
“August 28, 2014 at 16:16
How do we envisage keeping the superconducting wire cold? Does it need to be kept shaded and a radiator provided? Is aAlex Tolley passive system sufficient, or do we meed active cooling? “
Actually I wrote an entire appendix in the paper to show passive methods could keep it cold enough. I found that using a rectangular cross section, with the narrow side facing the sun and reflective and the wide sides facing space and black (emissive), would keep it more than cold enough.
“Appendix A Thermal Design for Temperature of 93 °K at 1 AU
Andrews and Zubrin selected YBa2Cu3O7 as a superconductor with good electrical and mechanical properties for use in a magsail in the inner solar system. Its critical temperature (Tc) is 93 K. It is shown below that this temperature can be achieved at 1 AU by appropriate geometry and thermal coatings without recourse to external cooling.
Consider a 1 cm length of ribbon cable 0.2 cm by 5 cm. The size is selected for convenience in calculation; the actual sail cross section would be much smaller but have the same proportions. The sail is painted black so emissivity is 0.95, except for the sunward face which is coated with silverized teflon with an absorptance of 0.09. The solar constant is 0.136 W/cm2 so the energy absorbed on the front face is:
.09 x .2 cm x 1 cm x .136 W/cm2 = .0024 W.
Neglecting the front and rear faces, the area of the top and bottom radiating faces is 10 cm2. The resulting temperature is
T = (Energy/(emissivity x Area x Stefan-Boltzmann Constant)1/4
T= (.0024 W/( .95 x 10 cm2 x 5.67 x 10-12 J s-1 cm-2 °K-4))1/4 = 81 °K
This is well below the required Tc of 93 °K. The temperature can be lowered further by making the cable even thinner to decrease the frontal area or by coating the front with a dielectric mirror with an absorptance of less than .01 instead of the .09 used above.* Thus a magsail of YBa2Cu3O7 can be made cold enough to operate at 1 AU. It is not necessary to go to the asteroid belt; the sail can be launched from space near Earth.
* (I was wrong about dielectric mirrors, which reflect only a narrow band of wavelengths. But silver has a reflectivity of 95-99%, or absorptance of 5-1%, almost 2-10 times better than the .09 I used in the calculations.)
The superconducting loop will work just fine for a first design. A statistical model is overkill, I would suggest trying a magnetohydrodynamic model first. Once we have that, we can see if it is good, and if not, whether we can achieve much improvement with more complex designs. I suspect the answer will be no on both counts, but I have been wrong before….
That is the problem here. We do not have a single ion, we have a bunch of them, a condition also known as plasma. We do not have to be too hypothetical about it: Magnetohydrodynamics or some other well understood method from plasma physics will apply. Plasma physics is one of those fields that are well developed, but often underappreciated by non-practitioners. Even so, we know quite well that magnetic fields are affected, and in many cases dominated, by the movement of plasma. As evidence please note the dynamics of solar eruptions and the magnetospheres of planets and stars.
I suspect the reason a proper model has not been published is that the results are disappointing, and few like to waste time going beyond preliminary calculations.
Alan Mole: Thanks very much for these well thought-out replies. You are right that recoil and superconductivity are easy issues. I assume you are working on a second post on the more difficult ones?
On beam temperature:
I don’t think this is correct. Why would the relative axial velocity be zero? To me, this seems even harder to achieve than zero transverse spread, and it is an extra requirement because it is not necessary to keep the beam together.
Mostly, I would love if you could comment on the magnetohydrodynamic effect of the beam on the magnetic field (wouldn’t it be shielded away?) and the objections raised by Bolonkin.
“I am not sure the calculations above are very realistic , in my book to accelerate 1 kg at 1000 G (i.e 10,000/s2) , you need a force or 22,000 lbs. And you must take into account the divergence of the beam , an unknown factor , and the yield of the impact of the useful particle on the sail , an other unknown factor.”
I believe my calculation is correct. One kilogram equals 2.2 lbs. To accelerate 1 lb to 1g requires one lb of force. (E.g. 1 lb weighs 1 lb and produces 1 lb of force against something holding it up. If the holder is removed the 1 lb drops at 1g. To accelerate 1 lb at 1000 g requires 1000 lbs of force. To accelerate 2.2 lbs requires 2200 lbs of force as I said, not 22,000 lbs as Galaxsi says. In the MKS system there is indeed a 10 times multiplier involved, but not in the English system. The transition to MKS will cost many mistakes!
In the paper I said I assumed 100% efficiency; Andrews said 60% I think and I assumed some improvement over 50 years. For lower efficiencies a larger beam force will indeed be required.
” Why would the relative axial velocity be zero? ”
Because we make it so. We send a beam of protons at .2 c (axial) and seperately a parallel beam of electrons at .2 c axial. (almost parallel; actually they would converge at a small angle, say 2 deg.) Thus the axial relative velocity would be zero, or as close as we can make it.
Please ignore my previous response — I was thinking of another case.
In the case of mercury ions neutralized by Benford’s method, all the mercury ions are accellerated to the same nominal speed. There might be some variation in axial speed, but most would be going at nearly the same axial speed relative to each other. In the idealized model all are moving at .2 c axially and have some radial speed in a random direction, but perpendicular to that their velocity is zero. If x is up, y is north and z is east, a typical atom would have a velocity of (0[.2c relative to the ground] , 210 m/s, 0.)
“Close as we can make it” is going to be much different from zero. It may even be much, much more than 210 m/s.