The Plasma Magnet Drive: A Simple, Cheap Drive for the Solar System and Beyond | Hacker News Reader

The Plasma Magnet Drive: A Simple, Cheap Drive for the Solar System and Beyond

centauri-dreams.org

185 pointscurtis45 comments

The article says that since the thrust is always away from the Sun, it can't be used for inward journeys. But if I'm thinking correctly, this isn't necessarily true.

If you're in a circular orbit, and run this for a while (short compared to orbital period), then turn it off, I think you'll be in an elliptical orbit with perihelion lower than your initial orbit. If when you reach perihelion there happens to be a convenient planet with an atmosphere, you could aerobrake and either land or maybe continue in a circular orbit closer to the sun than your original orbit.

Still reading up how it works, but if we could somehow bend the direction of the resulting force, ever so slightly, it would be enough to enable the spacecraft to thrust partially retrograde, which achieves lowering of the orbit.

At the very least, thrusting radially away from the sun while moving towards it during an elliptical orbit would reduce the energy of the orbit.

Figure 5 demonstrates how an array of two or more plasma magnets can deflect solar wind to achieve the same maneuverability as a solar sail.

Can't wait to see a cubesat demonstration vehicle blow past Mars!

Or perhaps, if you can only accelerate away from the sun, you just aim for a gravity well at aphelion and turn off as you approach, swing round it and float back towards where you want to go. You then turn the engine on to decelerate when you near your destination?

(full disclosure: never played KSP)

I'd imagine this is the easiest solution for improving mission flexibility.

If the only rules are "only accelerate away from the nearest star", that doesn't preclude using a planetary gravity field to also adjust your orbit.

E.g. Making small course corrections in interstellar space (or simply mission planning exactly enough) so that you hit a planet's gravity well on insertion into the destination solar system, thereby slinging yourself into a more manageable eliptical orbit (and potentially burning something else at the right point to change your orbit)

I'd also be really curious on how something like this works in the area of interaction between a planetary magnetosphere and the "sail" (e.g: https://en.m.wikipedia.org/wiki/Magnetosphere#/media/File%3A... ). Looks complicated!

The field generated by the drive is in the realm of 50 nanoteslas, and Earth’s geomagnetic field for example is in the microtesla range. I don’t think it really matters though, because the magnetic field around a planet redirects solar wind, and that alone would be enough to screw up this drive. In my opinion this is something you use for the interstellar leg of the journey, and you would avoid planets while the drive is on. That isn’t really a problem though, because other than a relatively short trip away from Earth, you’d be avoiding planets and building up momentum. To make this work, you’d be going at a hell of s clip relative to any other planets you’d come across, and you’d be a tiny mass to boot.

You’d probably just sail on past, with a deviation in course, but not orbit or a 180. For any gravitational maneuver, you could rely on momentum and shut down the drive, restarting for the return leg.

Instead of aerobraking, you're probably better off doing a momentum transfer gravitational "negative" assist - transfer a chunk of your momentum back to the planet.

I'm not an expert on plasma physics, but - couldn't do the solar equivalent of "tacking" - deform your magnetic field so that the solar wind flux in front is less than the flux behind, leading to a tangential acceleration, which could be used to achieve orbits that fall back into the sun?

I don't think that would work. Sailboats can sail into the wind similarly to how an airplane can have a glide ratio greater than one. The wind presses on the sail, and the boat transfers that to the water in such a way that it is easier for the boat to go forward than be pushed backwards. Of course it still slips back some, but it goes forward faster than it slips backwards.

why wouldn't this work with a plasma field? If you inflate your leading edge to be larger, your net solar wind flux is going to be larger and a greater amount of force will be transferred to the craft from the leading edge, slowing you down. Even though you're being pushed OUT by the solar wind, your tangential velocity relative to the sun will be reduced, which, even if it results in a greater net orbital energy, will result in a more elliptical orbit (and thus a trajectory that sends you inwards).

I agree. Tacking requires "levering" against a fulcrum (the the centerboard on a water yacht and the skates on an ice yacht. Imaging trying to tack on a frictionless surface - you'd simply get swept away from the wind.

Anything we can get out of our gravity well will have an orbit around the sun. If it didn’t you wouldn’t bother with a high impulse/low thrust engine like this.

Your momentum is what you’d be pushing against.

Specifically, using a force vector with a substantial component (greater than 50%) in the opposite direction of travel. It would push you out but also slow you down.

This kind of vectoring should be straightforward with a solar sail because it is flat. But if a magnetic field is perfectly spherical then there aren’t any angles. You’d have to make a fairly flat magnetic field to pull that off.

Claim 5 is a bit puzzling to me :

A key feature of the plasma magnet is that the diameter of the magnetosphere increases as the density of the solar wind decreases as it expands away from the sun. The resulting expansion exactly matches the decrease in density, ensuring constant thrust. Therefore the plasma magnet has a constant acceleration irrespective of its position in the solar system.

I'm not 100% sure about the science here, but the "exactly" looks a bit magical to me. If I understand the concept correctly, when the sail is closer to the sun, the particles "pressure" on the sail will "compress" and reduce its effective area.

Without having studied the equations, but having seen similar phenomenon occur in other work my guess would be that the equation to determine the total area of the magnetosphere (let's say A = M(V,r) where A is the area and V is applied voltage and r is the distance from the sun) includes a component based on the density of the solar wind at that position.

The component is likely in the denominator because as solar wind density goes down we'd expect the magnetosphere size to increase.

Now, the solar pressure is something like P=S(r) where P is the pressure and r is the distance from the sun and S contains some geometry and solar power terms.

If we look at M(V) we could probably then find an M'(V) s.t. M(V,r) = M'(V) / S(r). To get the force produced by the drive we'd take M(V,r) * S(r) which cancels out all radius terms.

Mathematically this would indicate that the thrust on the magnetosphere would be invariant for all r within the solar system (after that point other terms within S() and M() that are based on solar output would likely start to break down)

From a different source...

(One possible barrier is a magnetic field of 50nT at or beyond 4 km. To create such a field using an electromagnet requires very large-scale engineering, for example a circular electromagnet 300m in radius carrying 10^5 amp-turns. While such an electromagnet is not impossible, it would likely be so massive that the relatively modest thrust coupled from the solar wind would provide accelera tion too slow to be of interest.

http://earthweb.ess.washington.edu/space/PlasmaMag/

So we just need some novel superconductors, a portable fusion plant, and then we’re set! /s

More depth: http://www.niac.usra.edu/files/studies/final_report/860Sloug...

Edit: Corrected formatting errors. Thanks for the heads up!

Well, it's a novel superconductor or a portable fusion plant.

Still, when you pasted it here, the 10^5 A-turns become 105 A-turns. For anybody that looks at it and thinks "hum... seems almost viable", it's just a formatting error :)

EDIT: Oh, but the proposed conductor is plasma composed of solar wind particles. It is reasonable to get a coil much larger than 300m with those.

So we just need some novel superconductors, a portable fusion plant, and then we’re set!

Almost anything you can make work with a suicidally unstable hypothetical power plant, you could also make work with a suicidally concentrated beamed power system.

> a circular electromagnet 300m in radius carrying 10^5 amp-turns.

> So we just need some novel superconductors, a portable fusion plant, and then we’re set! /s

Those numbers are for when you're not using the Plasma Magnet.

And that's one tenth the number of amp-turns you have in a typical MRI, but on a bigger loop. It wouldn't require special power sources.

For more history on this, see the magnetic sail proposed by Dana Andrews and Robert Zubrin in 1988:

https://en.wikipedia.org/wiki/Magnetic_sail

They found that it wasn't very efficient for acceleration but could be used for braking (for example by a light craft propelled by laser to another solar system).

Also tacking works by angling the force to increase or decrease orbital velocity. This assumes that plasma drives can be used for acceleration, and I think that they probably can with the right geometry/technique and were abandoned prematurely. And they obviously can when used with something like nuclear power where avoiding the need to carry propellant is more important than efficiency.

The coolest part about decelerating with such a system is that when you encounter the solar wind and deflect it at high speeds, you're going to generate EM radiation by accelerating the ions in the wind. So, under the right circumstances, the inhabitants of the solar system you're braking into will see an enormous glowing bubble in space as you arrive.

Formatting problems are making it start out pretty much unreadable for me on Firefox on Mac [1]. Whatever is laying out the text and sidebar is using a width that is much larger than the width of the white area, so part of the text and all of the sidebar end up out in the black right margin. Since the text is black, this doesn't work well.

Workaround: narrow the width of the white area. As soon as the window shrinks to that width, it changes to a format that appears to be designed for narrow devices.

The narrow layout correctly matches the width of the text display to the width of the white area.

[1] 58b014

Not changing your angular momentum around the Sun limits the usefulness of this drive by a lot. Sure, you can change the shape of the ellipse that you travel. But anything that you encounter far away from the Sun will be going very fast compared to you, and you'll go very fast compared to anything that you encounter near the Sun, making actual visits very unwise.

As an interstellar carrier it's perfect though.

Boost a limited-duration (aka small mass) craft at unsustainable acceleration to dock with an interstellar transiting ship, then detach and decelerate on the other end.

All the while you're able to continue increasing the interstellar ship's velocity. Limited only by the max velocity you can feasibly intercept.

By being propellantless, it essentially takes mass considerations out of the interstellar transit system (given enough time).

This would definitely be more useful for one-way probes, or as a drive mode for some far-future generational voyage. Maybe this could even be combined with some form of charged particle collection as well, a low-grade ramscoop.

As I understand figure 5, an array of theses devices should allow momentum changes.

The CSS is badly broken for me.

Might regenerative braking at the destination star be possible? If so, nearly all of the stored energy could be depleted in acceleration, leaving just enough to bootstrap deceleration.

How would regenerative braking work in space without friction?

Yes, but. Yes, you can get energy out of slowing down with any conductor passing through a magnetic field, but spaceflight uses so much energy that current electrical energy storage is essentially as irrelevant to propulsion as clockwork would be.

So I think you are saying that only nuclear has the required energy density for acceleration.

Even so, it would be nice to carry much less of the stuff, or a faster decaying material, if regenerative braking provides enough power to almost completely power the magnetosphere upon arrival.

Mainly I’m saying batteries currently suck. If you can make a nuclear battery (Hafnium isomer, perhaps), that would work, but it would probably also make M2P2 obsolete.

I've heard about solar sails for decades. Why have we never created one? Is there something limiting its development besides money?

The short answer is that we have:

http://www.planetary.org/explore/projects/lightsail-solar-sa...

does anyone have better details on the plasma magnet, i still am not specifically wrapping my head around it's construction. looked at sources and still can't make sense of it

Shouldn’t they be able to tack “upwind” to go towards the sun?

"Tacking" (more technically, the non-downwind vector of a segment of tacking) only works because sailboats are pushing against a resisting vector, neutralizing one directional component of the wind vector (usually a centreboard).

See: http://asianyachting.com/aymasters/l1tech&man.htm

You can't neutralize a component, but a solar sail vessel would not need to: diverting a fraction of the force retrograde (without neutralizing the rest) would be enough to turn a stable orbit into a spiral towards the sun.

Edit: But the way I understand the plasma magnet drive it would not be able to divert the force vector at all. It could still travel in both directions by modulating the throttle after using other means of propulsion to "replace" some orbital speed with solar drag, but it would never match the speed of its orbital peers. Maybe useful as a complement to conventional solar sails ("solar spinnaker")?

To put it another way, my reading of the article is that it only provides an acceleration vector directly away from the nearest star.

That can be used for propellant-massless acceleration (leaving solar system) and deceleration (entering solar system), but cannot power any other maneuvers.

(But of course auxiliary drives of a different type could)

As I understand it, that's because the field is spherical. If you place a sphere in a river it can only go straight downstream. However if you combine two or more spheres in a line and angle that line with respect to the flow, it's possible to move diagonally downstream to reach either shore.

True, but the magnitude of any locally-tangential component to a spacecraft would be relative to the angular difference in the spheres' locations, relative to the nearest star. No?

Which is to say, impractical for any substantial radius from the solar system's center.

Right, it wouldn't exactly turn on a dime. :D Still, being able to turn at all dramatically increases the usefulness of the drive.

The US doesn't even produce any plutonium anymore and NASA has to buy fuel from Russia and then deal with protestors when they try and test power plants that would be able to last long enough and be durable enough to get us to Mars. Activists have halted much of the worlds energy research.

Penguins have a gland above their eyes that allows them to process saltwater into freshwater.

The US does in fact produce plutonium 238 now, not much but production is being scaled up[0]. From what I understand, most of the issues with developing new nuclear power sources stem from funding shortages. NASA canceled their Prometheus nuclear reactor program, which was to provide hundreds of kilowatts of power for a Europa orbiter, because the Constellation program needed more money. Most recently, NASA canceled the Advanced Stirling Radioisotopic Generator because development costs had risen more than expected[1].

Although there might be some encouraging results soon, NASA is supposed to be finishing testing of the new Kilopower nuclear reactor at the Nevada test site soon.[2] Next step after this breadboard test is building an integrated system. If they can get to that point, they will have solved much of NASA's problems with plutonium. A nuclear reactor can use uranium fuel which is much simpler to procure than Pu-238.

[0]https://www.space.com/36217-plutonium-238-nuclear-spacecraft...

[1]https://en.wikipedia.org/wiki/Advanced_Stirling_radioisotope...

[2]https://www.nasa.gov/directorates/spacetech/feature/Powering...

As of a few years ago, the DOE started making Plutonium again in partnership with NASA [0]. And as of last year that appeared to be still happening [1]. And this Plasma magnet still would need something like an RPS to get past Jupiter, according to the article:

>Using a steady, nuclear power or beamed power source, such a craft could accelerate to the heliopause, allowing interstellar precursor missions, such as Kuiper belt exploration and the FOCAL mission within a short time frame.

[0]https://www.popsci.com/plutonium-238-is-produced-in-america-...

[1]https://rps.nasa.gov/news/21/a-step-forward-in-reestablishin...

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