
The Plasma Magnet Drive: A Simple, Cheap Drive for the Solar System and Beyond - curtis
https://www.centauri-dreams.org/?p=39048
======
radford-neal
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.

~~~
clort
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)

~~~
ethbro
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...](https://en.m.wikipedia.org/wiki/Magnetosphere#/media/File%3AStructure_of_the_magnetosphere-
en.svg) ). Looks complicated!

~~~
QAPereo
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.

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Zarathust
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.

~~~
iaw
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)

------
QAPereo
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/](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...](http://www.niac.usra.edu/files/studies/final_report/860Slough.pdf)

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

~~~
marcosdumay
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.

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zackmorris
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](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.

~~~
thatcherc
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.

------
tzs
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

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btilly
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.

~~~
ethbro
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).

------
XR0CSWV3h3kZWg
The CSS is badly broken for me.

------
lovemenot
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.

~~~
ben_w
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.

~~~
lovemenot
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.

~~~
ben_w
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.

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

~~~
goodcanadian
The short answer is that we have:

[http://www.planetary.org/explore/projects/lightsail-solar-
sa...](http://www.planetary.org/explore/projects/lightsail-solar-sailing/)

------
random_user456
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

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

~~~
ethbro
"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](http://asianyachting.com/aymasters/l1tech&man.htm)

~~~
usrusr
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")?

~~~
ethbro
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)

~~~
mLuby
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.

~~~
ethbro
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.

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

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radworker
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.

~~~
gene-h
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...](https://www.space.com/36217-plutonium-238-nuclear-spacecraft-
fuel-production.html)

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

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

