
The Dipole Drive: A New Concept in Space Propulsion - cl42
https://arc.aiaa.org/doi/abs/10.2514/6.2019-1122
======
ScottBurson
Previously discussed:
[https://news.ycombinator.com/item?id=17428751](https://news.ycombinator.com/item?id=17428751)

I chewed on this idea for several days, eventually coming to the same
conclusion others had: it won't work. This poster came up with a correct
explanation much faster than I did:
[https://news.ycombinator.com/item?id=17432975](https://news.ycombinator.com/item?id=17432975)

Zubrin has taken a familiar approximation -- that the field near a flat
charged plate is uniform -- and forgotten that it's only an approximation, and
that the keyword is "near". As particles leave the vicinity of the drive, they
will be attracted back toward it, reversing the momentum transfer.

~~~
amluto
Agreee, although I think the arguments and calculations in that thread are a
bit silly. If you take any finite distribution of fixed charges, and you fire
a test particle from infinity at the charge distribution, one of two things
happens: either it hits one of the charges, or it goes back out to infinity,
possibly in a different direction, with the same velocity it started with. The
idealized field distribution that accelerates everything going through does
not exist in electrostatics.

So either some time-varying fields are needed or interactions between space
plasma particles is needed.

Also, the efficiency calculation doesn’t account for particles hitting the
grids and thus creating a short-circuit current. Which has its own issue: in
the original paper, 500W is going somewhere, but there is no explanation of
how, exactly, any current flows in the circuit, which is inconsistent with any
power at all being used.

~~~
ScottBurson
> in the original paper, 500W is going somewhere, but there is no explanation
> of how, exactly, any current flows in the circuit

I puzzled over that a bit too. I think the answer is that as protons are shot
out the negative side and electrons out the positive side, they create a field
that counteracts the field between the grids. Maintaining the field strength
thus requires continuously adding charge to the grids. In the hypothetical
case of infinite grids (being charged by an infinite number of generators),
the areal charge density on the grids would increase without bound, so that
the charge density difference, and therefore the field, across the infinite
plane halfway between the grids would remain constant. Exactly how this would
translate to the finite case I'm less clear on, but I suppose the behavior
would be similar, at least shortly after the drive was first turned on. Sooner
or later, though, I think the voltage across the gap would increase to the
point that any generator you could actually build wouldn't be able to push any
more charge onto the grids.

~~~
amluto
> Maintaining the field strength thus requires continuously adding charge to
> the grids.

I'm not sure what you mean. The whole system is running at steady state. For
simplicity, pretend that the spaceship is very massive or is otherwise
accelerating at negligible rate so that the spaceship's frame is inertial. [0]
In the spaceship's frame, there is some electron current density at all points
in space, some proton current density at all points in space, and some
distribution of charge carriers on the grids. If we further assume an infinite
work function on the grids, then there is no field emission and no plasma
charges are captured, so there is no transfer of charge between the plasma and
the grids. But then there is no current on the grid, so the driver circuit
outputs no power, and the engine can't possibly do any work.

So either my analysis is missing something or the paper is wrong. Or both.

[0] This could be achieved exactly by firing an equal-but-opposite-thrust
conventional rocket in the opposite direction. It doesn't really matter for my
argument anyway, I think.

[1] Or the grid could be made of metal filaments wrapped in perfect
insulators, but then there's an issue of charges building up on the outside of
the insulators. In general, the grid _will_ capture some charge from the
plasma, and this will be a parasitic loss on the system. I would certainly
believe that a pair of _solid_ parallel finite metal plates flying at high
speed through a plasma generates a force, but that's not what's being
described in the paper.

~~~
ScottBurson
Consider what happens when you've just turned the thing on and it has just
barely expelled its first proton through the negative grid. As long as the
proton is close to the grid, it may as well be on the grid: it cancels out the
contribution of one electron to the field between the plates, which means the
voltage across the gap is incrementally reduced. This reduction in voltage
allows one electron to flow out of the generator into the grid.

Of course, as the proton moves away, its contribution to the field between the
plates diminishes at a rate depending on the ratio of its velocity to the
radius of the grids. (This is why the infinite-grid case is easier to think
about: the contribution doesn't diminish at all.) Eventually — and depending
on the proton velocity and grid size, "eventually" might mean milliseconds —
its contribution becomes negligible. At that point a steady state will have
been reached, and (as you intuit) it won't be possible for the generator to
push any more charge onto the grids without increasing its output voltage.

------
T-A
General note to naysayers: before you present yet another "obvious" argument
why Zubrin's proposal can't possibly work, you may want to check how it
applies to this:

[http://news.mit.edu/2018/first-ionic-wind-plane-no-moving-
pa...](http://news.mit.edu/2018/first-ionic-wind-plane-no-moving-parts-1121)

[https://www.nature.com/articles/s41586-018-0707-9](https://www.nature.com/articles/s41586-018-0707-9)

It's essentially the same thing, except it has to ionize the medium (air) on
its own rather than just pick up the ions from its surroundings.

(Yes, it does so in front of the vehicle, so on average stripped-off electrons
get less work done on them by the electric field than positive ions, but the
electrons would carry less momentum than the more massive ions even at kinetic
energy parity, with the difference going to the vehicle, as Zubrin correctly
says - please let's not beat _that_ dead horse again.)

------
samcheng
I found a non-paywalled discussion here:

[https://www.centauri-dreams.org/2018/06/29/the-dipole-
drive-...](https://www.centauri-dreams.org/2018/06/29/the-dipole-drive-a-new-
concept-for-space-propulsion/)

------
peter_d_sherman
6 milli-Newtons per kW is not a lot of force relative to the electricity
consumed... but this is sort of the early alpha version of this technology. I
expect future versions of this to be greatly improved. So, all in all, a
brilliant set of ideas with a lot of future potential!

------
amluto
Better link: [https://www.centauri-dreams.org/2018/06/29/the-dipole-
drive-...](https://www.centauri-dreams.org/2018/06/29/the-dipole-drive-a-new-
concept-for-space-propulsion/)

------
fxj
The dipole drive uses the protons and electrons of the solar wind for
propulsion. They are reflected in an electric field and the paper claims that
velocities higher than the solar wind speed can be achieved.

But take the proposal with a grain of salt, because the whole mechanism highly
depends on the plasma density of the solar wind which is rather small in the
interplanetary space. It really works well only close to planets or stars
where the plasma density is high.

------
ThomasBarb
Should be an interesting read. Can't reach the site. Is it up or down? Tried
loading it several times but it didn't load for me.

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amingilani
I feel as though this is one of those points where a picture is worth a
thousand words.

------
jcims
Isn’t this basically electrohydrodynamics?

------
stareatgoats
> the dipole drive can be used to accelerate a spacecraft at velocities
> greater than that of the solar wind [0]

and

> [the Dipole drive] therefore offers potential as a means of achieving ultra-
> high velocities necessary for interstellar flight.

Sorry, but I couldn't come up with anything else than: warp-drive!! (if it can
deliver on that promise that is). I have long hoped/expected we would crack
the challenge of propellant-less acceleration through space, not sure if this
is it though.

[0]
[https://news.ycombinator.com/item?id=20666722](https://news.ycombinator.com/item?id=20666722)

~~~
Dylan16807
Nothing that accelerates at a tiny fraction of a percent of 1g is within miles
of a "warp drive", no matter how high the max speed is.

~~~
taneq
The point of a warp drive is to bypass the speed of light limit by warping
space. Acceleration is kind of irrelevant.

~~~
peter_d_sherman
Theoretically, in addition to the warp drive, you also need some kind of
secondary propulsion, no matter how slow, to move the ship through the warp
bubble/vortex/portal/curved space, etc.

Later versions of warp will be able to move the bubble/vortex/portal of curved
space itself -- around the ship and from front to back, thus obviating the
need for motion relative to the warp field.

Technology-wise think of the steps like this:

1) Figure out how to create a warp bubble

2) Figure out how to move it, relative to what's projecting it

Without #2, secondary propulsion is required. #1 is the first technological
achievement, #2 would follow upon that. Until #2 is achieved, secondary
propulsion is required...

#2 is a little bit wierd to do, because once you've changed your location in
space, you've changed the location of what's projecting the warp field -- you
need to include that in your warp field projection calculations for #2... and
that's IF it could be done...

In fact, the initial version of warp might look something like this: Ship A,
stationary in space, projects the warp bubble, Ship B goes through it. Why?
Because otherwise you've got all kinds of relative motion that would need to
be compensated for...

Of course, all of this is theoretical...

~~~
taneq
Then again, if you can actually generate a field that warps spacetime, you can
probably arrange for the spacetime inside the warp bubble to be curved such
that it causes objects inside to accelerate.

------
osamagirl69
The fact that the abstract does not mention laser driven sails, the currently
most viable approach to relativistic propulsion (ie, the current breakthrough
starshot plan), does not give me much hope in this researchers credibility in
the rest of the paper.

Furthermore, the performance of this engine is abismal. To put it into
perspective,a modest ion engine would produce about a million times more
thrust per power. While the ion engine of course requires carrying fuel, and
is thus governed by the tyranny of the rocket equation and not directly
comparable in performance, this engine still requires a power source and thus
effectively requires fuel anyway.

Also worth mentioning is that the least efficient possible massless thruster,
a simple blackbody radiator with reflector to aim all of the photons behind
you, is only about 1000x worse than this unit in terms of propulsion force per
power, and essentially infinitely better than the unit in terms of propulsion
force per mass, since this structure is incredibly massive where as you can
usually generate blackbody radiation directly from you power source.

~~~
vidarh
The author is Robert M. Zubrin. To write it off because the abstract doesn't
mention a specific technique seems a bit silly, especially given that Zubrin
personally knows a lot of the people involved in Breakthrough Starshot and
surely is aware of it, including its massive caveats.

It's worth noting that the paper specifically discusses the need for methods
that are not limited in travel direction, and in that respect Breakthrough
Starshot has the same caveats as a solar sail and so treating laser driven
propulsion separately is pointless.

Another major difference that is so obvious as to not require a mention of
every specific alternative is that it is talking about a propulsion method
that avoids the need for us to provide a source of thrust.

Laser driven systems doesn't require the ship to _carry_ propellant, but we're
still paying the cost to provide thrust, while this design is basically 'just'
relying on there being a source of protons and electrons with a speed
differential relative to the current speed and direction of travel of the ship
that it can 'scoop up'.

That makes for fundamentally different tradeoffs relative to classical sail
based propulsion.

