
The Dipole Drive: A New Concept for Space Propulsion - sohkamyung
https://www.centauri-dreams.org/2018/06/29/the-dipole-drive-a-new-concept-for-space-propulsion/
======
gus_massa
I almost believe in this ... but no.

It's just a parallel plate capacitor, with holes in the plates that allow the
surrounding plasma to pass trough. The question is:

 _Why a plate capacitor with holes in the plates inside a diluted plasma is
not a perpetual mobile?_

Let's assume that initially the device is with speed 0 inside a huge bath of
plasma that is also at speed 0. We charge the two plates of the capacitor
instantly and magically. In this model and the calculations the ions never hit
the plates because they have holes, so the plates never discharge. There is
some energy in the electric field of the capacitor.

Now the device is operational and it starts to move, the ions also move. Now
the devices and ions have some kinetic energy.

* If we believe the whole idea the ions just go away, and the plates never discharge, and new plasma enters the device, so it continues to accelerate forever. So the total energy of the system increase and increase, that is something impossible.

* If we don't believe the whole idea, the problem is that the ions accumulate outside the plates in some kind of ion cloud, even if they never hit the plates. Initially the effect is small, but eventually the charge of the ion clouds compensate the charge of the plates and the plasma inside the device feels no net force, so the device moves at a constant speed. Essentially, some part of the energy that was stored in the electric field was transformed into kinetic energy, but there is only a finite amount of it, so the device must reach a maximal velocity.

\---

There is some complications, because in the Solar system the plasma is not
static, because there is solar wind. So, just imagine that you pick a
reference frame (i.e. starship) that is moving at the same speed of the solar
wind. In this reference frame (i.e. starship) initially the plasma looks
static and the giant capacitor is moving. When it is turn on, there is some
energy in the electric field and some kinetic energy. After a while, the
charges are redistributed, and the speed of the giant capacitor changes, but
only a finite amount, it can't continue accelerating forever. If you see the
scene from the normal reference frame (i.e. the Sun) the device is initially
static and after a while it is moving with a cloud of ions around it, but the
total change in speed is the same.

I guess there is some drag, and the final speed in the starship reference
frame is 0 and the final speed in the Sun reference frame is the same of the
speed of the solar wind.

\---

About other comments:

* I agree that the momentum transferred to the protons and electrons is different. With some assumptions the ratio is 43, but in a moving device in a moving solar wind the calculation is more complicated. The exit speed of the electrons is 100 times faster than the solar wind, so it makes almost no difference. The exit speed of the proton is roughly the speed of the solar wind, so the calculation must be fixed, but there is a net effect that is no 0. (Until the ions cloud accumulate enough charge.)

* In an ion thruster the first step is to ionize the gas. Here they start with an ionized gas. The difference is that in a ion thruster they separate the positive and negative field.ions, so they waste some energy in the separation and then they accelerate only the positive ions. The separation of the ions is an active process, not just a static electric

* Hat tip for sandworm101 for the comment [https://news.ycombinator.com/user?id=sandworm101](https://news.ycombinator.com/user?id=sandworm101)

\---

About the article:

> _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._

This makes no sense. The acceleration of the electrons create some thrust in
the wrong direction, but it's not like a drag, it's like a thruster in the
wrong direction.

> _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. The relationship of jet
> power (P) to mass flow (m) and exhaust velocity (c) is given by: P = mc^2/2_

I'm not sure that mixing thrust and energy is a good idea, but anyway they
assume that the efficiency is 80% and use the usual formula to get the speed.
The problem is that assuming that only a 20% of the energy will be wasted is
optimism, not a real calculation. How much energy is lost due to the collision
of the ions and the plates? How much to recharge the plates? How much is lost
in real drag? How much is lost as heat? ... There are a lots and lots of
factors, and then you have to consider the nasty ions clouds.

~~~
adrianmonk
Why are you even talking about it as a perpetual motion machine? I don't see
any claim anywhere in the original article that it is. He even talks about
nuclear reactors used to power it.

You're refuting a claim that doesn't even exist.

And in doing so, you're also assuming the particles around it have "speed 0"
when the explanation clearly describes individual particles as having non-zero
velocity so that they can enter the space between the two plates on their own,
as it is claimed that there would be no net electric field outside of them.

~~~
gus_massa
They never claim it is a perpetual motion machine. They just invented a
machine and if it works as intended it would be a perpetual motion machine, so
we know it will not work as intended. [1] They don't claim that it is a
perpetual motion machine, if you analyze the consequences of their claims you
conclude that it is a perpetual motion machine. I guess they didn't notice
this, it's not an obvious consequence in the usual reference frame.

In my description I change the reference frame because it's much easier to do
the calculations. In the reference frame that moves at the same speed of the
solar wind the plasma has speed 0, but it doesn't mean that all the particles
have speed 0. It means that the number of particles that are moving in one
direction is (almost) equal to the number of particles in the opposite
direction. [2] It's like the air inside a quiet totally sealed room that is
left alone for a few days. The air "doesn't move" but the oxygen and nitrogen
molecules are moving very fast. The speed that they have is related to the
temperature. But in bulk they don't move and if you put some small pieces of
paper or strings they will not move (unless they are really tiny). If you have
some container like a glass inside the room the molecules that are inside the
container will change constantly, but the number of molecules inside the
container will (almost) no change. The problem in my version of the experiment
is not that the device will get empty.

[1] Assuming we don't need to change the current accepted laws of physics to
explain this. It would be even more groundbreaking.

[2] There are more accurate technical description but this is good enough.

~~~
adrianmonk
Am I missing something, or isn't this (at least while between the plates) just
the same phenomenon as the electric fields that are used to deflect electron
beams in some CRTs?

[https://en.wikipedia.org/wiki/Electrostatic_deflection](https://en.wikipedia.org/wiki/Electrostatic_deflection)

Obviously CRTs don't contain perpetual motion machines. So there must be some
way in which energy is lost in the plates.

(Note that when I say it's the same phenomenon, I'm talking about a force
acting on charged particles. The initial direction of the particles is
different, though. In this dipole drive, the charged particles are initially
moving roughly perpendicular to the plates, while in a CRT the particles are
moving parallel to the plates. But that shouldn't be relevant to the energy
needed to accelerate them.)

~~~
gus_massa
In a CRT, when the electrons hit the screen they go and complete the circuit,
so the negative charge doesn't accumulate.

Also, when the electrons are extracted from the cathode it gets a small
positive charge, but this charge also circulate and is not accumulate.
Moreover, it's the charge that eventually neutralize the charge of the
electrons.

I think there are no special wires to make this happen, the current I very
small and the charge is redistributed by the supporting parts.

A good comparison would be to use a CRT where the screen is split in two
perfectly isolated parts. After a while the half you are sending the electrons
two become too charged and it deflects the electrons.

~~~
adrianmonk
> _when the electrons hit the screen_

I am NOT talking about the main electron gun. I am talking about the
deflection plates.

Just to be perfectly clear, a CRT has three active components:

1\. Electron gun to create a beam of electrons aimed in the direction of the
screen.

2\. Vertical deflection to move the beam up and down.

3\. Horizontal deflection to move the beam side to side.

Here, you are talking about #1, and I'm talking about #2/#3.

~~~
gus_massa
If you have an ideal CRT and don't want to adjust where the electron bean
lands, you don't lost any energy in #2 and #3. You can replace #2 and #3 with
some perfectly isolated plates with a fixed amount of charge that have the
same charge forever.

If initially the position of the beam is in the center of the path , a charged
#2 and #3 will change the direction and in a smaller amount the velocity of
the electrons. The energy to change the velocity comes from the energy in the
electric field, not from the plates.

You can say that this energy is energy of the electric field or that it is
potential energy of the electrons in the electron beam. Both descriptions are
accurate and give the same result. Just remember to not count it twice.

Now the question is where does the energy from the field or the potential
energy of the electrons came from. The answer is that the repulsion has a
small but not 0 effect outside the main part of the capacitor and this creates
an electric field outside. To align the beam at the center you must overcome
this small force, so you must add some energy to the electron (in this case
it's easy to think in the potential energy of the electron). Perhaps the
comment from ballenarosada is more clear
[https://news.ycombinator.com/item?id=17432975](https://news.ycombinator.com/item?id=17432975)

In a normal CRT the electron bean is created from a high voltage source so it
moves fast, and the electrons initially move inside wires, so you usually
ignore the small potential field of the capacitor far away. If the charges
move slowly and freely then you must consider the effect of the electric field
outside the capacitor.

------
ballenarosada
This won't work. The basic idea about the protons gaining more momentum than
the electrons is valid. But the dipole creates an opposite field outside of
the charged plates. Protons will be decelerated until they pass the first,
positively charged plate, then accelerated through the plates, then
decelerated back towards the negative plate.

This is all clear if you consider the ions falling through a potential field.
The potential is 0 at infinity, positive at the first plate and negative at
the second. An incoming ion starts at 0 potential, climbs a big hill to get
through the first plate, then falls down below 0. Then on the way out it has
to climb back to 0 potential at infinity. So the ions gain energy inside the
plates but lose it all back on either side.

~~~
T-A
[https://physics.stackexchange.com/questions/12309/electric-f...](https://physics.stackexchange.com/questions/12309/electric-
field-outside-a-capacitor)

~~~
ballenarosada
The electric field outside an infinite capacitor is zero. For a finite
capacitor, there is a nonzero field. The importance of the infiniteness
assumption can't be understated--such a capacitor cuts the universe in half,
and every point of one half has the same electric potential.

On the other hand, if the capacitor is finite, then the surface integrals over
the plates are not equal.

~~~
T-A
> The importance of the infiniteness assumption can't be understated

No, but as you have shown it can be grossly overstated. The field outside a
finite plate capacitor falls off as a power of distance >= 2 (details depend
on the geometry), while the field inside it is constant. It can therefore
safely be ignored for a first order estimate of the effect.

If you want to get fancy and claim that higher order corrections invalidate
Zubrin's argument, you need to actually prove it. Also, don't forget to
include other effects like plasma shielding.

~~~
ballenarosada
Well, before breaking out a higher order analysis, I'd like to at least see a
real first order analysis. The argument from potential at infinity is
dispositive, but let's do some practice anyway:

Let A be the area of the capacitor, and dr the distance between the plates.
Let c be the appropriate electrostatic constant for the coulomb force between
a proton and the charge density on the plate. At a point a distance r from the
capacitor, the field effect from the negative side is, ignoring curvature
effects, about

cA/r^2

The repelling charge from the other plate will be about

cA/(r+dr)^2 = cA/(r^2 + 2rdr + dr^2) ~ cA(1/r)(1/(r+2dr))

So the difference between the coulomb forces, i.e. the net force, will be
approximately

(cA/r)(1/r - 1/(r+2dr)) = (cA/r)((r+2dr - r)/(r^2 + 2rdr)) ~ cA(2dr/r^3)

So the net force drops off approximately as the third power of the distance,
to a first order approximation. Integrating over the radius, we have that the
potential goes as -1/r^2, with the approximation breaking down near r=0.

Actually inserting appropriate constants of integration would make this
argument robust, but would also just reduce to the argument from potential at
infinity. Either way it's clear that the effect can't just be ignored out of
hand.

~~~
T-A
According to your derivation, the net force grows linearly with capacitor
area. Alas, the external field of a plate capacitor with infinite area is
exactly 0. You can look up the correct way to do a multipole expansion in any
introductory EM textbook, or google up nice a exposition like [1].

What really matters here is that with the force on the charge falling off as a
power of distance, even if you integrate force * displacement from the screen
out to infinity (which you shouldn't do in a plasma, because [2]), you get a
finite contribution which can be made arbitrarily small relative to the work
done inside the capacitor, where the force is constant, simply by increasing
the size of the capacitor.

[1]
[http://student.ndhu.edu.tw/~d9914102/Teaching/EM/Paper/data/...](http://student.ndhu.edu.tw/~d9914102/Teaching/EM/Paper/data/Electric%20field%20outside%20a%20parallel%20plate%20capacitor_Project%20Paper.pdf)

[2] [https://en.wikipedia.org/wiki/Electric-
field_screening](https://en.wikipedia.org/wiki/Electric-field_screening)

~~~
ballenarosada
If you read the exposition you linked, equation 14 gives an expression for the
field which is linear in the area. Again, it's pretty important that the
capacitor be infinite in extent, otherwise it behaves differently.

What's the dimension that you're proposing to increase of the capacitor? The
total work done across the capacitor will be fixed regardless of distance
across.

~~~
T-A
Eq. (14) is just the expression for a dipole. Keep reading to see the full
solution in the simplest case (circular plates), Eq. (19).

Since you insist: your derivation goes wrong right at the start by, as you
say, "ignoring curvature effects", i.e. by considering radial distance only.
By doing that, you are effectively imposing spherical symmetry; you are not
doing parallel plates, you are doing concentric spherical shells. That makes
the whole exercise pointless, since it's obvious from symmetry alone that such
a device could never produce a net thrust: there is no preferred direction for
the thrust to act along.

To answer your final question, just look at Eq. (19): make the plate radius
(R) larger.

That answer should also be perfectly obvious from the limit case of infinite
plates. A correct derivation for the general case must reproduce that result
in that limit. Yours does the opposite; it gets worse the larger you make the
capacitor. In the infinite limit, it is infinitely wrong.

~~~
ballenarosada
You're right, my derivation is mistaken for failing to take the z component of
the force. EQ. 19 is more like it although you'll note, also goes as 1/z^3.

You're also right that all that is moot from sheathing. But an ion still
begins it's journey out at the bottom of a large potential well; one which is
particularly steep because of the debye length, but still just as deep.

------
dojomouse
This seems basically the same as an ion engine without the need to BYO ions,
relying on the existing ion/electron mix in plasma being sufficient and
suitable. Which makes it seem fairly credible. Potential for much higher
efficiency than ion engines too if a plentiful supply meant you could
sacrifice thrust/ion.

Cathode Ray tube comparison below isn't entirely valid, as the goal of a CRT
isn't to generate net thrust.

~~~
dnautics
I would say that a good comparison (of course the math is different, since one
is monopole-monopole and another is monopole-dipole) would be saying it's like
a continuous gravitational slingshot except with solar wind ions and
electricity. With a gravitational slingshot, you use gravity to steal kinetic
energy from the larger body (usually the planet doesn't notice); with this you
use an electric field to steal kinetic energy from the passing wind.

------
tbabb
> 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.

Does this follow? I thought the force of an E-field on a particle is
proportional to charge and field strength-- so the electrons are lighter, but
will be accelerated with the same force to a much higher velocity. So wouldn't
the reactive force be the same for both?

~~~
nickparker
The particles are passing through a fixed distance (between the screens),
wherein they experience a fixed force due to the electric field.

An electron will shoot through that distance in no time flat and accrue very
little momentum. A proton will plod through and transfer far more.

For a 250V potential difference with 20m between screens, you can check it
yourself here:

[https://www.wolframalpha.com/input/?i=(1+electron+charge+%2F...](https://www.wolframalpha.com/input/?i=\(1+electron+charge+%2F+mass+of+proton+*+250+V+%2F+20+m+\)+*+sqrt\(2+*+20+m+%2F+\(1+electron+charge+%2F+mass+of+proton+*+250+V+%2F+20+m+\)\)+*+mass+of+proton)

[https://www.wolframalpha.com/input/?i=(1+electron+charge+%2F...](https://www.wolframalpha.com/input/?i=\(1+electron+charge+%2F+mass+of+electron+*+250+V+%2F+20+m+\)+*+sqrt\(2+*+20+m+%2F+\(1+electron+charge+%2F+mass+of+electron+*+250+V+%2F+20+m+\)\)+*+mass+of+electron)

The ratio of those two values is 42.8.

This also makes it easy to see why heavier ions full of neutrons are
_awesome_. They give a ratio of 5343.

~~~
ananonymoususer
Thank you. I was going to post the same concerns about the result being
electrons accelerated much more quickly out the front, thus no net propulsion.
I am glad I read the comments first.

------
andbberger
Huh. I don't see any obvious pitfalls here, the physics and principle of
operation are quite simple.

I am struggling to come up with an explanation as to why I haven't heard about
this before other than sneaky DOD stuff.

Who would like to bet me a large sum of money that there are currently
operation military sats employing this......

EDIT: as epicureanideal pointed out, commenters in the article raised valid
points discrediting most of the author's analysis. +/\- charged ions get same
net kick in the field, comparing proton mass to electron mass is a red
herring.

Overall concept still passes sniff test, just needs further analysis. It's
going to be all about the angle - you need one ion species to spend more time
in the field than the other to get a net thrust

A magnetic scoop with a normal ion thruster might be more practical.

EDIT 2: After further consideration, my concern here is what happens outside
the plates. This works for infinite parallel-plate capacitors - you just need
to have the field strong enough so one ion species undergoes more net
acceleration. In practice the field outside the plates will not be zero and
that could spoil everything

EDIT 3: I know nothing. E&M is hard. There will be a current around the
outside as you're pumping positive ions to one side and negative to the other.
This will induce a magnetic field affecting the ions being pumped through the
field. I have no clue what the net effect will be.

~~~
epicureanideal
Some of the commenters on the linked website claim it violates physics
principles. Like many people here I took several university physics courses
and it seemed plausible to me, but I don't have confidence in what seems
plausible to me actually working. There are a lot of "oops, I didn't think
about that" types of things when making calculations in physics, from my
memory...

Note: After reading post by nickparker here, note that most of the commenters
on the linked website who were claiming mistakes with the math seem to be
wrong. Nickparker here posted links to what seem to be valid calculations
showing the math works out.

~~~
ss2003
It's very good example of why we need real space testing of propulsion
concepts. Once we have a bunch of engineers actually living in interplanetary
space, where they can do easy experimentation, we will see some rapid
developments.

~~~
mnl
Not really, actually we understand pretty well electromagnetism,
electrodynamics, plasma physics and numerical methods for those. We can
simulate these things, so there's no need to reinvent the wheel going full
Faraday at the 21st century. A few trained physicists on Earth would do, yet
people get so excited wilfully ignoring all this.

------
tigerlily
I guess there's only one thing left to do. Experiment!

~~~
ridgeguy
Yes.

Seems like this would be a fairly straightforward proof of concept experiment
to fly outboard on the ISS. Basically a couple of charged screens with force
transducers on the screen mounts. It would seem this is within the bounds of
Figure 1, The Dipole Drive Accelerating Within a Magnetosphere.

Wish this paper had been published a year or two before SpaceX launched Elon's
Tesla. They could have tested a new interplanetary drive.

------
ArtifTh
Looks similar to Ionocraft[0], but only for low density plasma instead of an
air.

[0]
[https://en.wikipedia.org/wiki/Ionocraft](https://en.wikipedia.org/wiki/Ionocraft)

------
acertainfool
Where are these charged screens coming from? Is it possible to just magically
have charged screens that remain at the same charge regardless of all
interactions or am I seeing a place where someone has to bring their own ions?

------
sandworm101
Throwing positives one way and negatives the other? Wont they then seek each
other out? Would not this thing create a little storm of ions to negate its
thrust?

~~~
namibj
Still, a propeller works, even though the positive and negative pressure seek
each other out.

------
abakus
Talk is cheap, show me the prototype?

