Author Topic: Stored ionized gas for ion drives.  (Read 12192 times)

Offline RGClark

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Stored ionized gas for ion drives.
« on: 10/14/2007 04:26 pm »
I copied below a post to sci.astro that suggests high ISP for ion drives can be achieved by using stored ionized gas rather than creating the ions during the mission.
 It uses this formula for the exhaust velocity of an ion engine:

http://upload.wikimedia.org/math/3/5/2/3524a067a25b1083a4ceadbaad42480b.png">

 Using SI units with the charge on 6 x 10^18 protons to make one 1 Coulomb,  a mass of 1.6 x 10^-27 kg for a proton, and V representing the voltage in volts, then the formula gives the speed on the ions in meters/second.


   Bob Clark


//////////////////////////////////////////////////////////////////////////
From: Robert Clark
Newsgroups: sci.space.policy, sci.astro, sci.physics,
sci.physics.relativity, sci.physics.fusion
Date: Thu, 20 Sep 2007 13:47:28 -0700
Local: Thurs, Sep 20 2007 4:47 pm
Subject: Stored ionized gas for ion drives

This page gives a formula for the exhaust speed of an ion engine in
terms of the charge on the ions and the voltage driving the ion flow:

Ion thruster.
http://en.wikipedia.org/wiki/Ion_thruster#Energy_usage

 The exhaust speed increases with the charge on the ions and decreases
with their  mass. You would think then that a light gas like hydrogen
would be ideal since heavier gases even when fully ionized would still
contain approximately equal numbers of neutrons as protons which would
not contribute to the charge but would approximately double the mass.
 Yet it is the heavier gases like cesium and more recently xenon that
are used. The explanation is that of the energy it takes to ionize the
gas used as fuel. The figure on this page shows the energy to ionize a
light gas such as hydrogen is relatively high compared to the heavier
gases:

Ionization Energies.
http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/ionize.html

 The figure gives the energy per mole which is high in itself. It is
even worse when you consider this on a per mass basis since the mass
amount of hydrogen would be so small compared to the amount of energy
needed to ionize it.
 So could we instead store the hydrogen or some other light gas
already in ionized form so we would not have to supply power to ionize
the gas, only to accelerate it?
 If you used ionized hydrogen, so you would be accelerating protons,
then using 6 x 10^18 protons to make one 1 Coulomb, and a mass of 1.6
x 10^-27 kg for a proton, and V representing the voltage in volts, the
speed on the ions (protons) would be about (10^4)sqrt(2*V) in meters/
second.
 If we made the voltage be 5,000 V we would get 1,000,000 m/s speed
much higher than any current ion drive. Also, there are power supplies
that convert low voltage high amperage power into high voltage, low
amperage power, even up to 500,000 V. The we could get 10,000,000 m/s
= 10,000 km/s exhaust speed.
 The question is could we get light weight means of storing large
amounts of ionized gas? Note that is this for space based propulsion
not launch from Earth. You would have a possibly large energy
generating station that remained in low Earth orbit to supply the
power to ionize the gas once the spacecraft was placed in orbit. The
power generator would be left behind in orbit. Then the volume of the
gas container could be large to keep the density of the gas low. This
would allow very thin container walls. Note the low density would also
allow the electrostatic repulsion of the positively charged ions to be
more easily constrained.
 A possible problem though is the charged ions contacting the walls
could lead to a loss of ionization. You might be able to use a low
level magnetic field to prevent the ions contacting the walls. Low
density of the gas would insure the strength of the magnetic field
required would be low. It might even be accomplished by thin permanent
magnets so you would not need to use extra power.
 Some questions: what would be the electrostatic pressure produced by
a low density highly ionized gas? What strength magnetic field would
you need to contain it?
 Note that with an exhaust speed of say 10,000 km/s, by the rocket
equation we could get the rocket itself up to relativistic speeds with
acceptable mass ratios.
 Then this would provide a means of testing relativistic effects on
macroscopic bodies.


    Bob Clark


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Single-stage-to-orbit was already shown possible 50 years ago with the Titan II first stage.
In fact, contrary to popular belief, SSTO's are actually easy. Just use the most efficient engines and stages at the same time, and the result will automatically be SSTO.
Blog: exoscientist.blogspot.com

Offline sticksux

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RE: Stored ionized gas for ion drives.
« Reply #1 on: 10/14/2007 05:07 pm »
Quote
RGClark - 14/10/2007  5:34 PM
Ionization energies of the elements.
http://en.wikipedia.org/wiki/Ionization_energies_of_the_elements

 You see for hydrogen it's 1312 kilojoules per mole. Since the atomic
weight of hydrogen is 1, this is 1,312,000 joules per gram or 1.3
billion joules per kilo.

One kilo of protons without their fellow electrons will have
one helluva big electric charge, and will want to fly apart
due to electric repulsion. I doubt you can contain those
in any kind of container. Also, how do you plan to store
compensating negative charge?

Offline khallow

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Re: Stored ionized gas for ion drives.
« Reply #2 on: 10/14/2007 07:20 pm »
So how much does the containment apparatus weigh? For example, storing 1 kg of propellant in 1 ton of containment vessel isn't feasible. Might as well use laser propulsion, I think.

Here's a couple other approaches. First, pay the energy cost of ionizing the plasma, but do a better job of seperating the neutral particles from the ionized ones. Then you get the ludricrous ISP at a cost of more energy and much lower thrust, but you have no serious need for storage of ionized particles.

Second, keep launching the current partial ionized plasma, but either accelerate it enough so that the energy of ionization is insignificant, or harvest the energy of ionization by recombining the ions with electrons before or shortly after ejection. Say you have two holes A and B. The positively charged plasma comes out of A and a stream of electrons come out of B. Two long parallel plates jut out in the direction of the original beam flows such that both A and B are equally distance between them. In each plate is a long stretch of wire running parallel to the the original beam flows. Since A is positively charged and B is negatively charged, then the electron beam will be deflected towards the other beam. This in turn induces currents  in the wires in the plates, but the original motion of the beams do not. So you can recycle some of the original ionization energy with minor drop in ISP (from part of the ion beam hitting the plates).
Karl Hallowell

Offline kfsorensen

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RE: Stored ionized gas for ion drives.
« Reply #3 on: 10/14/2007 07:26 pm »
Salts are nice ways to store ions at room temperature and pressure without net electric charge.

Offline GraphGuy

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RE: Stored ionized gas for ion drives.
« Reply #4 on: 10/15/2007 04:42 pm »
Quote
vanilla - 14/10/2007  2:26 PM

Salts are nice ways to store ions at room temperature and pressure without net electric charge.

Yes, but last time I checked you can't get the ions out of a salt without putting alot of energy in.  The net charge of the salt is zero which is why salt doesn't zap you when you touch it.

Interesting idea overall, but how you could hold that much of a positive (or negative) charge is kind of a showstopper (IMHO, I write software for a living).

Let's just put a 1MW nuclear plant in space and let it drive an ion drive.  Seems simper to me and I like nukes in space (where they belong).

Offline pfdietz

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Re: Stored ionized gas for ion drives.
« Reply #5 on: 10/15/2007 09:22 pm »
Storing a non-neutral plasma in a Penning trap is a nonstarter.  The energy density is lousy even if you're storing antimatter this way; the stored energy of ordinary ions (per unit of mass) is far worse.

There IS a potential way of storing ions, though, using so-called 'superacids' or 'superbases'.  These are chemicals that are very strong electron acceptors or donors (due to delocalization of the charge over a large molecule, usually.)  If the acid/base is sufficiently strong, it could have a significant ionization in the gas phase, even at low temperature.  If the ion is also fairly massive, this will have the beneficial effect of increasing the mass/charge ratio of the ions -- and the maximum thrust/area of an ion engine scales as the square of this ratio, for fixed grid spacing and exhaust velocity.

Offline pfdietz

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Re: Stored ionized gas for ion drives.
« Reply #6 on: 10/15/2007 09:23 pm »
I should add, no known superacids/bases are strong enough for that, but they continue to look for them.

Offline kfsorensen

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RE: Stored ionized gas for ion drives.
« Reply #7 on: 10/16/2007 12:05 am »
Quote
GraphGuy - 15/10/2007  11:42 AM

Quote
vanilla - 14/10/2007  2:26 PM

Salts are nice ways to store ions at room temperature and pressure without net electric charge.

Yes, but last time I checked you can't get the ions out of a salt without putting alot of energy in.  The net charge of the salt is zero which is why salt doesn't zap you when you touch it.

These guys at MIT have figured out how to build an ion engine based on ionic fluids (salts).

http://web.mit.edu/dept/aeroastro/www/labs/SPL/research_electrospray.htm?section=research

They use alternating current to successively draw off the cation, then the anion, then the cation...you get the idea.  No ionizer or neutralizer needed.

Offline meiza

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Re: Stored ionized gas for ion drives.
« Reply #8 on: 10/23/2007 11:12 am »
Or just use heavier atoms: strip one electron and accelerate the resulting positively charged ion. You get more thrust for the same energy.

Why would it have to be hydrogen anyway? I don't see the rationale...

Offline meiza

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Re: Stored ionized gas for ion drives.
« Reply #9 on: 10/23/2007 07:33 pm »
200,000 m/s has been achieved with Xenon. (DS4G) And more could be done with lighter stuff like Krypton or Neon. (It's cheaper and more available too!)
Current energy sources don't make it useful to use higher exhaust velocities, you are more optimal if you put a bigger fuel tank but a lighter power source, and lower ISP.

And why store the ions; if you have such a power source (antimatter) that you will use million m/s exhaust velocities, you can strip the hydrogen atoms of their electrons then too, on the fly.
You will need million m/s exhaust velocities for interstellar trips, true.

Offline kkattula2

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Re: Stored ionized gas for ion drives.
« Reply #10 on: 10/24/2007 02:05 am »
Quote
pfdietz - 16/10/2007  9:22 AM
... If the ion is also fairly massive, this will have the beneficial effect of increasing the mass/charge ratio of the ions -- and the maximum thrust/area of an ion engine scales as the square of this ratio, for fixed grid spacing and exhaust velocity.

I think this is the key reason for using heavy ions. Even if H+ has 10 times the exhaust velocity of Xe+, you get (131^2) /10 = 1716 times as much thrust from the same area of grid.

Grid is fairly heavy, so unless Isp is the overiding factor, and mission durations of centuries are acceptible, hydrogen is NOT a good idea for an ion engine.  :(

Offline sticksux

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RE: Stored ionized gas for ion drives.
« Reply #11 on: 10/24/2007 01:09 pm »
Quote
RGClark - 23/10/2007  5:14 AM
Quote
khallow - 14/10/2007  3:20 PM
So you can recycle some of the original ionization energy with minor drop in ISP (from part of the ion beam hitting the plates).

Thanks for the serious responses.
This page shows that the energy content from antimatter-matter combination is tremendous:

Energy density in energy storage and in fuel.
http://en.wikipedia.org/wiki/Energy_density#Energy_density_in_energy_storage_and_in_fuel

It's on the order of a billion times higher per kilogram than the energy of burning hydrogen in air. So even if the containment for the kilogram of say charged antimatter particles was a ton that would still be a million times better than the energy content of hydrogen.
 To put that in perspective, the space shuttle external tank holds about 100,000 kg of hydrogen and 600,000 kg of oxygen. The energy content of that 700,000 kg total of hydrogen and oxygen would be matched by about 1/10th of a kilo, 100 grams, of antimatter storage, *including the weight of the containment system*.

1. How much does it cost to create 100 grams of antimatter? And since you cannot create it fast, how much will extended storage cost while you are accumulating produced antimatter?

2. 100 grams of antimatter, if containment fails, will result in 5-50 kiloton nuclear explosion. With such "interesting" failure mode in mind, good luck convincing people to allow production of antimatter here on Earth, and launching it to space.

Quote
If you do a web search on non neutral plasmas you'll find there has been extensive research on storage of positrons as well as electrons. I believe the storage density of the positrons now is close to that of the electrons.

I am unsure what are you trying to say. Positrons are far too light to produce any useful thrust, and also are too light to be useful as a method of storing antimatter.

Quote
500 watts per square meter, this would require less that 30 square meters for solar cells.  
 Not terribly bad for this. But the greatest amount of energy would be required to drive the protons at the high exhaust speeds.  The power required to drive a propellant at mass flow rate m and velocity v is (1/2)mv^2. So to drive the protons at a flow rate of 10 milligrams per second at 1,000,000 m/s would take (1/2) x (10^-5kg/s) x (1,000,000m/s)^2 = 5 megawatts. At 500 watts per square meter of power using solar cells, this would take 10,000 meters or 100m by 100m.
 Someone will give me the weight for solar cells nowadays but I think that would be prohibitive for a small unmanned spacecraft. One weight I've seen for space solar cells was 4 kg per square meter. So for 10,000 square meters this would be 40,000 kg.
 The thrust produced is the mass flow times the exhaust velocity so in this case would be (10^-5kg/s) x 1,000,000m/s = 10N, so appropriate only for small to medium sized unmanned craft in this case.
Bob Clark

The biggest problem with any kind of ultra-high Isp propulsion is not in high energy requirement, the main problem is efficiency requirement. In your example: you use 5MW to accelerate ions. At 95% efficiency (very generous assumption), you have 25KW converted to heat. Not good.

When Isp goes down, energy and energy efficiency requirements both go down, amount of propellant expended goes up. It just a matter of choice of ions. Energy-starved? Pick xenon. Have big freaking nuclear reactor and high-efficiency ion drive on board? Argon or neon, then.

Offline meiza

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Re: Stored ionized gas for ion drives.
« Reply #12 on: 10/24/2007 01:45 pm »
I don't think even nuclear reactors can achieve high enough power densities that you could justify using lighter than xenon propellants... You want to have a low ISP.

Offline kkattula

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Re: Stored ionized gas for ion drives.
« Reply #13 on: 10/24/2007 02:07 pm »
Nasa have a study for a positron propulsion system.  Basically using positron annihilation to heat up H2 to NERVA temperatures and Isp (900). But without the heavy and radioactive reactor core.  An advanced ablative engine might hit 5000 Isp. Of course you would need a heavy confinement system.

They say $US 250 million for 100 grams of positrons.  Enough for a Mars mission.

http://www.nasa.gov/mission_pages/exploration/mmb/antimatter_spaceship.html


Offline sticksux

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Re: Stored ionized gas for ion drives.
« Reply #14 on: 10/24/2007 03:21 pm »
Quote
kkattula - 24/10/2007  3:07 PM
Nasa have a study for a positron propulsion system.  Basically using positron annihilation to heat up H2 to NERVA temperatures and Isp (900). But without the heavy and radioactive reactor core.  An advanced ablative engine might hit 5000 Isp. Of course you would need a heavy confinement system.

They say $US 250 million for 100 grams of positrons.  Enough for a Mars mission.

http://www.nasa.gov/mission_pages/exploration/mmb/antimatter_spaceship.html

No, they say "A rough estimate to produce the 10 milligrams of positrons needed for a human Mars mission is about 250 million dollars using technology that is currently under development"

In other sources (http://www.engr.psu.edu/antimatter/Papers/NASA_anti.pdf) I find
$25 million per milligram cost for antiprotons, which is is agreement with above.

Offline khallow

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Re: Stored ionized gas for ion drives.
« Reply #15 on: 10/24/2007 04:09 pm »
Quote
sticksux - 24/10/2007  8:21 AM

No, they say "A rough estimate to produce the 10 milligrams of positrons needed for a human Mars mission is about 250 million dollars using technology that is currently under development"

In other sources (http://www.engr.psu.edu/antimatter/Papers/NASA_anti.pdf) I find
$25 million per milligram cost for antiprotons, which is is agreement with above.

Hmmm, given how much positrons are necessary, I wonder if it might be possible to fire a cloud of positrons at a vehicle and have some sort of electromagnetic catcher system on board direct them into the propellant? Depends on whether it's any more efficient than other means of heating H2 to the necessary temperature, I guess.
Karl Hallowell

Offline GraphGuy

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Re: Stored ionized gas for ion drives.
« Reply #16 on: 10/24/2007 06:09 pm »
Quote
khallow - 24/10/2007  11:09 AM
Hmmm, given how much positrons are necessary, I wonder if it might be possible to fire a cloud of positrons at a vehicle and have some sort of electromagnetic catcher system on board direct them into the propellant? Depends on whether it's any more efficient than other means of heating H2 to the necessary temperature, I guess.

Translation into English please?  It sounds like you are wondering if you could fire a microgram of antimatter at a spaceship and have it divert it to the fuel.

A couple of obvious problems with this:

Earth's magentic fields deflect charged particles, positrons have charge
Magic positron gun doesn't exist (none that could fire that much antimatter and not melt)
Little room for error, hope the crew cabin doesn't get hit by a few micrograms of anitmatter

Offline kkattula2

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Re: Stored ionized gas for ion drives.
« Reply #17 on: 10/25/2007 05:03 am »
Quote
sticksux - 25/10/2007  3:21 AM

Quote
kkattula - 24/10/2007  3:07 PM...
They say $US 250 million for 100 grams of positrons.  Enough for a Mars mission.

No, they say "A rough estimate to produce the 10 milligrams of positrons needed for a human Mars mission is about 250 million dollars using technology that is currently under development"

In other sources (http://www.engr.psu.edu/antimatter/Papers/NASA_anti.pdf) I find
$25 million per milligram cost for antiprotons, which is is agreement with above.

My bad. I read the arcticle a long time ago and someone mentioned 100 grams earlier.
10 milligrams is better. 10,000 times better. :)

I wonder what positron storage density is currently achievable?

Offline sticksux

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Re: Stored ionized gas for ion drives.
« Reply #18 on: 10/25/2007 01:45 pm »
Quote
khallow - 24/10/2007  5:09 PM
Quote
sticksux - 24/10/2007  8:21 AM
In other sources (http://www.engr.psu.edu/antimatter/Papers/NASA_anti.pdf) I find
$25 million per milligram cost for antiprotons, which is is agreement with above.
Hmmm, given how much positrons are necessary, I wonder if it might be possible to fire a cloud of positrons at a vehicle and have some sort of electromagnetic catcher system on board direct them into the propellant?

Very unlikely. I'm afraid you will just mini-nuke the LV in the "best" case.

Far more workable solution with some resemblance of the above is laser launch. With all megawatt range laser beam tracking / focusing R&D done in ABL program, it becomes actually feasible. Especially that you can start small by doing experimental boost phase laser assist on existing LVs. Maybe. I'm no expert.

Offline khallow

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Re: Stored ionized gas for ion drives.
« Reply #19 on: 10/26/2007 01:55 am »
Quote
GraphGuy - 24/10/2007  11:09 AM

Quote
khallow - 24/10/2007  11:09 AM
Hmmm, given how much positrons are necessary, I wonder if it might be possible to fire a cloud of positrons at a vehicle and have some sort of electromagnetic catcher system on board direct them into the propellant? Depends on whether it's any more efficient than other means of heating H2 to the necessary temperature, I guess.

Translation into English please?  It sounds like you are wondering if you could fire a microgram of antimatter at a spaceship and have it divert it to the fuel.

You are correct.

Quote
A couple of obvious problems with this:

Earth's magentic fields deflect charged particles, positrons have charge
Magic positron gun doesn't exist (none that could fire that much antimatter and not melt)
Little room for error, hope the crew cabin doesn't get hit by a few micrograms of anitmatter

The first problem is not a problem. You want to alter the trajectory of the positron cloud so it goes to a particular spot (or alternately misses your ship altogether).

Second, I just realized that there's no reason to deliver the positrons all at once. If you did and you couldn't immediately use them, then you'd either have to store them or discard them. Either is a bad idea since you don't want to do a lot of either. Instead have a beam shoot much smaller amounts at the ship continuously.

Third, have the crew huddled behind a shield. After all, they'll need to be shielded from electron-positron annihilation anyway.

Quote
sticksux - 25/10/2007  6:45 AM

Very unlikely. I'm afraid you will just mini-nuke the LV in the "best" case.

Far more workable solution with some resemblance of the above is laser launch. With all megawatt range laser beam tracking / focusing R&D done in ABL program, it becomes actually feasible. Especially that you can start small by doing experimental boost phase laser assist on existing LVs. Maybe. I'm no expert.

Another one chiming in that this is a Bad Idea.  :) Laser launch will probably be more practical anyway given that solar sails are probably going to be commonplace.
Karl Hallowell

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