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Author Topic: ion propulsion  (Read 3723 times)

Offline theCoolScientist

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ion propulsion
« on: 22/01/2015 06:00:37 »
is the only reason for using xenon in ion propulsion engines,  its inertness?what do you think?


 

Offline CliffordK

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Re: ion propulsion
« Reply #1 on: 22/01/2015 10:16:43 »
I'm not sure if being inert is vital.  However, it needs to be heavy, and it needs to be a gas. 

One could use lead which would have the advantage of ease of storage, but it would have to be vaporized as part of the ionization, which means heating it to 1,750C.  Also, it would tend to condense on anything it touches, or perhaps even precipitate out to form pellets of space junk.  Inert gases would not
 

Offline chiralSPO

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Re: ion propulsion
« Reply #2 on: 22/01/2015 14:11:48 »
I think cesium (caesium, Cs) is also used for ion propulsion. It is slightly heavier than Xe (133 g/mol vs 131 g/mol), and althrough it requires more energy to vaporize, it requires significantly less to ionize, overall costing less energy. Wikipedia lists heat of vaporization as 12.64 kJ/mol, and heat of ionization as 1170.4 kJ/mol for Xe; and 63.9 kJ/mol to vaporize Cs, but only 375.7 kJ/mol to ionize it.
 

Offline CliffordK

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Re: ion propulsion
« Reply #3 on: 22/01/2015 21:42:37 »
Does one have to use "heavy" ions?

For example, H2 or He is easy to vaporize, even at low temperatures, and Hydrogen should be relatively easy to ionize.  The difference is that one would have to use quite a few more light ions than the heavier ions.  Are higher exit velocities possible?

Another type of ion propulsion might be designed to use whatever is cheap to get into space.  For example attempting to capture material from a comet or asteroid to ionize and use for propulsion.
 

Offline chiralSPO

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Re: ion propulsion
« Reply #4 on: 23/01/2015 14:55:55 »
I too have wondered why rare heavy ions were the best choice. It shouldn't matter how much each individual ion weighs, if x grams of propellant are ejected from the back of a rocket at a velocity of y m/s, the thrust should be the same.

After thinking about this some more, I think the issue is that one doesn't want the ions to interact with each other--the charge/space density should be fairly small or inefficiencies will arise. Therefore, given a constant thruster size (space and time related by a constant), to have a high mass/second and low charge/space, ions with a high mass/charge ratio would be optimal.

Following this logic, might it not make more sense to use molecular propellants than atomic ones? One can chose a molecule of arbitrary molecular mass, ionization energy, and have some control over the volatility.

Alternatively, a readily ionized substance might be ideal. Sulfuric acid, for instance could be separated into H+ and HSO4 ions. A strong electric field could accelerate the heavy anions backward to generate thrust (and the light cations would go forward).

Another idea would be to use a salt where the anion and cation have (as close as possible) the same mass and charge. For instance the barium salt of dibasic t-butylphosphonic acid (the first such salt I could come up with; the most common isotope of Ba2+ has a mass of 137.91 g/mol, and (CH3)3CPO32 would have a mass of 136.0 g/mol). An electric field (maybe pulsed)would accelerate the ions away from each other, perpendicular to the direction of flight, and a magnetic field perpendicular to both the E field and the direction of flight would be tuned to the right strength to bend the particle paths 90 to generate the thrust.

EDIT: H3C-O-CH2-CH2-PO32 would be within 0.2 g/mol of Ba, but isn't as easily made as (CH3)3CPO32
« Last Edit: 23/01/2015 15:00:39 by chiralSPO »
 

Offline evan_au

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Re: ion propulsion
« Reply #5 on: 23/01/2015 20:40:09 »
Quote
Sulfuric acid, for instance could be separated into H+ and HSO4 ions. A strong electric field could accelerate the heavy anions backward to generate thrust (and the light cations would go forward).

It is necessary to electrically neutralize the emitted ions, otherwise the spaceship and ion trails will rapidly build up a high relative electric charge, making it increasingly hard to accelerate future ions. Thus the anions and cations must be accelerated in the same direction, not opposite directions.

While it is quite easy to pump electrons through most metals (from one part of the rocket motor to another), it is quite hard to pump H+ ions, let alone (CH3)3CPO32.

The neutralisation also loses some thrust.
  • The mass ratio of an e- to Xe+ is about 0.0005 to 131.
  • The mass ratio of H+ to HSO4 is about 1 to 97
  • So you will lose much more thrust neutralising H2SO4 than Xe
  • The mass ratio of Ba2+ to (CH3)3CPO32 is about 1:1, so you get zero thrust (as I understand it) .

There is also the small matter of the corrosiveness of H2SO4, and the importance of keeping the propellant as a fluid despite the very low temperatures sometimes experienced on a spacecraft.
« Last Edit: 23/01/2015 20:42:32 by evan_au »
 

Offline CliffordK

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Re: ion propulsion
« Reply #6 on: 23/01/2015 20:47:06 »
I would think that the higher the charge to mass ratio, the more velocity one could impart on the ions. 

Assuming Energy is cheap, and matter to expel is expensive (plus the weight of carrying and accelerating the unused propellant), then one would absolutely want to maximize the velocity the ions are being expelled at.

Certainly simple electron guns have been around as long as CRT TVs, but the thrust of a simple electron gun would be minimal whereas one could use something like HCl ==> H+ + Cl to give relatively simple ions.

Can one ionize H2 ==> H+ + H
 

Offline chiralSPO

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Re: ion propulsion
« Reply #7 on: 24/01/2015 14:29:05 »
I would think that the higher the charge to mass ratio, the more velocity one could impart on the ions. 

Assuming Energy is cheap, and matter to expel is expensive (plus the weight of carrying and accelerating the unused propellant), then one would absolutely want to maximize the velocity the ions are being expelled at.

That's true, but since the speed limit is c for any of these ions, electron to C60, one should be able to get greater momentum with each particle, the heavier they are. Presumably it would be easier to impart great velocity to the lighter ions than the heavier ones. However, it may be easier to impart greater momentum to the heavier particles, and since it is momentum that is crucial for thrust, perhaps that is the reason heavier ones are used...


Can one ionize H2 ==> H+ + H

Probably, but it would be fairly energy intensive compared to the mass involved. H is a pretty high energy species, and H2 is non-polar and essentially non-polarizable. It is much easier to ionize HCl because it is pre-polarized and Cl is much "happier" than H (for those who object to my personification of ions: H electron affinity is about 73 kJ/mol, vs Cl electron affinity of 349 kJ/mol; given bond dissociation energy of 436 kJ/mol for H2, and 432 kJ/mol for HCl, and 1312 kJ/mol ionization energy of H: H2→ H+ + H would take at least 1675 kJ/mol and HCl→ H+ + Cl would take at least 1395 kJ/mol)
« Last Edit: 24/01/2015 14:32:09 by chiralSPO »
 

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Re: ion propulsion
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