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Author Topic: How would steam-powered ion thrusters work?  (Read 2198 times)

Offline Skyli

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How would steam-powered ion thrusters work?
« on: 29/10/2013 17:09:39 »
Like any young man in those tender years just before turning 60 I'm fascinated with space-ships, particularly their engines and, very particularly, ion engines.

I've been following the NEXT thruster development at NASA (http://www.space.com/22916-nasa-ion-thruster-world-record-test.html) and made some observations (at least I hope that they're not assumptions!).

An ion thruster has a degree of "power to weight" about it in as far as the heavier the isotope then the more thrust it will deliver for a given exhaust velocity. A nice heavy ion like Xenon (Atomic Weight ~131) provides a nice reactive kick at velocity x while a little Oxygen ion (Atomic Weight ~16) provides little more than a tenth of that force for the same velocity. However, it takes a lot more energy to achieve that velocity with a Xenon ion. I do not know if the relationship between exhaust velocity and input power is linear and would welcome some feedback here. Would it only take a tenth of the power to accelerate an Oxygen ion to this velocity? Of course one would then only get a tenth of the thrust too.

Another observation was the relative absence of easily available Xenon.

Would it be possible to use Hydrogen and Oxygen created from electrolysis instead of Xenon? If the two gasses were ionised separately to remove all electrons before the resulting ions were accelerated then they would, effectively, be inert so there would be little risk of explosion. The Hydrogen ions would be travelling 16 times faster than the Oxygen ions but, eventually, they would grab a few electrons from the ether (lovely word!) and leave a very diffuse trail of steam in their wake.

The engines would be more complicated in as far as there would be two ionisation chambers and the thrust would be less. However, the power requirements would also be less. Furthermore water is plentiful.

I imagine it would be possible to ionise water vapour directly, in which case it would break down into separate Hydrogen and Oxygen ions anyway as there wouldn't be any electrons to form a bond. In this case the energy efficiency of first electrolysing water and then ionising the resultant elemental gasses versus simply ionising water molecules comes into play. Again, I do not know how these compare. Is it easier to ionise two elemental gasses than the compound such gasses constitute?

From the side-issues it's easy to see that I'm a dabbler, not a scientist, but I would be very interested to know if such an idea holds water (sorry, couldn't resist that). Is a steam-powered ion-thruster viable?
« Last Edit: 01/11/2013 22:21:39 by chris »


 

Offline CliffordK

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Re: Steam Powered Ion Thrusters
« Reply #1 on: 31/10/2013 00:42:58 »
How strong of ionization does one need?  Stripping all the electrons?

Perhaps one could simply ionize the water using Acid/Base chemistry to H3O+ and OH-.  Of course, neutral water has the minority of the molecules in ionic form.

Or, perhaps one could use the free radical atomic forms:
H+ & O2-

Is there a benefit of running the ion  engines at room temperature vs running them at thousands of degrees?
 

Offline evan_au

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Re: Steam Powered Ion Thrusters
« Reply #2 on: 31/10/2013 10:25:45 »
I get the impression that they were using Xenon with just a few electrons knocked off - not Xe54+.

With many inner shells of electrons, it is relatively easy to knock 1 or 2 outer electrons off a xenon atom (ionisation energy 1170 kJ/mol), and then accelerate this quite massive atom.

Oxygen (ionisation energy 1314 kJ/mol) and hydrogen (ionisation energy 1312 kJ/mol) hold onto their electrons more tightly; after spending the energy to strip off the electron, you have a much lower mass to accelerate. The ionisation energy is effectively wasted, reappearing as the blue glow in the exhaust gas as electrons recombine with the accelerated ions.

Another problem is that oxygen and hydrogen are quite reactive and can react with the engine components, while Xenon is quite inert.

See: http://en.wikipedia.org/wiki/Ion_engine#Propellants
 

Offline Skyli

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Re: Steam Powered Ion Thrusters
« Reply #3 on: 31/10/2013 12:32:10 »
Thank you gentlemen, I had not realised that the ionisation energy was such a significant factor or that ionisation energies of different shells are so diverse. Up until now I always thought that the vast bulk of the energy went into accelerating the resultant ion (and I've used the same Wiki article a half-dozen times!).

But water is plentiful, cheap and safe and, as CliffordK points out, easy to ionise chemically.

So, for this idea to fly (!), one would need to look at other ways of ionising water and a design that keeps those ions from touching any engine components. The second part isn't too difficult and there are ion engines out there that effectively accelerate ions from within a magnetic cocoon. However, if a chemical ionisation stage is introduced then the "fuel" is probably no longer safe and easily obtainable, which defeats the whole idea.
 

Offline AndroidNeox

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Re: Steam Powered Ion Thrusters
« Reply #4 on: 31/10/2013 19:00:22 »
I'm hoping we'll find lots of fissionable materials in the Asteroid belt. I think fission drive, spewing the most massive ions using its own nuclear power to vaporize it. They should be highly efficient.
 

Offline Skyli

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Re: Steam Powered Ion Thrusters
« Reply #5 on: 01/11/2013 09:07:54 »
Aye, AndroidNeox; I've been blown out of the water with this one. Unfortunately, if we want a steady stream of nice, heavy nuclei then our choices are limited to chemicals, radioactivity or huge power requirements. Nothing really better than what we already have.

What about coming at this from the other direction? Rather than accelerate positive nuclei, are there any substances that readily accept additional electrons (at least more "readily" than they lose them) and would hold that negative charge long enough to be accelerated?
 

The Naked Scientists Forum

Re: Steam Powered Ion Thrusters
« Reply #5 on: 01/11/2013 09:07:54 »

 

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