An ion engine fuel?

Alex Terrell wrote:

Sorry - I misunderstood. This seems feasible. However, I think you'd
want to make it for more than just electric engine fuel. A 200m long
gun would keep acceleration down to 1,000g.

-Electronics, even vacuum tubes, have been used in
artillery shells since the Second World War.
-Laser-guided Copperhead artillery shells withstand 10,000 g.
-Experimental circuits developed for railgun
launch withstand 100,000 g.

Simple electronics are limited to a few 000g. The system might need
slightly heavier masses, but how about a vertical tube firing out the
top an equatorial mountain (6,000m) - each projectile is about 10kg -
enough for aerodynamic stability at 2km/sec.

If you increase the projectile mass
100 times, you have to increase
the capture tube mass 100 times.

You'd sinter the regolith, or do the processing on the moon and fire up
silicon, aluminium, titanium and Sodium (which I think can be used as a
plasma fuel).

I do not know much about chemistry, but
I do know that many chemical processes
require the presence of solvents, which
are hard to find on the Moon. I do not
know of any chemical plant that is located
far away from deposits of a solvent (water).

If you work with solvents in a closed
environment, you have to purify them
before you can reuse them. Removal of
some impurities is difficult. For example
cheap grades of hydrogen peroxide are
contaminated with hydrocarbons, which make
them unstable; they spontaneously
break down into water and oxygen. There
is no easy way to remove the hydrocarbons.

Having looked this up now, it seems MPD thrusters work at high power
ratings (100KW) and can use lithium as a fuel. That means they might
also work on sodium, which is some 0.3 to 0.5% of the lunar regolith,
and has similar chemical properties to lithium.

The magnetoplasma dynamic (MPD) thruster is
also known as the Lorentz Force Accelerator (LFA)
thruster.

The MPD thruster can operate on ammonia, hydrazine,
methane, hydrogen, nitrogen, the noble gases, and
alkali metals (Li, K, Na). Lithium propellant has
the best performance below 10 MW and hydrogen has
the best performance above 10 MW. Gaseous propellants
have not produced high efficiencies at moderate
specific impulses. I like sodium (Na) because
it is cheap and has low melting point. It can be
used as coolant and chemical rocket fuel.

wrote:
In , on 01/24/05
at 08:44 PM, (Henry Spencer) said:


In article ,
Christopher M. Jones wrote:

Mercury might be the best choice for applications which
rely on off-Earth resource utilization, since Xenon is
hard to come by without a terrestrial atmosphere handy...

How much xenon or krypton would be trapped on outer solar
system icy bodies, I wonder?

Quite a lot, I think. Krypton and Xenon gases are
routinely found as trace elements in meteorites, for example.


Unfortunately, the operative word is "trace". They are relatively
uncommon elements for fairly fundamental reasons. What's needed is a
mechanism that would concentrate them somewhere; none is known. --


But as trace elements go, noble gases might be just about the best bet for cheap, simple,
quantitative extraction from bulk asteroid/comet/lunar material, [...]

Actually, I was thinking of outer planetary bodies where Xe/Kr might
exist in liquid or solid form. Their melting points are 161 K and 116 K,
respectively, so one could imagine ore bodies of the elements being
deposited by fluids undergoing temperature changes (in cryovolcanoes,
perhaps).

The elements might also be heavily concentrated in the atmospheres
of small bodies on which lighter gases have undergone Jeans escape
(but in which, for some reason, erosion by solar wind has not
removed the trace gases.)

Paul

In , on 01/26/05
at 07:00 AM, "Paul F. Dietz" said:

Actually, I was thinking of outer planetary bodies where Xe/Kr might
exist in liquid or solid form. Their melting points are 161 K and 116 K,
respectively, so one could imagine ore bodies of the elements being
deposited by fluids undergoing temperature changes (in cryovolcanoes,
perhaps).

The vapor pressures of these guys are still pretty high even at those T's
though. It's hard to imagine a concentrating mechanism, since there aren't
any reactions involved.

The elements might also be heavily concentrated in the atmospheres
of small bodies on which lighter gases have undergone Jeans escape
(but in which, for some reason, erosion by solar wind has not
removed the trace gases.)

Paul

Quite possible. We just don't know right now. I was just responding to the
term "trace" that Henry latched onto. In the field of economic geology, if
the price is right, even ore with "trace" quantities of the desired
element can be valuable. For gold, it's quite profitable with ore at the
ounce per ton level. That's trace. All platinum group element ores will
have trace quantities of the metals (often they are by-products from base
metal ores). It all depends on how valuable the target is, and how cheaply
you can extract it. For gold, you pile up a big hill of ore and leach it
with a cyanide solution. For noble gases, even with lunar/asteroid
material, a roaster would do fine. If you're processing large quantities
of such material anyway for whatever reason, it might pay to extract the
noble gases.

--
Chris M. Hall, Associate Research Scientist
Dept. of Geological Sciences, University of Michigan

Perfection is the enemy of the good

wrote:

The vapor pressures of these guys are still pretty high even at those T's
though. It's hard to imagine a concentrating mechanism, since there aren't
any reactions involved.

Ah, but there are purely physical effects, like dissolution/precipitation.
Both these gases will dissolve in cryogens with lower melting points
(argon, nitrogen). Perhaps nitrogen eruptions on Triton or Pluto produce
solid deposits of these trace gases?

Paul

In , on 01/26/05
at 06:36 PM, "Paul F. Dietz" said:

wrote:

The vapor pressures of these guys are still pretty high even at those T's
though. It's hard to imagine a concentrating mechanism, since there aren't
any reactions involved.

Ah, but there are purely physical effects, like
dissolution/precipitation. Both these gases will dissolve in cryogens
with lower melting points (argon, nitrogen). Perhaps nitrogen eruptions
on Triton or Pluto produce solid deposits of these trace gases?

Well, again maybe, but precipitation will require the solubility limit
being exceeded, and that usually means pretty high concentrations. I'm not
saying it isn't possible but you still have to find a mechanism that
encourages the deposition of one gas at the exclusion of others to get a
concentration. As Henry pointed out, there's not a lot of this stuff
around.

One interesting thing is that there are concentrating mechanisms on Earth
that aren't exploited. Shales have relatively high concentrations of Xe
and cold water dissolves Xe preferentially to the other noble gases. If
there really was a big market and a good price for Xe, I suspect methods
other than cryogenic processing of the atmosphere might start to be looked
at.



--
Chris M. Hall, Associate Research Scientist
Dept. of Geological Sciences, University of Michigan

Perfection is the enemy of the good