From: Carl Feynman (carlf@atg.com)
Date: Mon Jun 09 1997 - 08:48:42 MDT
At 05:21 PM 6/8/97 -0700, E. Shaun Russell wrote:
> Though I am by no means well versed in technology or engineering, I
>have been giving a bit of thought lately to the advent of ionic propulsion
>drives. Does anybody know about the current technological progress of ipds
>and how they are projected to be used? Even a layman's explanation of a
>theoretical ipd would be nice.
>
The higher the velocity with which a rocket pushes out its exhaust, the more
oomph it gets for a given amount of fuel. Ion rockets push out their fuel
very fast, much faster than chemical rockets, so they are much more sparing
of fuel than chemical rockets. Unfortunately, they cannot produce nearly as
much thrust for the same size rocket engine. So whereas the typical
chemical rocket runs through tons of fuel in a few minutes, blasting to the
stars on a collosal column of flame, the typical ion engine slurps up a tiny
amount of fuel over hours or days, and produces an invisible wisp of very
hot exhaust that provides almost no force. But a tiny force accumulated
over a long period can result in a higher final velocity than a mighty blast
that doesn't last very long.
An ion rocket is typically about the size of a coffee cup (they range from
demitasse to commuter mug sizes). At the bottom of the cup is a little oven
or electric arc that heats a liquid metal propellant. This vaporizes the
propellant and knocks off one or more electrons of the metal atoms,
producing positively charged ions. Across the top of the cup is a grid,
like a window screen, that is negatively charged relative to the oven. The
positive ions are attracted to the negative grid, so they whiz towards it
very fast. Most of them miss the wires in the grid and pass through the
holes, whereupon they shoot off into space, bewildered.
Ion engines need a source of electric power, like solar cells or a nuclear
generator. They use high voltages, on the order of 1000V, so they need a
gizmo to convert low-voltage to high-voltage electricity. All this adds
weight, complexity and unreliability to the spacecraft. On the other hand,
when they fail, they don't blow up, as chemical rockets are prone to.
Ion rockets have been commercially available for about 30 years. They are
mostly used for satellite stationkeeping, i.e. making small adjustments in
orbit to keep satellites where they should be. In theory they shoud be good
for interplanetary missions, but nobody wants to risk their mission with a
new technology, so it hasn't been tried.
There is reason to believe that, like most electrostatic gizmos, ion engines
get better as they get smaller. Eric Drexler came up with a (vague) design
for a teeny-tiny solar powered ion engine, a few microns across. They would
be deployed in vast arrays that would look and act like cloth. You'd make a
parachute-shape out of this cloth, on the order of a kilometer across, hang
your payload and fuel tanks from the parachute strings, and off you'd go.
Such a device would have incredible oomph. It would make travel to other
planets quick and easy. He alluded to such a design in an article in the
Journal of The British Interplanetary Society, but I guess the design didn't
meet his standards of impeccably detailed feasibility studies, because he
hasn't published anything about it since. I did some analysis on this idea,
and it seem feasible provided you can suppress arcing between the grid and
the ion emitter, which gets into areas of plasma physics I don't understand.
--CarlF
Amateur Astrophysicist
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