Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wasatch!cs.utexas.edu!rutgers!rochester!dietz
From: dietz@cs.rochester.edu (Paul Dietz)
Newsgroups: sci.space
Subject: Re: Where the hell are electric-ion thrusters????
Keywords: physics errors
Message-ID: <1989Aug29.124837.23692@cs.rochester.edu>
Date: 29 Aug 89 12:48:37 GMT
References: <8908241857.AA02943@fermat.Mayo.edu> <1989Aug25.183710.3054@utzoo.uucp> <4256@utastro.UUCP> <6091@lynx.UUCP> <4271@utastro.UUCP>
Reply-To: dietz@cs.rochester.edu.UUCP (Paul Dietz)
Distribution: usa
Organization: U of Rochester, CS Dept, Rochester, NY
Lines: 36

In article <4271@utastro.UUCP> terry@astro.UUCP (Terry Hancock) writes:

>	It uses electrostatically accelerated mercury (cesium would
>by bad news, by the way, it's both very reactive, and radioactive),
						   ^^^^^^^^^^^^^^^
>mercury will just give you heavy metal poisoning if you ingest it.  

I don't know what planet you're from, but here on Earth cesium
occurs naturally as Cs-133, which is most certainly *not* radioactive.

A reason to use mercury is its higher atomic mass.  This means that
(for the same ionic charge) you can use a higher voltage to reach the
same exhaust velocity.  Higher voltages are good because current
density through an ion engine is limited by space charge effects, and
this limit is strongly dependent on voltage (assuming constant
separation between the accelerating grids).  The more current, and the
higher the atomic mass, the higher the thrust per unit engine area.

>	50 lbs thrust is TOTAL B.S.  for an (electrostatic) ion drive,
>certainly any one tested.  The specifications for the 30-cm are:

Perhaps he was thinking of a magnetoplasmadynamic (MPD) engine.

>	The cost effectiveness of this depends also on the expense of
>mercury (anyone know what Hg costs?), which ought to be fairly rare,
>given its atomic weight (>Fe).  Probably cheaper to use kilos of Hg
>than kilotonnes of hydrogen, though.

Looking in an issue of the NY Times I had sitting around, I find that
mercury was listed at $275 for a 76 pound flask (look on the
commodities page), or about $3.62/lb.  In other words, the cost of
putting the mercury into orbit is about a thousand times the cost of
the mercury itself.

	Paul F. Dietz
	dietz@cs.rochester.edu