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From: rnm8s@hagar3.acc.Virginia.EDU (Rory Mcleod)
Newsgroups: sci.space.shuttle
Subject: Ion Engines
Keywords: Ion Engines, Chemical Rockets
Message-ID: <1991Mar24.191051@hagar3.acc.Virginia.EDU>
Date: 25 Mar 91 00:10:51 GMT
References: <1991Mar7.142311.10412@vaxa.strath.ac.uk> <6963@mace.cc.purdue.edu> <1991Mar11.201910.8476@casbah.acns.nwu.edu> <1991Mar12.003321.13988@zoo.toronto.edu> <3356@phred.UUCP> <1991Mar19.235853.6842@zoo.toronto.edu> <3361@phred.UUCP> <1991Mar22.043009.5544@zoo
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Reply-To: rnm8s@hagar3.acc.Virginia.EDU (Rory Mcleod)
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In article <1991Mar22.043009.5544@zoo.toronto.edu>,
henry@zoo.toronto.edu (Henry Spencer) writes:
|> 
|> What I'm looking for ON a solid rocket motor is dust, plus a museum
placard
|> saying "obsolete form of space propulsion system, abandoned in the
early
|> 1970s when ion rockets became practical".  We've known for twenty
years how
|> to build rocket systems that perform far better than any solid
rocket
|> motor ever will.

	Ion engines cannot be used to launch payloads into space.
A JPL/Hughs designed 30-cm diam HG bombardment thruster has a maximum 
thrust of 0.370 Newtons.  The mass of this same engine is almost 250 kg.
It can not even lift its own weight (on the Earth).  Ion engines can
only be used on spacecraft already in orbit.  Typical uses would be
for station keeping and unmanned interplanetary transfers. ( this
last use may be to what H. Spencer was alluding )

	An ion engine was successfully used on the NASA satellite
SERT-2.  The NASA-Lewis team shut down the engine on Apr 19, 1981, 
11 years, 74 days after the satellite was launched.  For more 
information read the Christian Science Monitor, Fri May 7, 1981.
I haven't kept up with the technology, so there could well be ion
engine
equiped spacecraft in orbit today, though I doubt it, even though
basic working designs are readily available, like most space systems, 
NASA or DoD will have to push to get it going before U.S. industry
will even take a look at it.  And without a real need (read: special
interest) Congress won't fund it.  ( If pro is the opposite of con,
then what is the opposite of Congress? )

	For the time being, and many years hence, chemical rockets 
- liquid, solid, and (hopefully) hybrid - will be the only method
of escaping Earth orbit.  Technologies to replace chemical rockets
for getting stuff into orbit will not be perfected until after the
turn of the century.  The most promising are the airbreathing
ramjet/scramjet engines being developed for the NASP, and perhaps
laser propulsion.  I know of no major project, though there may very
well be one, for the development of laser propulsion.  Mass drivers,
which have been discussed at length on sci.space (where this probably
belongs), are best for transporting materials from airless bodies, like
the moon.

Rory McLeod
Dept of Mechanical & Aerospace Engineering
University of Virginia