MadSci Network: Physics
Query:

Re: What is the future of ion engines?

Date: Mon Sep 13 22:15:19 1999
Posted By: Troy Goodson, Staff, Spacecraft Navigation, Jet Propulsion Laboratory
Area of science: Physics
ID: 937259278.Ph
Message:

By the nature of the ion rocket engine, it cannot produce a large force.  
You can (almost) place all rocket (and jet) engines into two simple 
categories: those which accelerate a lot of material a little AND those 
which accelerate a little material very much.  The ion engine falls into 
this later category, as do many electric propulsion concepts.

The ion engine only accelerates a very small amount of material and 
currently, such engines only produce thrusts around one hunredths of 
newtons (that's about 4 hundredths of an ounce of a push).  Some produce 
more thrust, some less.

You may be interested in reading about some other electric propulsion 
concepts, like the arcjet, the pulsed-plasma thruster, nuclear-electric 
propulsion (very different from nuclear propulsion!), the plane diode, and 
Hall-effect thrusters.  I'm sure that there are more ideas than I know 
about.  One thing that most have in common is the low-thrust 
characteristic.  They just can't push hard.


The benefit, as I'm sure you've read, is in the fuel consumption.  These 
engines are much more fuel-efficient (and much weaker) than traditional 
chemical rockets.

It sounds like you've read a lot on this already, but you may not be aware 
that this already has commercial use in Hughes Space & Communications XIPS

See also, 
NASA: Ion propulsion

You may have noticed that I haven't outright said "NO, you can't move a car 
with an ion engine" because, traditionally, those who claim something can't 
be done turn out to be wrong.  However, as it stands, ion engines are only 
for space travel, where the lack of friction allows a little push to go a 
long way.

[Moderator note:  Ion engines are thrifty with mass but are not energy 
efficient.  This is just fine for spacecraft because the reaction mass is 
limited and energy can be obtained from solar cells or nuclear thermoelectric 
generators.  The ions travel at a very high speed which means that they have a 
high kinetic energy.  For a given amount of thrust an ion engine might have ten 
times as much energy in its exhaust compared to the exhaust from a chemical 
rocket.  Here on Earth we have plenty of "reaction mass", usually the Earth 
itself, and the problem is a lack of energy, so we have exactly opposite 
conditions to those where the ion engines do well.]


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