Asri-unix.1374
net.space
utzoo!decvax!cca!OTA@S1-A@sri-unix
Thu Apr 29 17:27:03 1982
Ion Engines and the like
From: Ted Anderson <OTA AT S1-A>
It should be noted since the subject of particle (large or small)
accelerators as rockets comes up, that there are important energy issues
that need to be considered.
First, the big objection that many people have with "mass drivers as
rockets" is that the specific impulse is way too low. What this means is
that the propellant (in this case the mass thats driven out the back end)
is not moving very rapidly, at least not very rapidly compared to the
exhaust of a liquid fuel rocket. This means that the mass efficiency of
the rocket is low. On the other extreme is the ion engine which has a
very high exhaust velocity (approaching the speed of light in some cases)
and hence a very high specific impulse and very high mass efficiency.
With high mass efficiency of course comes low energy efficiency. Thus you
trade off mass efficience for energy efficiency. This is because the
thrust is proportional to the momentum of the exhaust (speed * mass) and
energy is proportional to the kinetic energy of the exhaust (speed^2 *
mass). Note that this is only a rough approximation to the real state of
affairs. What this says is that energy spent heating up your exhaust
material (to vaporize or ionize it) you are wasting that energy since it
does nothing to increase your thrust. An ion engine always has to ionize
its exhaust, hence its name. The reason it can win anyway is that it
spends a lot more energy accelerating the ion than ionizing it. A mass
driver that vaporizes the mass its driving must put a lot more energy into
accelerating the mass than vaporizing it or its wasting its energy.
Specific impulse is a good way to compare the efficency of a rocket
engines of similar types. Ion engines and the various types of mass
drivers are basically similar does anyone have specific impulse figures
for these engines? Ion engines typically have ISPs (Impulse (SPecific))
of around 10000 seconds. How do these various mass drivers compare?
Ted Anderson
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