The Lunar Elevator System

Message text written by INTERNET:spacesettlers@yahoogroups.com
>Now the drawback with the rotovator is that to keep it properly
balanced it requires an equal amount of mass coming down to the
lunar surface to match the material we send up from the surface,
otherwise rocket fuel must be used to boost the rotovator back to
proper orbit. An ion thruster wouldn't work because it would have to
restore the orbit within the 132 minute period of one rotation in
order to maximize our use of the rotovator to its fullest potential.
<

This is an excellent post. I do have a quibble with the statement that an
ion thruster would not work for momentum replacement.

It's true that ion engines require a lot of time to deliver significant
delta v because the thrust to mass ratio for a spacecraft is small.
However, in the case of the rotovator, we need only match the payload and
so do not need to deliver a large delta v to the rotovator.

For example, if we have ion thruster producing an acceleration of
.1cm/sec^2, the delta v gained in 132 minutes is approximately 8
meters/sec. If the mass ratio is 20 and the payload delta v is 1.6
meters/sec, the payload pickup delivers 80 meters/sec to the rotovator,
which is ten times what the ion engine delivers in one orbit. However, if
we increase the mass ratio by ten while maintaining the same acceleration,
the ion engine and payload delta v's will be the same. In practice the
acceleration should go up because a higher percentage of the rotovator will
be devoted to propulsion. Two way traffic will also improve the picture
because then only the difference in momentum needs to be made up.

Whether it will be done this way depends on the economics, with a tradeoff
between the capital cost of the propulsion units versus the propellant
savings from using high specific impulse. I suspect the first ones will
not use ion propulsion, but later ones will.