From: vince@offshore.ai (Vincent Cate)
Newsgroups: sci.space.policy
Subject: Re: Settle the moon first using tethers
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Ruediger Klaehn <klaehn@gamemakers.de> wrote in message news:<2ga44vFe479U1@uni-berlin.de>...
> Vincent Cate wrote:
>> If you are sending stuff to the moon you can power the lifting of moon
>> rocks with the energy from the stuff going down to the moon.  If you
>> are not going to the moon you need some other source of power to get
>> the moon rocks into space.  
>>
>Getting rocks from the moon into space is not really that hard. The escape
>velocity of the moon is something like 2500m/s, so you don't need all that
>much energy. 
>
>In the beginning you would pick up moon rocks using a rotating tether in
>lunar orbit that uses electric propulsion (you proposed this method for a
>lunar sample return mission, didn't you?). 

All true.  It is far less energy than getting off Earth.  Getting
a few tons of rocks is not that hard if you are willing to 
wait a few months.  I like using regolith as reaction mass.  
You pick up some extra regolith and toss it backwards after you are 
spinning faster from winching in.

However, a tether for the moon only needs to be like 3 times the mass
of the payload.  If it is a momentum exchange tether it could do 
an exchange every rotation, which could be very short, or at least
every orbit (about 2 hours).  

On the other hand, if it is being reboosted by solar power and
electric thrusters you would either need to wait a long time between
uses or have lots of mass in solar power and thrusters (many times
the tether mass).   The more cargo you are trying to move the more 
important this difference would be.  Moving megatons from Earth 
to the moon would be much easier than moving megatons from Earth
to HEO.

>(*) I am not sure wether an electical mass driver as proposed by O'Neill is
>optimal for this purpose. You would want an accelerator that requires as
>little preprocessing as possible for the mass. 

If you just hurl rocks without any guidance it could be hard to catch them.
Actually, to me this seems really hard.  Even if you are just tossing
from the moon's surface to L2.

>A rotating tether anchored to the moon on a tower would work just fine: you
>would hang a bag of rock on it, then slowly spin it up and reel out the
>tether until the tip velocity is high enough. Then you would just release
>the payload at the right moment and spin down the tether again.

For a picture of a "sling on a tower" see:
     http://www.islandone.org/LEOBiblio/SPBI123.HTM

If you had a non rotating tether hanging down over the moon so it would
pass near your sling, and your sling was spun in a synchronized way, 
you could transfer from your sling to the hanging tether.  I am not set 
on what the best way to do this transfer is, but it seems better to do 
something here, even if it is a toss/catch, than to try to catch after 
flying long distances unguided.

  -- Vince

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Vincent Cate                           Space Tether Enthusiast 
 vince@offshore.ai                      http://spacetethers.com/
 Anguilla, East Caribbean               http://offshore.ai/vince
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You have to take life as it happens, but you should try to make it
happen the way you want to take it.    - German Proverb