Re: Propellantless propulsion system

From: Del Cotter (del_at_branta.demon.co.uk)
Date: 01/23/05 

Date: Sun, 23 Jan 2005 16:42:41 +0000
To: sci-space-tech@moderators.isc.org

On Sat, 22 Jan 2005, in sci.space.tech,
Perplexed in Peoria <jimmenegay@sbcglobal.net> said:

>"John Smith" <user@example.net> wrote
>> As a bit of whimsy - how about two long thin contra-rotating masses,
>> with an electric motor at the pivot point that makes them contra-rotate.
>> (Motor is powered by sun.) Two chopsticks spinning against each other.
>
>More whimsy, but with a serious point: Consider a propellantless
>launch system consisting of a huge roll of toilet paper. Set the
>roll rotating. Set it rolling with a large horizontal velocity.
[...]
>But here is the hidden point in this whimsy. Since angular momentum
>is conserved as the roll unwinds, it must be the case that the
>ultimate escaping cardboard tube is rotating faster than the original
>roll consisting of tube and paper. The moment of inertia has decreased,
>Right? (It took me a long time to figure this one out. Dooooh!)

I think the system you're describing is mathematically equivalent to a
whip. As a wave travels down the length of the whip, a conserved amount
of momentum is stuffed into a smaller and smaller mass, until the end
breaks the sound barrier. *Crack!*

But I think you could almost get the same benefit from a rocket being
followed by a slow "vacuum cleaner", provided the rocket's exhaust was
carefully tuned to leave the nozzle at zero speed in the solar system's
frame of reference. It's all quite a bit of effort just to recover some
mass; I'd go and get more mass, since it's probably cheaper to do that. 

-- 
Del Cotter
Thanks to the recent increase in UBE, I will soon be ignoring email
sent to del@branta.demon.co.uk.  Please send your email to del2 instead.

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