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From: Henry Spencer <>
Subject: Re: Pinwheels and tethers (was: O'Neill colonies)
Date: Sat, 12 Jul 1997 17:58:51 GMT

In article <>,
<> wrote:
>Pinwheels are hideously dangerous!  ...  If it
>drops in orbit, even a little bit, it's going to start hammering on
>the surface of the Earth below, which is going to upset its very
>carefully planned geometry, which is going to make things worse... the
>end result is total devastation along the equator and no pinwheel.

Loss of the pinwheel, yes... but "total devastation" is an exaggeration.
Compute the mass of a typical pinwheel sometime, and the length of the
equator, and then remember that much of it will hit the atmosphere hard
enough to burn up.  While a major failure in a pinwheel or a beanstalk
is certainly spectacular and messy, it's not all that dangerous, even
if you happen to live on the equator (which is mostly ocean).

>Uh-huh. You can have a pinwheel on *your* planet, if you like. Me,
>I'll stick to conventional beanstalks.

What happens if your beanstalk takes a meteorite hit and breaks?

>Also, people have been talking about using tethers for generating
>power. The thing people haven't remembered is that a high electrical
>potential is not useful unless you can close the circuit... you need a
>connection from one end of the tether to the other...

Correct.  You close the circuit through the planet's ionosphere, using
a plasma contactor on each end of the tether.  This is not terribly
difficult.  Plasma contactors are a bit complicated, but in fact, simply
leaving a few kilometers of each end of the tether uninsulated might be
sufficient.  (The idea has been seriously studied, and looks workable.)

>The current tests have used an ion beam to complete the connection...

No, there's no need for a beam -- the ions are there already, provided
by nature.  You just have to make electrical contact with them.

>And, as people have said, there are more efficient ways of converting
>thruster fuel into electricity...

Actually, the most valuable role of a power-generating tether (near Earth,
at least) is not for energy production but for energy *storage*... and
it's hard to find more efficient ways of doing *that*.  Energy storage is
hard; lightweight energy storage is very hard.
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |

From: Henry Spencer <>
Subject: Re: halfbaked launch system idea
Date: Sat, 12 Jul 1997 18:09:23 GMT

In article <5q1nfe$42c$>, pete <VINCENT@reg.Triumf.CA> wrote:
>Basically, the idea is to split the task between a launch vehicle and
>an orbitting tether. The rocket tosses the payload out of the
>atmosphere with suborbital velocity, say about 8k km/hr. This saves
>a whack of propellant. Now, before it can fall back, the payload connects
>with the low end of the tether...

This idea has actually been around for quite a while, and looks workable.
The exponential nature of the rocket equation, and the relatively modest
exhaust velocities of existing rocket systems, means that slicing off even
a small part of the total velocity requirement for reaching orbit can make
it considerably easier to build your rocket.

Phil Chapman has pointed out that it can even make sense to have a
*chemical-rocket* tug come and snag your payload.  More sophisticated
schemes like tethers and pinwheels (rotating tethers) are even better.

>My chief suspicion is that the orbitting mass may lose velocity
>from the climbing payload, and the amount of fuel to keep it
>in orbit will equal that saved in the launch. What d'yall think?

There isn't a free lunch here, because the payload's orbital energy has to
come from *somewhere*.  But if you've got a relatively massive object to
anchor your tether, you can use high-efficiency low-thrust long-duration
thrusters like solar-powered ion engines, which are much more efficient
than chemical rockets.
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |

From: Henry Spencer <>
Subject: Re: Electrodynamic tethers
Date: Sat, 25 Oct 1997 22:17:18 GMT

In article <62p7ts$>,
David S. Patterson <> wrote:
>  There is something I don't get about these electrondynamic tethers.  They
>are always described as a wire with current in it, crossing Earth's magnetic
>field.  So what does this circuit look like?  In order to run a current, it
>seems to me I need a closed loop.  Do they count on picking up ions from
>space around them, and if so, does this limit the current?

Yes, the circuit is closed through the surrounding ionosphere.  The ends
of the tether need gadgets called "plasma contactors" to make good
electrical contract.  Just what limits the current is not well understood;
the TSS flight was getting more current than it expected before its
tether broke.
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        |

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