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  #24  
Old February 13th 05, 08:41 PM
Henry Spencer
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In article .com,
wrote:
the problem is not the solar cells but the structural dynamics of
enormous lightweight solar arrays...
...Solving either in the next 20 years is conceivable, but not
a small project...


But if you have lots of ion thruster, you can have lots of solar
arrays, each flying independently, powered by four ion thrusters.


If they're connected by cables or structural elements, they are *not*
flying independently, and the dynamics problems have *not* gone away.

If anything, the dynamics are worse. The problem is more complicated,
with a multitude of thrusters thrusting at slightly different levels in
slightly different directions, plus the dynamics of what happens if a
thruster shuts down (ion thrusters do shut down temporarily, now and then,
due to things like grid shorts).

Since the acceleration is so low, even a light weight latice could take
the strain.


Lighter, i.e. more flexible, structure makes the dynamics problems worse,
not better.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |
  #25  
Old February 16th 05, 12:18 AM
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Henry Spencer wrote:
In article .com,
wrote:
the problem is not the solar cells but the structural dynamics of
enormous lightweight solar arrays...
...Solving either in the next 20 years is conceivable, but not
a small project...


But if you have lots of ion thruster, you can have lots of solar
arrays, each flying independently, powered by four ion thrusters.


If they're connected by cables or structural elements, they are *not*
flying independently, and the dynamics problems have *not* gone away.


As long as the electrical cable is not taut, they are flying in
formation, but exerting no force on each other. They are therefore
structurally independent of each other. This resolves the structural
dynamics issues of the solar array, as the array is now smaller, and
hence more rigid.

If anything, the dynamics are worse. The problem is more

complicated,
with a multitude of thrusters thrusting at slightly different levels

in
slightly different directions, plus the dynamics of what happens if a
thruster shuts down (ion thrusters do shut down temporarily, now and

then,
due to things like grid shorts).

But again, operating on a smaller, and hence more rigid array. Any
worsening of dynamics is more than made up for by having reduced static
loads, as long as said dynamics avoid harmonic frequencies.

Of course, there's a new problem of formation flying and not getting
cables crossed.

Since the acceleration is so low, even a light weight latice could

take
the strain.


Lighter, i.e. more flexible, structure makes the dynamics problems

worse,
not better.
--

Lighter does not necessarily mean more flexible. My suggestion is a way
to reduce structure sizes, which tends to increase stiffness. In any
event, with the sorts of forces involved, a 1km by 1km array shouldn't
be too troublesome, as long as harmonic frequencies are avoided.

Have you tried the wobbly bridge over the Thames in London? Opened in
2000, then closed for a few years whilst they added damping. It no
longer wobbles - much less fun.

  #26  
Old February 16th 05, 12:20 AM
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Why is arcing more of a problem in a vacuum than on Earth. I assume
vacuum tends to be a poor conductor.

Is arcing a problem with a straightforwad voltage step up - transmit -
voltage step down system?

  #28  
Old February 17th 05, 08:21 AM
John Schilling
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writes:

Henry Spencer wrote:
In article .com,
wrote:
the problem is not the solar cells but the structural dynamics of
enormous lightweight solar arrays...
...Solving either in the next 20 years is conceivable, but not
a small project...


But if you have lots of ion thruster, you can have lots of solar
arrays, each flying independently, powered by four ion thrusters.


If they're connected by cables or structural elements, they are *not*
flying independently, and the dynamics problems have *not* gone away.


As long as the electrical cable is not taut, they are flying in
formation, but exerting no force on each other.



The electric cable can only be "not taut", if the cable is massless
and/or the spacraft are not accelerating. Otherwise, the cable is
under at least enough tension to supply the force accelerating the
mass of the cable. While that force may be small, if the other
part of your system is a swarm of middling large but featherweight
gossamer structures, it's enough. Henry is right, this is a Hard
Problem. Almost certainly harder than making one huge solar array,
and that's not easy.

There may be some merit to a swarm of modestly sized flying solar
arrays using *beamed* power transfer, whether microwave or laser,
and I have seen the idea proposed here and there. But it also is
rather beyond the state of the art.


--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
* for success" *
*661-718-0955 or 661-275-6795 * -58th Rule of Acquisition *

  #29  
Old February 17th 05, 08:27 AM
John Schilling
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writes:

Why is arcing more of a problem in a vacuum than on Earth. I assume
vacuum tends to be a poor conductor.


A *perfect* vacuum is a poor conductor, but there is no such thing.
The sort of vacuums you find in space, depending on exactly where
in space you are, often have just enough free electrons or ions to
carry a modest current and nothing to slow them down.

And a modest current at high voltage, represents a high power. A
high power that, with nothing to impede it, will concentrate at
whatever bit of your high-voltage circuit you expose to it.

At which point, you find that a high concentration of power on a
metal surface, boils off lots of electrons and ions. Now see step
one...


Is arcing a problem with a straightforwad voltage step up - transmit -
voltage step down system?


Yes. The high voltage parts have to be well and thoroughly insulated,
or you have to be real sure you're only going to operate in the parts
of outer space that have real good vacuums.

Unfortunately, most of our experience with high power systems in space
is with communications satellites, which necessarily operate in the
parts of outer space that have vacuums with a particularly high density
of stray electrons.


--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
* for success" *
*661-718-0955 or 661-275-6795 * -58th Rule of Acquisition *

  #30  
Old February 20th 05, 03:04 PM
Earl Colby Pottinger
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(John Schilling) :

writes:

Henry Spencer wrote:
In article .com,
wrote:
the problem is not the solar cells but the structural dynamics of
enormous lightweight solar arrays...
...Solving either in the next 20 years is conceivable, but not
a small project...


But if you have lots of ion thruster, you can have lots of solar
arrays, each flying independently, powered by four ion thrusters.


If they're connected by cables or structural elements, they are *not*
flying independently, and the dynamics problems have *not* gone away.


As long as the electrical cable is not taut, they are flying in
formation, but exerting no force on each other.



The electric cable can only be "not taut", if the cable is massless
and/or the spacraft are not accelerating. Otherwise, the cable is
under at least enough tension to supply the force accelerating the
mass of the cable. While that force may be small, if the other
part of your system is a swarm of middling large but featherweight
gossamer structures, it's enough. Henry is right, this is a Hard
Problem. Almost certainly harder than making one huge solar array,
and that's not easy.

There may be some merit to a swarm of modestly sized flying solar
arrays using *beamed* power transfer, whether microwave or laser,
and I have seen the idea proposed here and there. But it also is
rather beyond the state of the art.


Don't forget to add in the problem if one of the thrusters fails. With a
large array of thrusters (100+) it would be silly to shut the rest down as
the loss of one thruster would make very little diffirence the flight
profile. But that thruster and it's array will for sure start tensioning the
cables.

Worse would be a thruster with a bad connection to it's power cables, the
engine could start to pogo, and what would be needed to fix the problem even
if the bad engine is turn off would very complex indeed.

Earl Colby Pottinger

--
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