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  #31  
Old February 21st 05, 09:34 AM
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Based on what you and Henry say, I figure that Satellite Solar power
segments, transported from L2 or L4/5 to GEO, should be about 1km on a
square.

The square would be a "semi" rigid truss, with a thin sheet of solar
film between the truss. Compared to the original designs, this puts a
large proportion of the mass in the sides of the square, which could be
10-30m wide truss. Eight plasma engines would attach themselves to the
outside of the square. Additional cargo would be attach to the truss
near the plasma engines. The whole thing would weigh some 1,000 tons.

With half the mass (500 tons) in the 4,000m long truss, that allows for
125kg / m of truss. that should enable sufficient rigidity, given a
combined thrust of only 1,000 N.

Moving Gigaton NEOs is a different matter.

  #32  
Old February 22nd 05, 09:15 AM
Marc 182
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In article , says...
(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.


Obviously you provide a way to cut-away a failed array. Was that too
much alliteration for a science newsgroup?

Marc
  #33  
Old February 22nd 05, 03:54 PM
Earl Colby Pottinger
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Marc 182 :

In article , says...
(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.


Obviously you provide a way to cut-away a failed array. Was that too
much alliteration for a science newsgroup?


And the cutaway takes place how without adding vibrations to the array? Yes,
this a science group, and people who do real science find that snappy answers
to real problems tends to create more problems than they solve.

If you had followed this thread for a while you would have noticed that it
was already pointed out that all high powered ion motors tend to jitch at
sometime during thier flight due to internal arcing. Using your model of
cutting free a failing engine means a large percentage of the array never
completes the flight.

Earl Colby Pottinger
--
I make public email sent to me! Hydrogen Peroxide Rockets, OpenBeos,
SerialTransfer 3.0, RAMDISK, BoatBuilding, DIY TabletPC. What happened to
the time?
http://webhome.idirect.com/~earlcp
  #34  
Old February 23rd 05, 09:27 AM
Marc 182
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In article , says...
Marc 182 :

In article ,
says...
(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.


Obviously you provide a way to cut-away a failed array. Was that too
much alliteration for a science newsgroup?


And the cutaway takes place how without adding vibrations to the array? Yes,
this a science group, and people who do real science find that snappy answers
to real problems tends to create more problems than they solve.


Thanks for an answer from a "real science" guy Earl. Snappy answers are
certainly no worse than condescending ones. Thanks again Earl. In a
loose array like this the pyro-cutaway can be positioned on slack
cables, no significant vibrations can propagate.

If you had followed this thread for a while you would have noticed that it
was already pointed out that all high powered ion motors tend to jitch at
sometime during thier flight due to internal arcing. Using your model of
cutting free a failing engine means a large percentage of the array never
completes the flight.


And all high powered ion motors flight tested in space being about 2?
Plus you can't realistically test these things on the ground in a vacuum
chamber. However, you do have a valid point here, if we were to fly
today's motors in a distributed array like this the whole thing would
quickly fall apart.

But we can assume that with flight experience the glitch rate would
decrease. At some point it would practical to accept motor loss just
like we accept radiation or dust accumulation losses on solar arrays, or
the slow fade of RTGs.

I'm not saying that this isn't a hard problem, it is dynamically and
computationally difficult. However, it lets you substitute station
keeping and software for physical structure you have to loft.

Free-flying arrays of spacecraft already exist. Free-flying arrays with
cables for power transfer should also be possible. I'm not sure that
anyone has addressed how you actually get large numbers of these things
connected together and on their way.

Marc
 




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