Re: Re: RE: starship-design: Re: Perihelion Maneuver

[kuo@bit.csc.lsu.edu (Isaac Kuo)]

KellySt@aol.com wrote:
>In a message dated 11/24/97 11:45:08 AM, kuo@bit.csc.lsu.edu wrote:

>>KellySt@aol.com wrote:
>>>In a message dated 11/18/97 4:03:33 AM, lparker@cacaphony.net wrote:
>>>>On Monday, November 17, 1997 11:23 PM, Isaac Kuo
>[SMTP:kuo@bit.csc.lsu.edu] 
>>>>wrote:

>>>>> No, it still needs a fresnel lens.  Without it, diffraction limits
>>>>> are too severe.  The beam needs to fall on a spot 1km wide at a
>>>>> range of 30,000,000,000km.  With a wavelength of 1mm, that requires
>>>>> an aperture around 30,000km wide.  Don't tell me you're going to
>>>>> make a microwave laser that big.

>>>Why?  I thought everyone was prewtty comfortable with a phased
>>>array emmiter cluster?  (See details in recent Fuel/Sail is dumb responce)

>>It doesn't work.  I came up with the idea (and if I wasn't the one
>>who suggested it, then I came up with the idea independently).

>>However, it doesn't work.  This is actually pretty obvious when you
>>do the numbers.

>>A cluster of in phase emitters can only acheive the emitted power/m^2
>>equivalent to a single emitter with an aperture of the same total
>>aperture area.  However, this cluster is less effective than that single
>>emitter in that this power/m^2 is emitted onto a smaller spot (the
>>rest is lost in sidelobes).

>>If you wanted an array to take the place of the single emitter, the
>>best way would be to have an array of emitters sitting "shoulder to
>>shoulder" in a hexagonal grid.  It still needs to be 30,000,000,000km
>>in diameter, with lenses flush against each other.

>Saying its not as efficent as a single emmiter, isn't the same as saying it
>won't work.

Well, okay...technically, helicopter can be lifted with a thousand
small propellers rather than a small number of large rotors.

However, using a thousand small propellers "doesn't work" in the
sense that it doesn't make the design any easier--and in fact makes
the design harder.

So no one would do it.

Simply put, the total area of the lenses needs to be at _least_ the
same area of the total area of the single lens.  However, this
assumes the array is sitting side by side with no gaps.  If you're
using a "swarm" of orbitting emitters, the array must be much
larger, with a lot of energy wasted in sidelobes.

>Given you couldn't build one single emmiter of the size needed
>to get the resolution needed.  We need a phased array system.

That argument might have some validity if the phased array system
were any easier to build.  It isn't.

It's like a poor man thinking he doesn't have a hundred dollar
bill so what he needs is a thousand quarters.

>(Also the
>phased array make construction repair and reuse of the beamed energy after
>boosting easier.)

>How inefficent is it?

It depends on how far apart the gaps are in the phase array.  First
encircle the array in a circle of minimum diameter.  The ratio of
the filled part of that circle to the total area is its maximum
efficiency at its diffraction limit.

For instance, if an array of total dish area 3.14km^2 is spread
over 200km in diameter, the maximum efficiency is only .01%.
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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