Re: starship-design: Beamed Power (was: Perihelion Maneuver) (fwd)

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

(this e-mail was meant to go to the entire list)

kuo wrote:
>From kuo Mon Dec  1 22:45:17 1997
>Subject: Re: starship-design: Beamed Power (was: Perihelion Maneuver)
>To: lparker@cacaphony.net
>Date: Mon, 1 Dec 1997 22:45:17 -0600 (CST)
>In-Reply-To: <01BCFE40.8D297E00.lparker@cacaphony.net> from "L. Parker" at Dec 1, 97 08:13:55 am
>X-Mailer: ELM [version 2.4 PL23]
>Content-Type: text
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>
>L. Parker wrote:
>>On Monday, December 01, 1997 7:50 AM, Isaac Kuo
>>[SMTP:kuo@bit.csc.lsu.edu] wrote:
>
>>> It doesn't work as advertised.  If you have a bunch of emitters, they
>>> are most effective when "shoulder to shoulder".  Spreading them apart
>>> in an attempt to decrease spot size will _reduce_ the amount of power
>>> reaching the target.
>
>>I don't recall having ever seen anybody claim that the array was as 
>>powerful or as efficient as a single huge emitter, then again I have never 
>>seen anyone suggest that we should build a single large emitter either. 
>
>The claim is that with an array of widely spaced emitters, you
>would be able to focus the beam over the interplanetary
>distances needed to accelerate something to high speed without
>a planet-sized lens.
>
>Unfortunately, it doesn't work that way.
>
>>Just exactly what point are you arguing? That we should build one large 
>>emitter? Or that we should give up the whole idea just because the 
>>efficiency of the array is less than a single large emitter?
>
>I'm arguing that light sail schemes for interplanetary launches
>require heroically huge emitters and focussing systems, and that
>you don't make the total job easier by breaking it up into
>smaller bits.
>
>In point of fact, I do agree that a number of smaller emitters
>will be easier to design and maintain than a single one.  However,
>the ideal number of emitters would be relatively small, like a
>dozen or a hundred or a thousand, and that they should be "shoulder
>to shoulder" flush against each other.  And unfortunately, the
>job of building them is not easier than building a smaller number
>of larger emitters.
>
>>> This actually isn't a concern.  The effect of thrust is inversely
>>> proportional to mass, and the emitters are VERY HEAVY compared to
>>> the thrust they emit in beams.
>
>>Isaac, this isn't like you, you didn't do the math!
>
>Well, it means me pinning down some assumed power/weight ratio
>for the emitters.  Let's say a 1kg sail is being accelerated
>at 100 gees.  This requires 1kg * 1000m/s^2 * c = 3x10^11 Watts.
>This is about the power generation capability of all the U.S.
>power plants combined.
>
>How lightweight can all the U.S. power plants combined be made
>in the future?  Let's say 100 thousand tons, including the emitter
>array and the reactor fuel.
>
>The reaction against the emitter array would accelerate it
>at one millionth of a gee.
>
>>> The real concern is whether you can build that the huge honking
>>> emitter (or emitter array) in the first place.  It's dizzyingly
>>> massive and big.  If you can build it, then it's not going to go
>>> anywhere just because of the (relatively) puny beam it emits.
>
>>Ibid.
>
>>Isaac, I can rarely fault the technical correctness of your arguments (you 
>>usually take the time to at least do the math), but you seem to take off on 
>>minor tangents that really have no bearing on the original topic. NASA, JPL 
>>and a host of others seem thoroughly convinced that the concept will work. 
>>All of these people are specialists in this field with published papers and 
>>the respect of their peers, I see no reason for us (a bunch of amateurs) to 
>>pursue this argument any further.
>
>Yes, the laser sail concept can work.  It's pretty dizzying how big
>a project it is, though.  You have to realize that so far every
>workable scheme to acheive .3+ c has involved exotic technology
>and/or massive engineering feats.  Multi-planet sized lenses are
>par for the course.
>-- 
>    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
> __|_)o(_|__
>/___________\ "Mari-san...  Yokatta...
>\=\)-----(/=/  ...Yokatta go-buji de..." - Karigari Hiroshi
>


-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
 __|_)o(_|__
/___________\ "Mari-san...  Yokatta...
\=\)-----(/=/  ...Yokatta go-buji de..." - Karigari Hiroshi