Digest Number 547

Hi Tango,
Just a quick note to respond to a couple of thoughts I saw on the
thread.

>
>Message: 2
> Date: Sun, 15 Sep 2002 19:50:35 -0000
> From: "victoriatangoman"
>Subject: Re: What's wrong with this idea?
>
>--- In spacesettlers@y..., Ian Woollard wrote:
>
[Snip]

>>You could try having multiple repeaters though, that way the
>>antennas
>>are much smaller... The issue with this is efficiency, as you'd
>>lose
>>power with each hop, but also, for any significant distance you're
>>talking kilometer antennas. And the distances from one side of the
>>earth, the long way is 100,000 km or so if you have multiple hops.
>>
>
>To address your concern I'll just throw an idea out without having a
>clue as to its feasability. We use microwaves to beam energy from
>SPS to rectenna because they travel through the atmosphere without
>too much attenuation, but is this necessarily the best form in the
>vacuum of space. Could the inter-SPS transfers occur as laser light?
>We'd still face the dreaded inverse square loss issue, but wouldn't
>laser light resolve the problem of kilometer long rectennas in space?
>
Laser light may be do-able, but I doubt you would really want to try
it. First, exactly how large do you think an SPS will be? Is a
kilometer long rectenna a signifigant added expense? I noticed that Ian
Woolards concern was with current day launch cost, but if it was built
on-orbit from space resources, this is less of a problem. Second, its
not just getting through the atmosphere that makes microwaves attractive
- rectennas are simple (basically just a wire mesh) and convert
microwaves into electricity very efficiently (about 70 to 80 percent -
perhaps more - compared to bleeding edge solar-electric cells which peak
at about 35 percent). The dreaded inverse square law is a problem, but
it is a problem for all beams - the trick is focus. If you want to use
laser tech, could I suggest masers? Of course all this brings us back
to potential weaponizing of the high energy density beams you are
tossing about....

>
>Message: 5
> Date: Sun, 15 Sep 2002 21:19:29 -0000
> From: "victoriatangoman"
>Subject: Re: What's wrong with this idea?
>
>--- In spacesettlers@y..., "Joseph L. Brown" wrote:
>
[Paste snipped portion of Victoriatangomans previous post, to which I
was responding]

>>>Usually, when we read of the SPS scenario, there is a SPS and a
>>>dedicated rectenna farm.
>>>
>>>To me, such a scenario negates two of the advantages of GSO, namely
>>>the wide footprint into which a SPS can beam power and the never
>>>varying power output of the SPS.
>>>
>> I am afraid that I don't really understand your concerns here;
>>of course a rectenna will be required to receive the microwaves the
>>SPS produces - are you concerned about the initial costs of the
>>rectennas? Or worried about some aspect of the feasibilty of a dedicated
>>rectenna facility? Also is GSO Geo-Synchronous Orbit (also called GEO
>>Geostationary Earth Orbit)? If so, how is the unvarying output of
>>the SPS a problem? Is it scheduling of power delivery that you are
>>concerned with - what do you do with the excess power? As I said,
>>I am just a little confused, please be more explicit.
>>
>
>This was just a preamble and a concise statement of fact with
>respect to the prevalent design architecture. This was a lead-in to
>the scenarios and questions that followed.
>
Um. Tango - this doesn't actually answer any of my questions

>>The low density beam has its own problems though; the
>>
>>beam has a large footprint, and so requires a large (potentially
>>expensive) area of rectennas; plus losses to water vapor in the
>>atmosphere may be higher. If the rectennas are simply built into
>>the roofing materials of the customer area, then perhaps they could be
>>made available cheaper, and eventually the whole area could be brought
>>on-line piecemeal for lower cost than a huge dedicated facility -
>>but how do you charge for the power? Or distribute it to clients with
>>high requirements? Also, doing this wastes some (potentially much) of
>>the energy in the beam by sending it off target - if you are
>>blanketing the entire San Francisco (for example) area with your transmission,
>>some of your signal is going to hit the ground.
>>
>
>The scenario you wrote about above is a completely new one to me. Is
>this something that you've though about or did you see it written
>somewhere? If so, do you recollect who wrote it and where I can rad
>their analysis?
>
Well, of course of I've thought about it. More seriously
though, this isn't my area of expertise - but I have been in love with
the idea of SPS ever since I became aware of it about a decade or so
ago. As a result very little of my post represents original (my own)
thinking - with the possible exception of roofing with rectennas - but I
cannot point to a specific source for it. Even the roofing rectennas
aren't terribly original; they were just an extreme veiw of the more
mundane point that rectennas are pretty "friendly" (since one is
essentially just a conductive wire mesh with the wires about a
millimeter apart) and that you could happily live under one - even the
'shading' of the area underneath would be minimal. Imagine having a
fine wire mesh suspended over your yard and house about five or six
meters up, and this mesh providing most of the power you require for
your home; the land under the rectenna is not 'dedicated' to just
rectenna farm.
As for my original sources, the only thing I can suggest is that you
do what I did - and search the web on 'Solar Power Satellite'. Sorry
that I can't do better than that....

>Relaying energy from one SPS to another should present few
>
>>difficulties; and the idea of using one SPS to cover for another
>>is
>>appealing. Hmm... actually - wouldn't the transfer between SPSs
>>use
>>high density beams? Wouldn't this create some of the same safety
>>concerns as transmitting a high density beam to Earth?
>>
>
>Yes, but that's an administrative and political issue, not a
>technical nor economic one. I'd be happy if it could be done
>economically and let the future decision makers worry about
>safeguarding it.
>
It's an administrative and political issue that demands a technical
and economic solution - don't sidestep this one, it's important. I
notice that later in this thread there is talk of 'defocusing' the beam
- essentially lowering the energy density per square meter of beam
cross-section to avoid weaponization issues.