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Re: starship-design: Massively Distributed Computing for SETI

In a message dated 3/18/01 8:52:13 AM, lparker@cacaphony.net writes:

>> Given the relative positions of the stars haven't moved
>> visibly in thousands
>> of years.  We can aim a fixed vector.  The only problem is
>> earth orbit around
>> the sun.  If beam array isn't a ring around the sun, I.E. if
>> its around a
>> point in orbit, the ship would have to follow the beam in a
>> helical course
>> around a direct vector from the sun, toward the target star.
>> No serious math needed, or really possible.  We can't aim the
>> beam because we
>> can't know where the ship is to aim it at.
>Actually, there is more to it than that.
>Lets start with the relative motion first.
>"Moved visibly" is not the same thing as haven't moved. All stars are in
>motion, they follow their own orbits about the center of the galaxy as
>I am
>sure you know. Our sun is in one such orbit, any target star is going to
>in another orbit traveling at a different velocity.
>We cannot simply aim the beam at the star, the star will not be there when
>the beam arrives. We must aim the beam at where the star will be when the
>beam gets there. This is a non-trivial task considering that all distances
>to stars are currently _estimated_.

I can't see how this wouldn't be a trivial problem?  Your not talking a high 
lateral movement, or any delta-V of the two stars.   On the scale of the 
galaxy the two stars are right on top of one another.  (A couple light years 
out of a 30-40 THOUSAND light year orbital radius.)

>Then you must add for the motion of the beam array in its orbit about Sol.
>As was stated by Kelly this induces a helical component, and if we are
>orbit about Earth or some other planet, it induces another helical
>component. Although it is possible for the ship to correct its course for
>helical movement of the beam source, this involves tacking the sails to
>maintain a steady course on a continuous basis and involves an element
>risk if we lose the beam entirely and it doesn't solve the other problem.

Already covered the other problem.  Tacking, or otherwise manuvering into the 
path of the beam is nessisary.  If you fly out of the beam you'll need 
secoundary motors to manuver back into the beam.  You'll need manuvering 
engines anyway for in systems work.

Note, I've always assumed beam dispersal would make beamed power useless out 
more then a few light months from the transmitters (hence my fuel-sail idea).

>That is actually the easiest problem to solve. The second problem involves
>Doppler drift. As the ship gains velocity, it begins to experience Doppler
>drift or "red shift". Unfortunately, the sail is composed of a material
>designed to reflect a particular wavelength of radiation. It may also
>reflect other wavelengths, but not with the same efficiency. Therefore
>beam transmitter must be capable of tuning the output across a range of
>frequencies to keep the energy received by the sail at a constant frequency.

I'm wondering how bad the dopler shift would disrupt the acceleration of a 
sail driven craft?  

>This means we must know the exact speed, course and distance of the sail
>all times, constantly update the targeting data for the sail, the beam
>source, and the sail's destination, all in four dimensions, one of which
>at least one harmonic component if not two to compute a continually changing
>target solution and frequency solution for the beam.
>Any way we approach this, the solution is going to be compute intensive,
>it involves many of the same types of calculations being done by the SETI
>team, which was why I found the article so interesting.

It is completly impossible that we would be able to precisely know the 
possition and sppeed of a ship months or years after the last view of it.  
Any system that has to depend on that would be to dangerous and unrelyable.  
So the system must be designed to not need to rely on that degree of 
precision to function.