Re: PROBLEMS WHEN USING A ELECTROMAGNETIC BEAM TO PROPELL A STARSHIP

[T.L.G.vanderLinden@student.utwente.nl (Timothy van der Linden)]

Kevin, thanks for your comments, I've a few questions regarding your
comments, they include phased arrays, efficiency and diffraction. Well, read
on and you'll see them.

Timothy



>Seems objective enough.  One main point though, I would use a MASER, not a
>laser while this increases the cost for the beaming station (conversion of
>sunlight into electricity ~ 10%)  Conversion of electircity into microwaves
>and microwaves into electricity are both about 90%.  Use of Masers, allows
>phased array solutions.

I tried to keep it in the middle (laser or maser), at some places I even
used EM-beams. I don't understand the explanation with your numbers above:
You say the cost of a beaming station increases because the conversion from
sunlight into electricity is ~ 10%, but masers don't need electricity too?
I can understand that phasing is easier with longer wavelengths, so I will
mention that somewhere in a discussion of what kind of EM-radiation to use.
How did you get the numbers of 90%, I'd like to add a source.

>> 1 The beaming station very likely needs to be build on a moving/rotating
>> object like a planet, moon or asteroid.
>
>possible solutions: 1) Multiply the number of separate beaming stations, thus, 
>while some small number might be deviating, most will be on track most of the 
>time.  2) use phased array antenna to increase response time of the beam.

I'm not sure about (1), because a beaming station already consists of many
smaller masers. Isn't this the same as increasing the size of the beam?

About (2) I don't understand how changing phase can deviate the direction of
the beam slightly. Lee already told me about it, but I don't understand what
physical phenomenon is behind it (is it interference?). Is there anyone who
can help me here?

>> 2 Staying inside the beam may not be that easy if the beam is narrow.
>
>Possible solution, slight tilting of the sail at the ship's end...

Yeps, forgot that one.

>> 3 A single laser/maser of many kilometres wide is unlikely to be build,
>> instead many smaller lasers will be used. It may be necessary to get them
>> all lined up and all in phase so that interference is decreased. All lasers
>> need to be powered and controlled separately.
>
>solution: Phased array 

I think I was seeing a phased array as a problem on itself here.

>> 4 Small apertures and long wavelengths of the lasers cause widening of the
>> beam, this is a well known effect. In normal situations a laser may look
>> like a point but over distances of several lightyears this widening may
>> become problematic.
>
>Solution:  Phased array.  By conecting two transmitters many hundreds of
>kilometers apart, one can simulate a single arperature with the same baseline.

I cannot believe this is completely true. More (phased) sources can indeed
decrease diffraction effects, but does one extra source have so much effect?
Does anyone know this for certain by head? Otherwise I'd need to calculate
it myself (which I don't like at the moment).

>> 6
>
>efficiency of masers is high?  Unknown by me.  I do know that microwave
>generation is efficient, but masers may not be.  Anyone have any data?

Yes, I don't know very well either, the only number I could find this far
was that of 1% but that book was already old. It would be nice to have
several numbers for varying wavelengths.

>A reason to use a Lineac decell stage instead of a retro mirror.

Yes, we know the retro mirror had many difficulties, but the idea is to big
to leave out.

>> 9 Red shift is especially important when the starship reaches relativistic
>> velocities, it causes that the momentum transfer of the photons decreases
>> and therefore decreases the efficiency.
>
>But the time dialation on the ship cancels this effect, does it not?
>That is to say that while the ship is recieving 1/7 (@ .99 C) of the energy,
>it also has 7 times more time to collect that energy (from the viewpoint of 
>the ship)

Yes that is true, but it still means that relative to Earth the efficiency
decreases. Since all energy has to come from Earth (and they have to pay for
it) that is the most important reference point. The intensity of the beam is
probably not constant anyway, I assume that when the ship is farther away
from the source, the losses are greater and the intensity needs to be increased.

>> A beaming station should be visualized as a large array (100-1000? square
>> kilometres) of large highly accurate powerful lasers and a lot of fusion
>> reactors.
>
>or square mega-meters of Solar cells

Yes, but that is another chapter.

>> 12a Especially when the accuracy mentioned in 1 and 2 is low, the size of
>> the sail may be big, it needs to have the strength to pull or push a
>> starship. Large sails may become very heavy (some scenarios predict many
>> factors of 10 heavier as the ship itself).
>
>I thought I covered this problem already.  The use of multiple attachment
>points, as many as one every 10 meters, for the  sail (As opposed to the
>circumfrence only) reduces the pull to a very small amount.

Yes, I know, but that doesn't decrease the weight of the sail, which may
become a crucial point in the whole design. I'll add this in the solution
section.

>>14.
>communication to earth should use another portion of the EM spectrum, like the
>visible portion perhaps.

Yes, but where do you put the sending/receiving antenna? If they are inside
the beam they are likely to be ionized.

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 Solutions:

>> 4 Increasing aperture and/or decreasing wavelength (eg. use IR or gamma
>> radiation)
>
>I think increasing the arperature will be far more productive than
shortening the 
>wavelength.

Maybe both can add their share, also see the comment above (by problem 4).

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Advantages:

>> 2 The ship doesn't need to carry an engine (this may not be true for the
>> deceleration phase). This may increase the simplicity of the ship and thus
>> the reliability. This implies that less personnel may be needed.
>
>3) Ship can accelerate continously, taking advantage of the time dialation
>effect. and providing the crew with a near normal gravity environment

Are other designs like fusion engines not able to do this? (in theory) The
advantages of time dilations are not clear to me (see also 9)

>4) the beaming array only needs to be on for about two years.  then it can be
>used to send out other ships, or the energy could be used to power mining
>operations in the trans astreroid portion of our own system.

I wonder if the mining will be cost effective if that much energy is needed.
But I will mention the possible re-use effect.

>5) while the return system will be costly, once built, it will allow much
>smaller ships to travel back and forth between the two systems.

Yes, this is an advantage that may simplify futere missions significantly.