Re: USING A ELECTROMAGNETIC BEAM TO PROPELL A STARSHIP

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

Kevin wrote:
>> 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?
>
>What I mean is that while there could be a way to turn sunlight to laser light
>without going through an in-between step like electricity, there is no way to
>do this for masers.  The sun's microwave output is just too dim compared to its
>visible output.

Huh? I got the idea that you preferred maser because it is so efficient, now
your saying that laser may not need an in-between-step, from which I would
conclude that laser is a preferred method.

Oh wait, you are pointing at two things here:
maser: needs an extra step, easy to convert to electricity
laser: may not need an extra step, less easy to convert to electricity
Did I get it right?

>> I can understand that phasing is easier with longer wavelengths, so I will
>
>it's not just the longer wavelengths, a lot of this technology is available
>today in the form of radar installations and radio telescopes.

I see, do you know if it is difficult to transfer this technology to shorter
wavelengths?
If it is not too difficult it would be worth trying, especially because the
technology that is available now will need a lot of redesigning (if not
completely new ideas) anyway. 

>>>> 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
>>
>> 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?
>
>Not really.  consider the following 16 element maser array

Ah, now I see, I already assumed every single maser had his own tracking and
aiming device. I'm not sure however how independent they would be, but I
assume one could build in a rule that not all maser-unit are allowed to move
at the same time.

>All I know is that by changing the phase of the separate elements (all of the 
>Number 4s above, one can change the direction that the EM rad goes off in.  I 
>would think that phase adjustments would be more difficult for optical systems 
>than for microwave.  The elements phases are not all changed the same way at 
>the same time, so yes, I think it is an interference effect, both constructive 
>and destructive.

But what has the upper hand? The constructive or destructive side... (This
was the reason for my inquiry.)

>> I think I was seeing a phased array as a problem on itself here.
>
>I think pahes array comes with a different set of problems, like not being
>able to completely cancel Em going off in some unwanted direction.  This ends
>up costing power but the advantages of finer control and reduction of self
>interference effects should more than make up for a slight power drain

I guess here you answered my question just above...

>>>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).
>
>Again, we already do this for space craft.  the Magellan probe to Venus for
>example. many of the radar pictures were made using synthetic arperature.
>Instead of multiple antennas, different images from closely spaced times along
>the same orbit were used.

I already thought this was the base of your misunderstanding. For what I
know (I'm not 100% certain) this method only increases resolution (in the
sense of larger angle deviations), but it does not decrease diffraction in
the same way.
The term interferometry points at the comparing of two parts of a wavefront.
The difference in phase is then used in calculations that increase the
resolution. I know it is a bit fague, but I don't have good literature about
it at the moment. (Feel free to attack me if you still feel you are right).

>maybe a better explanation...  the probe sends out a long pulse of radar the 
>probe picks up a reflection from the surface, and, as it moves in it's orbit, 
>the signal changes.  Computers combine the multiple reflections into one that
>looks like it came from an antenna as big as the orbital "footprint"
>
>radio telescopes on opposite sides of the globe routinely are connected to
each 
>other and the result is an antenna as wide as the earth.  Called Very Long
>Baseline Interferometry


>>>>9 Red shift is especially important when the starship reaches relativistic
>
>Okay, so it is an increased cost, and not a technical difficulty

It may become a technical difficulty if the shift becomes too large, then
the reflectivity (and absorption) may change for the worse. While it may not
matter much that some energy is lost, it may matter that a small part (even
<0.001%) of high intensity radiation enters the crew space filled with
sensitive computers etc.

>> 12.
>> 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.
>
>Agreed.  I like Kelly's idea of using 6Li for the sail, very elegant.

Unless we need a sail for the way back home...

>> 14.
>> Yes, but where do you put the sending/receiving antenna? If they are inside
>> the beam they are likely to be ionized.
>
>I think a visible laser tube (say CO2 for example) which would be mostly glass
>or other non-ionizable material, would be immune to this effect.  If it
>becomes a real problem, you can shoot the visible light laser through the
>~ 1cm holes of a Microwave shield.

That would be a solution and a reason to use maser ;)

>> Advantages:

>> >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)
>
>I think the problem with the fusion rockets, is that in order to accelerate 
>continuously, the need planatary sized fuel tanks.  Kelly's top speed is about
>.4C and that's taking advantage of every trick in the book.

I agree, but it is only partly true, since you also need to decelerate using
onboard reaction mass (not necessary fuel). If you accelerate too much, that
amount of reaction mass needs to get bigger in order to be able to stop the
starship.

>The main advantages of 
>Kelly's hybrid fusion/maser design are
>1) lowered cost
>2) decell stage is independent of earth.
>
>The advantage of time dialations, is that _all_ stars (within reason of
course) 
>tend to be nearly the same time away (as seen by the crew) and much savings of 
>food and other supplies can be acheived.  Also, regardless of the actual
(earth-
>based) time of flight, earth (or the return masers) only need to send out
about 
>a two-year long pulse of energy to sustain the flight 

Oops, last time I told you that the Doppler effect was cancelled for the
people in the starship, I think I was mistaken there.
Note the following expression for the Doppler shift, after the arrow I've
rewritten it so that you can see that there is more than f/gamma in it.

                 2
       Sqrt[1 - b ]                             f       1
f'= f --------------  (where b=v/c) --> f' = ------- -------
          1 + b                               gamma   1 + b


That extra term makes it so that the time-dilation cannot make up the energy
drop.

So to make up that drop Earth would need to increase the intensity of the
beam. Steve, Rex, am I right here?

Timothy