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RE: starship-design: Beamed Power (was: Perihelion Maneuver)

On Monday, December 01, 1997 10:45 PM, Isaac Kuo [SMTP:kuo@bit.csc.lsu.edu] 

> 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.

I must have missed that part, I agree that the lens or emitter size is 
firmly established by the focal length of the beam and that you must have 
one or the other. Was someone suggesting that you don't need the lens? I 
stated that I didn't believe the lens would work, not that we didn't need 

> 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.

Well, that is basically the same as what I said. It is an extremely 
complicated and difficult undertaking - either way you do it.

> 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.

True, I can just see some of the engineering papers now - "Some Concerns 
Regarding Thermal Convection Currents and Thermal Blooming Phenomenon in 
Macro-scale Solid State Devices". Whew!

> The reaction against the emitter array would accelerate it
> at one millionth of a gee.

Assuming you are correct and that I have not misplaced a decimal, at the 
end of one year you will therefore have a velocity opposite the beam path 
of 30 meters per second...not counting cumulative effects of the Sun's 
gravity from breaking your orbit.

Actually, I don't think you are _quite_ right. "For every action, there is 
an equal, and opposite reaction..." IF you are providing 100 g (100 * 10 
m/sec^2) = 1,000 m/sec^2 of thrust to the spacecraft then you are also 
providing the same amount of thrust to the emitter. What is different is 
the total delta v on each system which is where your mass calculations came 
in. But they are irrelevant, because in order to maintain station 
(disregarding steering and attitude correction) it STILL requires ANOTHER 
1,000 m/sec^2 of counteracting thrust. This thrust must come from somewhere 
which was my disagreement with the whole scheme. Unless...

We must either:

a) leak approximately half of the beam the other way (probably a little 
less, because some thrust will be provided by the emitter's power reception 
system (but all such thrust may not be in the proper direction),

b) provide a sufficient supply of reaction mass to thrusters on the emitter 
to do the same thing (no way, might as well put them on the spacecraft in 
this case), or

c) build the emitters into a solar sail which is capable of balancing 
against BOTH the sun's gravity AND the thrust of the emitter.

The latter is by far the most elegant and offers some interesting benefits, 
such as built in collectors and a potential zero reaction design.

> 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 achieve .3+ c has involved exotic technology
> and/or massive engineering feats.  Multi-planet sized lenses are
> par for the course.

I also agree that it will work. However, my final conclusion is somewhat 
different. I don't think ANY of the concepts so far proposed are really 
_workable_. Possible with heroic effort yes, workable, no. Of course, I am 
being very pessimistic because of too many years in an intensive QC 
environment and I know just how hard it is to design complex systems to be 
(relatively) reliable.


                                                      (o o)

Ernst Eduard Kummer (1810-1893), a German algebraist, was rather poor at
arithmetic. Whenever he had occasion to do simple arithmetic in class, he
would get his students to help him. Once he had to find 7 x 9.

"Seven times nine," he began, "Seven times nine is er -- ah --- ah --
seven times nine is. . . ."

"Sixty-one," a student suggested. Kummer wrote 61 on the board.

"Sir," said another student, "it should be sixty-nine."

"Come, come, gentlemen, it can't be both," Kummer exclaimed. "It must be
one or the other."