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

[kuo@bit.csc.lsu.edu (Isaac Kuo)]

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

>> 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 
>one.

Kelly thought that an array of widely spaced emitters could
provide the needed focussing without needing any lens.  I myself
came up with that idea a while ago, but when I seriously thought
about it I realized it didn't work.

>> 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.

Well, in the example I gave I was thinking of acceleration over a
period of 2 weeks, not a year.

>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.

Yes, it's the same _thrust_, which is a _force_.  This force
causes acceleration by the formula A=F/M (close enough at
low speeds, anyway).

It's as if you were on roller skates, and you pushed against an
aircraft carrier at dock.  Both you and the aircraft carrier
experience the same force, and both you and the aircraft carrier
are pushed back with the same momentum.  However, you go flying
back visibly while the effect on the carrier is imperceptible.

>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.

No you don't.  First off, thrust is measured in Newtons (kg m/s^2).
In the example I give, the thrust level is 1000 Newtons.  The
effect on the 1kg sail is to accelerate it by 100 gees.  The
effect on the 100,000 ton emitter array is to accelerate it
by one millionth of a gee.

Small enough so that you just shouldn't give a damn about
"station keeping".  You're in the middle of deep interplanetary
space, so who cares if your array is gently pushed back impreceptibly
(small enough so that it won't affect the operation of the
emitters).

And even if you did care about station keeping, you could do
it with a _very_ mild increase in overall mass, even with
chemical rockets.  Assuming use of a very crude 100sec Isp
chemical rockets, adding 2% mass in rocket fuel would
provide a counteracting acceleration of one millions of a
gee for 2 million seconds (over 20 days, well over the
length of the lasing mission).

>This thrust must come from somewhere 
>which was my disagreement with the whole scheme. Unless...

As you can see, even if you absolutely had to counteract
the one millionth of a gee, it wouldn't take much effort.

>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),

No, this isn't needed.

>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

Actually, a sufficient supply of reaction mass is very easy to
do, as I showed.  In contrast, if you had put all those reaction
rockets on the spacecraft, it would have provided something
less than 10km/s of delta-v (the exponential costs of additional
delta-v kicking in).

>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.

Not worth the bother.  The force of the sun's gravity actually
overwhelms the force from the sun's light and emitter.  (It
does so at any range, because both drop off with 1/r^2.)

>> 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.

I mean "workable" as opposed to "it can't ever work because there's
an inherent flaw".

An example of an unworkable scheme would be to use chemical rockets
to acheive .3+c.
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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