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

["L. Parker" <lparker@cacaphony.net>]

On Tuesday, December 02, 1997 8:33 PM, Isaac Kuo [SMTP:kuo@bit.csc.lsu.edu] 
wrote:
>
> 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.
>
Not unless someone repealed Rayleigh's Criteria...

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

Oh, I was thinking of a longer term.


> 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.
>
> 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.

Oops, now I'm confusing thrust and velocity...but the original statement is 
still valid. It takes an equivalent amount of counteracting _thrust_ to 
cancel out the change (however miniscule) imparted by the emitter. In a 
phase locked system, as you yourself pointed out to Kelly, the emitters 
MUST be stable in relation to each other, so station keeping is essential. 
(I'm still not confident that it can even be done.)
>
> 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).

It would take thrusters capable of generating 1,000 Newtons...

> 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.)

Sorry, but it has already been shown that a light sail can "stand still" 
with respect to the Sun. In other words it can remain stationary in space 
or even accelerate directly away from the Sun. I will be happy to furnish 
references if you don't believe it.

> 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.

Yuck! Now that is unworkable.

Lee

                                                      (o o)
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"Science has 'explained' nothing; the more we know the more fantastic the
world becomes and the profounder the surrounding darkness."

-- Aldous Huxley, 1925