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

L. Parker wrote:
>On Tuesday, December 02, 1997 8:33 PM,
>Isaac Kuo [SMTP:kuo@bit.csc.lsu.edu] wrote:

>> 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.

Yes, but since the emitters is so damn huge and heavy, the additional
mass needed to produce this thrust turns out not to be so much (in

>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. 

Yes, they need to be stable in relation to each other IN A PHASE
LOCKED SYSTEM.  No, this does not mean that they need to keep
station with their original positions.  Since all the emitters
are being pushed back by the same (miniscule) amount, they
theoretically aren't moved off station with respect to each other.

Actually, with microwaves, it's possible to get around the
station keeping requirements altogether by not locking the
phases.  Instead, each emitter has a receiver in the rear,
and a carefully adjusted fixed time delay between this receiver
and the emitter, which merely amplifies the received signal.
In effect, the received signal is pumping a sent signal which
has a carefully chosen time delay.

Far behind the emitter array is a single emitter which sends the
pumping signal.  By adjusting the time delays appropriately,
the emitted signals are in phase even though the emitters may
not be placed precisely.

Realistically, this is the only way to get the thing to work
even if there weren't any reaction force (just because of
assorted vibrations).

>> 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...

Which, assuming an utterly pathetic thrust/weight ratio of 1/10,
would require rocket thrusters weighing a total of 1 ton.
On a 100,000 ton emitter array, this is barely noticeable.

(Thrust/weight ratios of over 300 are not uncommon for chemical

>> 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.

It's possible with something which is of sufficiently low
"density" (mass divided by sail area).  I calculate it to
be around 17 grams per square meter.

However, with anything as "heavy" like a solar collector and a
beam emitter included, I don't think such a low areal density
would be acheived.
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
/___________\ "Mari-san...  Yokatta...
\=\)-----(/=/  ...Yokatta go-buji de..." - Karigari Hiroshi