Re: Problems with beaming

[Kevin C Houston <hous0042@maroon.tc.umn.edu>]

Lee:
> Hi Timothy,
> 
> My point was that ALL of the currently stated plans have design limitations,
> many or most of which are being ignored in our enthusiasm.

Kevin interjects:
Yes lee, but you keep bringing up _engineering_ difficulties.  We are 
still talking about Physical Possibilities/impossibilities.  The 
limitations you point out (and they are important) haven't been ignored, 
just postponed.  after all, no use talking about collimation strategies 
if the maser-induced momentum is going to consistently outpush the engine 
thrust (which happily is not the case for MARS)

> RAYLEIGH'S CRITERIA:
> 
> Where
> theta=diffraction limited beam convergence angle
> r=separation between light source, S, and sail
> ds=sail diameter
> 
> The beam of radiation just fills the sail, thus minimizing energy that
> bypasses the sail. For a diffraction limited radiation source
> (monochromatic) and theta a small angle, Rayleigh's Criterion that
> determines the required diameter of the radiation transmitter is:

First, there is no way to make the beam "just fill the sail" this is a 
theoretical minimum energy waste condition.  The problem (assuming for 
the moment that we can have a n-1000 Km wide Maser generator) is that the 
beam will "jitter".  I have put forth a 'back-of-the-envolope calculation 
that suggests that this deviation could be no smaller than 650 Km in any 
(random) direction. 

There are three solutions that I can think of to get around this problem:
1) make the beam radius 650 km larger than the sail.
2) make the sail radius 650 km larger than the beam.
3) increase the beam density so that the time averaged beam provides 
enough power even if it does jitter away.

all three of these waste energy.  I've no idea which one wastes less.

> 
> Simply put, the problem is not the same one every one has been 
> discussing here.  
> FIRST, the beam WILL NOT be wider than the sail for any practical mission 
> and to do so is WASTEFUL. 

I'd be happy to hear any reasonable alternative.  NB 'not going' is not 
considered reasonable

> SECOND, for reasons previously discussed, the power requirements for 
> beamed propulsion are EXORBITANT so lets not be wasting any energy in 
> "bypass". 

Now, here is where we differ in philosophy,  I'm of the "if your going to 
be throwing power around like it was water (and an oceanful at that) who 
cares if a volume equal to the great lakes gets wasted"


> THIRD, there is an absolute upper limit at which beamed propulsion becomes 
> unfeasible due to the required aperture size. I think every one would 
> agree that a 400 km aperture is impressive even with phased array 
> technology, that being the aperture required for yellow light (a solar 
> pumped laser) and a mission to TC. 

Impressive yes, impossible no, impractical ... maybe (not sure yet)

Assuming some phased array, it is only required that the longest distance 
between two transmitters be 400 Km (or whatever it works out to for Masers)
and given that we are going to have to build the array on mercury, 
putting the maser transmitters around the 45 deg south Latitude (pointing 
toward the south) would give an effective maser transmitter of 2400 Km.
we routinely phase couple radio telescopes here on earth with that same 
distance between them.
 
> Note that a maser would require a larger aperture or array than the 
> yellow light laser, so this discussion applies equally to both pure 
> solar sails, laser driven sails, and the MARS design. 

>'Lest I be thought overly pessimistic,

Better a disappointed pessamist than a dissapointed optimist  ;)

Kevin