Re: starship-design: Laser or maser?

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

On Thu, 31 Oct 1996, Timothy van der Linden wrote:

> Kelly wrote:
> 
> >If you have to find a position for the array that has constant direct line
> >of sight at all times.  Your limited to a small polar region.  The farther
> >the target star is from true north or south the narrower the polar region.
> >Also the platforms must be in direct line of sight so they can adjust for
> >one anothers movement (yes platforms mounted on the Moon will more around a
> >bit).

Platforms need not be in direct line of sight, they must merely have an
electrical connection to each other.  this is especially true of Maser arrays
(more on this later)

> 
> No, that polar region is not small, it is rather big.
> If a star deviates 45 degrees from the rotation axis of the moon/planet, it
> can be seen downto the 45th longitude. (There is a small band in wich the
> Sun and other planets will move along (on Earth that is between +23 and -23
> degrees).)
> 

As I recall, Tau Ceti is in the southern hemisphere.  Anyone know what 
the angle is?  (I think the proper term is ascention)

> >>I wonder are semiconductor lasers more efficient and cheaper than ordinary
> >>lasers. What will be the costs compared to masers which cannot be made in
> >>semiconductors?
> >
> >Can't remember.  I thought free electron lasers had the highest efficency?
> >
> >>Also on the receiving side, what are the pros and cons?
> >>On the receiver-end we not only need a mirror, but also an absorber. Are
> >>photocells better than skottky diodes?

No, Schottky diodes are more efficient and cheaper (eff ~ 90%) Photocells
are only about 15% efficient, and that is for super high end cells.  While
this is not a problem in the collection of solar energy, (we can always 
make the solar array bigger without increasing the energy costs) such 
a low efficiency would be disastrous for the ship.

> >
> >Why an absorber?
> 
> During the deceleration phase, energy is used to accelerate repulsion mass.
> (I know, you proposed some ingenious reflecting system, so that the
> EM-radiation directly accelerates the repulsion mass, but somehow that seems
> to ingenious.)

I have to agree with tim on this one kelly, a direct conversion to 
electricity to run the lineac seems the simplest.  perhaps a plasma
mirror will have to be considered if the wavelength is going to 
be so short (because the conversion eff will be so lousy) but until
we rule out microwaves totally,  I'd like to keep the conversion to
electricity.


> >>What are the disadvantages of a reflective sheet compared to a wire 
mesh? Is > >>it only the weight?
> >
> >Increased drag and more limits in materials I think?

Along with drag, there is erosion.  A wire mesh would allow interstellar
particles to slip by, while a reflective sheet would be under constant 
bombardment by hydrogen atoms blowing by at nearly the speed of light

> 
> Yes maybe, however the drag may be far less than the increased efficiency or
> better focusing with a smaller array.

efficiency will be less with visible light, but focusing will be better.

> 
> We (also others than me and Kelly) should try to find some more pros and
> cons about laser and maser.


Pro for lasers:
Cheaper to make lasers than masers.
Easier to focus.
Smaller arrays.
Each photon has greater energy.

Cons for lasers:
Terrible conversion efficiency (both ends)
Heavier sail with smaller holes (microscopic holes might reduce some mass)
bigger redshift effects. 
	a 7-fold decrease in wavelength for laser light will take it out 
	of the visible spectrum.  but for Masers, a 7cm beam is still
	usable.  (that is, a beam that starts out from earth with a 
	wavelength of 1 cm ends at the ship with a wavelength of 7cm)


Pros for masers:
Easier to convert to electricity (diodes vs solar cells)
lighter sail with big holes (about 1 cm)
Smaller redshift effects.
	a 7-fold decrease in wavelength for a maser beam, can still
	be rectified by a diode (wavelength of maser from earth ~1cm
	when it reaches ship, it will be 7 cm.  this is still in the 
	microwave region.  Even a radio beam could be rectified by
	a diode and converted to electricity.)
emmitters can be linked electriclly, rather than optically for lasers

Cons for masers:
need larger arrays.
harder to focus.
photons have lower energy.
masers more expensive than lasers


can anyone add to this list?  or dispute what I've said?

--
Kevin Houston     http://www.urly-bird.com/