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On Wed, 28 Feb 1996, Brian Mansur wrote:

> High Kevin,  It's Brian.

Just what are you implying, I'm not high!  (right now)  ;)

Oh!, I get it.  Hi yourself.

> I've finally had the time to research a few of the old newsletters about the 
> maser design, trying to understand it better.  I have a few questions that I 
> was hoping you would have the time to answer.
> 1. Kevin said that the RM mass weight will go up from what was originally 
> hoped for on the decel phase to TC.  Kevin told me in  a previous e-mail 
> that exhaust speeds between .75 and .8c were to expected and that was down 
> from the envisioned .9996c.  Have you been able to figure just how much more 
> RM we will have to carry?

Not yet, but it is on my list of things to do over spring break.  I'll 
start re-vamping my ship model (in spreadsheet form) in the next few weeks
in a previous mail, I said that I don't think we will need that much more 
RM, because the engine won't be running the whole time like I originally 

> 2. I'm concerned about the size and weight of the maser sail.  Actually I'm 
> concerned about anything that has a diameter of a sizable moon.  Do you have 
> any good idea as to how much the sail will weigh?  After all, if it gets 
> much above say 100,000 tons (E5 tons), it will probably be too heavy to 
> carry enough RM to stop.

The sail can have a large area to weight ratio, because it can have large 
(~2cm) holes in it.  Like chicken wire, only not as thick.  Because the 
microwaves have such a large wavelength, they won't even "see" the holes.

> 3. This maser thing.  What exactly is a maser?  A microwave beam generator? 
>  Is there a way to reflect these microwaves with good efficiency?  Perhaps 
> we could somehow rig a reflector to detact from our ship and reflect back 
> the beam and do away with a linear accelerator.

A MASER is a Microwave lASER.  The reason to use a maser instead of a 
'normal' microwave generator is the same reason you'd use a laser instead 
of just reflecting sunlight.  Masers stay pretty well focused even over 
very long distances.  Robert L. Foreward in his book "Flight of the 
dragonfly" (recommended reading) describes going to barnard's star (~6 LY)
using just this system.  the sail is segmented into two pieces:  the 
inner sail and the outer (or ring) sail.  the areas are in the ratio of 
1:3.  The problem with this, is that it puts an effective upper limit on 
the ships speed.  This is because as the maser bounces off the larger 
ring sail, that sail speeds up drastically.  If that sail should approach 
the speed of light, then it stops reflecting.  The second problem is that 
the power beam has to be turned up by a factor of three (or more) halfway 
through the trip.  Foreward got a lot of dramatic suspense out of having 
some political problems spring up (funding cutbacks etc.) that almost 
stopped (or rather failed to stop) the mission.  A 12 LY long power cord 
is one thing, having to leave it plugged in for forty years is quite 

> 4. Has anyone figured out just how long the accelerator needs to be since it 
> has to be linear?  More to the point, can we keep the linear accelerator 
> short enough and, therefore, light enough to produce relativistic exhaust 
> velocities?

That is purely a function of the acceleration inside the core.  if the 
ions are accelerated at a measley 1 G, then our core must be nearly 1 LY 
long.  on the other hand, if we cram the magnetics in, I hope to be able 
to keep it under 10 Km

> 5. Can we even produce the magnetic fields in an accelerator necessary to 
> get an exhaust velocity of .9996c for .62kg/sec. or even a .75c exhaust 
> velocity using say a 1km long accelerator?  My understanding is that field 

1 Km is probably too short.

> generators that confine magnetics fields have a tendency to blow up.  I hope 
> I'm wrong but I thought you might know if this concern applies to the 
> designs we've discussed.

hopefully, this is one of the areas that will benefit most from new tech.