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Re: Laser Aperture Size

At 6:59 PM 3/13/96, Brian Mansur wrote:
>>From Brian
>> The Fresnel lens has already been discounted on the basis that it
>> chromatic aberrations into the beam, rendering it largely useless. A
>> reflecting optical system would work better.
>> If you must do it with mirrors, there is no need to get complicated, just
>> build a staged design (kind of like onion rings) where the main ship
>> large chunks of sail to function as a deceleration stage, then a slightly
>> smaller chunk as an acceleration stage to leave TC, and decelerates back
>> into Sol on the unreflected beam. Each stage would function as a mirror to
>> propel the remaining ship and sail sections in the appropriate direction.
>> This method would probably still require a Fresnel lens though.
>>This is the design put forth by Robert Forward, the main problem is that
>>you can't get anywhere near light speed, because then the doppler shift
>>from the retreating sail will reduce the energy to near zero.
>And we can't figure out how to get the darn mirrors keep form, aim, and who
>knows what else!
>> As much as I like the sail concept, I still think the RAIR concept is more
>> promising. Especially if you can boost fuel into its path to increase its
>> cruise velocity. I like the idea of using sails for the fuel, though.
>>I disagree.
>At this time, I disagree as well.  Both require HUGE amounts of power to
>work because both have to get up relativistic speeds to be of any use.  The
>RAIR needs 5E16W/s (if my recalculations on Kelly's paper are correct) just
>as the sail does if we want to get up  to 1/3c.  Both need to get that power
>from someplace.

In the explorer ships case the problem is you'ld need to mine or
manufacture millions of tons of fusion isotopes to fuel it.  I'm not sure
if that much is avalible.  But if it is the power plant should be straight
forward.  The laser launchers have a reaction mass problem, but that isn't

>> I think the simplest mission in terms of hab space design is
>>going to be a 1g thrust to midpoint, then a 1G thrust to target.  The big
>>advantage, is the time dialation for the crew.
>Problem is Kevin, as you pointed out above, that the doplar shift shoots the
>electric bill through as you try to get speed near c.

You have to have a rotating hab section anyway since the ship is going to
be in the star system for a couple of years.

>Another problem I may have just spotted is the that the effects of impacts
>with interstellar matter increases as you approach a light-speed.  Think of
>it.  Your apparant mass (internal energy, something like that) keeps going
>up even though you keep pushing your speed infintely closer to c.  While you
>are only going .999c (I'm making these numbers up to make a point), your
>impact with that speck of dust feels as if you were going 1000c.

Drag could start geting very noticable at high light speeds.  Probably not
enough to us for breaking.   .....  Wait a minutte has anyone worked up
drage figures for these huge sails?  If they were electrically charged and
reflected all the mass in their path they'ld be slaming into hundreds of
tons a months.  At high relativistic thats got to generate some usefull

> Am I making any sense?  Do I even know what I'm talking about?  I get this
>idea from word about a superfast particle known as the "Oh My God" Particle
>(although I detest the name on the basis of religious convictions).
> Apparantly, if it were to hit Earth's atmosphere at its speed of something
>like .99999999999999999999c, it would be as if a brick were in its place.
> And that is for a particle.  Think of what a starship would be feeling from
>every interstellar hydrogen atom at that speeds.
>So while the time dialation advantage is enticing, its tradeoffs seem to
>make it impractical.  By the way, the design I'm working on for the Argosy
>easily accounts for the acceleration and cruise phases by using a dumbell
>centrifuge with hab modules for cruise phase and then stacking the hab
>modules on top of each other come time for accel/decel phase.  I know Kelly
>is skeptical of a dumbell setup for erosion reasons, but we may have a way
>of  protecting against that too.  Read about it in my Argosy paper when I
>send it to the group after break.

Look forward to seeing it.  If you like I'll work it into a simple web page
for inclusion in LIT?



Kelly Starks                       Internet: kgstar@most.fw.hac.com
Sr. Systems Engineer
Magnavox Electronic Systems Company
(Magnavox URL: http://www.fw.hac.com/external.html)