Re: starship-design: Re: Perihelion Maneuver

[kuo@bit.csc.lsu.edu (Isaac Kuo)]

L. Parker wrote:
>On Monday, November 17, 1997 11:23 PM, Isaac Kuo [SMTP:kuo@bit.csc.lsu.edu] 
>wrote:

>> No, it still needs a fresnel lens.  Without it, diffraction limits
>> are too severe.  The beam needs to fall on a spot 1km wide at a
>> range of 30,000,000,000km.  With a wavelength of 1mm, that requires
>> an aperture around 30,000km wide.  Don't tell me you're going to
>> make a microwave laser that big.

>Unless, I totally misremember, a Fresnel lens is a diffraction device not a 
>reflection device. It won't work with microwaves unless you are going to 
>make it out of about a zillion miniature waveguides...

Actually, as far as I can tell it would works.  Yes, some diffraction
is involved, but primarily it works by constructive interference.
Concentric rings of reflective material reflect (or absorb, it doesn't
really matter) wavefronts out of phase with what is desired.  In
effect, it turns a single wavefront into a bunch of wavefronts which
happen to constructively interfere like a phased array radar in the
direction of the target.  The losses are minimized by having a very
large distance between the source laser and the lens.

The clever thing about this approach is that it's very insensitive to
the "flatness" of the lens.  A mirror would have to be kept flat to
within less than a millimeter, whereas this lens can be "bumpy", and
even rippling, without affecting the quality of the beam.

Another feature of the fresnel lens is that by having a large distance
between the source laser and the lens, the width of the rings is
maximized.  For a source laser 1000 times as far away than the diameter
of the lens, the thinnest rings (the outer ones) would be 1000
wavelengths wide.

Unless you're familiar with optics, VLBI, and/or phased array radar
concepts, it's a little difficult to explain how all this works.

>> Huh?  Isaac Newton could have given you the tidal forces with good
>> accuracy.  It's a simple application of the equation for gravitational
>> force.  The Sun's mass is 2x10^30 kg.  At a distance of 1,000,000km
>> from its center, the tidal force on something 1km wide is about
>> 4x10^-4 newtons per kilogram.  For a 20g Starwisp, this would be
>> 8x10^-6 N.  I'll admit this is less than I originally thought it
>> would be, but it's still enough to rip apart the flimsy Starwisp.

>Are you maybe confusing gravitational force with tidal force or the Roche 
>limit?

I'm talking just about the tidal force, which is derived from the
equation for gravitational force.  You know, the force that is
proportional to the inverse of the cube of the radius, which tends
to pull things apart/scrunch them together?

>> >> No, it uses a fission reactor powering an ion rocket.  This is an
>> >> actual _proposal_, which you can see on NASA's web page.  In other
>> >> words, something they could actually build and fly, if given the
>> >> funding.

>> >Yep, it is _one_ of NASA's _proposed_ designs. Look at some of the other
>> >
>> >engines JPL is currently working on for the same mission. I assume the
>> >one
>> >you saw is the Xenon based Ion engine. Look at Dense Plasma Focus, and
>> >Antimatter Catalyzed Microfission/fusion also. NASA and the USAF are
>> >funding development. BTW, Icarus was/is a solar sail design for that
>> >mission.

>> Anyway, it falsifies your claim that fusion "would" be used for TAU.
>> Obviously, it would only be used if a fusion drive were developed
>> for it.  Which it probably isn't.

>Huh? What claim?

You said "interestingly enough, TAU would use a fusion drive..."

That implies that _all_ TAU designs feature a fusion drive.

>> I am not aware of these other concepts, but I'm sure they are not
>> proposals for actual probes.

>Yep, they sure, are. On NASA sites even, with NASA funding. Just like the 
>ion thrusters...

You have a different definition of "proposal for actual probe" than me,
apparently.  I mean something which could actually be built and flown,
with a proposed budget, timeframe, and everything.  Something like
Galileo or Mars Pathfinder or Space Station Freedom.

Fission reactors and ion thrusters exist and are technology mature
enough to build a probe with.  We can't even guess when a practical
fusion reactor _concept_ will be developed, much less understood
and/or built.  (There's no certainty that MTF or MCF or ICF or even
one of the other currently conceived of fusion reactor concepts will
ever produce more power than they consume.)

>> Maybe not, but IMO it's clearly sufficient for an interstellar probe
>> flyby mission, which is all it's really good for anyway (my rule
>> of thumb is that anything good enough to use for decelerating at
>> an unprepared target system is good enough for the acceleration
>> run.  Conversely, anything only good for the acceleration run
>> doesn't really help if you want to stop at the target system.)

>I'll buy that, but see your earlier argument _against_ the flyby mission.

My argument against Starwisp specifically is that it's not clear
it will ever be worth it, seeing as its sensor suite is so limited.

Something with the capabilities of Voyager would easily provide much
more detailed and reliable information on a nearby star system than
we could expect from future telescopes.

>> IMO, 10%c is sufficent for interstellar flyby missions.

>I wouldn't want to wait that long to get my data back.

What, 50 years?  By the time we start throwing around interstellar
probes, I'll bet average human lifespans are comfortably over 100
years.
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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