starship-design: Re: Perihelion Maneuver

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

L. Parker wrote:
>On Monday, November 17, 1997 11:15 AM, Isaac Kuo [SMTP:kuo@bit.csc.lsu.edu] 
>wrote:

>> He never bothered with the engineering difficulties of making
>> and stabalizing such a huge fresnel lens, though.

>Not to switch sides or anything, but I don't believe the fresnel lens will 
>even work for several different reasons. His use of the fresnel lens was 
>for a larger, manned proposal. The Starwisp received ALL of its 
>acceleration while still within range of the original...uhh, maser.

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.

Let me repeat that last figure:

Starwisp with a 1mm wavelength requires a lens 30,000km wide.

3 times the diameter of Earth.

No matter how you cut it, this is clearly way beyond today's technology.

>> No it didn't.  He just didn't bother with that part of it.  He
>> also didn't bother with figuring out how much the Starwisp would
>> get heated by the laser, which is an even more basic issue.

>I will agree with the tidal force issue, primarily because we don't know 
>what it really is now and certainly didn't then.

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.

>The heating issue however, 
>was studied, both from laser/maser and solar. The references in the 
>previous post were the relevant studies.

Not by Forward.

>> 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.

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

>> I'm not attacking sails--however I am pointing out that the additional
>> boost you could get with a solar flyby for an inter_stellar_ mission
>> is not ever going to be worth it.  Such a sail will need to be
>> powered by one of:

>> 1. Laser
>> 2. Particle beam
>> 3. Nuclear bomb track

>> Whereas the small additional help a solar boost could give is far
>> outweighed by the disadvantages.

>Frankly, I don't really believe that a solar sail will be very useful at 
>all. I don't consider 0.3 c sufficient for interstellar travel.

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.)

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

>My only 
>point was that accelerating to 0.3 c in a matter of weeks (assuming it is 
>possible) is not a trivial boost, wherever it came from.

I agree that it's not trivial.

However, before, you were implying that you could get .3c essentially
"for free" because of the Solar flyby maneuver.  Which is nonsense.

>> A powered sail, OTOH, is really only applicable to interstellar
>> missions (or as anti-vehicular weapons).

>A sail as a weapon, Hmm, what an interesting idea.

Perhaps, but it's also one which has a very low rate of fire, is
highly visible, and relatively easy to counter.  About its only
selling point is that it has much longer range/much greater
speed than other possibilities.
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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