# RE: starship-design: Re: Perihelion Maneuver

```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...

Its irrelevant anyway, a Fresnel lens cannot be built that will function as
advertised, so if you are correct about the aperture (and I suspect you
are, but I will run the equations when I have more time) then you can
forget a maser powered sail. Or a maser powered anything for that matter.

> 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?

> >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.

<SIGH> No, not by Forward, at least not in the citations I provided. But I
provided citations to the people who did.
>
> >> 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?
>
> 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...

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

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

> 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.

No, I presented a carefully chosen sample of a proposed mission that did
make effective use of Hal's idea. I implied nothing that has not been
published in scientific journals. (Which, I will grant proves nothing about
the validity of said idea.) I specifically exempted his main point as
impractical because it did not provide sufficient delta v to make
interstellar travel feasible.

I am beginning to think we are now discussing whether interstellar
bubblegum is feasible.

(o o)
--------------------------------------------------oOO--(_)--OOo---------
Up the airy mountain,
Down the rushy glen,
We daren't go a-hunting
For fear of little men;

William Allingham, Ireland, 1850

```