starship-design: Perihelion Maneuver

["L. Parker" <lparker@cacaphony.net>]

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

> You mean .3c, of course.  And this _requires_ the use of a high
> power laser (yes, a coherent microwave beam is a microwave laser).

Umm, yes I did. Sorry about that. Gee, we used to call those Masers...am I 
giving away my age?

> Forward's proposal would take 2 weeks, which I wouldn't call
> "a few days".

I noticed that several places in his papers. He was/is pretty good about 
"few" meaning "several dozen or more". Sorry, I was just quoting.

> Actually, it is proposed to withstand 100 gees.  There is no attempt
> by Forward to demonstrate how this could be acheived.  Theoretically,
> negligible structural strength is required so long as the laser
> pushes absolutely evenly across the entire 1km of the Starwisp.
>
> In practice, even the slightest error (which realistically there
> will be) will rip apart the Starwisp as soon as the beam is turned
> on.

Well in one paper he uses 115 g's and in another he uses 400 g's.

> You must realize that none of these proposals are complete, and
> pretty much all of Forward's proposals have huge gaping holes.
> At first, he proposed using a ring of microwave beams emitting
> in phase in order to acheive the aperture needed using VLBI.
> Obviously, he didn't know that it wouldn't work.  Later he came
> up with the fresnel lens concept, which would work (but reduce
> beam power by about 50%).

I will grant that the lack of hard engineering specifications in his (and 
other) papers from that era probably indicate only cursory engineering was 
performed. Either way, I don't have the time to duplicate it. Perhaps there 
will be new data available next spring...
>
> 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.

> Not all data is classified.  The costs of these these things are
> often reported in news reports and AvLeak articles.  For instance,
> you've probably heard a gazillion times how much a Tomohawk cruise
> missile costs, seeing as they mention it every time they get used.

I've read Aviation Leak for over twenty years. This data is still 
classified.

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

> Umm...yes it needs a fresnel lens.  It's huge multi-kilometer diameter
> fresnel lens made up of consentric rings of reflective material.

Not for microwaves...and I still don't think a fresnel lens will work.

> 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.

> 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. 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.

> 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.


Lee

                                                          (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