[Runequest] Fact check

Styopa styopa1 at gmail.com
Mon Apr 8 03:09:51 EST 2013


All credit to Niven.  He's an intelligent fellow, and it's not casting any
aspersions on him to simply say that the math behind a ringworld is
extraordinarily complex, a nearly-unique frame of reference, and one could
spend - especially in 1970, when Babbage's Children were barely toddlers -
much of one's life down the rabbit-hole of computations trying to "get it
right".  Personally, I'm much happier that Mr. Niven DIDN'T do so, and
instead actually wrote the book, physics-warts and all.

Frankly it's a fun exercise, and has provided some entertaining discussions
between myself, and people who know more about physics than me - actual
rocket scientists, some of them.  One (probably more usefully) spends most
of his day working with the physics of golf balls.  (Well, I often do as
well, but not to the MATHEMATICS of them.  Mine is a
more....experimental...approach, using lots of profanity.)

The fact is that (unless for some reason my posts didn't make it to these
lists) I've proven with reasonable accuracy that our Armored Troll would
fall the 500 miles in 406-some seconds (in a vacuum), and would hit at
000's of miles per hour.  Certainly, he'd hit terminal velocity before
that, but he would MOST CERTAINLY impact with a most-terminal of velocities
- probably several hundred miles per hour.

Oh, and re the 'atmosphere' and 'spill walls' - yes, the point observed is
accurate.  The height of the walls DIRECTLY impacts the atmospheric
densities on the surface of the ring.  Assuming as physics-shorthand that
the 1g force in the revolving frame of reference is uniform, the

I'm NOT an atmospheric scientist, nor do I know any.  I'd speculate however
that:
- in this very odd frame of reference, I couldn't find any data on the
surface cross section of liquids, but I *suspect* that it would be a
shallow parabola, possibly a hyperbola.  But for this I'm just going to
assume it's uniform in effect at the surface, because in any case I suspect
a varying pressure effect laterally would anyway be homogenized by the air
shifting around anyway.  For this model, assume the cross-section of the
atmosphere height is horizontal (to someone on the ring-surface).
- ANYWAY, I checked against atmospheric height and surface pressure for
Venus, Mars, and Earth... just to see.  It turns out there's a fairly
straight log-function that describes the relationship.  For Mars (atmos
height ~25km) = surface pressure is 600 pascals.  Earth (~100km)
101kilopascals. Venus (~250km) 9 megapascals.  Assuming that the
atmospheric behavior is the same when extrapolated linearly, a 500km-high
side wall means probably 100 megapascals....or the pressure of water at
10000m depth (yes, 10km).
The point of this atmospheric diversion?
To illustrate only that the 500mile spillwalls DON'T necessarily mean it's
atmosphere 'all the way up'.  In fact, given the quality of the Ringworld
Engineers, it would make sense that they would OVER engineer the spillwalls
to defend against pretty much anything that would 'cost' the structure
airloss, if they could, so 500 mile sidewalls (800km) still still might
only mean 100km atmosphere height.

Simple enough...except for our trollish friend.

Because this means that at least 7/8ths of his 'fall' from the edge (from
the PoV of the surface), or of his fling-sideways (from the PoV of an
outsider watching), is in vacuum....meaning he's going to 'hit' atmosphere
at 000's of mph, relative.

Taking into account the atmosphere, then, suggests that he would fall most
of the way without atmospheric hindrance, until he hits atmosphere at
something like Mach 7, and really doesn't enjoy the next few moments.


On Sat, Apr 6, 2013 at 9:02 PM, Sven Lugar <vikingjarl at gmail.com> wrote:

>  Ringworld is based on Larry Niven's book of the same name. I came to know
> him fairly well back in the days when we both going to Cons & to English
> Regency Dancing events. He always struck me as very stringent in using his
> science in books. I remember an occasion of him discussing the physics of
> it & also mentioning that it was based on the work of Dyson (of the "Dyson
> Sphere concept"). So I suspect that he is probably correct in his
> assumptions to make the concept work. Perhaps someone who is more versed in
> these things can shed some light on this.
>
> Sven
>
>
>
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