From: kasprj@isaac.its.rpi.edu (Jim Kasprzak)
Subject: Re: Ringworlds
Reply-To: kasprj@rpi.edu
Organization: The Big Wedge 
Date: Sat, 29 Aug 1992 20:16:44 GMT

In article <1992Aug29.061737.16642@nwnexus.WA.COM>, elf@halcyon.com (Elf Sternberg) writes:
|> Niven's system works,
|> but one thing that always bothered me: Niven had TWO systems for
|> manipulating the relationship between the sun and the ring.
  
 Not exactly. See below. I'm including your whole interpretation of it
because this seems to confuse a lot of people, including myself when I
first read the book. I'll try to clear it up.

|>  One was
|> the attitude jets around the rim of the ringworld.  This was the ONLY
|> system he had for his world that didn't destroy 20% of the ring.  These
|> jets would collect solar energy and distribute it in an impulsive
|> fashion, pushing the ring back to center.
|> 
|>    The other method used the ring as a giant magnet to create a HUGE
|> solar flare to knock the ring back into position.  This method always
|> bothered me for several reasons, not the least of which was it would
|> KILL 20% of the ring population.  If the 'knock' was powerful enough to
|> push the ring back where the jets weren't, why did the jets have to
|> power to stop the ring from sliding in the _other_ direction?  It also
|> seemed to that the HUGE (approx 200,000km) electricity networks he had
|> buried in the ring were more than adequate to generate _gentle_ EM
|> fields that, fired in a computer-controlled sequence, like the attitude
|> jets, would, with the magnetic field of the sun, be more than
|> sufficient to keep the ring in place.  This method has less moving
|> parts too!
 
 Here's how the Ringworld Attitude Control System (a registered trademark of
Protector Engineering Services) is designed to function:
 
 First, picture the Ringworld system as seen from "above": a circle with the
sun at the center. The ring drifts a little off center, and one side of it is 
now closer to the sun. 
 
 Radiation detectors placed along the ring detect an increase of the solar 
flux above acceptable levels.[1] Too much solar energy coming in means that
that section of ring is too close to the sun, so it's time to fire the 
attitude jets. 
 
 The attitude jets scoop up the solar wind, ignite it in a fusion reaction, 
and spit the exhaust back at the sun.  This pushes the ring back towards 
the heliocentric position.[2] If the ring drifts just a little off course,
the jets only have to apply gentle thrust, and there's no need to use the 
magnetic grid. The jet kicks on as the solar flux exceeds a certain level, 
increases thrust as the radiation increases, and cuts out as it drops back
down to safe levels. 
 
 Our picture of the Ringworld system now includes an arc of the ring where
jets are firing, the brightest ones at perihelion (closest to the sun), and 
the dimmest at the spinward and antispinward extremes of the arc. As the 
ring rotates, the jets antispinward of perihelion are firing up, the ones 
spinward of perihelion are throttling down. As this process continues, the
ring is pushed closer to the proper position, so fewer of the jets reach 
the threshold radiation level. The arc narrows, the thrust at perihelion 
decreases, and when the ring is perfectly centered, no part of the ring is
receiving more energy than normal, so no jets are firing.[3,4]
 
 Okay so far? That's just the system to correct for normal drift. Now 
imagine something whacks the ring _way_ off-center. The attitude control 
jets need more thrust to re-stabilize it in time to avert disaster; the 
solar wind alone isn't providing enough fuel. This is where the superconductor
grid comes into play. The sun's magnetic field is manipulated to increase
flare activity, and thus solar wind output, on the side of the sun facing 
the close section of the ring. This gives the attitude jets more fuel to
burn and allows the drift to be corrected before the sun's gravity makes
it even worse.
 
 Now, what I've described is for a perfectly functional system, one with 
the detection system and attitude jets all in place and working nominally.
Use of the system in this manner would _not_ cause mass deaths on the ring.
Protectors wouldn't design it that way. 
 
 However, in _The Ringworld Engineers_, the system is far from perfectly
functional. Only a small arc of the ring has any attitude jets on it - twenty percent of the circumference. Considering how far off-center the ring has 
drifted already, this is nowhere near enough to push the ring back to its 
normal position if the system operated within its safety constraints. So
the few jets that are working have to accept _much_ more fuel input, which
means much more flare activity. This is what will kill the inhabitants of 
that section of the ring - the section with the attitude jets is getting 
bombarded with solar activity every time it passes perihelion. 
 
 Note that nowhere is the solar wind itself used to push the ring. I don't 
think the solar wind could generate anywhere near enough thrust by its 
impact alone to make a difference unless you raised it to ludicrous levels -
something on the order of flinging huge chunks of sun at the ring.
 
 In summary: there is only one attitude control system, and both the jets and
the magnetic grid are designed as parts of it. The mass deaths caused in 
_The Ringworld Engineers_ come about as a result of using the system in a way
unintended[5] by its designers.
 
 Notes:
 
 [1]: Niven never mentions these detectors, but their existence is the 
simplest way to assure that the system works. Note that these "detectors"
could be mechanical devices or living protectors; given what we know of 
the Pak, I wouldn't be surprised if the original design had a childless
protector stationed at each ramjet. 
 
 [2]: The system would also work in "reverse", if the jets at aphelion 
fired _away_ from the sun when the radiation dropped _below_ a certain 
level, but that wouldn't be as efficient.
 
 [3]: Note that this system is self-correcting, since if the thrust 
generated on one side pushes it too far and it drifts off-center in the
other direction, the jets on that side will start firing. Also, since 
the Ringworld is naturally unstable, it will be very rare that no jets
at all are firing. Under normal conditions there should always be some
low-level corrections going on.
 
 [4]: I just realized that you could "fool" a mechanical system into 
firing when it wasn't supposed to by bombarding the detectors with 
simulated solar radiation. A living protector wouldn't fall for this, 
of course. Maybe a better system would be to link all the detectors 
and have the thrust vary based on the differential between the closest
and the farthest points. Anyhow, I think that the last surviving 
protectors would have set up something reasonably foolproof if they'd
had time.
 
 [5]: But perhaps not unanticipated. The fact that the attitude jets 
have the capacity to handle fuel intake at a level far above safe solar
activity levels implies that the protectors designed for emergencies, 
which seems unlike them. They prefer to slap together an improvement as
the need arises. 
    
 
Organization: Masters student, Materials Science and Engineering, Carnegie Mellon, Pittsburgh, PA