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I've worked on this for D20 Future, but I figure it could appear in charted space as well. From a remote future or a parallel universe this appears in the Spinward Marches.

Location: Trin’s Veil Subsector Hex 0407 – Spinward Marches
Appearance Date: GM’s discretion (Suggested 1105)
Analysis: On the 3rd Month of the year 1108 (or whenever the GM chooses) a gravitational ripple moved across the Hammerium-Robin Trade Route knocking all starships out of jump space in its path, a massive rescue operation had to be organized by the local Imperial government as many ships were stuck in interstellar space without the necessary jump fuel to get out. Adding to the problem was the fact that the exclusion ripple was still expanding outward at the speed of light. A massive fleet carrier was dispatched to the area and was forced to spend the next 4.3 years trapped in the exclusion bubble until the gravity wave ripple dissipated enough to allow a jump out of the area. Hammermuim itself was cutoff for 3 months from interstellar trade. The cause of this disturbance was the sudden appearance of the Ringworld Star. Apparently disgourged by a giant wormhole or jump drive. Where it came from is a mystery. The star type is an exact duplicate of Earth’s Sun, one hypothesis put forward is that the ringworld traveled back in time from a point in the future. Interestingly a plot of Sol’s future orbit in the galaxy would place the era at least 50,000 years in the future. The presence of humans and other Earth creatures on the ringworld’s surface would seem to support this theory.

Radius: 149,604,905 km (92,960,000 miles)
Width: 1,609,347 km (1,000,000 miles)
Period of rotation: 8.838 days
Rotational speed: 4,431,599 km/hour (2,753,663 miles/hour); 1,231 km/sec (764.9 miles/second)
Angular speed: 40.733 degrees per day
Centrifugal Force: 0.976-g (due to Sun’s gravitational pull opposing centrifugal force)
Angular Size of the Sun: 0.53 degrees
Period of daylight: 12 hours
Period of darkness: 12 hours
Total length of day: 24 hours
Twilight: morning twilight begins at 5:49:24 am and ends at 6:10:36 am every day
Twilight: evening twilight begins at 5:49:24 pm and ends at 6:10:36 pm every day

Shadow Squares
Number of Shadow squares: 10
Radius of Shadow Square ring: 57,904,308 km (35,980,000 miles)
Period of rotation: 76 days
Rotational Speed: 199,303 km/hour (123,841 miles/hour) 27,037 kmph above orbital velocity (16,800 mph above orbital velocity)
Rotational Speed: 55.4 km/sec (34.4 miles/sec) 7.52 kmph above orbital velocity (4.67 mph above orbital velocity)
Angular speed: 4.733 degrees per day (0.642 degrees above rate at orbital velocity)
Relative angular speed of ringworld to shadow squares: 36 degrees per day
Angular Length of each shadow square: 18 degrees.
Angular length of gap between squares: 18 degrees.
Length of each shadow square: 18,191,175 km (11,303,450.4 miles) east-west
Length of each gap between shadow squares: 18,191,175 km (11,303,450.4 miles) east-west
Width of shadow Squares: 1,348,633 km (838,000 miles) north-south

The Sun
Diameter: 1,393,695 km (866,000 miles)
Temperature: exterior (photosphere): 5,968 degrees C or 6,241 degrees K(10,800 degrees F)
Gravity: 273-g

Climate at Sea level: Tropical

Encounter Table A – Near Civilization
D100 Result
01-40 Character Encounter
41-00 Animal Encounter (see Appropriate Terrain)

Encounter Table B – The Frontier
D100 Result
01-20 Character Encounter
21-00 Animal Encounter (see Appropriate Terrain)

Encounter Table C – The Wilds
D100 Result
01-10 Character Encounter
11-00 Animal Encounter (see Appropriate Terrain)

Character Encounters
01-05 Aarakocra
06-10 Yazirian
11-15 Weren
16-20 Vrusk
21-25 T’sa
26-30 Sesheyan
31-35 Dralasite
36-40 Robot
41-45 Lizardfolk
46-50 Hill Giant
51-55 Ogre
56-60 Hobgoblin
61-65 Centaur
66-70 Gnome
71-75 Halfling
76-80 Dwarf
81-85 Human
86-90 Orc
91-95 Aleerin
96-00 Elf

Animal Encounters
D100 Result Creature (number appearing)
01-08 Helicoprion (1d12) see above and Dragon Issue 318
09-12 Liopleurodon (1d2) see above and Dragon Issue 318
13-18 Elasmosaurus (1d2) D&D Monster Manual
19-25 Cryptoclidus (1d4, on 1: 1, else: 2d20) D&D Monster Manual II
26-29 Giant Octopus (1)
30-37 Porpoise (1d2) (1d20)Monster Manual
38-45 Medium-size Shark (2d6-1) Monster Manual
46-51 Large Shark (2d6-1) Monster Manual
52-55 Huge Shark (2d6-1) Monster Manual
56-59 Giant Squid (1) Monster Manual
60-67 Baleen Whale (1) Monster Manual
68-75 Cachalot Whale (2d6-1) Monster Manual
76-83 Orca Whale (2d6-1) Monster Manual
84-00 Special-Aerial

Animal Encounters
D100 Result Creature (number appearing)
01-08 Baboon (1d4- on 1: 1d4, on 2-4: 2d20) Monster Manual
09-16 Camel (1) Monster Manual
17-24 Donkey (1) Monster Manual
25-32 Giant Lizard (1) Monster Manual
33-00 Special

Animal Encounters
D100 Result Creature (number appearing)
01-06 Compsognathus (1d12) see below and Dragon Issue 318
07-10 Tyrannosaurus Rex (1d2) d20 Menace Manual
11-14 Triceratops (1d8) D&D Monster Manual
15-18 Deinoychus (1d6) D&D Monster Manual
19-22 Megaraptor (1d6) D&D Monster Manual
23-25 Allosaurus (1d2) D&D Monster Manual II
26-29 Ankylosaurus (1d2 or 2d20) D&D Monster Manual II
30-35 Black Bear (1d2)
36-43 Boar (1)
44-51 Eagle (1d2)
52-59 Leopard (1d2)
60-65 Ape (1d6)
66-71 Brown Bear (1d2)
72-77 Elephant (1d30)
78-83 Giant Lizard (1)
84-92 Monkey (10d4)
93-100 Wolf (1d10+6)

Animal Encounters
D100 Result Creature (number appearing)
01-08 Pachycephalosaurus (1d12) Dragon Issue 318
09-16 Weasel (1) Monster Manual
17-24 Quetzalcoatlus (1d6) Monster Manual II
25-32 Allosaurus (1d2) Monster Manual II
33-40 Ankylosaurus (1d2; on 1:1d20, on 2:2d20) Monster Manual II
41-48 Seismosaurus (1d20) Monster Manual II
49-50 Spinosaurus (1d2) Monster Manual II
51-00 Associated Terrain i.e Forest/Hills, Plains/Hills, Desert/Hills

Animal Encounters
D100 Result Creature (number appearing)
01-08 Dimetrodon (1d8) Dragon Issue 318
09-16 Parasaurolophus (1d12) Dragon Issue 318
17-24 Crocodile (1d12) Monster Manual
25-28 Giant Crocodile (1d12) Monster Manual
29-30 Tiny Viper Snake (1) Monster Manual
31-32 Small Viper Snake (1) Monster Manual
33-34 Medium Viper Snake (1) Monster Manual
35-36 Large Viper Snake (1) Monster Manual
37-38 Huge Viper Snake (1) Monster Manual
39-46 Toad (10d10) Monster Manual
47-54 Allosaurus (1d2) Monster Manual II
55-62 Ankylosaurus (1d2 x 1d20) Monster Manual II
63-70 Seismosaurus (1d20) Monster Manual II
71-72 Spinosaurus (1d2) Monster Manual II
73-86 Associated Terrain i.e. Forest/Marsh, Plains/Marsh, Desert/Marsh
87-00 Aquatic

Animal Encounters
D100 Result Creature (number appearing)
01-08 Pteranodon (1d10) Dragon Issue 318
09-16 Quetzalcoatlus (1d6) Monster Manual II
17-24 Eagle (1d2) Monster Manual
25-32 Giant Eagle (1d12)
33-00 Special Aerial

Animal Encounters
D100 Result Creature (number appearing)
01-04 Diplodocus (1d3 x 1d6) Dragon Issue 318
05-08 Giganotosaurus (1d6) Dragon Issue 318
09-12 Stegosaurus (1d10) Dragon Issue 318
13-16 Triceratops (1d8) Monster Manual
17-20 Tyrannosaurus (1d2) Monster Manual
21-24 Allosaurus (1d2) Monster Manual II
25-28 Ankylosaurus (1d2 x 1d20) Monster Manual II
29-32 Seismosaurus (1d20) Monster Manual II
33-34 Spinosaurus (1d2) Monster Manual II
35-42 Baboon (1d10 x 1d4) Monster Manual
43-50 Bison (1d6 x 1d6) Monster Manual
51-58 Cheetah (1d6) Monster Manual
59-66 Dog (1d12) Monster Manual
67-74 Elephant (1d6 x 1d6) Monster Manual
75-82 Horse (1d6 x 1d6) Monster Manual
83-90 Lion (1d10) Monster Manual
91-98 Rhinoceros (1d12) Monster Manual
99-00 Special Aerial

Animal Encounters
D100 Result Creature (number appearing)
01-08 Cat (1) Monster Manual
09-16 Dog (1d8 Monster Manual
17-24 Riding Dog (1d8) Monster Manual
25-32 Heavy Horse (2d10-1 Monster Manual
33-40 Light Horse (2d10-1 Monster Manual
41-48 Giant Lizard (1d4 Monster Manual
49-56 Pony (2d10-1 Monster Manual
57-64 Rat (10d10 Monster Manual
65-66 Tiny Viper Snake (1) Monster Manual
67-68 Small Viper Snake (1) Monster Manual
69-70 Medium-size Viper Snake (1) Monster Manual
71 Large Viper Snake (1) Monster Manual
72 Huge Viper Snake (1) Monster Manual
73-80 Toad (10d10) Monster Manual
81-00 Special Aerial

Special Aerial
Animal Encounters
D100 Result Creature (number appearing)
01-25 Giant Eagle (1d12) Monster Manual
26-50 Giant Owl (1d6) Monster Manual
51-75 Quetzalcoatlus (1d6) Monster Manual II
75-00 Pteranodon (1d10) Monster Manual II

I may have to adapt where it is or how it makes an appearance, but I'll be borrowing it for sure.

Thanks for posting it.
This ringworld may have been built by humans and related species in the far future, that's why there are so many humanoids on it. Since it is capable of traveling through time, we get a fairly good idea of where the dinosaurs came from.

I want it to be as flexible as possible. The ringworld material is incredibly strong. The main structural components are mono-atomic filaments. Each filament is made up of one atom, but it is a very very long atom, it is so long that each one can wrap around the Sun at a radius of 1 AU. The nucleus of each filament atom is what gives the material its strength. These filaments are arranged in a mesh and they have other normal atoms chemically bonding with them to fill in the spaces between making a floor of the ringworld. This ringworld floor material is superconducting at very high temperatures and nearly indestructable.

The floor is shaped into a landscape with rock and dirt piled on top. Bedrock is bonded to the ringworld floor material, on top of that is placed loose rock and then soil. The average thickness of the material placed on top of the ringworld floor is 100 meters. (roll 4d6 x 10 meters to determine the thickness at any given spot) This material also includes water in many places. (roll 3d6 x 10 meters to determine the depth of the water and roll an additional d6 x 10 meters to determine the thickness of the rock and soil at the bottom of all bodies of water.)

I mention the above because there is a new class of vehicles used exclusively on Ringworld, these are magnetic vehicles, they replace the grav vehicles that do not function here. The standard Mag Vehicles are the Magnetic Air/Raft, The Magnetic Pressurized Air/Raft, the MCarrier, the Magnetic Speeder, and the Magnetic Belt. Each one of these vehicles performs the same as their corresponding grav vehicles on a planets surface except that these magnetic vehicles have a maximum altitude of 250 meters above the ringworld floor. Since the thickness of the rock and dirt on top of the floor varies, the maximum altitude of the Mag vehicles above the ground also varies. Magnetism unlike gravity is a short range force. Grav vehicles do not work as there is no gravity to negate. I think this is a nice feature as it forces the PCs to interact more with creatures on the surface.

The humanoids have names corresponding to creatures in D&D, they have no magic of course, and those creatures that are listed as having Dark Vision have Low Light Vision instead. There is no Underdark, only separate individual caves in places. Orcs, for instance are nocturnal creatures that hunt at nightm but they live on the surface for the most part. Some creatures truly are alien, they originate from some of the many places and times the ringworld has visited. The current tech level for most of the inhabitants of ringworld is rather primitive. all metals are recycled and there are not much of those. The ringworld itself is self-aware but it chooses not to communicate and it does not interfere with the goings-on on its surface. What the ringworld wants is hard to determine, but when it detects a threat to itself such as an asteroid or comet, it brings its gigantic lasers on line and vaporizes the threatening object.

Most spaceships don't threaten the ringworld unless they exceed 1,000 tons of displacement while approaching too fast and coming too close while doing so. If a starship doesn't look like a threat to the ringworld, the ringworld ignores it. If a starship is tiny, such as under 1,000 tons of displacement, the ringworld ignores it even if it does approach too fast.

The magnetic vehicles by the way cost the same as their corresponding grave vehicles, everything except their maximum altitude is the same as the corresponding grav vehicle listed in the T20 Handbook.
The Hinterworlds Special Supplement for MegaTraveller, free inside a Challenge magazine, details the sector the Leenitakot (1432 Hinterworlds) system is in.
The ringworld is mentioned, but very few details are given.
Well that's another Alien Ringworld. Larry Niven's Ringworld is alien too, then he has to explain eventually why these Aliens visited Earth and picked up some humans and dumped them on their ringworld, its much simpler if you assume that humans built the ringworld in the first place. All the humanoids you find there except for the blatantly Alien ones descended from visitors, are partially human. An Orc, for instance is partially human and you can crossbreed and Orc with a human to make a half-orc because of that, the same deal is with elves. These creatures gets the same racial ability adjustments that the D&D books say they get, but then I give them a Traveller Class if they are of a high tech level, basically anything above Tech Level 4.
Tech Levels 0 through 4 get D&D classes, the D&D classes don't come with magical abilities, but you can have a Wizard who can't cast spells. A Wizards does have at least one advanced technological item in his possession that he know how to use, and in his community that makes him regarded as a wizard. Wizards are very adept at figuring out how to use technological items, its fair to say that a wizard's level indicates how many tech items he has in his possession that he knows how to use.

A Cleric has medical equipment in his possession that allows him to heal people, the cleric claims that the effects of his devices are the work of his god. His low tech followers don't know any better so they tend to believe him. A clerics greatest asset are his followers, his most devoted followers tend to be fanatical. The D&D classes work fine in this context, they use the same hit dice for example, hit dice determine stamina and their constitution scores determine their lifeblood. Soc and Edu scores can be treated as non-ability scores for the time being, the D&D classes don't use them and they have their own set of skills and feats which they use from the players handbook. I'll make things simpler. D&D Armor doesn't have an AR rating which deducts damage dice inflicted by Medeval weapons, what it instead does is make the person harder to hit with those medeaval weapons. Futuristic weapons tend to hit them as if they aren't wearing any armor. Iron or steel armor and swords tend to become projectiles when a magnetic vehicle is hovering nearby, such tends will be attracted along the field lines of magnetic force and hit the bottoms of magnetic vehicles, fortunately metal is scarce on Ringworld, but the vehicle designers have armored their magnetic field coils to protect them from flying swords, shields, and nails should any be encountered.
I don't think you could form a mono-atomic filament, as the components of atoms like to orbit the nucleus. But, you could use a mono-molecular substance.
According to the liquid drop model the sub-atomic particles behave a bit like the particles in a liquid and can slip and slide over each other.

Google for an animation of what happens to the nucleus of an atom that undergoes fission.

What Tom is suggesting is that a massive atom is constructed and its nucleus stretched out into a cable.

Now that's science fiction at its best IMHO ;)
OK. For me, that is one level of disbelief suspension too far.
I think people are trying too much to explain how ringworlds (RW) work here. Put bluntly, is that information ever going to be relevant?! I mean, you got a ringworld full of fantasy races here, seems a bit redundant to be expounding on the exact structure of the ring and what material it's made of etc. Despite our pretenses, all ringworlds are just made out of advanced Armwavium and built by Ancients Beyond Our Ken (TM). ;)

You'll probably have a more productive time by saying that the RW is just the setting, and if anyone can't handle that then there's no point in them reading on. That leaves you to develop the races, important locations, rationales for the tech (eg your clerics with healing tech, and your magnetism-based transportation mechanisms) without worrying about satisfying the physicists
Originally posted by Malenfant:
<snip>... Despite our pretenses, all ringworlds are just made out of advanced Armwavium and built by Ancients Beyond Our Ken (TM). ;)

Well I thought it was funny
Malenfant has a point here. Is it possible to build a massive atom into a ring that's 1 AU in radius? I don't know. I don't believe any physicists do either. Super-heavy elements tend to be unstable and have very short half lives and a macroscopic atom is way beyond that, but I can't say that its impossible either. This atom I talk about is only macroscopic along 1 dimension bent into a ring, in all other dimensions its a normal sized atomic nucleus. The nucleus is basically a long chain of protons and neutrons rather than a ball at the center of an atom. This chain of protons and neutrons would have a tremondous positive charge, therefore it would attract a cloud of electrons that orbit in tube shaped electron shells completely around the the entire length of the ring-shaped nucleous. One reason that this material would be a superconductor is that electrons flow very easily along just one atom. Going from one very long atom to another is only slightly harder, so for all practical purposes electricity would flow through this material without resistance. Another useful property of superconductors is that magnetic fields repel them. The Mag vehicles I talk about have superconducting coils inside of them of a more normal sort. The way to make these vehicles levitate is to generate electricity and let if flow around these super-conducting coils. These coils can hold alot of electrical current without resistance, so all the vehicles have to do is charge up their superconducting magnetic coils and once it generates an intense enough magnetic field it will repel the ringworld floor material and rise up off the ground.

The mag vehicle can just hang in the air expending very little power simply to replace the tiny amount of current lost due to electrical resistance. The mag vehicle can shut off its engines and so long as current continues to flow through its magnetic coils it will continue to float. If the vehicle wants to land it has to demagnetise its coils. To do this you simply heat the coils till they are no longer superconducting or you ground the circuit.

The height limitation comes from the fact that although the superconductor is very advanced, their is a limit on the amount of current that can flow through the coils. These coils are the best that the Imperium is capable of making. A regular grav vehicle can be converted to a mag vehicle with a little mechanical work.

The magnetic levitation phenominon is real, the superconductors themselves and the ringworld floor material is speculative. Also I think a mono-atomic filament, if one were possible, would also be a thermal superconductor. The normal atoms surrounding it would bunp into it and the giant atom would distribute these vibrations along its entire length at perhaps near the speed of light since the nuclear material is so tightly packed within the nucleaus. This means that its hard to blast a hole through this stuff since the heat of a laser or bomb would be quickly carried away along the entire length of the ringworld.

These are my thoughts anyway. To the laymen, this material is practically indestructable, the big exception being if it were to come in contact with antimatter.

Time travel is speculative too, but the ringworld decides whether to travel in time not the characters, also I suspect it takes a tremendous amount of energy, the ringworld would take some time to build up enough energy to make another time jump, it could be 100 years or it could be longer. The point is the ringworld collects things from different times and the PCs encounter those things on the ringworld rather than traveling in time themselves. Its not even a sure thing whether the future it comes from is inevitable or just one of many possible futures. The circumstances of its departure from that future and its reasons why are unknown, perhaps the PCs will find this out by adventuring on its surface.
Actually Ghost, I think you have an interesting idea for the mono-atomic approach. Color me convinced (at least enough to get on with the game).

Now, about the time travel.... ;)