# A riff on the 100D / 1000D limit thing

#### Geir

##### Emperor Mongoose
Okay, for most readers this will be either Duh! or Huh? but for that tiny sliver of people in between, this is an answer to a question nobody asked:

Somewhere in the World Builders Handbook I wrote up a suggestion about using a body's mass instead of diameter to determine '100D' for things like stars, because both smaller red dwarf stars and the assorted menagerie of dead star types: white dwarfs, neutron stars and, of course, black holes, they have stellar-ish masses but much, in some cases very, very much, smaller diameters. This goes for a variety of brown dwarfs and such as well. But that's not exactly correct, now that I think about it more.

If the j-drive 100D and m-drive D1000 limits (for, let's be frank, totally fictional drives that can behave in totally fictional ways) are based on the strength gravity, like the local 'curvature' of space or some such, then diameter is actually less wrong than mass. Gravity for a body is related to diameter x density, but mass is related to the cube of diameter. So for instance, if you double the density of an object without changing its size, then gravity is also doubled, but its mass is increased by a factor of 2 cubed, or 8. So using mass as the 'D' limiter would kick the mass-derived D-limit out by four times too much for the fictional curvature involved - because, like, space is all 3-dimensional and such.

(Side note: a black hole has a singularity with all of the mass in it, so it's infinite density in an infinity small volume: this does not cancel gravity out or make it infinite across all of space and time. It just means you should probably use the event horizon as the diameter and avoid the migraine of dealing with infinities.)

When Traveller came out, at least until Book 6: Scouts came out in 1983, all you had to go on for a world was diameter, so 100D had to be diameter and not mass-based (or density based).

Technically (if that's a thing for something made up), you could now (since 1983) add in density (and for instance the 'D100' limit for Mars would be about 71 Mars diameters based on density, but not the 10 Earth diameters {about 20 Mars diameters} that it would be if based on mass), but that's a lot of work for not a lot of gain. It's a game, not a simulation.

So even though it's not exactly correct (assuming the functionality of these fictional space drives is actually based on 'curved space'), it sure is easier to treat every normal planet and star as 100D, and only deal with dead stars - white dwarfs, neutron stars, and black holes - as exceptions, and then use 100 Sol diameters (about 139 million kilometers) x mass in terms of Sol for these really dense things.

Anyway. Just wanted to get that off my chest. It ends up being pretty much what I had said in the first place, but the reasoning behind it has changed. Maybe I'm just dense.

If it's gravity based, you'd assume gravity would be the primary factor that effects performance.

However, it's in the invisible spectrum, it would have to be stated in every subsector map.

Of course it's just a massive simplification.
JTAS#24, Jumpspace, p34:
___ Entering jump is possible anywhere, but the perturbing effects of gravity make it impractical to begin a jump within a gravity field of more than certain specific limits based on size, density, and distance. The general rule of thumb is a distance of at least 100 diameters out from a world or star (including a safety margin), and ships generally move away from worlds and stars before beginning a jump. The perturbing effects of gravity preclude a ship from exiting jump space within the same distance. When ships are directed to exit jump space within a gravity field, they are precipitated out of jump space at the edge of the field instead.
It was never all that exact...

When Traveller came out, at least until Book 6: Scouts came out in 1983, all you had to go on for a world was diameter, so 100D had to be diameter and not mass-based (or density based).
Technically density was always a thing, if you could be bothered:
LBB2'77, p26:
PLANETARY TEMPLATES
___ To express the effects of gravity in a scenario, a representative template may be
created for any world once the diameter of the planet (in thousands of miles) is known. If desired, the planetary density (expressed as a fraction of Earth density) may be included in the computations. Additionally, the specifications for certain standard worlds, and for the planets of the Solar System are provided in tabular form.

Have you just re-discovered the tidal force explanation of the 100D limit by any chance?

All puns should be intended
Yeah, they are. I'm not that subtle. I tried being subtle once, but I was so good at it nobody noticed. So I stopped.

Have you just re-discovered the tidal force explanation of the 100D limit by any chance?
Could be, and maybe m-drives surf: there has to be a gradient to get any 'traction'. (which would also mean, perhaps if you where going perpendicular to the field {approximately} then the m-drive would sputter, too, but that's way too complex).

Oh, yeah, for giant stars, the whole thing gets wonky based on diameter or on density-adjusted diameters, so it's best to use mass as the '100D' proxy.

I just spreadsheeted out a comparison between Alpha Centauri A and Arcturus, which both have about the same mass and the answer for Arcturus makes no sense. Actually neither does the Alpha Centauri answer when compared to Sol, so maybe just use 139 million kilometers (0.93 AU) times mass for all stars... again, what I started with, but in this case for no good reason - the numbers likely make little sense because the density of the stars isn't constant throughout, but now we're way out into 'too much math' land.

(The sun's average density is only about a quarter of Earth's so in that case for Sol it should be closer to 25D, but then you can't even make it to Jupiter on ten times as much or 250D {instead of 1000D} - so I'm going to stop poking at this thing before it blows up in my face.)

How would a giant sloughing off its atmosphere (e.g. Betelgeuse) affect this?
And what counts as the surface of a star in order to determine its diameter, anyway? Photosphere? Chromosphere? The non-spherical corona, just to make things harder? Do CMEs affect these?

And does it switch to 64D and 512D in Aslan space?

And does it switch to 64D and 512D in Aslan space?

It would be the other way around: 144D and 1750D, respectively.

It would be the other way around: 144D and 1750D, respectively.
Sorry, I was trying to keep the human perspective whilst questioning the arbitrary decimal-based number, but you are correct: If those are the actual physical constants, Aslan would be working around 144D and 1750D in their computations.

Sorry, I was trying to keep the human perspective whilst questioning the arbitrary decimal-based number, but you are correct: If those are the actual physical constants, Aslan would be working around 144D and 1750D in their computations.

You are correct in that sense, of course: how convenient for base-10 users that the cut-off value for a universal physical property just happens to be a multiple of 10.

The actual mystery is the universality of Terran norm gravity in factoring acceleration.

FWIW, In my games (your games are yours), I put in enough of what I know of science and the complexities of physical systems to allow players and important NPCs who roll well to fiddle with the limits. I figure even systems that have a hard transition boundary often experience turbulence and local variation, so why not?

So they can say, "We've got the enhanced system weather data, so I plot the insertion to take advantage of [star]'s axial-based cyclical solar wind irregularities. That should let us drop in 0.3 au inside the expected jump shadow... if we get the timing right. Give us the jump on them, as it were."

Note 1: Some of us like technobabble
Note 2: I currently just ignore the 1000D limit, but reserve the right to invoke something like it for dramatic reasons/spectacularly bad rolls.

Okay, for most readers this will be either Duh! or Huh? but for that tiny sliver of people in between, this is an answer to a question nobody asked:

Somewhere in the World Builders Handbook I wrote up a suggestion about using a body's mass instead of diameter to determine '100D' for things like stars, because both smaller red dwarf stars and the assorted menagerie of dead star types: white dwarfs, neutron stars and, of course, black holes, they have stellar-ish masses but much, in some cases very, very much, smaller diameters. This goes for a variety of brown dwarfs and such as well. But that's not exactly correct, now that I think about it more.

If the j-drive 100D and m-drive D1000 limits (for, let's be frank, totally fictional drives that can behave in totally fictional ways) are based on the strength gravity, like the local 'curvature' of space or some such, then diameter is actually less wrong than mass. Gravity for a body is related to diameter x density, but mass is related to the cube of diameter. So for instance, if you double the density of an object without changing its size, then gravity is also doubled, but its mass is increased by a factor of 2 cubed, or 8. So using mass as the 'D' limiter would kick the mass-derived D-limit out by four times too much for the fictional curvature involved - because, like, space is all 3-dimensional and such.

(Side note: a black hole has a singularity with all of the mass in it, so it's infinite density in an infinity small volume: this does not cancel gravity out or make it infinite across all of space and time. It just means you should probably use the event horizon as the diameter and avoid the migraine of dealing with infinities.)

When Traveller came out, at least until Book 6: Scouts came out in 1983, all you had to go on for a world was diameter, so 100D had to be diameter and not mass-based (or density based).

Technically (if that's a thing for something made up), you could now (since 1983) add in density (and for instance the 'D100' limit for Mars would be about 71 Mars diameters based on density, but not the 10 Earth diameters {about 20 Mars diameters} that it would be if based on mass), but that's a lot of work for not a lot of gain. It's a game, not a simulation.

So even though it's not exactly correct (assuming the functionality of these fictional space drives is actually based on 'curved space'), it sure is easier to treat every normal planet and star as 100D, and only deal with dead stars - white dwarfs, neutron stars, and black holes - as exceptions, and then use 100 Sol diameters (about 139 million kilometers) x mass in terms of Sol for these really dense things.

Anyway. Just wanted to get that off my chest. It ends up being pretty much what I had said in the first place, but the reasoning behind it has changed. Maybe I'm just dense.
100D is definitely easier to handle than figuring the gravity well of different celestial bodies.

Replies
11
Views
316
Replies
4
Views
241
Replies
19
Views
663
Replies
10
Views
1K
Replies
29
Views
1K