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100 Diameter Limit

H

hemulen

SOC-12
Just wondering...the 100D limit for Jump occurs due to being near to large masses (e.g. planets).

But what about asteroid/planetoid belts? There's no way that there will be anything like the mass of a planet in the same volume of space, so would that mean it's feasible to jump out sooner (I guess that you may not want to jump in to such an area, given the possible inaccuracies of arrival position, even given the wide spaces between asteroids).

cheers,

Mark
 
Also, what about stellar bodies? What orbits are within a star's 100D limit? (I know there are different sizes of stars, but I don't know if any of them are big enough to worry about. Guess I could go digging around the net.... )

Thanks,
Flynn
 
Good point. Solar diameter is around 7 x 10^5 km, so 100 diameter limit is at 7 x 10^7 km.

Mercury orbits at around 5.8 x 10^7 km
Venus orbits at around 1.08 x 10^8 km

So Mercury is right out, but Venus is ok.

Hm, could be interesting if your mainworld is in a close orbit, you might have to travel a long way out before you can jump safely...

An even odder thought, what about neutron stars? Typical size is 15km, but with about 1.5 solar masses. But can you jump out safely at 1500km? I think not, baby puppy! You don't want to get anywhere close to 1500 km of a neutron star...but they are somewhat of a special case.

cheers,

Mark
 
It seems to me that GURPS: Starports or Far Trader mentioned the jump shadow of the star (or gas giant) the mainworld was orbiting and provided a table of typical values incase you didn't want to specifically calculate it for everyworld.
 
This is one of those long-standing Traveller canon debates (and if we can keep this topic alive long enough Chris Thrash will probably surface to provide all the pertinent canon citations). The current Official interpretation is that the jump-limit is based soley on diameter -- not mass, density, gravitation, tidal force, or anything else. The only exceptions are that in order to exert a 100D jump limit the object exerting the limit must be larger than the ship attempting the jump, and that in order to exert a 100D jump limit an object must have 'significant' mass -- planetoids and iceteroids are specifically included, comet-tails and giant 10km-diameter soap bubbles are specifically excluded.

Furthermore, the stellar diameter definitely does count, and if the mainworld is within 100D of the star you have to maneuver out before you can jump (this is specifically canonical as of Tarsus, I believe). For some stars (including Antares) this leads to a situation where the Habitable Zone is so far inside the star's 100D limit that ships would have to maneuver out from the mainworld for several weeks before they could jump. This is another one of those 'dirty little canon secrets' that AFAIK has never been officially addressed (like near-C rocks and the absent 3rd dimension).
 
Ok, that sounds alright. I would prefer to go with some sort of mass-based idea myself, but I can see that the 100D limit and the 2D thing (especially) are useful from a game point of view, i.e. it gets too difficult to work out WTFIGO.

cheers,

Mark
 
If you like a mass based approach to the 100D limit question use Tidal Force (GM/R^3). I did up a spreadsheet at one point, and set the safe tidal force limit such that a normal density world (about 5 gm/cm^3) would have about a 100D limit. Iceball worlds (like comets) end up with a safe limit of around 70D. Gas Giants are between 30D and 60D. Main Sequence stars end up around the same 30D to 60D. Some of the larger red giant stars, the tidal limit is within the star itself. Very disconcerting if you also use the 100D limit as the default breakout point.

I also found most starships have a safe jump limit of around 100m to 700m or so.
 
The spreadsheets on using the tidal force sound like the could be very interesting.

I checked and GURPS:Trav:FarTrader was the one I remembered having several pages devoted to 'Jump Masking' which is what they call the effect of the stellar primary's 100D limit covering either the originating and or destination planet OR cutting into the most efficient course for a given destination.

The information is on pg.59-61 of GURPS: Far Trader and includes a table that allows you to randomly determine (based on stellar type of the primary) if a desired jump point is masked and if it is approximately how much travel time is added in order to get free of the masking.
 
Originally posted by Ghunkugoe:
Can you post the spreadsheet somewhere that we can al grab it? Or at least post here the petinent formulae?
Thanx
Click to expand...
Spreadsheet is unreadable without much interpretation.

Tide = 2*G*M/R^3 (Calculate tidal forces)

G=6.67E-11 (Gravitational Constant)
Me= 5.98E+24 (Mass of earth)
Ms= 1.99E+30 (Mass of Sun)

Mw = (Density * Diameter(kMi)^3 / 2750)* Me (Mass of world is Density * Diameter cubed * Mass of Earth) in Kg.

Some sources give stars masses in terms of solar messes which is where Ms comes in useful.

Density varies from 0.7-1.3 for gas giants and about 2-7.7 for planets.

Diameter is given on p375 of the T20 book. The formula above assume diameter is in miles however and you will have to convert.

Tide = 4E-13 (Force at safe distance)
T/2G = 3E-3 (Tidal force / 2 * G, a constant)
As stated above, the tidal force is calculated so the force around an average density planet comes out at about 100 diameters. To calculate the safe distance for a given world of size D and density R

Distance = cube root (Mass / T/2G)
This distance is in meters from the center of mass. You will have to convert to your favorite units.

In reality tidal force is G * M1 * M2 * L / r^3, that is it depends upon both masses, the distance between them, and the distance from the center of gravity of the object you are measuring. In the plane perpendicular to the vector between the two masses, the force in toward the center of the mass, along the vector the force is either toward or away from the other mass. We assume for the purposes of this disussion that M2 and L are both one, since we want unit quantities for the Tidal forces.

The reason to use Tidal force is because it is a measure of the rate of change caused by gravity. That is, how much is the local planet or star warping space in the vicinity of the starship.
 
Originally posted by tjoneslo:
The reason to use Tidal force is because it is a measure of the rate of change caused by gravity. That is, how much is the local planet or star warping space in the vicinity of the starship.
Click to expand...
I'd say that the reason to use tidal force is that for the same density it is directly proportional to the diameter, exactly like Traveller jump limits. Thus the current rules can be easily justified as a mere simplification.

I really wish Marc could be persuaded to reconsider his stand on the subject, especially since the 100 D jump limit of stars (which will be the most affected by the change) has not really shown up much in the Traveller background. How many canon system writeups do we have where the mainworld is located deep inside the stellar jump limit? I can't recall any, although there may well be one or two (Tarsus?).


Hans
 
Thank Thrash, that was very illuminating. In most cases, it seems that it is more than likely that any ship would trael above/below the ecliptic of the current system they are in inorder to be able to get the best possible angle/solution for their plot.
In many ways I am glad that the 3rd dimesion isn't taken into account for some of this, though it could make the universe more compact and give more routes, it sure would make jump a bit more difficult...
 
There were a lot of thoughts on the 2D vs 3D on the TML among others. I'll try to track down my favorite so I can give proper credit, unless someone else can recall where it was before I find it. In essence the explanation is that the 2D map represents a cross section layer of the galaxy. There are other systems above and below this layer but they seem to be inaccessible via normal jump tech though ships that misjump and are never seen again might have crossed that boundry and now be stuck in the next layer (a great place for your very own Traveller universe). In MTU I use a thickness of +/- 6 parsecs as the max you can jump from the boundry middle. So a system 7 parsecs above or below the middle of your current residence boundry is inaccessible except by catastrophic misjump. The "normal" d6 x d6 1-36 parsec misjump just bounces you around within the boundries.

I've also used and seen the easy out (no slight intended) that the 2D map represents the "motor club road map" version of space showing only the major centers and approximate lay of the routes. Real astrogators use a much more complex holographic 4 (or more?) dimensional "map" to plot jumps.

Anyway just a couple more ideas, though not really related to the topic.
 
The Neutron Star gravity field is so strong that at 15000 km it will pull the ship and your body apart by
streching it. So I think some considering of mass is needed also. You can have planetary neutron object also( the mass of than planet in afew metter object).
 
The gravity of the Earth at the 100 diameter limit is 1 / (200 Earth radii squared) or 1/40,000 of one earth gravity at the surface. A better rule would be that the starship must remove itself to a distance where the gravity is 1/40,000th of a G, before it can safely activate it's jump drive. Perhaps it also means that it must shut down all of its artificial gravity sources and everything aboard the spaceship must be weightless before the jump drive can be activated.
 
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