Jump Limits and Tidal Force
- Thread starter atpollard
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M
Malenfant
Guest
It means using an object's mass (and therefore its gravity) to define the jump limit, rather than setting it by its diameter (i.e. 100D), which makes no sense at all.
Usually when people talk about using "tidal force" they mean that you define the jump limit as being the distance at which the local gravity (or rate of change of gravity) is below a certain value. In practice it means that way a one solar-mass white dwarf's jump limit would be the same as a one solar-mass main sequence star's jump limit which would be the same as a one solar-mass red giant star's jump limit, even though the WD has a much smaller radius and the red giant has a much larger one.
In some cases (e.g. supergiants like Antares) it means that the gravity-based jump limit is actually inside the star, which means a ship can come out of jump anywhere outside it - and for Antares it makes a big difference, since canonically its jump limit would be at 1000 AU.
IIRC Anthony and Rancke are big advocates of the actual "tidal force", which is the differential of the gravity field (ie the gravity gradient) at a given distance. Personally I'm a big advocate of using gravity alone, since it's easier to figure out. Either version gives you roughly similar results that are very different to the limit calculated by diameter.
Usually when people talk about using "tidal force" they mean that you define the jump limit as being the distance at which the local gravity (or rate of change of gravity) is below a certain value. In practice it means that way a one solar-mass white dwarf's jump limit would be the same as a one solar-mass main sequence star's jump limit which would be the same as a one solar-mass red giant star's jump limit, even though the WD has a much smaller radius and the red giant has a much larger one.
In some cases (e.g. supergiants like Antares) it means that the gravity-based jump limit is actually inside the star, which means a ship can come out of jump anywhere outside it - and for Antares it makes a big difference, since canonically its jump limit would be at 1000 AU.
IIRC Anthony and Rancke are big advocates of the actual "tidal force", which is the differential of the gravity field (ie the gravity gradient) at a given distance. Personally I'm a big advocate of using gravity alone, since it's easier to figure out. Either version gives you roughly similar results that are very different to the limit calculated by diameter.
Andrew Boulton
The Adminator
The JTAS essay actually said,
For gameplay, 100d works much better than gravity simply because for most objects you know the diameter and you don't know the gravity.
M
Malenfant
Guest
For gameplay, 100d works much better than gravity simply because for most objects you know the diameter and you don't know the gravity. </font>[/QUOTE]But that makes no sense at all, because those "specific limits based on size, density and distance" aren't the same at 100D. If you go 100 diameters from a variety of objects of different sizes and masses and densities, there's nothing one object's limit has that is in common with another's - local gravitational field strength is different there, and any other fields would be different too.
Also, two objects with the same size and different densities would still have the same 100D limit, so it's obviously not based on size and density and distance.
It may be canonical but it makes no physical sense whatsoever.
And actually for Stars you usually know the mass as well as the radius. For planets all you know is the size though (without further calculation, for which you need to know the density too)
But tidal force gives you the same effect for all bodies witrh the same density (i.e. if the jump limit for a 1,000 miles world with a density of 5.5 is 100,000 miles, then the jump limit for an X thousand miles world with density 5.5 will be X hundred thousand. That is, the effect described in the rules. So it would be easy to explain the old rule as a simplification. After all, the density of most terrestrial worlds hover in the same neighborhood.
Granted, you'd get two other figures for stars and for gas giants, but how many published Traveller adventures actually use the solar jump limit? Gas giants... yes, there are some published star systems where it would make a difference. But the Regina system is stuffed up and in need of a retcon anyway
.Hans
Border Reiver
SOC-14 1K
It's a game. Not a Science Fair project.
I'm NOT going to spend time calculating a planets mass, density & radius and then calculating how far out its gravity will have an effect on other bodies.
The 100D was quoted as being a rule of thumb. It works for gameplay, is simple and that's all. End of.
I'm NOT going to spend time calculating a planets mass, density & radius and then calculating how far out its gravity will have an effect on other bodies.
The 100D was quoted as being a rule of thumb. It works for gameplay, is simple and that's all. End of.
M
Malenfant
Guest
And yet the same people who complain about this would probably gladly spend time calculating in-system travel times, working through complex character generation, designing vehicles etc.
You don't even need to calculate anything, I'm pretty sure there are tables out there (if I didn't actually put them up myself) showing rough masses for planets of a given size.
Thank you all:
That was close to what I suspected it meant.
For reasons of basic simplicity, I tend to just use the target world in my game to determine the jump limit – the game benefit of the extra realism seldom seemed worth the extra work. Of course, that is just my game.
I could see a supplement that went into that level of detail for those interested, but I would stick to a simple rule of thumb for the base rules. I might be willing to accept a three tiered rule of thumb like 100 diameters for a rock world, 50 diameters for a gas giant, and 1 AU per solar mass for a star (or whatever the exact values might be). That would seem the upper limit on complexity for a basic set of rules (perhaps a sidebar to the 100-diameter rule).
Again, thank you all.
[POST EDITED BASED ON NEW DATA]
That was close to what I suspected it meant.
For reasons of basic simplicity, I tend to just use the target world in my game to determine the jump limit – the game benefit of the extra realism seldom seemed worth the extra work. Of course, that is just my game.
I could see a supplement that went into that level of detail for those interested, but I would stick to a simple rule of thumb for the base rules. I might be willing to accept a three tiered rule of thumb like 100 diameters for a rock world, 50 diameters for a gas giant, and 1 AU per solar mass for a star (or whatever the exact values might be). That would seem the upper limit on complexity for a basic set of rules (perhaps a sidebar to the 100-diameter rule).
Again, thank you all.
[POST EDITED BASED ON NEW DATA]
Constantine, just as a matter of curiosity, how much of a difference would going by tidal force mean for worlds with densities +/- 1 of Earth normal -- i.e. worlds with densities from 4.5 to 6.5? (That's assuming that the 100 diameter figure corresponds to a density of 5.5).
Hans
Border Reiver
SOC-14 1K
Yup, that's me.
M
Malenfant
Guest
In fact, check out this thread:
Jump masking based on gravity
It's got a table that I made that shows exactly this.
Jump masking based on gravity
It's got a table that I made that shows exactly this.
I have a somewhat related question. The gravity of a planet or star is calculated from the center of mass. If a planet was composed of loose pebbles, its gravity would still be calculated from the center of mass. Has anyone ever bothered to figure out what the Jump shadow is for the entire galaxy? How close to the center of the galaxy can you jump?
I was just curious if anyone else had ever wondered about that and bothered to figure it out.
I was just curious if anyone else had ever wondered about that and bothered to figure it out.
M
Malenfant
Guest
In fact, I think I just realised what's been bugging me about this "tidal force" thing ever since I first heard of it... it isn't actually tidal force at all.
The strength of a local gravity field is GM/R^2. But I don't think that GM/R^3 - which is used to determine this so-called "tidal force" - actually has any physical meaning in and of itself. What you're actually trying to find out using that formula is just the rate of change of the gravitational field strength at a given distance from a mass (i.e. the differential of GM/R^2 - which is actually equal to -2GM/R^3).
If it really is "tidal force" then you'd also need to know the size of the spacecraft because the actual tidal stress on an object depends on the distance between two points from the centre of gravity. I'm a bit rusty on this, but I think that to calculate this you do need to use GM/R^3 but you need to calculate the difference between one end of the object and the other (using two different values of R).
So what you're really after here if you use GM/R^3 on its own is something akin to the rate of change of the local gravitational field, which in itself doesn't really have any meaning. If you said "the value of (GM/R1^3 - GM/R2^3) must be less than a certain value" (where R1 and R2 are the distances of each end of the object from the central mass) then that would have more physical meaning, because that is saying the tidal differential across the object has to be below a certain value for it to be able to jump. But that's a lot more complex to calculate, which is why I prefer sticking to the straight gravitational field strength to determine the jump limit.
The strength of a local gravity field is GM/R^2. But I don't think that GM/R^3 - which is used to determine this so-called "tidal force" - actually has any physical meaning in and of itself. What you're actually trying to find out using that formula is just the rate of change of the gravitational field strength at a given distance from a mass (i.e. the differential of GM/R^2 - which is actually equal to -2GM/R^3).
If it really is "tidal force" then you'd also need to know the size of the spacecraft because the actual tidal stress on an object depends on the distance between two points from the centre of gravity. I'm a bit rusty on this, but I think that to calculate this you do need to use GM/R^3 but you need to calculate the difference between one end of the object and the other (using two different values of R).
So what you're really after here if you use GM/R^3 on its own is something akin to the rate of change of the local gravitational field, which in itself doesn't really have any meaning. If you said "the value of (GM/R1^3 - GM/R2^3) must be less than a certain value" (where R1 and R2 are the distances of each end of the object from the central mass) then that would have more physical meaning, because that is saying the tidal differential across the object has to be below a certain value for it to be able to jump. But that's a lot more complex to calculate, which is why I prefer sticking to the straight gravitational field strength to determine the jump limit.
Andrew Boulton
The Adminator
I was just going to suggest that. I think it's a good compromise.
Andrew Boulton
The Adminator
Yup, that's me.
M
Malenfant
Guest
Yup, that's me.
Border Reiver
SOC-14 1K
Tables work for me. Mostly.
Mal, I like the tables you came up with, but I'd like to see them nicely formatted in a web page and perhaps in a PDF. Would either you be interested in doing this or would you mind if someone else did it? (And if you want explicit credit in a footnote or endnote, let me know the particulars). You can reach me off-forum at kaladorn at gmail dot com.
I like the idea that in some systems, the stars will make jump limits quite distant, forcing people to jump in and move towards the mainworlds. Also, if you do in-system hopping towards the inner system, a 'jump exit' might become very hazardous, so travelling in the inner system might be considered more perilous.
Also, how do you handle binary or trinary systems? Do you just add the gravitational effects from each of the stars? I assume that a point exists between two binaries where the gravitational attraction balances, should I not be able to jump there? Is there some sane way to simplify the math or physics here to produce a table or a formula with a variable or two in it?
I'd like to turn this into a short treatise of some sort that covers the sorts of gaming situations. For those who say 'DO NOT WANT!', that's fine for them. But to me, Traveller is (a few mcguffins aside) a hard-sci game and I would love to use an answer to the jump limit that has some sort of physical interpretation.
I like the idea that in some systems, the stars will make jump limits quite distant, forcing people to jump in and move towards the mainworlds. Also, if you do in-system hopping towards the inner system, a 'jump exit' might become very hazardous, so travelling in the inner system might be considered more perilous.
Also, how do you handle binary or trinary systems? Do you just add the gravitational effects from each of the stars? I assume that a point exists between two binaries where the gravitational attraction balances, should I not be able to jump there? Is there some sane way to simplify the math or physics here to produce a table or a formula with a variable or two in it?
I'd like to turn this into a short treatise of some sort that covers the sorts of gaming situations. For those who say 'DO NOT WANT!', that's fine for them. But to me, Traveller is (a few mcguffins aside) a hard-sci game and I would love to use an answer to the jump limit that has some sort of physical interpretation.
shadowdragon
SOC-12
gravity equation:
F~(M1*M2)/d^2
M=mass
d=distance from centerpoint to centerpoint
not sure if the one posted earlier works the same as this one or even is the same one, didnt look quite right to me as i glanced thru.
F~(M1*M2)/d^2
M=mass
d=distance from centerpoint to centerpoint
not sure if the one posted earlier works the same as this one or even is the same one, didnt look quite right to me as i glanced thru.
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