GT: Far Trader: Jump Masking

[Malenfant]

Originally posted by Jamus:
the trick is to find a happy medium between hard-science and playability<word?> first and formost Traveller should be fun and sadly for most people doing advanced mathematics is not really fun.

It's not "advanced mathematics" if the numbers are precalculated and provided on a table.

 

[far-trader]

Just curious Malenfant, not trying to push your buttons, but I wonder how you rationalize calculating a specific gravity based limit on a however defined density when the Size is a rather loose and wide 2d6-2 roll for diameter in thousands of miles?

By the way has the "representing planetary size" as "the diameter... stated in thousands of miles" bugged anybody else much. I mean being that the game went out of it's way at the time to be metric, here was one big abstraction that relied on imperial measure. It always bugged me and made it awkward. Sure the table gives the conversion but couldn't it have been done strictly metric to start with? I dunno, just one of those things I've wondered about off and on over the years. Maybe someone knows the answer. I've always kind of thought that a Size code = radius in thousands of kilometers looked better but I never sat down to figure if it would work well.

 

[Malenfant]

OK... this is interesting.

Take the Earth. If we use a Gravity Limit of 0.01m/s2, the Jump Limit is around 200,000km from the centre of the planet.

If we take the OTU 100D limit, the 100D Limit is at 1,275,600 km from the centre of the planet.

If we calculate the Gravity Field for Earth at the 100D limit (GM/R^2) we get 0.000245 m/s2. This is a Small Number (smaller than the 0.01 m/s2 I've been using previously).

If we calculate the "Tidal Force" (GM/R^3) for Earth at the 100D limit, we get a even smaller number of 1.93e-13 (erm... I think the units are m/s3? I'm really rusty on this). This is a Very Small Number, which we shall call F.

If we take the gravity strength at Earth's 100D and the Tidal Force at Earth's 100D and use those to calculate the "Gravity Limit" and the "Tidal Limit" for each object by using D = SQRT(GM/g) and D = CUBERT(GM/F), then we get the following table (all distances are in km). So now you can directly compare 100D, a grav jump limit of 0.01m/s2, a grav jump limit of 0.000245m/s2 and a tidal force limit of 1.92e-13.

</font><blockquote>code:</font><hr /><pre style="font-size:x-small; font-family: monospace;"> m r 100d g (0.01) g (0.000245) F-limit
Moon 0.01e 1738 347600 19966 127557 274820
Mercury 0.05e 2439.7 487940 44645 285225 469935
Mars 0.1e 3390 678000 63137 403369 592081
Venus 0.8e 6052 1210400 178579 1140901 1184163
Earth 1e 6378 1275600 199658 1275566 1275601
Neptune 15e 24624 4924800 773271 4940247 3145902
Uranus 15e 25362 5072400 773271 4940247 3145902
Saturn 100e 58232 11646400 1996577 12755663 5920814
Jupiter 1j 69911 13982200 3559287 22739447 8704945
Sol 1s 696265 139253000 115205847 736023156 88411153
Antares 15s 750000000 1.5E+11 446190326 2850605425 218040653</pre>[/QUOTE]Hopefully this is correct, I'm in a bit of a rush so I can't check this. But although Anthony has said that using a Tidal Limit gives results close to the 100D limit, I'm not seeing that here. Maybe he can elaborate on how he figures that (or point out where I've done something wrong

).

 

[Malenfant]

Originally posted by far-trader:
[QB] Just curious Malenfant, not trying to push your buttons, but I wonder how you rationalize calculating a specific gravity based limit on a however defined density when the Size is a rather loose and wide 2d6-2 roll for diameter in thousands of miles?

Not sure I follow you. The gravity limit is based on the object's mass, not density. It doesn't matter if it's a 10,000 km wide ball of low density ice or a 1,000 km wide ball of rock and metal with the same density, the mass is the same, and so is the gravity.

By the way has the "representing planetary size" as "the diameter... stated in thousands of miles" bugged anybody else much. I mean being that the game went out of it's way at the time to be metric, here was one big abstraction that relied on imperial measure. It always bugged me and made it awkward. Sure the table gives the conversion but couldn't it have been done strictly metric to start with? I dunno, just one of those things I've wondered about off and on over the years. Maybe someone knows the answer. I've always kind of thought that a Size code = radius in thousands of kilometers looked better but I never sat down to figure if it would work well.

It's annoyed me a bit. What annoys me more is that we're given a diameter, rather than a radius.

 

[Anthony]

Very early editions of Traveller were not metric. Those 1.5 meter squares used to be 5' squares, those 14 cubic meter dtons used to be 500 cubic feet, that 3 meter deck height was 10'. Most of the non-metric aspects have been hidden or masked over the years (can you actually think of any sensible reason, for pure metric, to not use 1 meter squares for mapping?), but it's hard to hide the 1600 km per size modifier.

 

[far-trader]

Originally posted by Malenfant:
</font><blockquote>quote:</font><hr />Originally posted by far-trader:
[QB] Just curious Malenfant, not trying to push your buttons, but I wonder how you rationalize calculating a specific gravity based limit on a however defined density when the Size is a rather loose and wide 2d6-2 roll for diameter in thousands of miles?

Not sure I follow you. The gravity limit is based on the object's mass, not density. It doesn't matter if it's a 10,000 km wide ball of low density ice or a 1,000 km wide ball of rock and metal with the same density, the mass is the same, and so is the gravity.</font>[/QUOTE]Ah, right, brain melt on my part. The question is silly, ignore it, and thanks for the wake

It's annoyed me a bit. What annoys me more is that we're given a diameter, rather than a radius.

Quite, I never really thought that made much sense either hence my thought towards correcting that and the units.

 

[far-trader]

Originally posted by Anthony:
Very early editions of Traveller were not metric. Those 1.5 meter squares used to be 5' squares, those 14 cubic meter dtons used to be 500 cubic feet, that 3 meter deck height was 10'. Most of the non-metric aspects have been hidden or masked over the years (can you actually think of any sensible reason, for pure metric, to not use 1 meter squares for mapping?), but it's hard to hide the 1600 km per size modifier.

Truly? Like 1st edition books 1-3? I know we had some in our group, at least I thought we did, but I don't recall that.

As you say it'd be odd to list it as 1600km diameter per size digit. I guess they didn't think 1000km radius per size digit made sense or it just didn't occur to them when converting the rest. Or maybe redoing the tables of travel times and such seemed too much work. It would make Earth only a slightly over average size planet (size 6 rather than size 8) and allow for more larger worlds making Heavy Gravity skills a little more useful. It would also make the small worlds not quite so small though only marginally. But then maybe it's just me thinks that would all be fine

 

[mike wightman]

Yep, who can forget the ease with which you can convert backwards and forwards on a scale of 1 inch equals 1 thousand miles
And ship combat turns were only 10 minutes long.

However, this all shows that the math can be done in the background and all that's needed in the game are the tabulated results, as Mal said earlier.

 

[far-trader]

Hmm, the memory fails in curious fashion. I remember 10 minute turns but not thousand mile scales

 

[Jamus]

does the gravity well of a planet really matter? seems like the stars gravity well or tidal influence or what ever would supercede any of the closer and most likely inhabited planets in a system.

 

[Anthony]

I remember thousand mile scales. If you accelerate at 1g for 10 minutes, the actual displacement is 1,097 miles, but it was rounded down to 1,000 miles.

 

[Malenfant]

Originally posted by Jamus:
does the gravity well of a planet really matter? seems like the stars gravity well or tidal influence or what ever would supercede any of the closer and most likely inhabited planets in a system.

Well, looking at the table I posted earlier, Sol's limits are (in AU):

100D: 0.93 AU
g-limit (0.01m/s2): 0.77 AU
g-limit (0.000245m/s2): 4.9 AU
Tidal Limit (1.92e-13): 0.59 AU

Remember, the 0.01 used in the first gravity limit was just an arbitrary number I picked.

The 0.000245 m/s2 gravity limit was picked because that's what earth's gravity field was at its 100D limit. Though in retrospect, since the only thing that is going to have a gravity of that strength at that distance is a planet with one earth mass, it's actually not a particularly relevant number to use as a limit.

Ditto for the tidal limit - the 1.92e-13 value is just what earth's tidal force is doing at the 100D distance. Other bodies with different masses aren't going to have the same value of tidal force at their 100D limits.

The only reason I used the value at earth's 100D limit was to have something to anchor the results onto the OTU 100D limits, but as you can see, the numbers diverge around that - the limits calculated for other bodies aren't close to the OTU 100D limits at all.


Personally, I feel it's better to use the gravity field strength as a limit rather than tidal force. The gravity strength is a more obvious quantity to calculate for a start, and something more generally meaningful to readers. It's also clear to me that it's not really useful to tie it to the OTU 100D distances in any way. A gravity limit of about 0.01 m/s2 (or maybe down to 0.005 m/s2) seems to get numbers that require a fair distance to travel across, that are very roughly comparable to the OTU 100D scales.

 

[Anthony]

The problem with the gravity strength is that for a planet beyond the jump limit of the primary, the planetary gravity limit is a very short distance -- 65,000 km (5 diameters) for earth, for example. That's dramatically shorter than the OTU distance.

 

[Malenfant]

Originally posted by Anthony:
The problem with the gravity strength is that for a planet beyond the jump limit of the primary, the planetary gravity limit is a very short distance -- 65,000 km (5 diameters) for earth, for example. That's dramatically shorter than the OTU distance.

Yes. Well, them's the breaks

(not sure where you're getting the 65k from though, I got Earth's gravity limit to be at 200k from the centre of the planet)

You can't have it both ways it seems. If you tweak the limit so that Earth's compares to its OTU 100D, the star limits get correspondingly huge. If you want the stars' limits to compare to the OTU 100D limit, then the planetary limits get correspondingly small.

Personally, I'd say screw the OTU limits and just pick a gravity value between 0.01 m/s2 and 0.005 m/s2 and use that. If it's 200k for the Earth, then so be it. While the Tidal limits are a little bit closer in order of magnitude (depending on what limit you pick; 1.92e-13 is a somewhat obscure value) it's an even more abstract concept to deal with than a gravity field. Sure, things will be different, but at least they'll be consistent.

 

[Anthony]

Originally posted by Malenfant:

You can't have it both ways it seems.

Sure you can. You just can't base it directly on the acceleration of gravity.

Incidentally, it's impossible for a ship in freefall to measure the acceleration of gravity with sensors strictly internal to the ship -- you can only measure it by looking at objects outside the ship. The tidal effect, however, can be measured directly.

 

[Malenfant]

Well, what value would you use for the tidal limit? The one I'm using? Or a different one?

And I'm not sure you're right about the inability to measure gravity inside the ship. We have gravimeters today after all (they're used for geological gravity surveys), they basically consist of a little widget connected to a sensitive spring IIRC, that measures the downward acceleration. Plus a Traveller ship has gravity sensors anyway, doesn't it?

 

[Anthony]

The reason you can measure gravity on the surface of a planet is because you are being supported by the planet. In free fall, the apparent strength of gravity is always zero.

 

[Jamus]

why not implememt this by stating that jumps take place at jump points outside the system and that ships would then spend time burning the m-drive to the target world.

 

[Malenfant]

Originally posted by Jamus:
why not implememt this by stating that jumps take place at jump points outside the system and that ships would then spend time burning the m-drive to the target world.

And how would this be different from the OTU method? This is what's already done - you need to spend X time going to/from the jump distance before you can jump out.

Or do you mean something like the Battletech approach? IIRC in Battletech you could only jump into/out of a system through two points, one above the star's north pole and one above the star's south pole. The distance depended on the star type.

 

[far-trader]

One problem I see with adopting a required stellar polar jump point is the Solomani discovery of jump. They serendipitously discover it while experimenting with a new manuver drive or something in the outer reaches of the solar system, not over the stellar pole. So going this route would require a rewrite of that.

As for the value of any gravity or tidal limit, since we are talking new values, why not make the whole thing a little more granular, rules wise.

Edit: Well that was my idea, but then it broke down going to the table

Three values for each Size code is enough to tabulate.


Make jump entry smooth and safe (no DM) at 0.001 m/s2 and then increase the danger as you increase the gravity influence (just some numbers out of the air, not sure what GURPS numbers to propose, and the others may be off some too):

</font><blockquote>code:</font><hr /><pre style="font-size:x-small; font-family: monospace;"> m/s2 d20 DM d6 DM

0.001 -0 -0
0.050 +5 +5
0.100 +15 +10</pre>[/QUOTE]