Gravity: Hg Lg Codes?

[maksimsmelchak]

Gravity: Hg Lg Codes?

*** Are there codes for low gravity and high gravity worlds? ***

Or are they entirely assumed by planet size irrespective of planetary core type or density... Just curious. Don't remember reading that on my last go-through of T5.

Thank you.

Shalom,
Maksim-Smelchak.

 

[77topaz]

No, there are no such codes. Oc and Bw give High-G as a homeworld skill, though, so I guess it's assumed by planetary size irrespective of other factors.

 

[robject]

Yeah, we don't generate or record gravity. You can assume a range based on size and atmmosphere... and I think T5 has some guidelines... but no, no codes.

 

[kilemall]

If there was to be such a correlation, I would look at the size of the planet vs. atmosphere, the smaller the planet with a higher atmosphere number the more dense and thus higher G the planet is.

Or even cheat some more- an atmosphere 6/diameter 8 means 1 G, less atmosphere means less Gs and vice versa. Exotic corrosive or insidious doesn't track on this or deal with multipliers like Jupiter's immense pressures, just a quick and dirty for most rocks.

The 4:3 ratio for diameter to atmosphere Earth baseline could also be used to yield whether a planet is metal-rich or poor. A 1:1 planet might be VERY metal dense, while something like the moon's 2:0 is 'lighter' and therefore not as heavy in metals.

Of course this all skips the magnetosphere, closeness to stars or large planets or tidal locks or other atmo phenomena, not to mention the effects of bio-induced atmospheric changes, but I'm looking for a quick and dirty for most adventuring.

 

[G. Kashkanun Anderson]

There is a very basic correlation between world size and gravity outlined in LBB-2 (Weight and Burden Section). World size 8 is 1G at the surface, and each Size above or below it increases or decreases the surface gravity by 12.5%. It specifically assumes an Earthlike density, and talks about applying a constant discussed in LBB-3 to account for different densities, but that constant is never mentioned anywhere else in the books (not even LBB-3 -- or at least not the first-run cuneiform tablet edition that I own).

The 12.5% rule is a decent-ish kludge for world sizes 6-A, but it breaks down pretty quickly for anything world size 5 or lower. Assuming Terrestrial densities, it would be more accurate to simply divide the planet's world size number by 8 and call that the surface gravity in G's. Maybe apply a DM of -1 first if you want to account for worlds with Martian or Lunar densities.

 

[77topaz]

If there was to be such a correlation, I would look at the size of the planet vs. atmosphere, the smaller the planet with a higher atmosphere number the more dense and thus higher G the planet is.

I don't think that is quite correct. Gravity is proportional to the mass of the planet, and, generally, the larger a planet is, the more massive it is. Thus, the larger the planet, the higher the G.

The mass of a planet of course also depends on the planet's density, and the density of a planet's atmosphere may correlate to the density of the planet itself, but it is certainly not the case that, for a given atmospheric density, one would expect a smaller planet to have higher gravity - rather the opposite.

 

[MichaelSTee]

I'm thinking of using a code based on gravity for my world-gen design.
My thoughts: First, I use the gravity table from Mongoose, NOT from GDW. After rolling for size, roll 2D6. If the roll is a 2, the planet has the gravity of a world one size code smaller. If the roll is a 12, use one world size larger. (So if a world is size 8, the gravity is code 8, which is 1.0g. Then roll for density changes: If the roll is a 12, the gravity becomes code 9, which is 1.25g.) If the roll is a 2 or 12, you can optionally roll again for a further +/- 1.
Then use a code Lg or Hg to note the change in expected gravity. This code also gives a +/- 1 to the atmo code, and to resources (higher gravity means more minerals...).

I'm still working on my world-gen design, but then I have been for a long time; it might never get finished. Such is the nature of procrastination But I'd like to see the possibility of large worlds with Earthlike gravity, like in Jack Vance's Big Planet.

 

[kilemall]

I don't think that is quite correct. Gravity is proportional to the mass of the planet, and, generally, the larger a planet is, the more massive it is. Thus, the larger the planet, the higher the G.

The mass of a planet of course also depends on the planet's density, and the density of a planet's atmosphere may correlate to the density of the planet itself, but it is certainly not the case that, for a given atmospheric density, one would expect a smaller planet to have higher gravity - rather the opposite.

A bad expression on my part- I was trying to convey that in situations in which the regular correlation that results in our standard atmo at size 8 being 1 G, in order to accommodate a result in which say a size 5 planet still has a 6 atmosphere, the size 5 planet must be more dense to come close to 1 G then a 'normal' density 5 planet would have.

This came up rather early in my system rolling when Prometheus at Alpha Centauri ended up with a far better atmosphere then it's size would indicate. It ended up being a planet with it's rock mantle blown off and metal core exposed.

 

[shaunhilburn]

If there was to be such a correlation, I would look at the size of the planet vs. atmosphere, the smaller the planet with a higher atmosphere number the more dense and thus higher G the planet is.

For solid bodies, mass and radius are logarithmically coupled; they are not independent values. You can determine the scaling exponent for any solid body taking x = log(r)/log(m). In our solar system, this yields values of x between 0.24 and 0.3.

     Large icy moons      0.239 - 0.242    0.240   Callisto, Ganymede, Titan
     Superterrestrials    0.265 - 0.275    0.270
     Plutinos, KBOs       0.271 - 0.285    0.280   Triton, Pluto, Charon
     Icy moonlets         0.265 - 0.290    0.278   Dione, Rhea, Mimas, Iapetus
     Large Terrestrials   0.257 - 0.283    0.281   Mars, Earth, Venus
     Small Terrestrials   0.292 - 0.298    0.295   Io, Europa, Moon
     Asteroids            0.292 - 0.324    0.309   Pallas, Juno
     Mercury              0.300 - 0.350    0.332

Mercury is the outlier here because of its over-sized core.


I produced the following table using this principle. It shows the various formulae used for each column. Units are earth masses, earth radii, earth gravity where Earth = 1. The first column is the Traveller size digit, and the second is earth radii based on Size/7.9175, because Earth is not precisely Size 8.

Calculated mass m, density d, gravity g of silicate or carbon planets with FeNi or FeS cores and 32.5% core mass fraction.
Note: Icy planets with silicate cores have 33% to 66% of the calculated mass and gravity.

                         r³×2^(r-1)  2^(r-1)    r×2^(r-1)
             S/7.9175    r³×2^r/2    2^r/2      r×2^r/2    10/(m/r)  HZ
     Size S  Radius r    Mass m      Density d  Gravity g  MMWR    Atmosphere
     ------------------------------------------------------------------------
      0.2    0.0253      0.0000      0.5088     0.013                0         semi-spherical
      0.3    0.0379      0.0000      0.5133     0.019                0         smallest spherical bodies
      0.4    0.0505      0.0001      0.5178     0.026                0
      0.5    0.0632      0.0001      0.5224     0.033                0
      0.6    0.0758      0.0002      0.5270     0.040                0
      0.7    0.0884      0.0004      0.5316     0.047                0
      0.8    0.1010      0.0006      0.5363     0.054                0
      0.9    0.1137      0.0008      0.5410     0.062                0
      1.0    0.1263      0.0011      0.5457     0.069                0
      1.5    0.1895      0.0039      0.5702     0.108                0
      2.0    0.2526      0.0096      0.5957     0.150                0
      2.5    0.3158      0.0196      0.6223     0.197      159.6     1-
      3.0    0.3789      0.0354      0.6502     0.246      106.1     1-        retains sulfur hexafluoride
      3.5    0.4421      0.0587      0.6793     0.300       74.6     1
      4.0    0.5052      0.0915      0.7097     0.359       54.7     1, B+     retains sulfur dioxide
      4.5    0.5684      0.1361      0.7414     0.421       41.3     1, A+     retains carbon dioxide, argon, hydrogen sulfide
      5.0    0.6315      0.1951      0.7746     0.489       32.0     2+        minimum habitable mass, retains oxygen, acetylene, ethylene
      5.5    0.6947      0.2713      0.8093     0.562       25.4     2+        retains nitrogen, ethane
      6.0    0.7578      0.3679      0.8455     0.641       20.4     2+
      6.5    0.8210      0.4887      0.8833     0.725       16.6     2+
      7.0    0.8841      0.6377      0.9228     0.816       13.7     2+        retains methane, ammonia
      7.5    0.9473      0.8195      0.9641     0.913       11.4     2+
      8.0    1.0104      1.0391      1.0072     1.018        9.6     4+
      8.5    1.0736      1.3021      1.0523     1.130        8.2     4+
      9.0    1.1367      1.6148      1.0994     1.250        7.0     6+
  A   9.5    1.1999      1.9842      1.1486     1.378        6.0     6+
     10.0    1.2630      2.4178      1.2000     1.516        5.2     8+        superterrestrial planet
  B  10.5    1.3262      2.9241      1.2537     1.663        4.5     8+
     11.0    1.3893      3.5125      1.3098     1.820        3.9     8+        retains helium
  C  11.5    1.4525      4.1931      1.3684     1.988        3.4     8+
     12.0    1.5156      4.9774      1.4296     2.167        3.0     8+
  D  12.5    1.5788      5.8775      1.4936     2.358        2.7     B+        maximum terrestrial planet size
     13.0    1.6419      6.9073      1.5604     2.562        2.4     B+        gas giant core, chthonian planet

 

[maksimsmelchak]

No, there are no such codes.
Oc and Bw give High-G as a homeworld skill, though, so I guess it's assumed by planetary size irrespective of other factors.

77Topaz,

*** What is "Bw"? ***

Thanks!

Shalom,
Maksim-Smelchak.

 

[ovka]

*** What is "Bw"? ***

BW = Big World, though I'm not sure where it says that Bw is a trade code or grants Hi-G as a homeworld skill.

BigWorlds are worlds with larger than expected Size. Occasionally, a satellite Mainworld in a system without Gas Giants requires a BigWorld as its primary.

BW BIGWORLD

Size  Diameter in Miles
B 11  11,000
C 12  12,000
D 13  13,000
E 14  14,000
F 15  15,000
G 16  16,000
H 17  17,000
J 18  18,000
K 19  19,000

World Size B+ is Big World

 

[maksimsmelchak]

BW = Big World, though I'm not sure where it says that Bw is a trade code or grants Hi-G as a homeworld skill.

*** Is that Mongoose? ***

Shabbat Shalom,
Maksim-Smelchak.

 

[Nathan Brazil]

*** Is that Mongoose? ***

Shabbat Shalom,
Maksim-Smelchak.

It is in the T5.09 World Generation Rules. There is no trade code/remark, but is clearly posted in the mapping chart pages. World Sizes B+ are listed as Big Worlds (example pg. 453 amongst others). Well B thru K. Those are diameter of 11000-19000.
Size L - Y are Gas Giants with specific diameters listed ranging from 20000-250000. Brown Dwarfs are considered Size Y (250000 mi diameter).
Note with the new world generation rules, it is possible to roll worlds of Size B thru F now. G thru K, not thru random rolling.

 

[Nathan Brazil]

If you apply RL to the UWP of planet, could you infer the density of the world and thus the gravity by having the Atmosphere Type and Size? I have an idea.

In the original Traveller 2300, there was an item in world generation called Minimum Molecular Weight (MMW). It was derived from diameter and density. The value gave the lightest element that would be retained in the atmosphere in large quantiies. So....

Could not an estimated density (and thus gravity) be derived by reversing formulas? You have Atmosphere Types and Size. You would have to have a minimum density to retain oxygen to have atmosphere types 3-9 (which all contain oxygen in large quantities).

I see there were some examples of something like that in a previous post by shaunhilburn. What do you think.

 

[77topaz]

BW = Big World, though I'm not sure where it says that Bw is a trade code or grants Hi-G as a homeworld skill.

Magnus von Thornwood used it that way in his PbP. It may have been one of his house rules, but I'm not sure, you'd have to ask him.

 

[kilemall]

Shaun, you are telling me composition means nothing and raw volume is everything for gravity?

I have a bit of a problem with that.

 

[77topaz]

Shaun, you are telling me composition means nothing and raw volume is everything for gravity?

I have a bit of a problem with that.

I'm not sure where you get that idea. Shaun's post includes a scaling factor between mass and radius (this factor would be proportional to density) and his table includes a density column as well.

 

[maksimsmelchak]

Yeah, we don't generate or record gravity. You can assume a range based on size and atmmosphere... and I think T5 has some guidelines... but no, no codes.

Rob,

*** What do you think are the most important factors in determining gravity? ***

Shalom,
Maksim-Smelchak.

 

[maksimsmelchak]

Shaun,

Thanks for all of that awesome data!

For solid bodies, mass and radius are logarithmically coupled; they are not independent values. You can determine the scaling exponent for any solid body taking x = log(r)/log(m). In our solar system, this yields values of x between 0.24 and 0.3.

     Large icy moons      0.239 - 0.242    0.240   Callisto, Ganymede, Titan
     Superterrestrials    0.265 - 0.275    0.270
     Plutinos, KBOs       0.271 - 0.285    0.280   Triton, Pluto, Charon
     Icy moonlets         0.265 - 0.290    0.278   Dione, Rhea, Mimas, Iapetus
     Large Terrestrials   0.257 - 0.283    0.281   Mars, Earth, Venus
     Small Terrestrials   0.292 - 0.298    0.295   Io, Europa, Moon
     Asteroids            0.292 - 0.324    0.309   Pallas, Juno
     Mercury              0.300 - 0.350    0.332

Mercury is the outlier here because of its over-sized core.

I don't have the column header here.

*** What do these numbers mean? 0.265? And how do they relative to standard gravity ratings (e.g. 1.0G, which correlates to Earth at 0.257)? ***

Thanks!

I produced the following table using this principle. It shows the various formulae used for each column. Units are earth masses, earth radii, earth gravity where Earth = 1. The first column is the Traveller size digit, and the second is earth radii based on Size/7.9175, because Earth is not precisely Size 8.

Calculated mass m, density d, gravity g of silicate or carbon planets with FeNi or FeS cores and 32.5% core mass fraction.
Note: Icy planets with silicate cores have 33% to 66% of the calculated mass and gravity.

                         r³×2^(r-1)  2^(r-1)    r×2^(r-1)
             S/7.9175    r³×2^r/2    2^r/2      r×2^r/2    10/(m/r)  HZ
     Size S  Radius r    Mass m      Density d  Gravity g  MMWR    Atmosphere
     ------------------------------------------------------------------------
      0.2    0.0253      0.0000      0.5088     0.013                0         semi-spherical
      0.3    0.0379      0.0000      0.5133     0.019                0         smallest spherical bodies
      0.4    0.0505      0.0001      0.5178     0.026                0
      0.5    0.0632      0.0001      0.5224     0.033                0
      0.6    0.0758      0.0002      0.5270     0.040                0
      0.7    0.0884      0.0004      0.5316     0.047                0
      0.8    0.1010      0.0006      0.5363     0.054                0
      0.9    0.1137      0.0008      0.5410     0.062                0
      1.0    0.1263      0.0011      0.5457     0.069                0
      1.5    0.1895      0.0039      0.5702     0.108                0
      2.0    0.2526      0.0096      0.5957     0.150                0
      2.5    0.3158      0.0196      0.6223     0.197      159.6     1-
      3.0    0.3789      0.0354      0.6502     0.246      106.1     1-        retains sulfur hexafluoride
      3.5    0.4421      0.0587      0.6793     0.300       74.6     1
      4.0    0.5052      0.0915      0.7097     0.359       54.7     1, B+     retains sulfur dioxide
      4.5    0.5684      0.1361      0.7414     0.421       41.3     1, A+     retains carbon dioxide, argon, hydrogen sulfide
      5.0    0.6315      0.1951      0.7746     0.489       32.0     2+        minimum habitable mass, retains oxygen, acetylene, ethylene
      5.5    0.6947      0.2713      0.8093     0.562       25.4     2+        retains nitrogen, ethane
      6.0    0.7578      0.3679      0.8455     0.641       20.4     2+
      6.5    0.8210      0.4887      0.8833     0.725       16.6     2+
      7.0    0.8841      0.6377      0.9228     0.816       13.7     2+        retains methane, ammonia
      7.5    0.9473      0.8195      0.9641     0.913       11.4     2+
      8.0    1.0104      1.0391      1.0072     1.018        9.6     4+
      8.5    1.0736      1.3021      1.0523     1.130        8.2     4+
      9.0    1.1367      1.6148      1.0994     1.250        7.0     6+
  A   9.5    1.1999      1.9842      1.1486     1.378        6.0     6+
     10.0    1.2630      2.4178      1.2000     1.516        5.2     8+        superterrestrial planet
  B  10.5    1.3262      2.9241      1.2537     1.663        4.5     8+
     11.0    1.3893      3.5125      1.3098     1.820        3.9     8+        retains helium
  C  11.5    1.4525      4.1931      1.3684     1.988        3.4     8+
     12.0    1.5156      4.9774      1.4296     2.167        3.0     8+
  D  12.5    1.5788      5.8775      1.4936     2.358        2.7     B+        maximum terrestrial planet size
     13.0    1.6419      6.9073      1.5604     2.562        2.4     B+        gas giant core, chthonian planet

Best regards!

Shalom,
Maksim-Smelchak.

 

[Spartan159]

One of the things I love here is I am constantly learning something either directly or from being forced to go researching things to better understand something posted.