Freelance Traveller

SGGs, LGGs, and Fuel Skim Runs

by Ken Pick

(Some of my ramblings regarding Gas Giants.)

At my first SF Con (NASFiCNorth American Science Fiction Convention. NASFiC is held in any year in which WorldCon (the World Science Fiction Convention) is not held in North America 1975), I first heard of a planetology book called Habitable Planets for Man (1964) [HPFM, henceforth] by Stephen H Dole of the RAND Corporation/think tank A couple months later in late 1975 I discovered a hardcopy in the library stacks at Cal Poly PomonaFormally, California State Polytechnic University, Pomona. It is part of the California State University system and used it for my first attempts at planetology and system design. Then Wayne Shaw (who I’d met at NASFiC) introduced me to D&D (at the time, the only FRP game in town).

A year and a half later came Traveller, again by the hand of my regular Game Master Wayne; the first time I leafed through Book 3, I noticed some Dole influence in the planet-generation rules (especially the Atmo & Hydro rolls). As HPFM was state-of-the art at this time and had this apparent link to Traveller, I have used it as a guide for “Old School” Traveller planetology ever since.

Traveller divides Gas Giants into Small Gas Giants (SGGs, now called “Ice Giants”) like Uranus and Neptune, and Large Gas Giants (LGGs) like Jupiter and Saturn. But what is the dividing line between SGG and LGG?

My answer comes from a graph on Page 34 of HPFM (reproduced below). The graph shows increasing density with mass until 3-4 Earth masses, then decreasing density with increasing mass until around 50-60 Earth masses, then increasing density with mass afterwards.

• The 3-4 Earth mass transition is the (admittedly fuzzy) border between a rockball/iceball and a Gas Dwarf. Depending on system conditions and orbits, you can have a Super-Earth or Hycean or Gas Dwarf through this transition zone. Planets in this transition zone are profiled in “Between Size A and SGG”, Freelance Traveller #108, Nov/Dec 2021.
• The border between Gas Dwarf and Small Gas Giant (SGG) is an arbitrary 10-12 Earth masses; above this size you will have a gas planet.
• The 50-60 Earth-mass transition marks the border between SGG and LGG; SGGs’ density decreases as mass increases, LGGs’ density increases with mass. For Traveller purposes, I place this border at 60 Earth-masses.
• For Traveller purposes, I have added a split between Large Gas Giant (LGG) and Very Large Gas Giant (VLGG) at 600 Earth-masses (two Jupiters, 10× the SGG/LGG transition), the point where surface gravity becomes too great for fuel-skimming.

One of the peculiarities of this mass-density relationship is that unlike rockballs, diameters will vary differently than mass depending on the upward or downward trend of the curve. This has effects on surface gravity.

Rockballs: In Traveller, all rockballs are assumed to have the same density:

• Diameter increases as the cube root of the mass. This gives a surface gravity proportional to diameter:
• Maneuver-1 ships cannot land/take off from a world above Size 8 (i.e. larger than Earth).
• At the 3-4 Earth-mass transition, surface gravity would be between 1½ and 2g.

On the downward SGG curve, diameter increases as the square root of the mass:

• 20× Mass (Neptune) = 4× Diam, Traveller Size 32
• 30× Mass = 5× Diam, Traveller Size 40
• 40× Mass = 6× Diam, Traveller Size 48
• 50× Mass = 7× Diam, Traveller Size 56
• 60× Mass = 8× Diam, Traveller Size 64
• At the 50-70x Mass transition (the “sweet spot”), surface gravity is actually under 1g, allowing even Maneuver-1 ships to skim safely.

On the upward LGG curve, diameter continues to increase but slowly until around Jupiter-size (300 Earth-masses) where it stops increasing with mass. At that size, the more mass, the more its own gravity crushes that mass together until the mass (and surface gravity) increases but not the diameter.

• 100× Mass (Saturn) = 9× Diam, Traveller Size 72
• 200× Mass = 10× Diam, Traveller Size 80
• 300× Mass (Jupiter) = 11× Diam, Traveller Size 88
• Over 300 = 11-12× Diam (diameter stays constant all the way up to M9v red dwarf).

Note that all these Gas Giant diameters and densities assume a Hab-to-Outer Zone solar flux. Diameters for Inner Zone GGs will be somewhat larger than the tables above due to thermal expansion from sunheat; a red-hot Gas Giant in a “sungrazer” torch orbit will be a “poofy planet” with a much larger diameter than in the tables.

Implications for Fuel Skimming

or, Just Because a System Has a Gas Giant Doesn’t Mean You Can Skim It

A subject of importance to cheapskate Free Traders

Not all Gas Giants can be skimmed for fuel; the “Gas Giant” rating on the UWP should properly be “skimmable Gas Giant(s) insystem”.

The ideal Gas Giant for skimming should have a relatively calm atmosphere, minimal contamination of other gasses than hydrogen, and a surface gravity lower than your ship’s Maneuver Drive rating. All these are functions of size.

Surface Gravity

The best skimmed fuel in the Galaxy won’t help if you can’t boost out of the Gas Giant’s gravity well. A ship’s Maneuver Drive must be equal or greater to surface gravity, and a Gas Giant’s size determines its surface gravity. Unlike rockballs, this is not a simple “the bigger the world, the more you weigh”.

On the downward SGG curve, surface gravity is between 1g and 1½g below 50 Earth masses, 1g at 50, and a little less than 1g at 60 (the point of minimum density):

• 10× Mass (Gas Dwarf) = between 1g and 2g; Maneuver-2 required
• 20-50× Mass = a bit over 1g; Maneuver-2 required
• 50× Mass = 1g; Maneuver-1 possible, but no safety margin
• 60× Mass = 0.8g; Maneuver-1 OK with small safety margin

On the upward LGG curve, surface gravity averages 0.8g for every 100 Earth masses at 200 and up (Saturn-sized LGGs are still on the transition curve up from minimum density); round this up to the next whole number for the Maneuver drive needed to skim and get away:

• 100× Mass = 1g; Maneuver-1 possible but no safety margin (Saturn)
• 200× Mass = 1.6g; Maneuver-2 required
• 300× Mass = 2.4g; Maneuver-3 required (Jupiter)
• 400× Mass = 3.2g; Maneuver-4 required
• 500× Mass = 4g; Maneuver-4 possible, but no safety margin
• 600× Mass = 4.8g; Maneuver-5 required (border between LGG & VLGG)
• 700× Mass = 5.4g; Maneuver-6 required (VLGG)
• 800× Mass = 6.4g; Forget It (VLGG deathtrap)
• Brown Dwarf = over 30g
• M9v Red Dwarf = over 100g (Trappist-1)
• There is a reason for robotic/unmanned fuel lighters in LGG systems.

The mass range of around 45-50 to 70-75× Mass straddling the SGG/LGG boundary is called “the sweet spot”; here a Maneuver-1 ship can boost clear with a safety margin.

Calm Atmosphere

Gas Giants are known for their violent atmospheres, supersonic winds, mega-storms that can slap a ship around and mega-lightning that can EMP unshielded electronics.

This is largely a factor of heat, whether external (from the sun) or internal (from within the planet itself).

• The closer to the sun, the more violent the atmosphere from external (sun) heat. Habitable Zone GGs have more violent atmospheres than Outer Zone GGs and Inner Zone GGs are guaranteed to have a violent atmosphere.
• The larger the Gas Giant, the more violent the atmosphere from internal heat. This often manifests itself in visible cyclonic storms pockmarking the planet, i.e. the “Jupiter with Smallpox” appearance common to VLGGs.
• Both these factors combine to influence how violent the atmo can get. The best chance for calm atmo is an Outer Zone GG between Neptune and Saturn in size.

Contaminated Atmosphere

There is a raw fuel worse than Unrefined Fuel: Contaminated Fuel. Atmospheres of Gas Dwarves and the smallest of SGGs tend towards high levels of helium and heavier gases, as they have not accreted enough hydrogen to dilute the contaminants. The sun’s overall “metallicity” (proportion of elements other than H & He) is also a factor; the higher the sun’s metallicity, the higher the planets’ atmospheres’ in general. And this Contaminates the skimmed raw fuel.

• Contaminated Fuel has twice the bad effects of Unrefined Fuel on a ship’s Power Plant and Jump Drive. Double the effect of Unrefined Fuel for Misjump/Catastrophic Misjump/Powerplant Failure.
• Contaminated Fuel refines into Unrefined Fuel, with all the effects of Unrefined. This Refined-Contaminated Unrefined requires a second refining pass to become Refined Fuel.
• There is also a possibility of even worse fuel: Severely Contaminated Fuel, one step beyond Contaminated which refines into Contaminated. Increase the effects for such higher levels of contamination and require additional refining passes. There may be a limit where refining is no longer effective, and the best the refinery can do is Unrefined Fuel. Do you feel lucky today?

Once again, just because there’s a Gas Giant insystem doesn’t mean you want to skim it. The UWP notation for the system should read “skimmable Gas Giant(s)”.