#RPG Rulings – Understanding Gravity in #TheExpanse Using #CepheusEngine #TravellerRPG

Season 5 of The Expanse TV series is streaming now, which means its that time of the TV season that I look once again at how I can take the hard-ish science fiction of James S.A. Corey and depict it in a roleplaying game campaign. This time I am focused on one foundational aspect of the setting that I took for granted before – gravity.

Getting Down with Gravity in The Expanse

Gravity in The Expanse is kept fairly realistic. The sources of gravity are what we expect from our current understanding of physics. Gravity is created by mass (planets, moons, planetoids, asteroids), spin habitats, or along the thrust vector of ships. Gravity is also a vital part of the cultures in The Expanse. From Belters who are tall and lanky from growing up in low-g but weak in normal gravity to Martian Marines who train at 1-g to be ready to fight on Earth, gravity is an important descriptor (discriminator?) between different factions. Gravity also has important impacts to space travel and combat; look no further than the need for “the juice” to withstand high-g acceleration or maneuvers or how it acts to limit human activities if one’s body is subjected to high-g stress for long durations (not to mention the catastrophic consequences of decelerating too quickly, yuck).

Courtesy pintrest.com

Rules of Gravity

I don’t presently own The Expanse Roleplaying Game from Green Ronin so I do not know the details of how they handle gravity in that setting. However, I am a long-time Traveller RPG player. These days I enjoy the modern incarnation of the Original 2d6 Science Fiction Roleplaying Game by using the Cepheus Engine rules set.

For my exploration of of gravity and The Expanse, I focused on five RPG rule sets or settings:

I really appreciate how all these various rule sets can work together to create an internally consistent, plausible setting. Although I use all these different rules sets and settings for reference, I will describe my rules interpretations primarily in Cepheus Engine terms for ease of integration across the various rules incarnations. My goal here is not to “science the sh*t” out of gravity in my RPG campaign, but to lean on a reasonable set of rules to provide good setting “flavor.”

Characters and Gravity

Characters in Cepheus Engine are described using three physical characteristics; Strength, Dexterity, and Endurance. The usual character generation method is to roll 2d6 for each characteristic. Each characteristic can range from 1 to 15 with 7 being a human average. Interestingly, when creating the physical characteristics of a character, there is no adjustment in the rules based on a low-gravity homeworld. Instead, an (assumed) low gravity homeworld grants certain default skills. Specifically, a homeworld with a Trade Code of “Asteroid” or “Vacuum” earn the Zero-G-0 skill (CESRD, p. 26)

The CESRD also has rules for alien species that are not specifically intended for human characters but I note them here as they may prove useful:

  • Notable (Characteristic): Some species are notably dexterous, intelligent, tough or strong. Characters from such races have a positive Dice Modifier when rolling for that characteristic (+2 unless otherwise specified), and their racial maximum for that characteristic is increased by the same amount. (CESRD, p. 44)
  • Weak (Characteristic): The opposite of Notable (Characteristic), some species are weaker, less resilient or less well educated than others. Characters from such races have a negative Dice Modifier when rolling for that characteristic (-2 unless otherwise specified), and their racial maximum for that characteristic is decreased by the same amount. (CESRD, p. 45)

The Clement Sector setting supplement Tree of Life: Altrants in Clement Sector defines an altrant as, “groups of humans which, from birth, have been altered thanks to advanced genetic manipulation, to have abilities different than a baseline human. These changes were most often made to allow humans to be able to perform tasks and live in environments which would be difficult or impossible for baseline humans.” If one ignores the “genetic manipulation” and instead views the change as “naturally evolved” then two Body Alterations found in this supplement may be useful:

  • Muscle Increase Package: Often referred to as the “Hercules” alteration, this procedure alters the body to make it as strong as it can possibly be. This alteration is the equivalent of years of strength training and will give the body the appearance of a successful bodybuilder. Taking this alteration gives the character +3 to their STR and -3 to their DEX immediately after the full alteration time period has passed. (Tree of Life, p. 45)
  • Vestibular System (Improved): This alteration improves the sensory system which provides the user with their sense of balance, spacial orientation, and balance. This gives the character a +2 DM to any physical task performed in gravity of less than 0.50 standard. However, the character will suffer a -2 DM to any physical task attempted in gravity of more than 1.25 standard (Tree of Life, p. 49)
Courtesy geekwire.com

The CESRD is also limited in what affect gravity has on characters. The skill “Zero-G” provides some guidance for actions in zero-gravity environments:

Zero G: The Character is acclimated to working and living in micro-gravity environments and freefall. The character is trained and familiar with the use of weapons and combat in such environments. In addition, the individual has been trained in the wearing, care, and maintenance of all types of Vacuum Suits and Combat Armor commonly used in these conditions.

CESRD, p. 57

[As an aside, looking back over the history of Traveller, the Zero-G skill, seemingly so foundational to a science fiction setting, has evolved in interesting ways. In Classic Traveller Little Black Book 1: Characters and Combat, one finds the skill Vacc Suit but not Zero-G. The CESRD has Zero-G but not Vacc Suit. T5 has both Vacc Suit (which it names as a Default Skill that all characters start with) AND Zero-G.]

Rules for “High and Low Gravity Worlds” are found in Chapter 12: Worlds of the CESRD:

High and Low Gravity Worlds: Worlds where the gravity is 0.75 or less are low-gravity worlds….Humans tend to find life on low-gravity worlds to be initially pleasant, but regular exercise regimes and medicinal supplements are required to prevent bone and muscle degradation. Those who spent too long on low-gravity worlds cannot tolerate higher gravities. Characters on low-gravity worlds suffer a -1 DM to all skill checks until they acclimatize, a process which takes 1D6 weeks. Characters with Zero-G skill at level 0 or better acclimatize instantly.

High-gravity worlds have a gravity 1.25 times or more than of Earth….Humans find high-gravity worlds unpleasant. Especially high-gravity worlds require the use of pressured or powered suits to support the human frame. Characters on high-gravity worlds suffer a -1 DM to all skill checks until they acclimatize, a process that takes 1D6 weeks.

CESRD, p. 168

Going back deep into the Classic Traveller lore, Module 2: Beltstrike includes rules for activities in zero and low gravity. Basically put, anytime the characters attempted to move or otherwise act in zero-g they had to make a saving throw of 10+ (on 2d6), applying modifiers as found on the Zero-G Activities Chart:

Classic Traveller Module 2: Beltstrike, p. 11

The more recent Orbital 2100 also provides rules for activities in zero-g environment. The task roll in Orbital 2100 is the spiritual successor of Beltstrike but greatly simplified:

Every crewman on DSV [Deep Space Vehicle] or orbital vehicle will have Zero-G skill-0 as standard. Higher levels of the skill are indicative of much greater experience of working in zero gravity. A crucial task, whether it is aligning and antenna or an EVA, shooting someone with a revolver or trying to shut an airlock door quickly to prevent an intruder forcing their way in, requires a skill check. For regular activities, skip the rolls entirely.

Avoid Losing Control in Zero-Gravity: Zero-G, Dexterity, Instant, Average (+0)

Apply the following DMs: Using a tool to repair/construct -2, Firing a gun -3, striking with tool, weapon, fist etc., or pushing/pulling -4, using a handhold +2

Losing control means that the task has failed until control reestablished, the character is tumbling! Roll again to regain control, but this time there are no DM’s, either positive or negative, except for those derived from Zero-G skill and Dexterity characteristic.

Orbital 2100 v3, p. 125

Before we get into making a character for The Expanse, let’s also consider what the spacecraft in book and TV series show us.

Ships and Gravity

The Expanse Canon

Way back in October 2016 and February 2017 I did two posts on how I viewed the depiction of spacecraft in The Expanse in Cepheus Engine-terms. To recap, there are three basic forms of Maneuvering Drive (M-Drive) shown in The Expanse; the “Teakettle,” the fusion torch, and the Epstein Drive:

Flying teakettle was naval slang for flying on the maneuvering thrusters that used superheated steam for reaction mass. The Knight‘s fusion torch would be dangerous to use this close to the Canterbury and wasteful on such a short trip. Torches were pre-Epstein fusion drives and far less efficient.

Leviathan Wakes, Chapter 3

By doing some backwards math I worked out that the “Teakettle” tops out at 2-G acceleration. Based on the upper limits of the instruments in Solomon Epstein’s ship, the fusion torch appears to have a limit of 7-G acceleration. In the novella The Drive, Epstein’s new drive pushes him at something like 12-Gs and in Season One of The Expanse the Rochinante pushes upwards of 17-G acceleration. As fast and exciting a high-speed run is, the mundane reality of travel in The Expanse is that ships usually plod along at a much slower cruising rate. Judging from the book Leviathan Wakes and the novella The Drive it appears that “cruising speed” is somewhere around 0.3-G acceleration. This ‘minimal acceleration provides just enough g-force to avoid the penalties of zero-g activities. This low-G acceleration is also important to note because it plays into the design of the ships.

Courtesy Syfy.com

Ship Gravity Using Cepheus Engine

One of the major “handwavium” technologies in Classic Traveller and now Cepheus Engine is that with the advent of the gravity-based maneuver drives you also get artificial gravity, known in the Traveller setting as “Compensators”:

  • Compensators. Integral to Maneuver Drives, Gravitic Drives, and Lifters is an inertial compensation component which counteracts the effects on acceleration on occupants of the ship. (T5 v5.10, Book 2: Starships, “How Maneuver Works,” p. 101)

In The Expanse there is no artificial gravity device for spacecraft so we need to find some rules to help us depict what happens with too much, or too little, gravity and what design decisions can be made to compensate.

Gel-y Juice

In The Expanse, there are two technologies for dealing with the crushing force of high-g acceleration. The first is acceleration gel; “Thirty minutes later, the engines kicked on, pressing him into the acceleration gel at a joint-crushing high-g burn for thirteen days, with one-g breaks for biological function every four hours.” (Leviathan Wakes, Chapter 46)

The second acceleration compensating technology is “The Juice:”

Going on the juice was pilot-speak for a high-g burn that would knock an unmedicated human unconscious. The juice was a cocktail of drugs the pilot’s chair would inject into him to keep him conscious, alert, and hopefully stroke-free when his body weighed five hundred kilos. Holden had used the juice on multiple occasions in the navy, and coming down afterward was unpleasant. 

Leviathan Wakes, Chapter 5

Standing Up In Space

The design of ships in The Expanse is also driven by the lack of an artificial gravity device. Fortunately, the setting of Orbital 2100 is in our near future and leans heavily into pre-gravitic spacecraft design similar to The Expanse and therefore can be used as a guide:

The biggest difference in space technology is the absence of anti-gravity….Not only are the drives different but the lack of on-board gravity means the crew must operate in zero-G throughout the mission. The only way to mitigate this is the installation of spin habitats, or rotating sections of the spacecraft, that ‘simulate’ gravity.

Orbital 2100, Chapter 5: Spacecraft Design, p. 37

For The Clement Sector setting, ship design is found in the Anderson & Felix Guide to Naval Architecture. Included in A&F is a module for designing Pre-Gravitic Drive Spacecraft. Here we actually get rules for continuous micro-gravity exposure as well as alternate methods of producing artificial gravity, both of which are also featured in The Expanse:

For every four week period of continuous micro gravity exposure there will be a one point strength and endurance characteristic loss that will require 1d6 weeks of recover in a one standard gravity environment.

The two main methods of producing artificial gravity are:


Producing “rear is down” gravity

A ship under acceleration will produce thrust gravity. In this instance the ship’s internal layout will need to be perpendicular to the axis of the ship or ninety degrees to the line of flight.

Centrifugal Rotation

Producing “out is down” gravity

For any type of method using centrifugal rotation to produce gravity, the internal layout must be aligned so that decks face inward towards the center of the rotation arc.

Anderson & Felix Guide to Naval Architecture, “Artificial Gravity,” p. 106
Courtesy syfy.com

Expanse-ive Characters

Having looked at many rules of Traveller and Cepheus Engine, how do I think characters from The Expanse could be portrayed?

Building a Better Belter

For Belter characters, at character generation I give each the Weak (Strength) and Weak (Endurance) trait from the CESRD alien species listing. I also give Belters the Vestibular System (Improved) alteration found in The Clement Sector supplement Tree of Life. Note that Belters grow up with the Zero-G skill so they instantly acclimatize when moving between different gravity world unless they cannot exercise or medicinal supplements are not available. To simulate the absence of such I ruled that they suffer loss of strength and endurance the same as if they were exposed to micro gravity for long periods.

I’ll point out here that the Belter career in the CESRD is perfectly adequate for generating a Belter character. However, if you have access to the Cepheus Engine supplement Uranium Fever: Asteroid Mining Rules for the Cepheus Engine from Stellagama Publishing (2018) the Independent Belter, Company Belter, and Planetary Miner careers are well worth the small cost of the product.

For Mars – Hu-ah!

I tend to generate and play Martian characters pretty much as a standard human. Being born, raised or living on Mars for any extended length of time automatically earns the Zero-G skill. Martian Marines, of course, are generated using the CESRD Marine career although I also draw upon materials found in The Clement Sector, in particular the sourcebook Hub Federation Ground Forces.


There are no specific rules in Cepheus Engine or Orbital 2100 for acceleration effects on characters. Looking at “Falling and Gravity “in CESRD (p. 164), we see that on a 1g world, a character suffers 1d6 damage per 2m of fall. The rules further specify that for higher g worlds, multiple the 1d6 by the planet’s gravity number. The Epstein Drive accelerates at 11-G which we can compute as 11d6 damage. The question is the time period in which this damage takes place. Falling is assumed to be instantaneous, but declaring 11d6 damage per combat round (every 6 seconds) does not seem to fit the events of The Drive. This seems excessive because an average character in Orbital 2100 (7 Strength/7 Dexterity/ 7 Endurance) only has 21 damage points until death. The “average” damage from 11d6 is 44, meaning an average character is dead twice over!

Perhaps we should assume the 11d6 damage takes place every space combat round  (1,000 seconds/16.6 minutes) instead? This better reflects the painful, but non-instantaneous death like Solomon Epstein experiences in The Drive. It still seems like an excessive amount of damage guaranteeing a quick character death.

Looking around for a solution, and not finding one in the rules, I suggest a “house rule” that acceleration couches (built with that acceleration gel) absorb some of the damaging g forces. In This New Ocean: A History of Project Mercury, acceleration couches in the Mercury spacecraft were designed to absorb 9G (assumed to be the maximum G at reentry). If we use couches to absorb, say, 10 of 11G, the character will have only 1g of damage (1d6) per space combat round. This means an average human may  last as long as six space combat rounds, or about 100 minutes, before succumbing to the strangling G forces. We could also say that prolonged exposure to high-g, defined as more than 1-G acceleration but less than the 9-G acceleration gel couch rating, will subtract one from the strength and endurance characteristic every 16 space combat rounds (around 4 hours) unless there is a four-round (1 hour) break in acceleration at 1-G or less. This fits with the time period in Leviathan Wakes when talking about acceleration gel.

These Boots Are Made For Walking

One important piece of equipment is The Expanse is Magnetic Boots. Characters with the Zero-G skill automatically can use Mag Boots; other need 1d6 hours to acclimatize. (I’m so tempted here to say that, based on canonical events in the TV series, female characters get an automatic -2 DM on the time roll, but that would be gender biased, eh?)

“Now you just walk around like you’re in pumps.”

Working In Space

When the player characters are in low-G or micro-G environments, I makes sure to use the Orbital 2100 working in space task check unless they are wearing Mag Boots (count as a handhold for the +2 DM) or the ship is moving with at least 0.3-G acceleration. I also enforce the A&F prolonged micro-gravity exposure rule.

Rochinante, Meet Broadsword and Azhanti High Lightning

Ships in The Expanse are built using what I call a “tower-ship” or “tail-sitter” design where the decks are arranged like floors in a building perpendicular to the axis of thrust. Classic Traveller and Cepheus Engine don’t have many designs to reference, but I will point out that the Azhanti High Lightning-class of cruisers (Classic Traveller Game 3 – Azhanti High Lightning) or the Broadsword Mercenary Cruiser (Classic Traveller Adventure 7: Broadsword) are built using a tower-ship/tail sitter design like the Rochinante. If you want to see a Cepheus Engine ship design that uses the tower-ship configuration I recommend you get Ship Files: Atticus Class Freelancer from Moon Toad Publishing (2017). This 100dTon ship is a tail-sitter not that much smaller than the Rochinante….

Courtesy Moon Toad Publishing

One thought on “#RPG Rulings – Understanding Gravity in #TheExpanse Using #CepheusEngine #TravellerRPG

  1. 0.3 g as a typical acceleration will keep the Martians happy, and be a buzz for the Earthers. Tough on anyone from Luna, but who’s *from* Luna, people just go there to work (!)

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