Friday, December 2, 2022

The Long Slow Black Road to the Stars: Interplanetary Travel for Mothership


Ailantd Sikowsky
The transhumanist TTRPG Ghost Ship has the most elegant and intuitive rules for interplanetary travel I've ever seen. The game sadly seems to be vaporware (the draft documents, untouched since 2017, are available for free), but I took the liberty to steal these mechanics and customize them for Mothership 1e, with a no-FTL, full interplanetary sandbox campaign in mind.

Transfer Ratings

Own work, based on Ghost Ship. Click here if Blogspot ate the image quality.

Each transfer from orbit to orbit has a Transfer Rating, ranging from 1 to 5. Trace your finger along the route you want to take, from your starting point to your destination. The single highest Transfer Rating on this route is the total Transfer Rating for your entire trip. During your trip, you will have to expend an amount of Fuel equal to this Transfer Rating, for orbital injection burns and deep-space maneuvers. You will spend the rest of your time coasting through space.

Special Maneuvers

The above method assumes a trajectory that tries to balance travel time and fuel expenditure reasonably. All interplanetary-capable ships have an astrogation computer able to compute this route without any roll or risk involved. When you're short on either time or fuel, special maneuvers come into play.

If you want to get somewhere and you really, really can't spare the time, you can strap yourself in and execute a Hard Burn. You expend 1 Fuel to bruteforce your ship into a less fuel-efficient, shorter trajectory. Your Transfer Rating decreases by 1 (to a minimum of 1) and everyone on the ship gains 1 Stress. You can do this only once every trip.

If you can't spare the fuel needed for your trip, you can take a more circuitous route. Timing your trajectory so that you encounter bypassing celestial bodies, you can exploit their gravity wells for a Slingshot Maneuver. Your Transfer Rating increases by 1 (to a maximum of 5) and your fuel expense decreases by 1 (to a minimum of 1). Likewise, you can only do this once on a single trip.


Spaceflight takes time, most of it uneventful coasting. Depending on your Transfer Rating, everyone on board will have a certain number of Downtime Actions at their disposal. These work in the exact same way as Shore Leave Actions (i.e. they can be used to train Skills, get medical care, work on a project etc.) with one exception: they cannot be used to relieve Stress.

If you want to spend your downtime on something that requires access to outside information, you may run into the problem of interplanetary bandwith limits. Response might be slow, the trickle of data across the radio even slower. If your Transfer Rating is higher than 3, requesting files from the outside becomes a separate Downtime Action.


Space travel is slow, monotonous and claustrophobic. Every day the same two or three capsules where elbow room is at a premium, every day the same five or so other people, every day the same big fucking black nothing outside the windows. Even for the shortest trips, it's not a pleasant time.

Most interplanetary ships are equipped with a cryobay where the crew can while away the months or years. You can decide to spend a Downtime Action in cryosleep - you won't be able to do anything else for this action. However, every Downtime Action not spent in cryosleep incurs 1 Stress. At the end of your trip, you can make a Sanity save to halve the amount of Stress you gained on your trip, rounded up.

An Example

While on shore leave on the Moon, the crew of the Cacomistle accept a job to stake out a claim on a mineral-rich asteroid in the Belt. They consult the system chart and trace out the route: Moon-Earth-Mars-Belt. The highest Transfer Rating on this route is 4, so the Warden decides the trip will take a year and four months. Since the mission is time-sensitive (gotta claim the asteroid before anyone else gets to it) the crew decides to execute a Hard Burn, decreasing the Transfer Rating to 3. The Cacomistle arrives to the asteroid after 8 months of travel, having expended 5 Fuel.

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