Saturday, November 6, 2010

Key Assumptions for a Spacefaring Game

I've been doing more brainstorming about a spacefaring game. These are some key assumptions I'd make about the setting in order to facilitate play.


ION DRIVES. The characters have access to highly efficient Ion Drive (or similar) technology allowing constant acceleration of typical spaceships from 0.1 to 1 G (or so). This makes interplanetary travel fairly rapid. For example, with constant acceleration at 1G, you can travel from Earth to Mars in a matter of days. This allows you to basically assume that players enter any tactical encounter with any reasonable relative velocity vector that they want. It could also be used in tactical encounters to generate very small thrust (say, 0.5 to 1 or even 1.5 Gs).

Reaction mass expended in an enhanced ion drive for interplanetary travel purposes is essentially negligible because we are assuming an incredibly efficient thruster (many orders of magnitude more powerful than exists today). The downside is that these drives require a lot of energy to function, from, say, a fusion reactor. Less powerful variants might be powered with fission or solar sources.

Here's a useful rule of thumb to calculate travel time: If you accelerate at A gravities to midpoint, then turnover and then decelerate at A gravities to your destination, elapsed time is approximately 4 SQRT (D/A) days, with D in astronomical units.

So to travel from Earth (1 AU) to Mars (1.5 AU) at constant acceleration of 1g, it would take: 4 * SqRt (0.5 / 1) = 2.8 days.

SOLAR SAIL. A less expensive (and lower energy) variant of the ion drive is mature solar sail technology. The advantage to this is that it is theoretically fast. It also requires very little energy and thus is probably less expensive. The downside is that it is not very maneuverable. This could be an option for players on a budget but it would prevent them from entering encounters with whatever velocity they want.

HOHMANN TRANSFERS: Interplanetary Travel without constant acceleration is done on Hohmann Transfers. Generally, for game play purposes, it is not necessary to understand how these work: it is a long time, and you can do it with limited thrust (generally two burns suffice; an initial and a final burn). Hohmann transfers are used for a few gameplay purposes:
  • Bulk cargo shipments from one orbit to another often use Hohmann transfers. For example, maybe the PCs get involved in an asteroid mining operation and need to get their bulk wares to somewhere else in the system inexpensively.
  • Sometimes a system might have a space station or other useful object on a Hohmann transfer orbit.
  • The PCs might be absurdly poor or limited on funds/local tech to old-school solid or chemical rocket technology.
  • Escape capsules might use Hohmann Transfer Orbits. This is a nice penalty for "death:" if you have to eject from your crippled spaceship, the escape pod has just enough thrust to put you into a transfer orbit. It basically means that the penalty for "death" is weeks or months of inactivity while you sit in your life raft and think about failure (or pay for a lift from an ion-drive equipped salvage ship).
You can estimate the time required for one relatively easily, however:

P^2 = A^3 (where P = time in years and A = distance in AUs)
Steps to solve (example is Earth to Mars):
  1. Add the distance of the first planet from the sun to the distance of the second planet from the sun (1 AU + ~1.5 AU)
  2. Find the semi-major axis of the transfer orbit by dividing this number in half. (2.5 AU / 2 = 1.25 AU)
  3. Solve the above equation. Use a calculator... (P = ~1.4 years)
  4. The equation gives you the time for a full orbit (i.e., Earth to Mars and back). Divide this by two to get the one way travel time. (0.7 years or around 8 months)

CHEMICAL DRIVES: Liquid and solid fuel rockets are still useful. They provide very high impulse. They are also cheap. This makes them great for tactical maneuvering on a "fighter" type spacecraft; generating 1G on an Ion Drive is not going to stack up favorably against a fighter maneuvering at 9Gs. Additionally, they are cheap and simple to operate.

The downside is that they require a lot of mass for fuel.


ION DRIVES. The Ion Drive can also be used for interstellar travel. In general, the amount of time it takes to get from one system to another with constant acceleration at >0.5G is the distance in light years plus one. So, to travel two parcsecs (about 6 LY) takes about 7 years with an ion drive. Mass consumption is significant at these distances and should probably be checked for each parsec of travel.

Over long journeys, relativistic effects can occur; for example, it traveling 3 LY (1 parsec), people on the planets age 5 years but those on the ship age only 4. if traveling 6 LY (2 parsecs), planet dwellers age 13 years and those on the ship experience only 13. The bottom line is that I don't expect PCs to travel this way most of the time, so it isn't a factor other than to understand abstractly; there may be "gypsies" that travel from system to system with ion drives at velocities which are high percentages of C that essentially accept that everything they once knew on a planet will be left behind.

SLEEPER SHIPS. Cryogenic technology allows passengers to be put into suspended animation. This allows a ship to burn up to a significant fraction of C (say, 0.05 to 0.1) but only twice (to accelerate then decelerate). Travel in this method takes decades or centuries to go even a few parsecs. The PCs might occasionally come across an old sleeper ship. Under very rare circumstances they might be forced to travel this way due to limited technology or funds. Travel in this manner might be a good way to basically start a new campaign.


There are three FTL technologies available for interstellar travel. Any or all of these may be unavailable in any given campaign. In particular, jump drives and warp drives work in a very similar manner and could easily be combined, i.e., there could be man-made wormholes. These technologies are useful within a globular cluster of systems or a spiral arm, about 20-30 parsecs in diameter.

JUMP DRIVES. Special jump drives allow travel between systems by exploiting "temporary" wormholes. The wormholes link nearby systems forming "space lanes." While temporary in astronomical terms, they generally remain stable for long durations in human terms. This allows the GM to add or remove space lanes occasionally. The wormhole is great because it allows travel that might normally be measured in parsecs to be completed in days. The downside is that both ends must be charted in order to safely jump. Additionally, these routes are predictable, which means that pirates and others often prey on them. Finally, the jump points are not always conveniently located.

More powerful jump drives allow longer jumps to occur, on the order of 1-9 parsecs. Regardless of the length of the jump, travel time is always the same (around a week). Ships in the wormhole cannot generally interact with anything outside the wormhole, and interactions within wormholes are very rare and unpredictable.

WARP DRIVE. This is based on the Alcubierre Drive. It works just like a wormhole except that the infrastructure to travel is man-made, not reliant on natural phenomena. Ships in warp drive cannot interact with the outside universe outside their warp bubbles.

HYPER DRIVES. Hyper drives allow a ship to "sidestep" into an alternative dimension (hyperspace) where travel at FTL speeds is possible. The advantage of hyperdrives is that unlike Jump or Warp drives, one can travel anywhere independently of Wormhole entrances or Warp Drive infrastructure. With a hyperdrive, it takes a number of days to travel somewhere equal to the number of lightyears + 1 (thus to go two parsecs takes a week). Hyper drive is popular with scouts and explorers (as well as pirates, recluses, or others who want to travel to obscure backwaters or avoid chokepoints and checkpoints). There are several downsides to hyperdrives:
  • Expense.
  • Fuel consumption; the vessel is constantly accelerating within the hyperspace bubble, requiring fuel expenditure.
  • Limited mass. There should be a non-linear energy cost to bring objects into hyperspace, preventing it from being useful for mass commerce.
  • Need for precise calculations. There is a significant chance, especially over longer jumps, that the ship will not end up exactly where intended. This effectively caps safe travel ranges at 1-9 parsecs per jump. The ship will likely need to drop out of hyperspace for navigational fixes on a regular basis.
  • Gravity Well sensitivity: Hyper Drives cannot be safely used close to stars or other major gravity wells. While some systems have Warp infrastructure or Wormholes at more convenient locations, a hyper-drive equipped vessel always needs to navigate to the outer edges of a system in order to safely jump. Of course, under extreme circumstances, a hyper jump can occur (for example, to escape), with unpredictable results...
  • Time. Hyperdrives can be faster for short trips of 1-2 parsecs, but anything longer is faster to do with a Jump or Warp drive.

The three technologies above are ideal for travelling distances of 1-9 parsecs at a go. They are good for traveling within one spiral arm or globular cluster of a galaxy. Travel across a galaxy requires movement rates in Kiloparsecs. Some obscure, rare, and expensive technology might allow such travel. As an example, the milky way is 30 kpc in diameter.

This technology should be rare and expensive with perhaps limited usage or requiring highly specialized ships. Alternatively, very rare natural phenomena such as the Deep Space Nine wormhole might allow such travel.


Travel between galaxies is exceedingly rare. Intergalactic travel should be a one-time event leading to dramatic changes in the campaign. Intergalactic travel is measured in megaparsecs. For example, the nearest other galaxy to the Milky Way, the Andromeda Galaxy, is 0.77 mega parsecs (770 kpc).


There are not many sentient alien life forms. The explored systems are decidedly humanocentric. As humanity has spread out, some humans have adapted to different local conditions, such as high or low gravity, but they are still decidedly homo sapiens. The exact reason for this is unknown.


The game occurs in a different "neighborhood" of the galaxy than Earth. Maybe it is a different spiral arm or globular cluster. In any event, due to the travel limitations described above, each globular cluster is fairly isolated with limited travel between them.

Alternatively, Earth has been destroyed by some cataclysmic disaster.

In any event, the theme should either be one of a fallen empire or of an isolated backwater sector.


Moore's Law states that computing power doubles every 18 months. Starting in the early 21st century, computing technology rapidly leveled off as in an S-curve. While it has advanced since that time, computing power is not exceedingly greater.

Additionally, there are strong taboos against various forms of artificial intelligence, especially AI for controlling weapons or other potentially lethal systems as well as any sort of interstellar FTL travel. Perhaps there is a history of Earth being destroyed by rogue Unmanned Systems. There is a chance that AI used for FTL travel pilotage will for some reason malfunction with potentially disastrous consequences.


There are a few ways to get stuff from the surface of a planet into orbit. They are limited. Generally, player spacecraft are not great at operating freely in an atmosphere or strong gravity well. The purpose of this is to keep the focus of action in space, not on planets. Travel to planetary surfaces should be the exception, not the rule.
  • Space Elevators: Very developed planets that regularly move large masses to orbit may have a space elevator established. This allows the PCs to pick up and drop off cargos from orbit.
  • Mass Drivers: Lower gravity planets may have large mass drivers to launch objects violently into orbit. While not good for fragile cargos, this method is highly efficient for bulk materials. A mass driver might even be able to place cargo into a Hohmann Transfer orbit.
  • Space Stations: Many planets may establish space stations as way points. For example, if traveling from a planet to its moon, it would make sense to have a station in Earth Orbit and another in Lunar orbit. Shuttles to and from the earth are designed to operate in atmosphere; those between the stations can be designed for pure vacuum operations; the final link to the lunar surface can be designed for lunar landing. If regular commerce occurs it is more viable to have a station, even a small one, and specialized rockets than to try and build a multipurpose space vehicle. Heck, even Earth today has a small manned station in low orbit.
  • Occasional orbital shuttles: The least populous and most backwards planets may just run an occasional shuttle to orbit. Cargoes might be launched with single-use rockets as needed.

In general, there are no faster than light communications. Interstellar FTL comms are limited to mail runs on FTL-drive equipped ships. Due to the prohibition on AI FTL drive operation, generally mail runs are carried on manned ships. This means that systems are relatively isolated from one another, making decisive local action important, and gives the PCs something important to do (carry mail and act as couriers). It also limits the scope/size of any sort of interstellar authority and forces decentralization.


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