Space Travel

Space Travel

As a game system, Fate doesn’t simulate the physics of the game universe. Instead, it relies on the players and GM to estimate the chance of success and failure by judging countervailing forces—attack and defense, speed and distance, stealth and alertness, and so forth—roll the dice, and then interpret the result together at the table.

This philosophy works best when people at the table have a strong sense of how things work in the fiction. So, knowing a little background about the physical details of space travel will help. To help with this, the following sections introduce some scientific vocabulary for space adventure, and provide information that will help GMs add verisimilitude to the game.

A Little Bit of Rocket Science

Let’s talk about rocket science for a little bit. Rockets work by virtue of Newton’s third law: for every action, there is an equal and opposite reaction. Throw some mass out of the back of your spacecraft—the reaction mass—and you go forward. The bigger the reaction mass, or the greater the energy with which you throw it, the more forceful your forward motion. With no friction or other countervailing forces to slow you down or bring you to a stop, once your spaceship starts to move, it will keep moving until it experiences other forces. Each time you throw stuff out the back, you accelerate a little more, so if you can do so over a long enough period of time, you can build up a pretty good head of steam.

Thrust is what rocket scientists call the force required to accelerate a spacecraft at a given rate. Thrust is measured in newtons, and one newton is the force required to accelerate one kilogram of matter at a rate of one meter per second squared (i.e., per second per second). A spacecraft with a high-thrust engine can accelerate quickly, while one with a low-thrust engine must accelerate slowly.

Specific impulse (_I__sp) is a measure of the efficiency of a rocket engine’s fuel use. A spacecraft engine with high specific impulse can accelerate for a longer period of time on a given mass of propellant than a spacecraft engine with a low specific impulse can.

Together, these two ideas are the main considerations in spacecraft engine design: how much force can you apply to get your spacecraft moving and to slow it down again, and how much fuel do you need to carry to reach a particular velocity?

The following table gives examples of various at-least-theoretical spacecraft engine technologies.

Engine Type	Low Specific Impulse	High Specific Impulse
Low Thrust	Water rocket, cargo jettison	Ion drive, lightsail
High Thrust	Chemical, atomic rockets	Fusion torch, Bussard ramjet

Together, thrust and specific impulse determine the total delta-vee, or change in velocity, available to a spacecraft. Space-mission planners talk about the “delta-vee budget” of the mission—how much energy the spaceship making the journey must expend in order to get from where it is to where it needs to be. The biggest part of any delta-vee budget is usually the launch from a planetary surface. If you’re starting from the surface of the Earth, you’ll need to expend a great deal of energy even to get to low Earth orbit (LEO). Once in orbit, however, getting anywhere will require much less energy, by at least an order of magnitude—although the further away the destination, the less practical a low-energy transfer orbit (the least energy-expensive trajectory between two planets) becomes, because of the time involved. But the limits have to do with the patience and durability of human travelers—how much food and other supplies they’ll need to bring along, how much radiation exposure they’ll suffer—rather than the physics of it.

In some cases, the launch from a planet’s surface can be a moment of high drama or techno-thriller style action. In other cases, it can be as routine as catching a flight at the airport, particularly if the planet has a skyhook, space elevator, or other surface-to-orbit transportation system. Generally, it will take a high-thrust, high-specific-impulse engine to get a spaceship with a crew of PCs into orbit without booster stages, a piggyback to higher altitude on a suborbital craft, or other assistance. This detail can be ignored if your craft is equipped with sufficiently advanced technology, such as antigravity or reactionless thrusters.

Low Thrust, Low Specific Impulse

Pressurized fluid can be used as a reaction mass; you’ve probably seen a toy water rocket pumped up and launched. These engines are not terribly forceful and not terribly efficient, and even with the most extreme sort of staging couldn’t be used to escape Earth’s gravity well and achieve orbit.

You wouldn’t design a rocket this way on purpose, but it’s got story potential. In Isaac Asimov’s short story Marooned Off Vesta, for example, a pair of astronauts in the Asteroid Belt save themselves from a slow death in outer space by poking a hole in their water tank and using the escaping fluid as reaction mass to slowly push themselves toward the safety of a nearby asteroid settlement. This is a low-tech contrivance, potentially useful for moving around in a low-gravity environment as an emergency expedient.

The same principle also applies to things like jettisoning cargo or other mass. Conceivably, such an expedient could be used to impart a very small force to a spacecraft. In Frederik Pohl’s novel Gateway, two spaceships—both trapped above the event horizon of a black hole—docked with each other and transferred all hands to one ship and then separated, boosting one ship away and the other deeper into the black hole’s gravity well.

High Thrust, Low Specific Impulse

Such engines are capable of relatively rapid acceleration, but carry a lot of fuel in proportion to their payload. Often, lifting a heavier payload out of a planetary gravity well and into orbit requires staging, which is simply the use of disposable booster rockets comprised only of fuel tonnage, the engine itself, and whatever structural support is needed. Chemical rockets use either ignitable solid fuel or combustible mixed-liquid fuel; solid-fuel rockets use up all their fuel in one burn, while liquid fuel rockets can be turned off or even throttled for variable thrust.

Additionally, there are various sorts of atomic-powered rockets; for example, nuclear thermal rockets expose a “working fluid” of low-mass particles like hydrogen atoms to the heat of a nuclear reaction and then expel the exhaust as an energetic reaction mass. These are by some accounts about twice as efficient as chemical rockets, but the most efficient of such engines produce highly radioactive exhaust in their wake, making them unsuitable for use in an atmosphere one cares about.

A typical mission profile for this sort of rocket involves an initial burn (the application of thrust) followed by a long period of coasting, followed by a decelerating burn to match velocity with or enter orbit around the destination. Intermediate, course-correcting burns may also be applied. In many cases, the spacecraft can use an intermediate planet for a gravity assist or slingshot maneuver, accelerating as a result of the planet’s gravity well as it speeds past. Likewise, the craft can decelerate via aerobraking by using a planet’s atmosphere, like a stone skipping across the water, but this causes significant structural strain.

Low Thrust, High Specific Impulse

In contrast to chemical and atomic rockets, which burn fuel rapidly during brief intervals of high acceleration, some spacecraft rockets will thrust slowly but steadily, building up momentum over long periods of time and then decelerating slowly as well. For example, an ion drive uses an electric field to accelerate charged particles—usually ions of a noble gas such as xenon or argon—as the reaction mass. This drive, however, cannot be used in atmosphere because the presence of other particles apparently interferes with its operation.

Although not technically rockets, lightsails can be included in this category because their fuel use amounts to zero and their thrust is microscopic. Instead of burning fuel, a lightsail uses photon pressure against its enormous but low-mass reflective surface to gain very small but continuous acceleration. Lightsails that rely only on light from the Sun are called “solar sails,” and in the Solar System they are most effective inside the orbit of Mars. Further out, a lightsail would need to be pushed by a beam from a large laser; a large enough battery of lasers could potentially propel an interstellar probe.

These sorts of spacecraft take a long time to reach their destinations compared to high-thrust vessels, but their low rate of fuel consumption means they are often better for long-distance journeys where an engine with low specific impulse would run out of fuel.

High Thrust, High Specific Impulse

These sorts of engines are capable of long burns at high acceleration. A fusion torch relies on hydrogen fusion to create high-velocity exhaust to accelerate continuously to the midway point, then reverses its orientation and decelerates continuously until it arrives at its destination, all other things being equal. A torchship’s exhaust is extremely hot and viciously radioactive, making it a dangerous weapon at close quarters—this was the “Kzinti lesson” in the Larry Niven short story The Warriors, where a warlike alien race with reactionless thrusters runs into pacifistic but quick-thinking human beings traveling via fusion torch. A Bussard ramjet uses an electromagnetic scoop to gather hydrogen atoms floating in interstellar space to power its fusion engine, although it does have to accelerate to scooping speeds by other means and do something to ionize the hydrogen so that it can be scooped up. And since there may not be as much hydrogen floating around in interstellar space as was once thought, for this to work interstellar societies may need to “seed” the spacelanes with deuterium.

This category also includes high technology low- and medium-plausibility “reactionless thrusters,” “impulse drives,” and the like that propel a ship without expelling reaction mass. Adding in acceleration compensation fields or artificial gravity plates enables spacecraft to perform impossible-seeming maneuvers, like accelerating at high gravity continuously or changing direction almost instantaneously. At very high technology levels, even the most advanced torchship might comparatively seem as if it were standing still.

Interplanetary Travel

During play, the biggest question about interplanetary travel is usually how long it takes to reach a destination. Without more specific information, you can use the following chart as a rough guide to travel time, based on the type of thruster used and the distance involved. If the goal is merely to cross paths with or fly by the target, the time required is considerably shorter, since no deceleration or matching of velocities is required.

If travel time is an important issue, players can often use Engineering or Science to create advantages that can be invoked when overcoming with Astrogation or Pilot to plot and execute the course. Succeeding with style can reduce the travel time by one step (Fate Core, page 197), and succeeding at a cost can increase travel time by one or more steps or prompt the need for additional fuel. Complete failure means that the trip is impossible; the ship lacks the delta-vee needed to make the trip.

Thruster Type	Close Approach	Interplanetary	Extreme Interplanetary
Low thrust, low specific impulse	several months	a few decades	a few centuries
High thrust, low specific impulse	several days	a year	a decade
Low thrust, high specific impulse	several weeks	a few years	several years
High thrust, high specific impulse	a day	several weeks	several months

Close Approach: Travel to a nearby interplanetary destination, such as a planetary satellite (Earth to the Moon, Europa to Ganymede) or an L-5 space colony (a space station at a gravitationally stable interplanetary coordinate).

Interplanetary: Travel to another planet under relatively favorable conditions—orbital proximity, matching velocities, and so forth. Earth to Mars or Venus will usually fall into this category.

Extreme Interplanetary: Travel to another planet under more extreme circumstances. This includes reaching the outer planets of a solar system or trying to match velocities with a rapidly moving destination. Earth to Jupiter or Saturn will usually fall into this category. For greater distances, increase the travel time accordingly, noting that a high-specific-impulse craft will scale up travel time more slowly than a low-specific-impulse craft within the limits of the former craft’s fuel supply, since its ability to accelerate over longer periods of time lets it go faster.

Interstellar Travel at Relativistic Speeds

Einstein’s principle of relativity means that as a spacecraft approaches the speed of light, it will experience relativistic effects. In your game, the most important implication of relativistic effects is that time aboard a moving spacecraft will pass more slowly than it will to an observer at rest with respect to the vessel.

Relativistic effects might be an important and intriguing conceit for your setting—for example, if you’re playing a game loosely based on Joe Haldeman’s The Forever War where interstellar soldier PCs come back for R&R to an Earth that is increasingly alien to them because of the time elapsed in interstellar travel, and where they find themselves dealing with an increasingly technologically sophisticated enemy as they get closer and closer to the alien homeworld.

Alternately, you may need to incorporate relativistic effects as an element of an otherwise typical sci-fi setting. For example, the PCs may find themselves trapped on a primitive world and have to make a desperate subluminal bid to return to FTL starfaring civilization. In such a case, the following table* may be helpful. It maps speed as a fraction of the speed of light (_c_) to the Fate ladder, and lists the elapsed times for those aboard the spacecraft and those back home as a function of the distance in light-years that the ship travels.

Rating	Speed	Elapsed Ship Time	Elapsed Rest-Frame Time
Terrible (-2)	.05_c_	light-years × 20	light-years × 20
Poor (-1)	.10_c_	light-years × 10	light-years × 10
Mediocre (+0)	.20_c_	light-years × 5	light-years × 5
Average (+1)	.50_c_	light-years × 2	light-years × 2
Fair (+2)	.60_c_	light-years × 1.3	light-years × 1.67
Good (+3)	.70_c_	light-years × 1.0	light-years × 1.5
Great (+4)	.80_c_	light-years × .75	light-years × 1.25
Superb (+5)	.90_c_	light-years × .5	light-years × 1.10
Fantastic (+6)	.95_c_	light-years × .3	light-years × 1.05
Epic (+7)	.99_c_	light-years × .15	light-years × 1.01
Legendary (+8)	.999_c_	light-years × .05	light-years × 1.0

• This table is an oversimplification, given that it does not worry about the time needed for acceleration or deceleration. Also, for game purposes, relativistic effects don’t really kick in until the ship passes .5_c_, even though physicists consider an object moving at .15_c_ or above to be experiencing noticeable relativistic effects.

So, for example, an interstellar spacecraft capable of achieving nine-tenths of the speed of light—traveling at Superb (+5) speed—travels for a hundred light-years at its cruising velocity. Aboard the ship, it will seem as if about fifty years have passed (“several decades” or “half a century” in Fate terms). Meanwhile, to those who sent the ship on its way, it will seem as if 110 years have passed before the ship covers those hundred light-years—over twice as long!

Add in life-extending technology like cryosleep or stasis fields for the travelers, and you’ll find that each time an interstellar spacecraft arrives at a world it has visited previously, radical changes may have taken place. In such a game, each interstellar mission might be a significant or major milestone, changing both the game universe and the characters.

Faster-than-Light (FTL) Travel

Faster-than-light travel is one implausibility that science fiction readers are quick to forgive, since we need it to get us to where the action is. Of course, physicists are hard at work trying to come up with ways to defy the laws of physics and get us to the stars for real. Here are some typical science-fictional methods of traveling faster than light.

Hyperspace

In hyperspace travel, a starship leaves normal space and enters a higher-order space with different physical laws but which spatially corresponds one-to-one to normal space. Once in hyperspace, a ship typically no longer needs its hyperdrive, so it activates a separate propulsion source, which may or may not be its normal space drives. Alternately, a ship may move within hyperspace along a vector determined by its velocity and heading when it left normal space; arriving at the desired location is then a matter of turning off the hyperspace field generator at the correct moment.

If the spaceship’s sensors do not reach into normal space from hyperspace, it may be risky to re-enter normal space close to a planetary surface or anywhere else where normal matter may be present in sufficient quantity or density. Hyperspace may be featureless or may contain obstacles and hazards that must be avoided or evaded. It may even be occupied by alien entities, some of whom may be hostile to human life.

To use hyperspace in a game, think about the following questions:

• What kinds of FTL speeds can a starship travel at in hyperspace?
• How does hyperspace affect human beings physically and psychologically? Does it have the same effect on various alien beings?
• Are there obstacles or hazards in hyperspace? How can these be detected or avoided?
• Can people in hyperspace see into normal space? Can people in normal space see into hyperspace?
• Does time in hyperspace pass at the same rate as time in normal space?
• What happens if something leaves the ship while it is in hyperspace? Does it pop back into normal space or remain lost in the void of hyperspace? More generally, does it take energy to keep the ship in hyperspace, or does it take energy to bring the ship out of hyperspace once it reaches its destination?
• How hard is it to successfully calculate a path through hyperspace to a destination? How big is the normal margin of error?
• How large does the hyperspace field generator need to be, relative to the starship? In other words, do starships need to be markedly larger than interplanetary vessels? Do they need to be significantly smaller?
• How much additional fuel does the field generator require to activate or operate?
• Can signals be sent through hyperspace?

Example Hyperspace Drive: The Wang-Chaudary Vortex Drive

The Wang-Chaudary Vortex Drive causes a ship to enter a dimensionless space where it exists as a sort of standing wave. The activation of the drive is accompanied by a Burst of Gravity Waves that can be destructive to nearby objects, and nearby gravity wells add to the complexity of the calculations needed to “plot a course” through V-Space. Typically, the drive is only activated in interplanetary space at the margins of a star system, where space-time is relatively flat, so reaching a safe distance typically requires a Months-Long Journey Out.

The Wang-Chaudary drive requires Enormous Energy Inputs and is typically powered by a dedicated Antimatter Reactor. The reactor is fueled with very expensive, specially made Antimatter Containment Bottles manufactured at large, well-guarded industrial complexes and available for purchase at starship docking facilities. (These bottles make for great MacGuffins.)

Prior to entering V-Space, the navigator calculates the necessary amplitude and frequency of the drive-wave; it takes more energy to remain close to the point of origin, making the V-Drive useful only at interstellar distances. Simultaneous translation, so that the ship appears at its destination at the moment it disappears from its origin, requires the lowest energy input, but navigation errors and engineering failures have been known to throw ships far into the past or future at locations far from the intended destination.

Once in V-Space, the ship is essentially Coterminous with All Time and Space. Passengers experience V-Space as a Brief Period of Disembodied Sensory Deprivation sometimes accompanied by Unpleasant Auditory and Visual Hallucinations.

While intergalactic travel is theoretically possible via the transdimensional vortex, in practice the error margins at those ranges are too great for reliable transit. A number of interesting and strange space-going cultures are said to be the product of errant V-Drive colony ships emerging in the distant past at the far reaches of the universe.

Space Warp

Warp drive involves folding or otherwise manipulating normal space so that the ship can cross interstellar space as if the distances involved were much shorter than normal. A warp ship never leaves normal space, but is moving at superluminal speeds. Distortions at the interface between warpspace and normal space affect the precision of these observations, and plotted courses must avoid significant gravity fields.

Questions to address include the following:

• How powerful is the warp effect? What effective rates of travel does it permit?
• Does the warp effect produce movement in and of itself, or must the ship maneuver through the folded space using conventional means?
• What happens when two warp fields overlap?
• How much fuel does the warp engine require?
• Does the warp field work at full strength within a star system or close to a planetary mass?

Example Warp Drive: The Millennium Drive

This drive enabled human beings to make use of a warp network created and maintained by a multi-species galactic confederation. Ships make warp transits when their warp engines interact with the warp fields created by confederation waystations. A ship can create a space warp if there is a waystation within a few light-years of the ship’s warp initiation point.

Warp sensors can detect ships moving in warp via quantum vibrations that have not as of yet been harnessed for communication. When two warp fields meet, they interfere with one another, sending both ships off on a new vector in a combined or shared field. This means that interception is possible, and even a failed interception can slow or strand the target ship.

For this drive, the approximate travel time is a time increment based on the travel distance, modified with steps of “half,” “one,” “a few,” or “several,” as in Fate Core. So, a hop of 80 light-years takes either several hours or about a day, while a 1,000 light-year trip takes about a week, and a 35,000 light-year journey takes a few months.

Travel Distance	Time Increment
10 light-years	hours
100 light-years	days
1,000 light-years	weeks
10,000 light-years	months

The warp drive is experimental for human beings, so using it requires the crew of a human warpship to succeed at two actions: powering up the warp drive using Engineering, and reaching the desired location using Pilot. A crew member can use Astrogation to create an advantage for the engineer or the pilot, or for both on a success with style.

Failing the Engineering action can mean damage to the ship due to power surges and equipment overloads, control difficulties for the pilot, or similar problems.

Failing the Pilot action can mean delays, navigation errors, or encounters with threats or hazards that a more skillful pilot might have avoided. The pilot has only a limited ability to affect the course of the starship, since it is in essence merely riding the space-time warp created when the drive was activated near the warp beacon.

The difficulty of a warp depends on the distance being covered. Round to the nearest power of ten. Anything above five light-years but below about 55 light-years is Average (+1) difficulty. The difficulty increases by one step per order of magnitude that the distance increases, so traveling a distance of 1,000 light-years faces Good (+3) difficulty while traveling a distance of 35,000 light-years faces Great (+4) difficulty. If a waystation is not sufficiently close, increase the difficulty or travel time.

Wormholes

Wormholes—which could be skinned as portals, stargates, or transit points—are pathways that connect distant points in space-time through a higher dimension. They can be thought of as a kind of hyperspace, but the endpoints of each hyperspace path are predetermined. Ships may be able to just enter the wormhole without special equipment, or a nearby control station may be needed to open the wormhole.

Questions to address include the following:

• How are wormholes detected and accessed?
• Does each wormhole connect only two endpoints or several?
• Do wormholes allow travel in both directions or only in one, so that there are “entrance” and “exit” terminals in any given system?
• Can a wormhole be located anywhere, or must it be near specific sorts of places, such as near a black hole?
• How extensive is the wormhole network?
• Are wormholes artificial or natural? If artificial, who maintains the wormholes? How do they determine whom they permit to use a wormhole?

A Wormhole Network

In the late 22nd century, high-energy physicists discovered a means to create artificial wormholes by using a negative-mass fluid. In a collaborative multinational effort along the lines of the Manhattan Project, a portal to the habitable exoplanet Proxima Centauri b was created in orbit around Earth, and a race to colonize Terranova began. The influx of resources through the wormhole from the planet and from a nearby planetoid belt initiated a new age of prosperity on Earth. The Autorité du Portail (AdP) was formed to build and maintain additional portals. Soon a network extending from Earth to other stars began to form.

A wormhole station appears as a gigantic spherical lattice that glows with the luminous blue energy of the negative-mass fluid that must be continuously powered in order to hold open the throat of the wormhole. The station creates a one-way link to a distant point that must be at least several light-years away, though greater distance requires more negative-mass fluid, to a practical maximum of perhaps a dozen light-years.

The AdP charges transit fees to ships using the portal network, using the funds to maintain and extend the network, and wormhole stations are generally regarded as neutral territory by the various factions that seek to exploit the resources of the star systems opened up by the network. The decision to open up a wormhole station is typically regarded as an investment, with the AdP relying on entrepreneurial types willing to pay for the construction of a return station out of the profits of their colonization and settlement efforts.