## Distance and Speed Units

Astronomical Unit (AU). The basic unit of interplanetary measurement, equal to the average distance between the Sun and the Earth. One AU is about 93 million miles.

Parsec (Pc): This is the unit of interstellar measurement used in GURPS Space. One parsec is about 3.26 light years or 206,000 AU.

Miles Per Second (mps): The basic unit of velocity and delta-V used in this book. One mile per second is approximately a Move of 1,800 yards per second.

# Maneuvers Drives

Maneuver drives are used to propel a spacecraft through normal space, and if powerful enough, can be used to lift off from a planet or maneuver through an atmosphere.

## Interplanetary Voyages

The distance separating two planets in a star system may be as short as the difference between their distances from their star or as long as the sum of those distances, depending on their orbital positions. For an approximation, assume the distance between two worlds is equal to the average orbital radius (distance from its star) of the world furthest from that star.

The table below shows the orbital radii of planets and other significant bodies in our own solar system. GURPS Space can be used to determine these statistics for other systems, or the GM can just make up numbers using our solar system as a guideline.

The table shows orbital velocity (how fast the planet is moving in its orbit around the sun) in miles per second and orbital period (the time it takes to orbit the sun) in Earth years. These two statistics are used when calculating Interplanetary Transfer Orbits (p. 38). Also shown are gravity (G) and escape velocity, two statistics important for determining if a spacecraft can take off or leave orbit.

**Solar System Travel Table**

Planet |
Orbit Radius |
Orbital Velocity |
Orbital Period |
G |
Escape Velocity |

Sun | - | - | - | 28G | 383 mps |

Mercurcy | 0.39 AU | 29.6 mps | 0.24 yr. | 0.38g | 2.7 mps |

Venus | 0.72 | 21.8 mps | 0.642 yr | 0.38G | 2.7 mps |

Earth | 1 AU | 18.5 mps | 1 yr | 1 G | 6.96 mps |

- Luna | 1 AU | - | - | 0.16G | 1mps |

Mars | 1.5 AU | 15.1 mps | 1.88 yr. | 0.38G 3.1 mps | |

Ceres* | 2.7 AU | 11.3 mps | 4.6 yr. | 0.03G 0.32 mps | |

Jupiter | 5.2 AU | 8.1 mps | 11.9 yr. | 2.36G 37 mps | |

Saturn | 9.5 AU | 6 mps | 29.5 yr. | 0.92G 22 mps | |

Uranus | 19 AU | 4.2 mps | 84 yr. | 0.89G 13.2 mps | |

Neptune | 30 AU | 3.4 mps | 165 yr. | 1.19G 14.6 mps | |

Pluto* | 40 AU | 2.9 | mps 24 yr. | 0.067G 0.68 mps | |

Oort Cloud (comets) about | 10,000 AU | 0.59 mps | 200 yr. | neg. | neg. |

- Ceres is the largest main belt asteroid; Pluto is a large Kuiper Belt object. Oort cloud statistics are for a typical long-period comet.

# Newtonian Space Flight & Delta-V

The top speed of a spacecraft that uses a reaction drive is really its “delta-V”: the maximum change of velocity it can perform before running out of reaction mass (rocket fuel, etc.). Each acceleration or deceleration “costs” a fraction of this delta-V.

The important spacecraft statistics are the acceleration of the reaction drive engines and the fuel tank’s delta-V reserve of reaction mass for those engines.

The GM will want to know how far the destination is, in miles or AU, as well as the escape velocity (in mps) and gravity (in G) of the origin or destination worlds.

*Example: The Princess of Helium is a passenger liner with a fusion torch drive. She’s presently in Mars orbit. She has enough reaction mass in her fuel tanks to give the ship a delta- V reserve of 55 mps, and her drive has a 1G acceleration using her fusion torch engines. She’s bound for Earth, which at this time we’ll assume is about 1.5 AU from Mars.*

## Getting into Space

To take off from a planet and reach a low orbit around a body requires a delta-V equal to 80% of the planet’s escape velocity. This is 5.6 mps for Earth orbit. The spacecraft’s acceleration must exceed gravity (1G, for Earth), or it must have wings (in atmosphere) or contragravity lifters.

To reach low orbit around a celestial body and then break orbit, escaping its pull of gravity, requires a delta-V equal to escape velocity. This is about 7 mps for Earth. The spacecraft’s acceleration must also exceed gravity (1G, for Earth), or it must be winged (in atmosphere) or have contragravity lifters.

A spacecraft that is already in low orbit uses delta-V equal to about 30% of escape velocity to break orbit. This is about 2 mps to leave Earth orbit. winged or contragravity lifter-equipped spacecraft with jet engines (or reaction engines with the ram-rocket design feature) in a very thin or denser atmosphere (p. B249) needs less delta-V to reach orbit or escape velocity. First calculate air speed (see Air Performance, p. 35) using only the jet engine or ram-rockets; then divide mph by 3,600 (giving air speed in mps); then subtract this from required delta-V.

If escape velocity of other planets is not known, values can be determined from a planet’s mass and radius relative to Earth. Multiply the above velocities by the square root of (Me/Re), where Me is mass in Earth masses and Re is planetary radius in Earth radii.

Stars and Escape Velocity: The sun’s escape velocity is 383 mps. For other stars and remnants such as neutron stars or black holes, multiply this velocity by the square root of (Ms/Rs), where Ms is the mass in solar masses and Rs is the radii in solar radius.

Time required is to lift off or break orbit:

T = dVx 0.045/A.

T is time in hours.

dV is the total delta-V required.

A is the spacecraft’s acceleration in G.

*Example: The Princess of Helium is orbiting Mars and wants to break orbit. This requires 30% of Mars escape velocity. The escape velocity of Mars is 3.1 mps (see table), so she needs 0.93 mps. She reduces the reaction mass in her tanks from 55 mps to 54.07 mps. She could accelerate at full 1G, but since her passengers are from Mars, she decides to use less than that: a gentle 0.5G, intermediate between Earth and Mars gravity. Accelerating at 0.5G, time required is 0.93 x 0.045/0.5G = 0.084 hours, or about 5 minutes. After a brief acceleration, she’s free of Mars’ gravity!*

## Space Travel with Reaction Drives

Orbital maneuvers, or interplanetary travel once a spacecraft has escaped orbit, require accelerating to the desired cruising velocity, coasting through space, then decelerating to the velocity required to orbit the destination. To plot a space journey for a reaction drive-propelled spacecraft, decide how much of the spacecraft’s delta-V reserve will be used to accelerate. This delta-V is the cruising velocity. An equal amount, minus the destination’s own escape velocity, must then be used to decelerate, unless the spacecraft is to fly past or impact the destination.

*Example: The Princess of Helium has just escaped Mars orbit and is boosting for Earth! She has a reserve of 54.07 mps of delta-V left in her fuel tanks. The navigator decides to accelerate to a cruising velocity of 25 mps, and as the spacecraft nears Earth, another 25 mps – 2.1 mps (Earth’s “to orbit” velocity) to decelerate.*

## Travel Time

The full travel time breaks down into acceleration, cruise, and deceleration steps.

1. Time to Accelerate

First, determine time the spacecraft will spend accelerating to the desired cruising delta-V. This is the acceleration time.

T = dV ¥ 0.0455/A.

T is time in hours.

dV is the total delta-V required for the acceleration and deceleration.

A is the spacecraft’s acceleration in G.

The spacecraft will normally spend the same time decelerating.

*Example: Since Princess of Helium has an acceleration of 0.5G and wants to reach 25 mps, it spends 25 ¥ 0.0455/0.5 = 2.3 hours accelerating to cruising speed near Mars. In the process, the occupants experience a gentle one-half of an Earth gravity acceleration. The ship will need to decelerate for the same time at the other end of its trip.*

2. Distance Traveled during Boost

An additional complication that is important during short voyages is the distance that was traveled during the acceleration and deceleration phases of the journey.

cD = T2 ¥ A ¥ 0.00042.

T is the acceleration time in hours as calculated above.

A is the acceleration in G. cD is the distance traveled in AU during constant acceleration.

It’s usually simplest to assume the deceleration distance is the same: double the distance.

*Example: Since Princess of Helium spent 2.3 hours accelerating at 0.5G, she traveled 2.32 x 0.5 x 0.00042 = 0.0011 AU. We double that to include the deceleration burn, for 0.0022AU.*

3. Cruise Time

If the distance traveled during the acceleration and deceleration phase is less than the total distance to the destination, the spacecraft will also spend time coasting. The time spent coasting (in zero-gravity, unless the spacecraft has spin gravity or artificial gravity generators) is calculated using this formula:

Time spent coasting (days) = tD ¥ 1,076/dV.

tD is the distance to the destination in astronomical units (AU) minus the distance traveled during boost (while accelerating and decelerating).

dV is the cruising delta-V in mps.

*Example: The distance from Mars to Earth is 1.5 AU. Princess of Helium has already traveled 0.0022AU while boosting to and decelerating from a speed of 25 mps, It therefore spends the following time between boost periods coasting in zero gravity: (1.5-0.0022) x 1,076/25 = 64.46days*

## Continuous Acceleration with Reaction Drives

A reaction drive vessel with enough delta-V can accelerate to midpoint, turn about, then decelerate, thrusting all the way, much like a reactionless drive craft: Delta-V (mps) required = (square root of [distance in AU/acceleration in G]) x 1,482 x acceleration in G. Voyage time (hours) = delta-V/(21.8 x acceleration).

## Special Case: Ramscoops

Spacecraft using reaction drives whose reaction mass is being provided by ramscoops can travel as if they had reactionless drive. However, they will generally first need to accelerate normally up to the ramscoop velocity (1,800 mps or more)!

## Travel with Reactionless Drives

Spacecraft with reactionless drives don’t need to worry about reaction mass. They can continuously accelerate as long as the drives have power, although most are limited to velocities a fraction below light speed (186,000 mps).

### Interplanetary Transfer Orbits

Planets and other celestial bodies don’t stand still. It’s possible to carefully time an interplanetary trip so that the two planets’ own orbits provide most of the necessary delta-V. Exact travel times will vary depending on the time of year. However, you can estimate the statistics of typical low-energy transfer orbit between two planets or other bodies (starting in orbit around one and ending in orbit around the other) using their orbital velocities and orbital periods. The Solar System Travel Table (p. 37) shows values for planets in our own system.

A transfer orbit involves maneuvers that require a delta-V equal to the difference in the two planet’s orbital velocities (in mps). Use the first step of the reaction drive travel rules to find the time spent maneuvering.

The majority of the trip is spent drifting in the low energy orbit. To find out how long this takes, add together the origin and destination body’s orbital periods (in Earth years) and divide by 4 to get travel time in years. Multiply by 365 for days.

### Blast Off!

A spacecraft using a reactionless drive can fly into space if it has anacceleration less than the planetary gravity, or is winged (in a very thin or denser atmosphere), or is using contragravity lifters.

To Orbit: T = ([Escape Velocity ¥ 0.8]-Air Speed) ¥ 2.8/-(A-G).

To Escape Velocity: T= (Escape Velocity)-Air Speed ¥ 2.8/(A-G).

T is time in minutes.

Air Speed is the top air speed in mps (mph/3,600) if the spacecraft has wings or contragravity. If not, treat as 0.

A is the spacecraft’s reactionless drive acceleration in G.

G is the world’s gravity; treat as 0 if the spacecraft uses wings or contragravity lifters.

### Breaking Orbit

A reactionless drive vessel that is already in orbit takes the following time to escape orbit:

T = (eV ¥ 0.3) ¥ 0.0455/A.

T is time in hours.

(eV ¥ 0.3) is 30% of the world’s escape velocity.

A is the spacecraft’s acceleration using reactionless drive.

### Space Travel Time (long voyages)

Once a spacecraft has broken orbit, the time in hours required to travel a distance measured in AU, including acceleration and deceleration, is shown below.

T = 68 x [square root of [D/A]).

T is time in hours.

D is the distance in AU.

A is the spacecraft’s acceleration in G.

The table below gives some typical times and distances.

(Peak velocity in mps will be: 10.9 ¥ T ¥ A).

**Travel Times**

AU |
0.0001G |
0.001G |
0.01G |
0.1G |
1G |
2G |

0.1 | 3.1 months | 1 month | 9 days | 68 hours | 22 hours | 16 hours |

0.2 | 4.5 months | 5.7 weeks | 1.8 weeks | 4 days | 31 hours | 22 hours |

0.5 | 7 months | 9 weeks | 2.9 weeks | 6.3 days | 2 days | 34 hours |

1 | 10 months | 13 weeks | 1 month | 9 days | 2.8 days | 2 days |

2 | 14 months | 4.5 months | 6 weeks | 13 days | 4.1 days | 2.8 days |

5 | 23 months | 7 months | 9 weeks | 2.9 weeks | 6.3 days | 4.5 days |

10 | 32 months | 10 months | 13 weeks | 1 month | 9 days | 6.3 days |

50 | 5.5 years | 23 months | 7 months | 9 weeks | 2.9 weeks | 2 weeks |

100 | 7.8 years | 32 months | 10 months | 13 weeks | 1 month | 2.9 weeks |

### Space Journey (short voyages)

For short distances, such as from the Earth to the Moon, or moving about within a planetary orbit, distances in miles are easier to calculate. The time in minutes required to travel a distance measured in miles, including acceleration and deceleration, is shown below.

T = 26 ¥ [square root of [D/A]).

T is time in minutes.

D is the distance in miles.

A is the spacecraft’s acceleration in G.

(Peak velocity in mps will be 0.18 ¥ T ¥ A).

### C-Fractional Velocity

High acceleration reactionless drives may just boost up to near-light speeds. The time required to accelerate to (or from) a fraction of light speed, c, is shown below:

T = Vc ¥ 8,300/A.

T is time in hours.

A is acceleration in G.

Vc is the desired velocity as a fraction of light speed (1 c = 186,000 mps).

## Lightsails & Magsails

Treat space sails as reaction drives but with no limit on the delta-V they can spend (though their top speed is limited as noted in their description). They get more acceleration the closer they are to a star. Multiply the base thrust by (1/D)2 where D is the average distance in AU from the star during the voyage.

This is a (somewhat unrealistic) simplification of actual sail flight. Most light sail or magnetic sails will accelerate into low-energy transfer orbits!

### Interstellar Voyages (measured in parsecs and years)

Time required for a long voyage at c-fractional velocity:

Ty = D x 3.261/Vc.

D is the distance in parsecs.

Ty is the travel time in years.

Vc is the cruising velocity as a fraction of light speed.

### Interplanetary Voyages (measured in AU)

T = 500 x D/Vc.

T is the travel time in seconds.

D is the distance in AU.

cV is as above.

### Short Voyages (measured in miles)

A ship can travel 186,000 miles x Vc every second.

# Atmospheric Flight

Spacecraft with wings, contragravity lifters, or if streamlined and with an acceleration greater than local

gravity, can fly in an atmosphere.

Use the rules on p. B466 for air travel. Use Piloting (Aerospace) skill rather than their normal spaceship operation skill. Ships using contragravity lifters use Piloting (Contragravity).

### Atmospheric Landings

In a very thin or denser atmosphere (p. B429), a streamlined winged spaceship can glide down for orbit, landing like an airplane, even without engines, as can any spacecraft with a soft landing system Roll against Piloting (Aerospace) as described on p. B214; add the spacecraft’s Handling modifier.

Failure means the approach wasn’t perfect and the ship must abort and double back, or land in the wrong place. In busy airspace, the pilot may get in trouble for violating regulations. On a critical failure, or any failure by more than its SR, it’s a crash landing that inflicts damage as per a very low velocity collision (p. 61) to one location. Roll: 1-3 front hull, 4-6 center hull.

Apply modifiers for terrain and weather. Spacecraft sensors can mean visibility is not a problem, but wind, hail, storms, ice, mountains, or built-up areas all give penalties: -1 (mountains), -2 (skyscrapers, strong winds, thin atmosphere), -3 (electrical storm), -4 (hail, very thin atmosphere), -5 (blizzard), -6 (hurricane).

### Vertical Landing

A spacecraft with acceleration better than the local gravity, or contragravity, can perform a vertical landing – “bringing her down on her jets.” This is the only way to land if a world has a trace or vacuum atmosphere!

Landing takes 20/(Acceleration-Gravity) minutes. A Piloting roll is needed every five minutes (minimum one roll). If the vessel is streamlined and landing in atmosphere, this is easier: halve the time required. Modifiers are as for atmospheric landing, though weather is ignored in vacuum. Use the space handling rather than air handling modifiers if landing in a vacuum.

If using a reaction drive, this requires spending delta- V reserve equal to 0.1 mps ¥ local gravity per attempt. Roll vs. Piloting skill. Success means a proper landing, failure means you may abort or suffer a crash landing (as detailed above), critical failure means you have a crash landing.

# Stardrives

Stardrives can work in just about any way the GM wishes. Thus, their performance depends on the intended scope of the campaign more than anything else. The GM should work out the mechanics of stardrive operation to suit the campaign. GURPS Space has detailed guidelines on the many different options and possibilities for GMs who wish to design their stardrives to suit a campaign. For GMs without GURPS Space, this section presents some ready-to-use examples.

## Starway Drive

The drive opens a portal allowing access to a parallel dimension commonly known as the astral plane in which differing laws of physics permit faster than- light travel. The astral plane is much like space and a vessel has the ability of taking a “short cut” through that dimension for a few days or weeks while traveling at faster than light speeds, then emerge back into the normal universe.

Typically, a ship entering the labyrinth of the starway must have some understanding of the route they must take or risk becoming lost among the overwhelming amount of passages that could become available . Its paths are woven into the fabrics of space and time where the soft empty space of the astral plane exists. Pathways could be large enough for entire fleets to pass through or small enough for vehicles or even persons. Once a ship has entered the starway through a temporary portal access back through that gate is impossible the ship must travel through the designated course and emerge at its desired location. The ship then at that point can turn around and travel the starway network back to its point of origin should I need to return.

Travel on the starway should be smooth and seamless as most of the network remains intact. However the millenniums of time have taken quite a toll on the ancient tunnels. Some areas of the tunnels have been collapsed or have been breached and flooded with astral energy. Other paths may have large space hulks lost to time floating while some paths lead to dead ends where fixed gates once connected them to the material universe.

The starway voyage, or "astral skip", lasts for distance in parsecs/FTL speed days. Then the spaceship emerges into normal space through another temporary portal at the calculated location.

Astral Skip accuracy typically is up to the GM, but often a successful Navigation (Starway Space) roll is required. Plotting Normally takes about 30 minutes and usually can only be done from the general area of space where the skip is being performed. Success gets the spacecraft as close as it can safely come to the destination. Failure may be a miss by 10^ of that safe distance x the margin of failure. Miss calculation could mean you've spent to much or to little time traveling on the astral. Critical failure may be by parsecs, an arrival someplace unpleasant, or an arrival closer than the safe distance with the drive disabled (damaged on a failed HT roll).

## Warp Drive

This has become the standard of space travel invented by the Star League. This allows the ship to travel and maneuver at faster-than-light speeds while still interacting with the normal universe. The drive created a gravimetrical field around the ship allowing for acceleration be multiplied by factors.

A typical warp drive ship has a speed in parsecs per day equal to its FTL rating. ( a speed of one parsec per day is equivalent to 220,000,000 miles per sec.)

Standard warp drives only faction in deep space. Some are automatically slowed to sub light speed when in closer proximity to stars (with in about 75AU of a sun-sized body) but still work, at speeds of light-per seconds per minute rather than parsecs/day.

## Jump Gates

Jump gates are large physical gates in space that connect two distant points, usually over interstellar distance. The popular aspect of these gates are that ships without FTL Warp drives may travel to remote parts of sector.

These gates are high maintenance and very expensive and only handful of them exist among the core worlds, and fewer among the colonial worlds. They also have a distant limitation of up to five light years. (As it becomes dangerous due to the mechanics of gate jumping). Typically, Solar systems on main travel routes will have two or more jump points leading to different destinations, creating a network of jump points routes that connect one system to another. There may also be cul-de-sacs systems - dead ends with only one jump point. Their size are often thousands of feet wide and high large enough that several ships can often travel between them at the same time.

To use a jump point, a ship travels to it, and passes through accelerator archways and perpells the ship speeds no warp drive could ever attain. Your ship will travel in straight shot to the other jump point where it will decelerate as it exits the gateways. Jump gates generally don’t' require navigation rolls, although a Navigation or Piloting roll may be required if entering jump point requires very precise course.