YAGS SciFi is all about spacecraft and travel between worlds, so having a system for defining spacecraft is important.

The statistics used are simplified from real world values to make the maths of designing craft easier. Mostly, this involves including all the constants in the basic statistics.

Displacement: Total size of the vehicle, in displacement tonnes (dT). Each dT is 14m³ (one tonne of liquid hydrogen), and by default is assumed to have an actual mass of 1 tonne, though this can vary depending on the configuration.

Mass: The actual mass of the vehicle in metric tonnes. This can get complicated, because mass can vary depending on both fuel and cargo load. Dry mass is the mass of the ship without any fuel. Wet mass is the mass fully fuelled. A loaded ship which is full of cargo will also have different masses depending on the amount and type of cargo.

Thrust: How powerful a drive is, per displacement tonne. The higher this is, the faster a spacecraft can accelerate. This is based on Newtons of force produced by the drive, multiplied by a magic number.

Impulse: How efficient a drive is. The higher this is, a spacecraft can accelerate for longer. This is based on the specific impulse (seconds) of the drive, multiplied by a magic number.

Delta-vee: The maximum velocity the ship can achieve after being fully fuelled. There will often be values for this based on unloaded and loaded mass.

The Total Thrust is the thrust of the engines times their total mass.

The Acceleration is the Total Thrust divided by the Ship Mass, in m/s².

Total Wet Mass is equal to the ship Mass, plus the mass of fuel times its density.

Full statistics for some future designs are available here:

• Chemical Rocket Engines

In Traveller, mass is measured in displacement tonnes, which is actually a volume based on one tonne of liquid hydrogen - equal to 14m³. YAGS SciFi considers this the ship's displacement, which is basically its volume.

Each displacement tonne is considered to be about 1,000kg in mass. A ship is mostly space, being a mix of pipes, electronics, living space, hull and structural support. There are two parts of the ship though which have much higher mass.

Some craft are Weak, in which case total dry mass (including engines, living quarters, cargo hold etc) is multiplied by 0.75. This also has a negative impact on the hull strength of the ship (base soak x0.50).

Some are Fragile, in which case their total dry mass is multiplied by 0.65. It improves performance, but they really suffer on punishment (base soak x0.25).

The first is cargo space, which when full can contain 10,000kg per displacement tonne. It is assumed that most cargo is efficiently packed and sealed in individual containers. Some types of cargo may have different mass ratings (such as metal ores, which could be 5 - 20 times this).

An empty cargo hold has a mass of 100kg per dT, since it is mostly empty space. When loaded with cargo, the mass of the cargo is in addition to this.

The second is fuel, which is often a liquid (sometimes solid) which completely fills the fuel tank with no space left over. Like cargo, fuel also has a density and so a full fuel tank increases the mass of the ship. Water counts as +14 per dt (14m³ of water has a mass of 14 tonnes, by definition), other fuels will count differently.

An empty fuel tank has a mass of 100kg per dT, again since it is mostly empty space.

Crew need supplies, which counts as cargo. 1dt of supplies has a mass of 5,000kg, and will last 1 crew member 1,000 days. This includes food, water, air and other necessities. It is assumed water and air is recycled as much as possible, but 'loss' is included in the expendables.

How much space crew need depends on the duration of the flight.

• Hours: ¼dT
• Days: 1dT
• Weeks: 2dT
• Months: 4dT

Cramped quarters are about half this. Luxurious can be from twice to five times this.