In space, the only real option for propulsion is pure thrust. Drives of these sort fall into two categories: reaction drives, which rely on Newton's third law of motion, and reactionless drives, which do not.

For all space vessels, the only thing that matters is raw acceleration. Divide your vessel's total thrust by your vessel's total mass; the result is your acceleration, in meters per second per second (m/s²).

Most of these drives can also be used on air, water or ground vessels. However, those with reactors (fission, fusion, antimatter or nuclear pulse) will probably have unfortunate effects on a planet's biosphere.

“For every action, there is an equal and opposite reaction.” Newton's third law is the fundamental concept behind the reaction drive: If you throw something out the back of your ship, the ship will go forward.

These work by burning a propellant of some sort, and then exhausting the resulting hot gasses and particles out the back of the ship. They are extremely simple in concept, and nearly foolproof as a result.

†per minute of burn-time

Mass is per Newton of thrust (and per minute of burn-time for solid rockets). Cost is per kilogram. Fuel is in liters per hour. Volume for all engines listed above, in cubic meters, is equal to their mass divided by 150.

Similar in concept to chemical rockets, thermal rockets heat up a reaction mass and then fire it out the back of the ship. The principal difference is that chemical rockets combust their reaction mass to impart energy on it, whereas thermal rockets use an external source of energy. Thermal rockets are less powerful than their chemical counterparts, but frequently more efficient.

Mass is per Newton of thrust. Cost is per kilogram. Fuel is in liters per hour. Volume for all engines listed above, in cubic meters, is equal to their mass divided by 100, and includes limited access space for maintenance to be performed, typically to the heating elements.

The following drive systems all employ the same principal, of imparting energy upon a reaction mass and then throwing it out the back, but the means of imparting that energy wanders around quite a bit. Ion drives use a magnetic repulsion system; nuclear pulse drives use a fission reaction, and direct the neutrons out the back. Conversion drives are the most exotic: mass is converted directly into energy, and used to propel additional mass.

Mass is per Newton of thrust. Cost is per kilogram. Fuel is in liters per hour. Volume for all engines listed above, in cubic meters, is equal to their mass divided by 75, and includes access space for maintenance to be performed.

These move directly into the realm of science fiction—if not science fantasy. Some principal, currently completely unknown to physics, allows these drives to directly impart thrust on a ship without the need of reaction mass.

No types or names are provided for these drives, for the simple fact that they are completely up to setting fiat. The closest to any functional concept we have is the idea that if gravity could be manipulated, it could be used to cause a ship to “fall” in the desired direction.

The best that can be assumed is that these drives will suck a lot of power from the ship's powerplant. The exception is the TR? reactionless drive, which is assumed to use some even more exotic principal to produce thrust, like magic.

Mass is per Newton of thrust. Cost is per kilogram. Volume for all engines listed above, in cubic meters, is equal to their mass divided by 75, and includes access space for maintenance to be performed.