For a typical chemical rocket, the fuel and propellant are the same thing. For Nuclear rockets the fuel and propellant are different things. The fuel powers the reactor, and the propellant is thrown out the back.

See:

• https://thephysicsofspacex.files.wordpress.com/2016/07/isp-upper-limits.pdf
• https://en.wikipedia.org/wiki/Liquid_rocket_propellant#Bipropellants

Density: 14
Max Impulse: 4.7

Liquid Oxygen and RP-1. Denser and requires less cryogenic cooling than liquid hydrogen.

Density: 4
Max Impulse: 5.3

Liquid Oxygen and Liquid Hydrogen. Very low density but high efficiency without being insanely dangerous. The cryogenic needs complicate things though.

Density: 10
Max Impulse: 4.6

Liquid methane and liquid oxygen.

Density: 1
Impulse: ~

Generally used as propellant in nuclear or plasma engines.

From: https://youtu.be/D4SaofKCYwo?t=2180

The “/a” and “/v” engines are variants designed for optimal atmosphere (sea level) and vacuum operation.

The environment is where the engine is actually being used. “/a” engines tend to operate in both environments, though are less suited to vacuum. Most “/v” engines perform self-disassembly at sea level.

Aerospikes work well in both environments.

The following drive types are based on real technology, simplified for purposes of the game.