What all practical propulsion technologies exist?

Question

We are all too familiar with chemical rockets that burn liquid fuel and control ejection of exhaust gases to achieve propulsion for the rocket.

What other (practically implementable) propulsion systems exist?

Answer 1

There are two broad categories of propulsion in use today/possible today. Those are engines that use reaction mass and systems that don't use reaction mass.

Propulsion with internal reaction mass:

• Chemical rockets: Chemical energy is released and this energy is used to accelerate the resultant components out of the rocket, thus gaining propulsion. This is most commonly used today and includes everything from solid rocket boosters to traditional fuel-oxidizer liquid fuel rockets and everything in between (eg Hybrid engines)
• Cold gas thrusters (reaction engines): Mechanical energy in the form of pressurized gas is stored in a pressure vessel. Since the gas is under pressure, it can be released to create thrust. This type of propulsion is often used in RCS systems of rockets and maneuvering systems of satellites because it is rather simple and long-lasting. It is less efficient than chemical propulsion
• Electrical propulsion: Using an electric or magnetic field, ions (charged particles of a gas like xenon) are accelerated as reaction mass. These are typically called "ion thruster" or similar. They are incredibly efficient because the reaction mass (the ions) reach an incredible speed, however due to their low mass, the thrust is not very high. This type of propulsion is good for satellites which need to perform station-keeping or deep-space probes where thrust is not as important as efficiency. These are by far the most efficient space propulsion technology currently in use
• Nuclear propulsion*: Very similar to chemical rockets, except the energy is provided by a nuclear reaction. Nuclear engines can be more efficient than ideal chemical rockets, but not as efficient as electric propulsion. None are currently in use, although they have been built and tested here on Earth (look up NERVA)

Propulsion without internal reaction mass:

• Solar sails: Several spacecraft have demonstrated functional solar sails. Essentially, a large reflective surface reflects sunlight and the mass of the reflected light gives the spacecraft a little push, acting as wind would to a sailboat. This method of propulsion is limited because--without very clever navigation--it can only be used to gain momentum away from the sun and it is very slow. Additionally, the further out in the solar system you go, the weaker the sunlight.
• Photon rocket*: Essentially a powerful flashlight or laser. Unfortunately, for such a thruster to produce meaningful thrust, the laser or flashlight would need to be hideously powerful. As the thrust achievable with today's light sources and spacecraft power systems is so low as to be negligible, this type of propulsion hasn't ever been used.
• Laser propulsion*: The same as a solar sail, except instead of using the sun as the source of "wind", ground or space based lasers shoot at the craft's sail in order to push it. This has similar limitations as the solar sail, as it can only fly away from the laser source, but it could potentially accelerate faster and for a longer time. The Breakthrough Starshot proposal suggests this technology could be ready to launch interstellar probes by 2036 (if work began in 2016) that are fast enough to reach the nearest stars in under a century.

Technologies I've marked with a star (*) are within our technological capability today but haven't been used due to lack of purpose/funding/motivation.