What are the primary propulsion systems used in rockets?

The primary propulsion systems used in rockets primarily include chemical propulsion, which is the most familiar and widely utilized method for launching spacecraft. This system operates by burning propellant, which releases gas at high speeds, producing thrust according to Newton's third law of motion.

Electric propulsion is another significant category, which employs electric or magnetic fields to accelerate ions or plasma, offering high efficiency and the ability to operate for extended periods, though at lower thrust levels than chemical rockets.

Gas heating can be a less common method, but it can be seen in specific rocket types where gas is heated to create a propulsion effect, particularly in certain experimental or hybrid systems.

Other options do not accurately capture the main types of propulsion. For instance, nuclear propulsion and combinations of propulsion methods like mechanical propulsion do not typically make up the primary propulsion systems used in most rocketry. Therefore, the combination of chemical, gas heating, and electric gives a more precise view of the fundamental propulsion technologies currently employed in rocket science.