Propulsion Cheat Sheet

The core ideas of Propulsion distilled into a single, scannable reference — perfect for review or quick lookup.

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Quick Reference

Thrust

The forward force produced by a propulsion system, generated by accelerating a working fluid or reaction mass in the opposite direction according to Newton's third law.

Specific Impulse (Isp)

A measure of propulsion efficiency defined as the thrust produced per unit weight of propellant consumed per second. Higher specific impulse means more thrust per kilogram of fuel.

Tsiolkovsky Rocket Equation

The fundamental equation of rocket propulsion relating a vehicle's change in velocity (delta-v) to the exhaust velocity of the propellant and the ratio of initial to final mass.

Combustion Chamber

The enclosed space in an engine or rocket motor where fuel and oxidizer react exothermically to produce high-temperature, high-pressure gases that generate thrust.

Nozzle Design (Convergent-Divergent)

A de Laval nozzle with a converging section that accelerates subsonic flow to sonic speed at the throat, followed by a diverging section that further accelerates the gas to supersonic speeds.

Propellant

The chemical substance or working fluid that is expelled by a propulsion system to generate thrust. Propellants can be solid, liquid, or gaseous and may include both fuel and oxidizer.

Turbofan Engine

A type of air-breathing jet engine that uses a large fan driven by a turbine to produce most of its thrust by bypassing air around the engine core, improving fuel efficiency and reducing noise.

Electric Propulsion

A class of propulsion technologies that use electrical energy to accelerate a propellant, typically an ionized gas, to very high exhaust velocities. These systems offer high specific impulse but low thrust.

Bypass Ratio

In a turbofan engine, the ratio of the mass flow rate of air passing through the fan duct (bypass stream) to the mass flow rate passing through the engine core.

Staging

The practice of dividing a rocket into multiple sections (stages) that are jettisoned sequentially after their propellant is expended, reducing the mass the remaining engines must accelerate.

Key Terms at a Glance

Afterburner:A component that injects and ignites additional fuel in the exhaust stream downstream of the turbine to produce extra thrust, commonly used in military jet engines.

Bipropellant:A rocket propulsion system that uses two separate propellants, a fuel and an oxidizer, which are mixed and burned in the combustion chamber.

Bypass Ratio:The ratio of the mass of air flowing through the bypass duct to the mass of air flowing through the engine core in a turbofan engine.

Chamber Pressure:The pressure of combustion gases inside a rocket engine's combustion chamber, a key parameter affecting engine performance.

Cold Gas Thruster:A simple propulsion device that expels a stored, pressurized inert gas through a nozzle to produce small amounts of thrust, often used for attitude control.

Compressor:The section of a gas turbine engine that increases the pressure of incoming air before it enters the combustion chamber.

Convergent-Divergent Nozzle:A nozzle shape that first narrows (convergent) then widens (divergent), used to accelerate exhaust gases from subsonic to supersonic velocities.

Delta-v:The total change in velocity that a propulsion system can impart to a spacecraft, used as the fundamental metric in mission planning.

Exhaust Velocity:The speed at which propellant gases exit the nozzle of a rocket engine, directly related to specific impulse and thrust.

Gimbal:A pivoting mount that allows a rocket engine nozzle to be tilted to change the direction of thrust, providing vehicle steering.

Grain Geometry:The shape of the internal bore of a solid rocket propellant grain, which determines how the burning surface area and thus thrust change over time.

Hypersonic:Speeds greater than Mach 5, at which aerodynamic heating and real-gas effects become dominant design considerations.

Ion Thruster:An electric propulsion device that creates thrust by ionizing a propellant and accelerating the ions with an electric field to very high velocities.

Mass Ratio:The ratio of a rocket's total mass at ignition (wet mass) to its mass after all propellant has been consumed (dry mass).

Monopropellant:A single chemical propellant that decomposes exothermically when passed over a catalyst, producing hot gas for thrust without a separate oxidizer.

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