How is the efficiency of a propellant combination quantified by measuring thrust per unit propellant consumed over time?

Specific Impulse (I_sp)

Specific Impulse, often abbreviated as $ ext{I}_{ ext{sp}}$, is the fundamental metric used to quantify the efficiency of a rocket propellant system. It directly measures how much thrust is generated for every unit of propellant mass consumed over a specific duration, typically expressed in seconds. A higher $ ext{I}_{ ext{sp}}$ means the engine can produce more thrust for the same amount of fuel, allowing a rocket to achieve greater velocity changes or carry a larger payload. This concept directly links the chemical composition of the propellant (e.g., the lightness of exhaust products like water vapor from hydrogen combustion) to the overall performance capability of the propulsion system.