What metric measures propellant efficiency, favoring liquid propellants?
Specific Impulse (I_sp)
Specific Impulse ($ ext{I}_ ext{sp}$) is the key metric used to quantify propellant efficiency, defined as the amount of thrust generated per unit mass of propellant consumed over time. Generally, liquid propellants achieve a higher $ ext{I}_ ext{sp}$ compared to solid propellants. This performance superiority often stems from the use of superior liquid oxidizers, such as liquid oxygen ($ ext{LOX}$), which create more energetic reactions when paired with appropriate fuels than the common solid oxidizer ammonium perchlorate. For example, comparisons frequently show liquid engines registering $ ext{I}_ ext{sp}$ values several seconds higher than comparable solid boosters in vacuum conditions, reflecting better utilization of the chemical energy stored in the propellants.
