Why are dense propellants like RP-1/LOX favored for first stages?

Answer

Smaller tank volume allows for a lighter overall vehicle structure at liftoff

Propellant density plays a crucial, practical role during the initial ascent through the atmosphere, where overcoming gravity drag is the primary challenge. Dense propellants, such as $ ext{RP-1}$ (kerosene) used with $ ext{LOX}$, pack more mass into a smaller volume compared to low-density options like liquid hydrogen ($ ext{LH}_2$). This high density permits the use of smaller, lighter propellant tanks relative to the total energy stored. A smaller tank size translates directly to a lower dry mass for the vehicle at liftoff, enabling higher initial thrust-to-weight ratios and more effective acceleration during the dense atmospheric phase, even if their vacuum efficiency ($ ext{I}_ ext{sp}$) is lower than that of $ ext{LOX}/ ext{LH}_2$ systems.

Why are dense propellants like RP-1/LOX favored for first stages?

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