In this lesson from Yale's Rocket Science for Everyone, astronomer Marla Geha explains the chemistry and engineering behind chemical rockets, covering the two primary propellant types: solid and liquid systems. The lecture describes how each type generates thrust, the tradeoffs between them in terms of controllability and storage, and why different missions favor one propellant system over the other. Moving from the course's earlier focus on orbital mechanics into propulsion, this lesson begins the section of the course concerned with how rockets actually generate the force needed to reach the orbits described in previous lessons.