Which capability do liquid engines offer that solids cannot provide?

Answer

Ability to be shut down and restarted at will

The defining advantage of liquid propellant engines lies in their commandability. Because the fuel and oxidizer are stored separately and actively pumped into the chamber, mechanical components like precision valves allow for precise control over the flow. This leads directly to the ability to throttle, meaning the thrust output can be modulated in real time to manage vehicle stress or optimize trajectory. Most significantly, liquid engines permit complete shutdown and subsequent reignition. This on/off capability is vital for complex missions, such as performing orbital maneuvers or coasting phases that require multiple engine firings separated by long intervals, features impossible to achieve with a self-contained solid grain motor.

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