Discussion
For a given chamber pressure, the thrust of a rocket engine is highest when
Answer: B
The thrust is highest when the rocket is operating in vacuum.
Rocket thrust is \(F = \dot{m}V_{e} + (p_{e} - p_{a})A_{e}\). For a given chamber pressure and nozzle, the mass flow, exit velocity and exit pressure are fixed, so the thrust depends only on the ambient pressure \(p_{a}\), and it is largest when \(p_{a} = 0\).
At its design altitude the nozzle is ideally expanded (\(p_{e} = p_{a}\)), which is the best that particular ambient pressure allows, but the thrust is still lower than in vacuum. At sea level the ambient pressure is highest, so thrust is lowest. A normal shock in the nozzle means a badly over-expanded nozzle with large losses.
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