Radio Cui Bono

The stage was 138 feet (42 m) tall and 33 feet (10 m) in diameter. It provided 7,750,000 lbf (34,500 kN)[10] of thrust at sea level.

A rocket engine experiences different thrust levels at sea level compared to higher altitudes primarily due to changes in atmospheric pressure and temperature. Here are the main factors that contribute to this difference:

Atmospheric Pressure: At sea level, the atmospheric pressure is higher than at higher altitudes. This affects the expansion of exhaust gases. At lower pressures (higher altitudes), the gases can expand more freely, which can lead to higher efficiency and potentially greater thrust.
Nozzle Design: Rocket engines use nozzles to accelerate the exhaust gases. At sea level, the nozzle is designed to optimize performance for the dense atmosphere. However, as a rocket ascends and the atmospheric pressure decreases, the same nozzle may become less efficient because it is optimized for a specific pressure. This is why many rockets have nozzles that can be adjusted (variable area nozzles) to maintain optimal performance at different altitudes.
Temperature Variations: The temperature of the exhaust gases can also vary with altitude, affecting their density and the speed at which they exit the nozzle. This can influence the thrust produced.

In summary, the combination of atmospheric pressure, nozzle efficiency, and the thrust equation leads to variations in thrust between sea level and higher altitudes. As rockets ascend, they typically experience an increase in thrust due to the lower ambient pressure, provided the nozzle is optimized for those conditions.

Where does the thrust of a rocket engine actually act?”

Good question, and perfectly asked. The answer is, “on the rocket.”

The reaction chamber in a rocket, where the thrust is produced, is a sphere. This means that the reaction of the burning fuel acts equally against all sides of the chamber except for one: the exhaust at the rear.

That MEANS that the thrust pushing *forward* on the chamber is *unbalanced,* and an unbalanced force causes acceleration! Aha!

So the thrust is literally acting on the rocket itself and nothing else. It’s the only thing there, really. The thrust escaping out the back is literally acting on nothing, and that’s *exactly* what we need it to do. It needs to act on nothing so that the unbalanced thrust at the front of the chamber pushes the rocket forward.