RSE #27
When a rocket lifts off the launch pad the thrust of the booster must be greater than the weight of the rocket. Actually a little bit more is helpful. For that much thrust you need really big engines. I porportion to it's size, the booster will have much larger engines than any other stage of the rocket. However, quiet soon after lift off, as the propellants are consumed, those big booster engines really build up a high level of acceleration. This can be hard on the passengers, upper stages, and delicate payloads. There are several ways to reduce this over acceleration problem.
One, throttle the engines back to produce less thrust. Throttling a rockt engine is not a good idea. First, it messes up the ISP of the engine, you loose efficency which is bad enough already. Second, it is very complicated to keep the mixture correct. Third, the engine can become unstable at certain throttle settings. Not a good idea, big bang.
Two, have a cluster of engines and turn some off. This is a simple and easy method to reduce thrust. Werner used it. The Saturn I turned off the inside four engines. The Saturn V turned off the center engine on both the first and second stages.
Three, Stage early, stage often, and use heavy Minimum Cost Design fracture resistant designed pressure fed propellant tanks. The engine size will always match closely the mass of the rocket, reducing acceleration. This is a cheap easy way to eliminate the over acceleration problem, and it costs less.