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Mostly it's moar stages. Each stage of a kerolox engine can get you about 5 km/s, and LEO is about 8 km/s, so two stages works pretty well. Velocity for LSEO (low super-earth orbit) might be 11 km/s, so you'd need a third stage, and each stage would be 3-5 times the size of whatever it's launching.

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It’s not that simple a falcon 9 upper stage on a 5g super earth would collapse from its own weight during takeoff.

It’s a double hit as your lower stages are also losing ~5g’s of acceleration due to gravity. So if you want to add 3g the entire rocket needs to be able to withstand an effective 8g, and you need a rocket engine + fuel to provide 8g’s worth of force. On top of this the time between each stage becomes extremely costly.

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