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If adding more engines doesn't increase the payload capacity, then why has NASA added a 6th RS-68 to their Ares V designs?
Because the mass of the Ares V increased beyond what is tolerable with five engines. The engines are only there for converting the impulse stored in the propellants into velocity. If you have not enough engines for flying a good trajectory (eg, too little acceleration, though enough to just barely defeat gravity), you could need much more fuel for compensating the less then optimal trajectory.
That is why you need as many engines as needed for using the propellants in the desired time (= get the right velocity at the right time) - but not more.