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taxiya said:

If I understand you right, you just described why I asked the question. How could FH which lacks the two extra upper stages send GTO payload of 3 times of one F9. In other words, due to lack of 2 extra upper stages, FH should not be able to send 26.7t to GTO IMO, but around 15t.

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It is the same way that theoretically the Delta IV Heavy had more than triple the GTO performance of a Delta IV Medium+ (5,2) with the GEMs removed, even if you accounted for the larger fairing the Heavy had.

You are thinking the two (payload-less) stages as adding more fuel and range. It's the exact opposite, they simply add more mass and drag to the equation.

Delta IV is again a good example. Delta IV Medium+ (4,2) has a GTO capacity of 6,150 kilograms. The bigger Delta IV Medium+ (5,2), due to the larger 5-meter (16 ft) DCSS upper stage gets 5,072 kilograms. Even when it has a lot more fuel.

gelgoog said:

In a two stage rocket the ideal ratio is the lower stage should be 10x bigger than the upper stage. That is why Falcon 9 has 9 lower stage engines. 9 is the closest number to 10 for which you can still build a symmetrical engine arrangement. They are using the same engine on both stages.

This ratio is broken on Falcon Heavy. The upper stage for it is underpowered. Period.

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It's the other way around btw.

Falcon 9 has a first stage weighing ~440 tons wet, and a second stage weighing ~110 tons. That gives you a ratio of around 4.
Falcon 9 has an abnormally large and overpowered second stage, and it stages low and slow so it has to do most of the work to get a payload to orbit.

Falcon Heavy has a first stage weighing ~445 tons wet, two boosters weighing about ~433 tons and an upper stage weighing ~110 tons. That gives you a ratio of around 12. Which is better for high energy orbits/missions.