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According to wikipedia, the Falcon 9 1.0 had ca. 10t payload capacity to LEO. The Dragon has a 3.3t payload capacity to the ISS with a dry mass of 4.2t that leaves 2.5t unused lift capacity.

Is this all used for fuel and propellant? That seems like a lot (that would mean more than 800m/s delta-v assuming the Draco has 300s isp, as it is "comparable" to the R-4D). The difference grows even larger with the Falcon 9 1.1 and (presumably) the full thrust.

So why is there this gap between the Falcon 9's performance and the Dragon's?

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  • $\begingroup$ Interestingly enough, Dragon rarely launches even close to that mass inside it. Turns out most ISS payloads are kind of fluffy and not dense enough to fill the Dragon's limited volume. Dragon has internal volume mass limits, and external cargo (stored in the trunk) limits. Not entirely sure where the extra mass neccasarily goes, but that is where Dragon keeps its mass. As of more modern Dragons on more modern Falcon 9's there is a bigger gap as Falcon 9 gets more capable. $\endgroup$
    – geoffc
    Apr 8, 2016 at 18:20

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First of all, the difference in capacity is actually greater than you included. Dragon launches without a fairing, thus there is an increased capacity as a result of launching without the fairing. The exact mass isn't known.

Okay, so what else might be different in the cargo capacity? Here's a few things:

  • The Dragon might not be structurally capable of carrying more than that weight given the vibration requirements imposed on launch.
  • Fuel mass (Unlikely, however, to be 2.5 tons)
  • The ISS orbit requires more fuel to achieve than the referenced LEO orbit. This is quite likely to be a major contributor, accounting for over a ton reduction in mass to LEO per the old Falcon 9 user's guide.

Bottom line, I think the loss in mass is partially due to the inclination, partially fuel, possible some other consumables (Port covers, etc), and the remaining portion being deliberately keeping the capacity lower to ensure there is sufficient margin in case of sub-optimal performance.

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