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I was reading a NASA study talking about the benefits of flying the Dragon spacecraft vs the Space Shuttle. The study mentions that both launch systems use only about 2/3 of their payload capacity when delivering supplies to ISS. Given that extra payload shouldn't cost any more, given that the rocket is the real cost, not the fuel, I don't understand why cargo wouldn't be maxed out?

My first thought is that, by reducing the mass, it makes it easier to maneuver and adds some extra margin of error? But it seems like, if that margin of error is necessary, shouldn't the "safe" payload be the official figure?

My other thought is that maybe the height of ISS means that the Dragon needs a smaller payload to make it there?

The problem with these two explanations is that, since the Dragon at least was designed to just go to the ISS, then you'd think that the maximum-designed payload would be the normal amount of stuff they would send?

If anyone knows the answer it would be much appreciated. Thanks!

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    $\begingroup$ Volume of payloads is also a consideration. If you fill the space with light but bulky goods, you won't hit the mass limit. The early shuttle days used a complicated pricing formula based on both mass and volume. $\endgroup$
    – Organic Marble
    Apr 22, 2018 at 3:55
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    $\begingroup$ Can you cite, or add a link to the NASA study talking about the benefits of flying the Dragon spacecraft vs the Space Shuttle? It would be helpful to see what statements you are referring to. Thanks! $\endgroup$
    – uhoh
    Apr 22, 2018 at 8:41

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Dragon was designed to use with Falcon 9 1.0, which LEO capability was around 10 ton, so it just does not have enough free space inside to put more goods.

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    $\begingroup$ Does dragon the use all of its volumetric capacity, or in fact is neither capacity used? $\endgroup$
    – uhoh
    Apr 22, 2018 at 8:20
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    $\begingroup$ If all available volume is used without hitting the weight limit, a larger capsule would enable more stuff to be carried up. $\endgroup$
    – Hobbes
    Apr 22, 2018 at 11:48
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    $\begingroup$ @uhoh it was designed for a smaller launch vehicle $\endgroup$
    – Pavel Bernshtam
    Apr 22, 2018 at 12:14
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    $\begingroup$ @Hobbes Oh, the 1st stage got bigger, but cargo mean density didn't increase accordingly. Got it, thanks! $\endgroup$
    – uhoh
    Apr 22, 2018 at 12:18
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    $\begingroup$ Confirmation that Dragon 2 will have more pressurized volume than Dragon 1: forum.nasaspaceflight.com/… $\endgroup$
    – Hobbes
    Apr 27, 2018 at 11:44

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