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If you compare the pressurized volume of all the space stations launched to their mass, their volume-efficiency (in this context volume divided by mass) seem to decrease over time.

In chronological order:

Salyut: (actually seven stations, but practically identical. Salyut 1 numbers are used)

  • Pressurized volume: 99 m³
  • Mass 18,425 kg
  • Cubic meters per metric ton: 5.37

Skylab:

  • Pressurized volume: 319.8 m³
  • Mass 77,088 kg
  • Cubic meters per metric ton: 4.15

Mir:

  • Pressurized volume: 350 m³
  • Mass 129,700 kg
  • Cubic meters per metric ton: 2.70

International Space Station:

  • Pressurized volume: 916 m³
  • Mass 390,378 kg
  • Cubic meters per metric ton: 2.35

Tiangong:

  • Pressurized volume: 15 m³
  • Mass 8,506 kg
  • Cubic meters per metric ton: 1.76

What is going on? This is quite the opposite of what one would expect from technological improvements.

What is the reason for the volume-efficiency of space stations dropping when the overall technological progression improves?

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    $\begingroup$ Launching an empty can is pretty easy but not all that useful. The more functional equipment you want per person in your space station, the higher the mass to pressurized volume ratio is going to be. $\endgroup$
    – Russell Borogove
    Feb 7 '16 at 22:51
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    $\begingroup$ Are you counting in all the truss and solar panel mass in your ISS numbers? That doesn't seem right. $\endgroup$
    – Organic Marble
    Feb 7 '16 at 23:15
  • $\begingroup$ @OrganicMarble Yes I count all that stuff as well. I know that from a strictly just-the-habitation modules view, it does not make sense. However, the question is more like why pressurized volume for the crew is not a priority any more. $\endgroup$
    – SE - stop firing the good guys
    Feb 7 '16 at 23:17
  • $\begingroup$ @OrganicMarble You are perhaps comparing the number to the Wiki's? Well, the mass without the shuttle attached is better. isslive.com/displays/adcoDisplay1.html $\endgroup$
    – SE - stop firing the good guys
    Feb 7 '16 at 23:19
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    $\begingroup$ It just seems like apples and oranges to me. If you compare pressurized cylinders to more complex structures like the ISS, the cylinders will always win on these grounds. $\endgroup$
    – Organic Marble
    Feb 7 '16 at 23:19
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The International Space Station is a far more capable vehicle than any of its predecessors. This comes at a cost, and that cost is mass. There's a problem here: Doubling the electrical power more than doubles the mass needed to produce that power.

Things don't scale linearly. You can see this in the animal kingdom. Given equal sized images of the skeletons of an elephant and a mouse, you can immediately tell which is which. Scaling a mouse to the size of an elephant would not work. An elephant-sized mouse would fail under its own weight.

While the ISS does not need to sustain its own weight, it does need to withstand things such as thermal stresses. A flimsy tin can cannot scale up to the size of the ISS, without structural changes. Another name for this is the cube-square law.

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  • $\begingroup$ "Doubling the electrical power more than doubles the mass needed to produce that power" That does not sound right. I have one square meter of solar panels, and I just add another. $\endgroup$
    – SE - stop firing the good guys
    Feb 8 '16 at 0:07
  • $\begingroup$ Does not scaling up that "flimsy tin can" give me more volume for the mass due to the cube-square law? $\endgroup$
    – SE - stop firing the good guys
    Feb 8 '16 at 0:09
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    $\begingroup$ @Hohmannfan - You need to add structure to support that extra panel, and that means you need to add structure to support that extra structure. You also need to add wiring, and that too doesn't scale linearly. Look at the huge mass of the ISS's truss system. $\endgroup$
    – David Hammen
    Feb 8 '16 at 0:23
  • $\begingroup$ @Hohmannfan - Look at a mouse versus an elephant. Which has more structure (i.e., bones) per unit volume? $\endgroup$
    – David Hammen
    Feb 8 '16 at 0:27
  • $\begingroup$ Counter example: If I have a cube with a side length of 1, my surface to volume ratio is 6:1. If I make the sides 2, the ratio is 3:1. More volume for the same mass. That is what the cube-square law is about $\endgroup$
    – SE - stop firing the good guys
    Feb 8 '16 at 0:34

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