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A twist on the Archimedes law. Any object in Earth's atmosphere should weigh less than in vacuum, by amount of mass of air it displaces. At 1.2 kg/m³ of air at sea level, that is a bit of a difference, especially in case of light materials like insulation foam.

Is this difference taken into account when weighing payloads, parts etc? Or is it simply sunk into allowable error margins?

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  • $\begingroup$ Did you see the comments here? Is the Paramagnetism of Liquid Oxygen Ever Considered. Organic Marble sez yes. $\endgroup$ – kim holder Mar 13 '16 at 2:44
  • $\begingroup$ @kimholder: I didn't notice it. I guess it answers it. $\endgroup$ – SF. Mar 13 '16 at 2:47
  • $\begingroup$ Well, more can be said, for sure. It's an interesting case. $\endgroup$ – kim holder Mar 13 '16 at 2:51
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Yes, we calculated the bouyancy for the External Tank in the code for the Shuttle Mission Simulator. I was dubious that it really mattered since you get out of the dense atmosphere so quickly, and made at least one run to see if I could tell. It's been a long time but my memory is that I could not tell any difference at Main Engine Cut-off...but I just may not have spotted it. It's all ancedotal anyway, sadly.

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  • $\begingroup$ It might matter not because of short time of flight in the atmosphere but long time of flight in void! You put a tank on a weight on Earth and read that it's 10 tons sharp at 1 ton/m³, then it quickly gets above the atmosphere and you have extra 12kg to orbit, that didn't show up on the weight due to the tank's buoyancy. $\endgroup$ – SF. Mar 14 '16 at 8:52
  • $\begingroup$ Our analysis tools were pretty primitive then, small effects could be easily missed. $\endgroup$ – Organic Marble Mar 14 '16 at 16:57
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I think it is very likely that the buoyancy of the rocket is taken into account. First, let us see what savings it gives in terms of mass:

What is the density of a rocket? That is actually quite difficult to find, but considering a kerosene/LOX rocket like Soyuz, it is mostly propellant anyway. Kerosene has a density a bit less than 1 kg/L, and LOX a little bit more. As the rest of the rocket is metal, that should average out a little over 1 kg/L. Continuing the rough approximation, that means that the Soyuz rocket displaces around 350 kg of air. Is that much? For a comparable number, the Space Shuttle External Tank was painted with reflective paint on the first two flights. Then they decided that it could actually be skipped, "saving approximately 272 kg"

Rocket science is all about small margins, and that means there is a fight for every kilogram. As the effect of buoyancy is within the magnitude of other things considered, it is almost for sure taken into account.

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  • $\begingroup$ In determining the density of the rocket why not just divide its total mass by its total volume? The volume can be determined by the physical dimensions of the rocket plus the dimensions of any bits which may protrude like nozzles & fins. $\endgroup$ – Fred Mar 13 '16 at 13:14
  • $\begingroup$ @Fred Total volume is what I am estimating, but do you now where I can find the numbers? $\endgroup$ – Hohmannfan Mar 13 '16 at 13:20

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