[simon_]

Could someone weigh in on the following quote? Sounds fishy / wrong to me:

To put this in perspective, a balloon that is one kilometer in diameter is capable of lifting about 700,000 tons, or the weight of two Empire State Buildings. Add a second balloon of the same size and the lift capacity of these two balloons increases exponentially: it’s now capable of supporting nearly 6 million tons of weight.

[Tossrock]

I assume they meant that if you were to double the diameter of the balloon, it would be a superlinear increase in lifting capacity (since volume is cubic in radius). However, it wouldn't actually be exponential, and two balloons added together definitely does not equal more than the sum of their individual lift capacity.

[jackcarter]

To be precise, it's 8x the lifting capacity (=2^3), which would be 5.6 million tons, or "nearly 6 million".

[kissickas]

OK, so just the journalist misunderstood then? Because doubling the diameter is in no way compatible with "add a second balloon."

[someone13]

Some back-of-the-envelope calculations suggest that first number is in the right ballpark. From [0], the lift generated by helium vs Earth air at STP is approx. 1.03 g/L. This suggests a lift of approx. 590,000 short tons on Earth:

http://www.wolframalpha.com/input/?i=%28%28%28%284%2F3%29+*+...

Then, if we assume that the gravity on Venus is 90% of Earth standard, you get about 656,000 short tons of lift. Note that this doesn't take into account the different air density on Venus.

No idea about the "doubling" bit, though :-P

[0]: http://www.chem.hawaii.edu/uham/lift.html

[titanomachy]

You're right, that doesn't make any sense. The buoyant force is equal to the weight of the fluid displaced. Doubling the volume of your balloons doubles the buoyant force and therefore doubles the weight you can carry. The relationship is linear, not exponential.

[_nb]

I'd agree, that doesn't sound right at all. Perhaps they meant doubling the size of the balloon? Doubling the diameter of a sphere indeed increases its volume many times more than double.