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According to James Cameron, the submarine barely had any space for him to move around etc., and as he approached the bottom of the Mariana Trench, the submarine shrieked down by a few inches (due to pressure).

Why was the challenger deep designed to be just big enough for James Cameron to fit in?

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  • $\begingroup$ larger = more expensive and the technical challenges increased : windows etc... $\endgroup$ – Solar Mike Sep 2 '17 at 18:45
  • $\begingroup$ Also, larger means heavier, which mean more expensive & more powerful winches would be required to lower & raise the submersible & possibly even larger & more expensive surface vessels to service the submersible. $\endgroup$ – Fred Sep 3 '17 at 3:07
  • $\begingroup$ the wall stress in a pressure vessel scales with size, so if you made it twice as big, the wall would need to be twice at thick. So then it would be 4x the weight (and material cost) roughly speaking, $\endgroup$ – agentp Sep 3 '17 at 19:12
  • $\begingroup$ @agentp Is there a formula that involves the material's size and thickness and outputs the amount of pressure it can stand or something? $\endgroup$ – Coto Sep 3 '17 at 21:21
  • $\begingroup$ Yes there is, you can apply the ASME VIII code here, it's for pressure vessels but I think it's the same here. External pressure is harder to calculate though ;) $\endgroup$ – Mech_Engineer Sep 4 '17 at 9:19
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The design of the submarine to be used in such purpose depends on the following (arranged from least important to most):

  1. Cost - The amount used to build the submarine. With respect to Solar Mike's and Fred's comments, the larger the submarine, the more money you will use to fund the project.

  2. Buoyancy - The larger the submarine, the harder it gets to make it remain submerged. Remember that buoyant force is equal to the weight of the fluid displaced by the object, therefore, having bigger submarine requires bigger engines, thus requires more fuel. Fuel efficiency is very important as depth is about 11 kms below MSL.

  3. Design strength - The walls of the submarine shall be designed to resist 107,910 kPa of pressure (gauge) because the pressure is directly proportional to the depth. Stress = PD / 4t. That means, as the diameter becomes bigger, the thicker the vessel should get to resist pressure. If the submarine becomes too thick, this goes back to item no. 1 (cost efficiency) and no. 2 (submarine now becomes very heavy and will need a lot of power to propel upwards)

Hope this helps.

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    $\begingroup$ According to the documentary, the submarine had weights that were controlled by the captain and were used to ascend the submarine back up. This was also there in case of an emergency: if the propellers/systems malfunctioned or something, the captain would drop the weights. $\endgroup$ – Coto Sep 5 '17 at 11:10
  • $\begingroup$ You also mentioned that the bigger the surface area, the more expensive it'd get, and the heavier it'd get. But since the volume increases too, more air would fit in the submarine and therefore the weight of the submarine (excluding additional weights) wouldn't be enough to submerge it to the bottom of the ocean (right?). $\endgroup$ – Coto Sep 5 '17 at 11:59
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    $\begingroup$ @coto as a back-of-the-envelope calculation mass and volume of a cylindrical pressure vessel will both scale with radius^2, so the buoyancy issue is a wash. $\endgroup$ – agentp Sep 5 '17 at 13:48
  • $\begingroup$ @agentp What's your point? That there will be no problem with ascend/descend control? Also, why did you use a cylinder as the shape of the pressure vessel? Is it because it's the closest to the Challenger Deep's overall structure? Also, was the entire capsule filled with air or another gas of a specific density? $\endgroup$ – Coto Sep 5 '17 at 20:30
  • $\begingroup$ yes a cylinder is a crude approximation suitable to give a rough order of magnitude approximation. $\endgroup$ – agentp Sep 5 '17 at 20:59

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