|Quoting tdscanuck (Reply 1):|
True, but submarines live primarily in compression (I think...astuteman?). Getting good fatigue (i.e. tension) performance out of welds has proven fairly tricky in the past.
I'm not astuteman - but yes - submarines live in compression - the pressure outside the hull is greater than the pressure inside. Oh - I'm sure you can over-pressure a sub at the surface, but not by much and not normally. And, no, they do not pressurize a sub hull when you dive - at least to any meaningful amount. (unlike a diving bell which does pressurize). If they did, a sub would have to follow decompression procedures when surfacing - just like a diver - or the crew would get the bends.
This is exactly opposite of an aircraft where the pressure inside is greater than outside.
This is one reason that one airport movie where the aircraft ends up underwater and 'holds' is so stupid. There ain't no way an aircraft fuselage would survive even a tiny amount of exterior pressure. It would crush like a paper bag. In firefighting, we use a rough guide of 2ft elevation per psi. That is not quite right - it is really ~85% of that - but a fire hose has pumping loss so that guide works.
An aircraft under 30 feet of water would have ~13psi - or 1872 lbs force per square foot (or for those metric folks - at 19m the aircraft would have 9100kg/sq meter)
Plus - the strength of the sub hull is far more than an aircraft. If you pretended that an aircraft could fly to vacuum and hold cabin pressure at sea level - the pressure on the aircraft fuselage would be about 10300 kg/sq meter. A sub at just over 30 feet has that much pressure.