Submarine pressure vessel design

[exceter]

I'm working on the design for a dry-hull model submarine. My plan is to cast the hull shape that I want out of aluminum because this type of casting is cheap, easy to make, and because aluminum has a good strength to weight ratio. I want the hull to be able to withstand an external pressure of at least 173psi(the pressure at the bottom of the deepest lake within driving distance of my house) but, with a safety factor, I'd prefer that it can withstand an external pressure of 250psi. The submarine will be cylindrical in shape with an inside diameter of 6-8" and a length of 4-5'. The problem is that I'm not entirely sure how thick to make the hull walls. I've done a lot of googling but what I find are a lot of references to pipe or pressure vessel design where the pressure vessel is subjected to internal pressure not external pressure. The trouble is that this type of pressure vessel will exhibit a completely different failure mode than the one that I'm trying to prevent. Consequently, I have a couple design decisions in front of me and I'm not entirely sure how to approach them.

1) How do I calculate the minimum hull thickness to get the strength that I'm looking for?
2) Do I need to place re-enforcing bulkheads on the interior of the submarine to prevent buckling and, if so, how to I calculate the spacing between them?
3) If I pressurized the interior of the submarine to match or exceed the external water pressure(to allow the computer to detect leaks and ascend to the surface) how would this affect my previous two design questions?

Any comments, thoughts, or suggestions are greatly appreciated!

 

[Enthalpy]

Yes, the thickness will result from buckling. Formulas exist for a cylinder but only if the cylinder is complete, hence you'd have to open sections of the hull, not split it into right and left or up and down.

Necessary formulas are in Dubbel, which has been translated in English - something like Handbook for Mechanical Engineering, then. Stiffeners may well be compulsory; it depends on the minimum uniform thickness you compute.

250 for 173 is not a serious safety margin. Even less so if you cast aluminium by yourself! You write "easy to make", it only looks so.

Don't but gas pressure inside, as a burst would be much more dangerous, and seals are more difficult with gas.

 

[exceter]

You've convinced me. Dangerous is bad. No internal gas pressure. :)

 

[exceter]

Also, I didn't mean to suggest that aluminum casting doesn't require skill and practise because it clearly does. I just meant that it's something that I can hypothetically do in my back yard without a lot of specialized equipment.

On the topic of the safety margin, since posting this question, I've realized that I'm probably going to end up with a hull that's thicker than what is perhaps technically necessary to withstand the pressures that I'm anticipating because of the difficulty that would be involved in making a sand mold for thinner walls. That being the case, I'm hoping that I won't have to worry too much about under-engineering the wall thickness. I'd probably have a tough time making a sand mold that would allow me to get the wall much thinner than 1/4" which, if my rough estimates are correct, should be more than sufficient tensile strength for 173psi at 6" diameter, and would leave me worrying about how to deal with the compressive stress. My current thought is to add ribbing to the interior of the casting by changing the shape of the sand core that I'll use in the casting. I'm still not sure how thick to make the stiffeners, as you call them, or what to use for spacing.

Is this the book that you are referring to?
https://www.amazon.com/dp/3540198687/?tag=pfamazon01-20

If so, I hope I can find the equations elsewhere as that book is kinda pricey.

 

[Enthalpy]

Billions of blistering barnacles! I certainly didn't pay that price for my German copy of the handbook. With the price difference you can nearly learn German...

http://www.amazon.com/s/ref=nb_sb_n...keywords=Dubbel+taschenbuch+Maschinenbau&tag=
German 2011 new for 80€
German 2004, new 55€, used 28€
found none affordable in English. Apparently it was printed once in 1994 only.

Try to browse with cylinder, "external pressure", and "elastic stability" or "buckling".
Sorry, I don't have my Dubbel here.

 

[CFDFEAGURU]

You should also consult the ASME (American Society of Mechanical Engineers) Section VIII Division 1. It has all the needed equations for stiffening and thickness calculations for designing for external pressure. You sill also need Section II-D for the material properties and allowable stress values and external pressure diagrams.

Thanks
Matt

 

[exceter]

Thanks for the tip, I'll definitely check that out! I've seen references to ASME before but I didn't look at it very closely because I thought that it was just a specification for pipes which are only subjected to internal pressure.

 

[CFDFEAGURU]

Your welcome. No, the ASME is not just for internal pressure on pipes. There are codes by the ASME that deal with just pipe. ASME B31.1 Power Piping and B31.3 Process Piping.

Thanks
Matt