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Re: [PSUBS-MAILIST] Calculations
What is Thijs's double end cap flying sauser craft? And when you're
speaking about stress concentrations what, for example, kind of problems
are you talking about ?
Thanks, - Brian
----- Original Message -----
From: "Michael Wright" <mwright@smallip.com>
To: <personal_submersibles@psubs.org>
Sent: Tuesday, July 08, 2003 5:34 PM
Subject: Re: [PSUBS-MAILIST] Calculations
> First, since air at mean sea level is roughly 14 PSI, a tank at full
> vaccume is the same as a tank with 1 atmosphere inside at 28 PSI outside
> (or 32 feet of water).
>
> Steel is about the same stregnth and modulus of elasticity in
> compression as in tension.
>
> Here's the thought model I use to envision the difference between
> internal and external pressure. Take an empty one liter plastic pepsi
> bottle. It can hold upwards of 55 PSI internal pressure. However you
> can collapse it with your lungs by suckin on it (far less than 2 PSI).
>
> This should easily illustrate that a tank rated for X psi internal
> pressure is not likely safe to use for X psi external pressure. A tank
> designed for internal pressure needs no structure to keep it' shape.
>
> The physics of the situation are relatively straightforward. If a tank
> deforms due to internal pressure it becomes a shape that is loaded
> entirely in tension. The force from the internal pressure keeps the
> round shape.
>
> If a tank deforms due to external pressure it gets less rather than more
> round, becoming an ellipse. As this deformation occurs the tank
> structure is loaded more and more in bending rather than compression.
> This leads quickly to a posetive feedback loop that leaves the tank
> looking like the soda bottle with the air sucked out.
>
> The posetive feedback nature of this process is what should scare anyone
> thinking of building an atmospheric submarine. If you exceed the
> capability of your internal structure to maintain the round shape, the
> hull will begin to oval, as it ovals it will be loaded in bending rather
> than compression, and quite rapidly the hull will collapse on it's self.
>
> So if you wanted to use a tank you'd have to build in all the structure
> to keep it from ovaling under external pressure. In doing so you'd have
> to make sure not to create stress concentrations in the skin.
>
> It seems to me that you'd rapidly spend more effort and more mass
> stiffening the inside of the hull than would be required to build
> Thijs's double end cap flyin saucer shaped craft (a shape that looks
> quite useful to provide space for a crew of two with a minimum of excess
> air space).
>
> I supose if one had access to a plasma or lazer cutter you could build a
> bolt together, laminated steel internal structure for the tube section
> that could be installed via the main hatch opening. All this would
> require 3d solid modeling to get the geometry and placement right but
> would be doable. The modeling will probably show that the end caps
> being spherical makes them less susceptable to the posetive feedback
> loop that would affect the tube section.
>
>
> At any rate, assuming that because a tank is rated to a certain internal
> pressure it will handle that external pressure is incorrect and likely a
> fatal mistake.
>
>
> michael
>
>
>
> Chris Jackson wrote:
>
> > Very good point. Logically, if it thick enough steel to be
> > able withstand internal pressure, it should mean that it is thick enough
> > to withstand an equal amount of external pressure.
> >
> > ----- Original Message -----
> > *From:* steve <mailto:steve@kobol.worldonline.co.uk>
> > *To:* personal_submersibles@psubs.org
> > <mailto:personal_submersibles@psubs.org>
> > *Sent:* Tuesday, July 08, 2003 4:52 PM
> > *Subject:* Re: [PSUBS-MAILIST] Calculations
> >
> > Hi All
> >
> > i noticed a lot of talk recently about propane tanks, the thread was
> > a bit dead but i gathered that the question of 'Is it worth using
> > that old propane tank in the garden', was dumped because the
> > work would probably be more involved to convert a tank than to just
> > buy the tube and end caps.
> >
> > My question though is, how does it compare with cost? If the cost
> > of the tank is say 1/2 the price would it be a viable option?
> >
> > I had the opportunity to check a propane tank up close recently; i
> > keep seeing psubs in all types of pressure tanks now...i think it's
> > called obsessive compulsive disorder.
> > Anyway, thinking as you do, that it would be a great size for a psub
> > i checked out the plate attached to one end.
> >
> > The plate read:
> > /*Manf: Robert Bros*/
> > */BS1500 Class 2/*
> > */210psi and Full Vacuum/*
> > */TP 310psi/*
> > */310 Gals/*
> >
> > A thought came to me, i remember on previous posts, a lot of talk
> > about the pressure vessel only being rated for internal pressure.
> > If it states that it can also withstand a FULL VACUUM, how does that
> > affect its suitability as a psub hull?
> >
> > I may be completely wrong here but, can vacuum stresses be compared
> > with external pressure stresses?
> > It seems to me that if the internal pressure was reduced to 1/2 ATM
> > then wouldn't that be the same as applying 2 ATM pressure to the
> > outside of a hull?
> >
> > i understand that once the pressure vessel used for a purpose it is
> > not intended for (ie. cut / welded etc.) the figures wouldn't be
> > valid anyway, but it makes me think
> >
> > anybody got any ideas???
> >
> > Regards
> >
> > Steve Bosworh
> > UK
> >
> >
> > P.S. I did check out the manufacturer but they apparently don't
> > exist anymore and the BS number is now obsolete and doesn't say
much.
> >
> > ----- Original Message -----
> >
> > *From:* Chris Jackson <mailto:trumpetrhapsody@comcast.net>
> > *To:* personal_submersibles@psubs.org
> > <mailto:personal_submersibles@psubs.org>
> > *Sent:* Tuesday, July 08, 2003 9:10 PM
> > *Subject:* Re: [PSUBS-MAILIST] Calculations
> >
> > After futher research and sudgestions, I have found that at 100
> > meters the water exerts a pressure of about 162psi, but what I
> > am not sure of is whether an air tank (decided on an air
> > tank instead of a propane tank due to price and comments I have
> > read about propane tanks) with a 165psi rating would hold up
> > to that kind of EXTERNAL force, since I assume the rating aplies
> > to the INTNERAL force rating.
> >
> > ----- Original Message -----
> > *From:* Chris Jackson <mailto:trumpetrhapsody@comcast.net>
> > *To:* personal_submersibles@psubs.org
> > <mailto:personal_submersibles@psubs.org>
> > *Sent:* Tuesday, July 08, 2003 1:12 PM
> > *Subject:* Re: [PSUBS-MAILIST] Calculations
> >
> > I have recently aquired more information, however I still
> > uncertain of several things. I found this link:
> > http://hotconnect.com/tank/vertair.htm and I am considering
> > the 36" by 98" tank made with carbon steel, but I am unsure
> > which pressure rating to use, the depths I
> > am considering will be in the 50-100 meter range, but
> > possibly shallower since this will mainly be used in
freshwater
> >
> > ----- Original Message -----
> > *From:* Chris Jackson
<mailto:trumpetrhapsody@comcast.net>
> > *To:* personal_submersibles@psubs.org
> > <mailto:personal_submersibles@psubs.org>
> > *Sent:* Tuesday, July 08, 2003 11:12 AM
> > *Subject:* [PSUBS-MAILIST] Calculations
> >
> > I am trying to do some calculations to figure out such
> > things as crush depth and required hull thickness. The
> > constants are that the pressure hull will be 36 inches
> > in internal diameter and 100 inches long, and what I
> > need to know is the relationship of hull thickness
> > versus crush-depth with these parameters. If anyone can
> > provide me results or information of a simple
> > calculation program, I would be much appreciative.
> >
> > Thank you,
> > Chris Jackson
>
>
>