Sonotube for submarine body / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Sonotube for submarine body

1 2 next

Move Thread

LAN_403

Andyman

Mon Sept 12 2011, 08:41PM

Registered Member #1083 Joined: Mon Oct 29 2007, 06:16PM
Location: Upland, California
Posts: 256

So me and 3 other students are working on a AUV project, this semester focusing on chassis construction. (Next semester is the logic)
The main body will be housing a mini ATX motherboard so we are going to be using an 8-10 inch tube. The dimensions will probably be around 10" x 30" (25x60cm). The design needs to function at a depth of 10 meters.
PVC pipe of this size would be very expensive, so we are currently considering getting a sonotube and encasing it in fiberglass sheet + resin to withstand the pressure and for waterproofing.
My question is, will a fiberglass shell be strong enough to withstand the pressures that occur at 10 meters? I know it has high tensile strength, but I'm not sure about the compressive strength; we definitely don't want the thing caving in.

Ash Small

Mon Sept 12 2011, 10:52PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

If the shell is thick enough it will.

I assume you are using 'S' glass and epoxy resin. Composite design is a whole subject in itself, not the kind of thing I can cover in one post after nearly a bottle of Merlot.

I'm assuming 'wet lay up', and probably no vacuum bagging. If you have access to vacuum pumps (of any kind) this is worth inverstigating.

Also the weave of the cloth. 'Crow's foot' or a similar weave is worth looking into if the ends are rounded.

You will 'probably' want progressively more layers towards the mid-section.

The major manufacturers all have links to design guidelines on their websites.

Hope this is of some help.

EDIT: You 'may' want to look into 'uni-directional' if you really need to save weight, but this is where it gets a 'bit more complex'. (probably not worth it if you are using wet lay up with no vacuum bagging)

Chopped strand mat and polyester resin 'will' do the job, but at the expense of extra weight and thickness. This will be easiest if weight is no problem, especially if you don't want to 'dig too deep' into the subject.

Patrick

Tue Sept 13 2011, 12:53AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

You need to do some engineering, the math that gives you a useful result, could also lead you down a path that costs more in cardboard and fiberglass than if you just bought a full price plastic tube. You are facing an optimization problem here.

Heres what you do: figure out a supplier for your fiberglass material, you need to know cost per volume.

second i presume the internal volume is important, mainbboards, motors, and batteries, and such, but including vertical and horzontal spanning beams and circular ribs would reduce the wall thickness while remaining crush resistant. A few internal braces could increase your crush resistance greatly, with a much thinner wall thickness of fiberglass.

The wall thickness fiberglass has its strength in the lengh and circumference directions, but the wall thickness is thin, therefore the strength is weak in these demensions without the internal braces.

If you try to make a perfect hollow cylinder with no internal braces (like a pipe), you may need a lot of fiberglass. Circumference times length adds up to a lot of SA. 10 meters deep is what? double atmosperic or 30 psi? i think.
SA times 30Psi could leave you with an expensive failure.

Doing this right could save you a lot of money, but doing it wrong could cost you everything.

Ash Small

Tue Sept 13 2011, 01:10AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

'To brace.....or not to brace, that is the question'

Patrick has some very good points here, but.....It does further complicate the design.

Bracing 'can' reduce wall thickness and weight, but can also lead to 'weak points' if you try to save too much weight/materials.

Depends whether you prefer simplicity or complexity.

Depends on your priorities.

Patrick

Tue Sept 13 2011, 01:17AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ash, welcome to the world of enegineering, How much ? too much ? too little? what the hell does this derivative imply? what!? We were suppose to integrate !?

Ash Small

Tue Sept 13 2011, 08:20AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

Ash, welcome to the world of enegineering, How much ? too much ? too little? what the hell does this derivative imply? what!? We were suppose to integrate !?

Yep.

Ok, some numbers. ~3 atmospheres pressure, 50psi ish, plus 'safety margin', X2? X3?...X1.5?

I think the first problem is internal access.

Some sketches might be useful here, along with some numbers, weight, cost, etc.

Have you considered buoyancy? commpressed air and ballast tanks is probably simplest, unless you just pull it back up.

Some more details are needed, will it have an umbilical cord or be untethered? etc, etc.

Andyman

Tue Sept 13 2011, 08:50AM

Registered Member #1083 Joined: Mon Oct 29 2007, 06:16PM
Location: Upland, California
Posts: 256

Thanks for the feedback guys! We are considering making it as close to neutrally buoyant as possible and "flying" it up and down with thrusters.
For bracing, I was thinking about those 50 gallon steel drums with the ribbing. If i form ribs along the circumference that should provide more crush resistance.
We were also thinking of using 2 slightly different sized tubes and slipping some rib lath sheet between them so you get a corrugated type thing

For this semester, it might be tethered, but ultimately it will be untethered and completely autonomous. We will be designing a watertight pressure hatch at some point.
Ideally, we are aiming for less than 45 Lb because you get super bonus points for that light.

Ash Small

Tue Sept 13 2011, 10:24AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

This may be of some use:

Also this site has some good information on mechanical properties of composites.

Dr. Slack

Tue Sept 13 2011, 12:00PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

We will be designing a watertight pressure hatch at some point.

There may be the opportunity to simplify the design and manufacture, build in some flanges for reinforcement, and get plenty of internal access if you make the watertight pressure hatches the front and back end caps.

That way, you are essentially bolting together a cylinder and two ends, each reinforced at their fragile edge by a flange. You have no compressive stress attempting to shear any fixings, as you would have if you cut a hole in the side of the cylinder and bolted a curved hatch over it. You could seal the flanges with a reasonable thickness of neoprene foam sheet, and the external pressure will only hold it together more.

If the motor goes in the back end-cap, and any sensors in the front one, then you have a very splittable assembly that allows you to work on several benches in parallel.

just a thought

Ash Small

Tue Sept 13 2011, 12:03PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

That would definitely be the ideal solution, Dr Slack, but I was trying not to design the whole thing for them.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob