Why is steel stronger for tanks but not for airplanes?

[divad]

many car engine blocks have gone back to steel fwiw.

it all has to do with the type and quantity of forces.
Diesel engines are typically no more than 25:1 compression ratio and I am unaware of an aluminum block diesel *I'm sure there is one, just don't know about it*. Gas engines don't really go above 17:1. That compression ratio is easily converted to pressure since the "1" is 1 atmosphere, 1 bar, or 14.7psi. So 25:1 is roughly 370psi. That's a LONG way from 3000psi in most aluminum tanks. Aluminum is quite soft so you need a bunch of it to be able to resist bending forces which is why aluminum tanks are about 3x as thick as steel tanks. Roughly offset by the density of steel being ~2.5x that of aluminum, but the aluminum tanks also have flat bottoms which adds a considerably amount of weight to them.

If we want the best comparison out there, we can look at the Luxfer AL72, against the Faber LP85. Same diameter, same length, one aluminum flat bottom, one steel round bottom.
Both 7" long, both 26" tall. AL72 weighs 28.4lbs, the LP85 is 31lbs. Due to the difference in the wall thickness, there is a difference in water capacity, the LP72 holds about 10 liters of water where the LP85 holds 13 liters of water. Pretty significant increase in volume which explains the capacity difference, but the tank is about 3lbs lighter, and if you made it round bottom, or made the lp85 flat bottom, it would be about 6lbs delta. So of comparable physical dimensions, the aluminum tanks are considerably lighter, but because there has to be more material to resist the forces, the capacity is reduced. With this specific example, the wall thickness causes the aluminum tank to have a capacity about 75% of the steel tank.

If we look at aerospace, bike frames, etc. there is a minimum thickness required to resist bending. This is why composites are so brilliant because you can make incredibly thin structures that are incredibly rigid. There is no need for the actual strength of the materials in terms of tensile strength, or in this case resistance to pressure, but you need the thickness for the rigidity. Aluminum being less dense than steel is lighter for comparable rigidity, fiberglass even better, and carbon fiber one of the best materials out there. We don't really care about rigidity in scuba tanks, but it is inherent to the material thickness required to maintain that pressure.

We can make tanks out of carbon fiber, and some do, that are idiotically light, but idiotically light also means idiotically buoyant so the weight advantage on land is instantly negated by the need to put ballast on your body so you can become neutrally buoyant.

That all make sense?

I was about to say that.

 

[tbone1004]

Just tossing this in the thread salad but...........
Cousteau used 40's Titanium tanks for a while.

correct, but similar to composite tanks, if they float so much because of displacement that you have to add lead back, why use a more expensive material to make it lighter in the first place?

@divad sorry....

@Rred strong does not necessarily equate to tough either, and many materials are incredibly strong, but not tough. Carbon fiber for example is immensely strong in terms of ultimate tensile strength, but the area under the stress:strain curve is very small so it can't do a lot of work which is why it shatters like glass and has to be laid up with multiple layers of tough materials underneath it in most applications to be durable

 

[Rred]

rh-
Actually, yes, I am familiar with the Vega and the 2300cc engines. The larger story is that Chevy was tasked with building a small high performance engine, which they had never done before. And the Cosworth Vega proved it could be done, same base engine.

What usually goes unsaid in the urban rumour mill, is that Chevy actually did a reasonable thing, they copied some design points from Porsche. Including the aluminum engine block and the surface hardening treatment of the cylinder walls. The engine was designed to operate with a 210F thermostat instead of the typical 170F thermostat, because that allowed for better performance. The flame front in the cylinder would not be quenched as quickly as in other engines.

And that worked perfectly well for Porsche! Someone forgot to tell Chevy that the average "cheapest GM product" customer was usually either a newbie, who didn't know how to take care of a car, or a cheapskate, who wouldn't spend money on maintenance. Result being that they did not keep up the oil--which is the primary coolant for the cylinder walls--and they burned up the engines.

The Chevy engineers were kinda upset that what worked so well for Porsche (and for the Cosworth buyers) was such a disaster with the general public.

I put many miles on one of the Vegas. Never had an oil or temperature problem. But then again, I knew how to keep the oil and coolant full. And if you ran it up mountain highways with the throttle floored? The windshield wipers woulc sometimes come up and wipe by themselves, from the air pressure at 95mph. Which was about all the two-speed "slushpump" automtic could do.(G)

Kinda like saying "Aluminum 80's are killer tanks, 90% of all dive fatalities occur with folks using them!"
Ture, yes, but not a causal relationship.

tbone-
I fully agree with you. In fact, my buddy used to wonder why I had picked a huge heavy steel tank as my second tank and preferred to dive with it. I showed him the ~ten# lead plate I stuck to the bottom of my Alu80, and asked him what's the big deal? Ten pounds on my belt, ten pounds on my tank, I'm still carrying the same ten pounds, I'd rather get some extra air with it.

 

[wnissen]

Ironically, my first job was at a Ford casting plant that ingested iron ingots and belched out engine blocks, camshafts, etc. At the time (1993), the folk wisdom at the plant was that the new aluminum engine for Ford's vaunted "world car" was a huge mistake, could never be made reliable, etc. I see from Wikipedia that it had cast iron linings, though. The Ford Contour was a flop, but not because of the choice of metal in the engine, which turned out to be a pretty good one.

There is a lot of good stuff here on tank composition. Apologies if I don't call out your particular contribution.

I love that equation, that is exactly what I was looking for. I will try to work through with the relative densities and strengths of steel vs. aluminum, taking displacement into account. It does make sense that the rigidity of aluminum makes it better for certain narrow applications (pun intended) but it is not stronger in the overall sense. (And, incidentally, that also explains the use of composites in bikes being even preferable to aluminum). And also the elastic properties of aluminum making it less able to be overfilled make sense now that I've looked at their respective stress curves. Thanks, all.

 

[Centrals]

https://www.heritageconcorde.com/airframe-materials
The heat encountered by Concorde causes the airframe to expand 7" in flight!

 

[lucca brassi]

1 and most important is young modul : compression vs extension before break

Young's modulus - Wikipedia
Young's Modulus - Tensile and Yield Strength for common Materials

2. second is more economical : density : volume vs mass

Density - Wikipedia