Comments about technological history, system fractures, and human resilience from James R. Chiles, the author of Inviting Disaster: Lessons from the Edge of Technology (HarperBusiness 2001; paperback 2002) and The God Machine: From Boomerangs to Black Hawks, the Story of the Helicopter (Random House, 2007, paperback 2008)

Thursday, March 22, 2012

Deepsea Challenger: Waiting or going?

Still waiting to hear what Deepsea Challenger is down to. Given the lack of word on the website's latest news page, friends on its Facebook page are speculating whether Explorer-Pilot James Cameron is on his way down, on his way up with a smile on his face, or just waiting for calmer seas. 

Following my first post, I've been wondering whether they might have gone ahead with an unmanned test dive for a full systems check; that would explain the lack of publicity. But that's just speculation. 

Meanwhile, here's more information on some of the ultra-deepwater tech in use. First, a photo of the Penn State Applied Research Lab's pressure facility where the Deepsea Challenger's pilot sphere underwent its stress test:
The four-ton cap to the pressure cylinder is moved by overhead crane, but the two employees twirl it into place by hand, using a two-man lever arrangement. From the website for ARL's high-pressure lab, here's a photo sequence. 
On the subject of big hunks of metal, here's a photo of the pilot sphere that ARL tested, early in its life, just after forging. 
We're told in this IEEE Spectrum article that it's "traditional steel." Well, maybe as in "traditional for submersibles." I doubt if it's the kind of mild steel used for girders. Higher-strength steel is essential to keep the size and cost of the craft in control. Syntactic foam is pretty expensive. I've seen figures of more than $10,000 for one cubic meter of standard syntactic foam, which would be enough to displace one ton of water.

The personnel cabins on the Russian Mir submersibles, which have to be the most time-tested manned submersibles in the world, use an nickel alloy called maraging steel. Here's the business end of a Mir in action.
Two of them will be busy during the upcoming Titanic season, taking two ticket-holders on each trip.

Maraging steel has been used for decades in submersibles. So it's likely that Deepsea Challenger used a similar nickel alloy for its Cameron-protective sphere. 

Now, a little more about the proprietary syntactic foam used for buoyancy on the craft, called IsoFloat. The Deepsea Challenge page on the sub's technology, including IsoFloat, says that it makes up 70% of the volume of the craft, and has twice the tensile strength of standard syntactic foam.

Standard syntactic foam is not known for its tensile strength (the ability to resist pulling forces). Rather, it's remarkable for its compressive strength, its ability to resist water pressure without crushing. That's due to millions of micro-spheres embedded in a plastic matrix. Here's a photo of the l'il wonders:
Without microspheres (glass, ceramic, or aluminum), there's no compressive strength down deep. Here's what happens to polystyrene foam when immersed to 9,500 feet, from the National Geographic Society's My Wonderful World site:

Here's an article about failure tests carried out on a standard syntactic-foam slab, illustrating how it fails in tension before compression. We're told that IsoFloat foam has twice the tensile strength of standard syntactic foam. 

Why is tensile strength relevant? As I mentioned in the previous post, about the last thing the team wants to see is the craft breaking in two when it's suspended over the ocean. Here's a photo from the project's Facebook page. (The lifting point is at the top, suspended from the red line; the two white straps in the foreground are taglines, to keep the load from swinging.) The lifting bags are also visible.

If that were to happen, the heavy end (the pilot sphere, trim-shot hoppers, and 1,100 pounds of ascent weights) would drop into the water. Even if the ascent weights came free as the power failed, no one would want to find out if there's enough syntactic foam still adhering to keep the pilot sphere afloat. 

So I'd guess the Cameron design team didn't put all their bets on the tensile strength of IsoFloat to hold things together. In this photo from Deepsea Challenger website, notice the white lifting straps that surround the capsule:
Presuming the straps weren't some kind of temporary arrangement during manufacture, it suggests to me that these straps assist in tying the crane's lifting point (at the top) to the sphere and the ascent weights (below). 

That could be done, for example, by gathering the straps into a longitudinal tube and running them the length of the craft, up to an embedded lifting frame.

Use of internal straps would still leave the IsoFloat as the structural beam the website says it is, because it serves as a backbone that unifies the flotation, the heavy iron, batteries, lights, camera arms, and lots of other equipment. But the foam beam wouldn't be the only key structural element. The Cameron team is big on redundancy, so I'd guess that there's structural redundancy also.

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