Unmanned sub touches deepest part of world's ocean(cbc.ca)
cbc.ca
Unmanned sub touches deepest part of world's ocean
http://www.cbc.ca/technology/story/2009/06/03/nereus-mariana-trench-dive.html?ref=rss
19 comments
"The water pressure at the bottom of the Mariana Trench is like having the weight of three SUVs on your big toe"
"The fiber optic tether transmits high bandwidth data only, not power...As the vehicle moves through the water, the tether is laid from the vehicle, avoiding any dragging of the tether through the water. This decreases the energy load on the vehicle and the strength requirements for the tether; hence its size and weight can be minimized"
http://www.spawar.navy.mil/robots/pubs/HROV%20Tether%20Ocean...
Cached: http://74.125.47.132/search?q=cache:jrgXAl_Pvf8J:www.spawar....
"The fiber optic tether transmits high bandwidth data only, not power...As the vehicle moves through the water, the tether is laid from the vehicle, avoiding any dragging of the tether through the water. This decreases the energy load on the vehicle and the strength requirements for the tether; hence its size and weight can be minimized"
http://www.spawar.navy.mil/robots/pubs/HROV%20Tether%20Ocean...
Cached: http://74.125.47.132/search?q=cache:jrgXAl_Pvf8J:www.spawar....
This is amazingly awesome. At the end it talks about how this device connects to the surface using cabling thinner than a human hair. Technology just blows my mind.
but [steel cable] systems would snap under their own weight in the pressure conditions of the Mariana Trench.
Does that make sense? I don't see how pressure can make things weigh more... but I guess 10km of steel cable will have some weight. I'd have thought you'd make the cable have the same net density as water, so it is effectively weightless.
Does that make sense? I don't see how pressure can make things weigh more... but I guess 10km of steel cable will have some weight. I'd have thought you'd make the cable have the same net density as water, so it is effectively weightless.
Materials act differently under different conditions. Lots of pressure can make some materials very brittle or even softer. Brittle materials plus the weight/stress from the water currents involved could be what they are referring to.
I don't know the specifics of how the steel/iron/other compounds would react to that pressure but that's a guess.
Edit:
Other sources leave out the pressure part, it looks like the weight of the cable would be the issue. I guess it's also why we don't have a space elevator yet.
I don't know the specifics of how the steel/iron/other compounds would react to that pressure but that's a guess.
Edit:
Other sources leave out the pressure part, it looks like the weight of the cable would be the issue. I guess it's also why we don't have a space elevator yet.
The problem is the weight of the cable. You can reduce the effective weight of the cable in water by, for example, combining polypropylene and steel strands (they experimented with this for some anchor cables in the late 70s early 80s) but the cable has larger diameter with greater drag on the moving robot. Maybe even enough drag to snap the cable anyway. Also, I am not sure how well the experiments turned out, I can see several other possible problems with the combined cable.
> I'd have thought you'd make the cable have the same net density as water, so it is effectively weightless.
It still has momentum. So it could be broken just by trying to pull it - even without the weight.
And you are right - "pressure conditions" is not correct. It's simply the weight of the cable.
It still has momentum. So it could be broken just by trying to pull it - even without the weight.
And you are right - "pressure conditions" is not correct. It's simply the weight of the cable.
I'd guess even changing its density wouldn't be trivial. Whatever material you choose it'll have to stand the pressure without compacting.
Poorly described, but I take it to mean that the pressure on the cables themselves from the water would cause them to be destroyed.
I don't see how. Accelerated rusting? Transition to a different crystal structure?
Maybe by simply stretching the wire by the force of water movement (like pulling a guitar string up really hard).
Crushing?
Crushing on an atomic level?
There is no void (air) inside the cable, and the pressure is even all the way around - the only crushing you can do is move the atoms closer to each other. (Compress actually, not crush, and iron is basically incompressible.)
There is no void (air) inside the cable, and the pressure is even all the way around - the only crushing you can do is move the atoms closer to each other. (Compress actually, not crush, and iron is basically incompressible.)
"moving the atoms closer to each other" is what I was talking about; under some circumstances, that could manifest as a shift to a different crystalline structure. (If you search for information on high-pressure crystalline phases of materials, you'll find a lot of interesting things.) "Iron is incompressible" is often a useful approximation, but no material is actually incompressible (that would entail, among other things, faster-than-light propagation of shock waves and infinite hardness) and many day-to-day materials are less compressible than iron. Glass, for instance.
Most metal objects do actually have voids in them, but you can manufacture metal objects that don't, and it's not clear that having voids in your cable would cause it to be damaged by pressure.
Most metal objects do actually have voids in them, but you can manufacture metal objects that don't, and it's not clear that having voids in your cable would cause it to be damaged by pressure.
The Sanmina/SCI fiber cable used is only .25mm diameter and 11 km of the stuff weighs only .173 kg (less than 1/2 pound).
The Woods Hole page on Nereus is at http://www.whoi.edu/page.do?pid=10076
The world has had the technology to go there since 1960 when a two man crew in the Trieste went to the Challenger Deep. I think the reason new subs don't have a crew is because it's cost prohibitive. Since the median ocean depth is 1/3rd of the depth... most of these subs aren't engineered for extremes. My guess is the next time a crew goes there it'll be funded by some rich explorer looking for a challenge (Richard Branson, etc). Hopefully he'll make some real innovations and share them with the community. In the meanwhile it's good to see a new robotic sub visiting.
http://en.wikipedia.org/wiki/Bathyscaphe_Trieste
http://en.wikipedia.org/wiki/Challenger_Deep
EDIT: Added a few sentences.