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Comment by stevecalifornia | Hacker News Reader

Does this thing have a tether to the surface or not? I would be blown away if it didn't have a tether, but that seems to be the case.

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Even without a tether, I'm mystified it didn't have an attachment that was designed to return to the surface and act as a beacon if anything went drastically wrong. Since all it would need would be a few sensors, a battery, and an antenna one imagines it would be somewhat easier to design than the main vessel.

They're going 4,000 meters/ 2.5 miles under the sea. Roughly 400 atmospheres of pressure. A quick google shows the longest commercial tether is 1,100 meters and is meant for undersea drones for power/ data.

Aside from the massive tether and engine that would need to pull it, I'd imagine it would rip the submersible to shreds at those depths

The Jason ROV has a six mile tether:

https://en.wikipedia.org/wiki/Jason_(ROV)

And the Titanic was discovered by camera sled being towed by a ship.

https://en.wikipedia.org/wiki/Argo_(ROV)

So your search might have been a bit too quick. It certainly is possible possible to build a tethered crewed submersible, though this vehicle wasn't.

Credit to js2 from this comment: https://news.ycombinator.com/item?id=36396161

Just as another data point, the Nereus[1] explored the Mariana Trench at nearly 11km depth, and had a comms tether (very thin optical fiber). According to the wiki, its tether was 40km in length total. So, it's not out of the question, I don't think.

No good for pulling it to the surface, of course, but reliable communication alone would be a great benefit.

[1] https://en.wikipedia.org/wiki/Nereus_(underwater_vehicle)

The tether is not connected directly, it's connected to a TMS that then has a smaller cable connected to the submersible.

Ex.

https://www.researchgate.net/figure/Schematic-image-of-a-wor...

https://f-e-t.com/subsea/vehicles/tether-management-systems/

There's also this one which looks like it can get to the depths of the Titanic:

https://nautiluslive.org/tech/rov-argus

So it was certainly possible to do this with a tether, but it would have been much more expensive.

They didn't really need an expensive and complex "umbilical cord" type of system, but a simple rope could pull the sub up in an emergency, and keep it from getting lost or separated from the ship.

This sub has 4 Innerspace 1002 thrusters that make for a max total of 342*4=1,368lb of thrust... I think a useful point of reference in deciding how much pull force would be needed for an emergency recovery tether.

I think it's reasonable that a 4mm dyneema line with a breaking strength of 4,000lbs would be sufficient for emergency surfacing, and safely provide much more thrust that it's own motors. The whole sub weighs 23k lbs and is approximately neutrally buoyant.

Total weight for 3 miles of 4mm dyneema is only about 120lbs, and for simple emergency use, in the spirit of OceanGates now infamous "scrappy thriftiness" could be pulled with the type of simple/cheap hydraulic winches used for pulling crab pots, anchors, or fishing nets. 3 miles of this line would have cost about $15,000.

How big of a ship would you need to even hold that tether?

At 4000m under the ocean, you'd need like 10000m of cabling at a minimum I'd assume? And then you'd need the tether + winch to be capable of supporting and lifting the sub at that depth.

I'd be amazed if any country let alone company has that capability outside of maybe the US military.

How do they control the unmanned subs that they use to survey the Titanic then?

They are not tethered?

A data link tether and one capable of pulling up a giant heavy craft are very very different

The vehicle that discovered the titanic was a sled pulled behind a ship on a tether. A thick steel cable can take a lot of load. https://en.wikipedia.org/wiki/Argo_(ROV)

Also, subs operate at close to neutral buoyancy. A tether does not need to support the weight of the vehicle, it only needs to resist the force of drag that the water exerts on the vehicle moving at whatever the max tow / lift speed is.

The vehicle that

I think a 4mm dyneema line would have been more than sufficient to pull this to the surface in an emergency, and would have been relatively cheap and light. It could safely 1-2 tons of force, which would rapidly lift a neutrally buoyant sub of any size…

This would cost about $3/meter.

Thethers and ships that can hold their position for thethered ROVs are very expensive. OceanGate didn't even have their own ship, they chartered one and that ship released their base platform from which the sub takes off.

It doesn’t have a tether.

It did not.

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