[jedc]

Former submariner here with a quibble on this:

> one of the primary reasons subs need to surface , in order to GPS lock

While getting a GPS position is helpful, the primary reason a submarine goes to periscope depth regularly is for communications. The Navy needs to send information to submarines and know that they'll get it and take action within a certain timeframe. That's by far the primary driver.

[eyko]

From layman to former submariner, a very silly question I've always wondered about and that I now have the rare chance to ask someone with some expertise: Do submarines mostly roam about or do they tend to stay relatively quiet/idle? I always imagined submarines constantly moving about but at the same time it feels like a waste of fuel.

[jedc]

Ohio-class submarines (the ones that carry missiles): when they're "on station" they're just tooling around staying as quiet as they can. There's a relevant phrase for them: "Three knots to nowhere"

Los Angeles and Virginia-class submarines are always doing something: doing exercises, transiting from one location to another, etc. And typically multiple things at once. While the boat is transiting from an exercise area to homeport, the team is doing engineering drills, or other kinds of training. Or the forward part of the boat is doing exercises with a carrier battle group while the engineering team is doing engineering drills. (There's ALWAYS engineering drills or maintenance happening.)

Fuel isn't a primary concern: a nuclear reactor is fueled for the life of the boat, so 30-ish years. That said, effective life of a reactor is something the Navy tracks closely, and depending on the life of the boat, the life left in the reactor, some boats are decommissioned as they get close to the end of their fuel life, and others get re-fueled. (And in the case of the USS San Francisco, who had recently been refueled before it hit an underwater mountain, they cut off the front half of the submarine and welded the front half of a recently-decommissioned submarine on, because the reactor and fuel was too valuable to go to waste)

[ateng]

What about diesel boats? They are exclusively attack subs but fuel is relatively limited.

[jedc]

The US doesn't have diesel boats anymore. Though other countries (like Australia) do.

[jwithington]

French (& American and Royal Navy) submarines are nuclear powered. No fuel necessary.

They do have measures of "nuclear fuel" remaining, but it lasts about 30 years (at least in the American boats) so generally doesn't impact day-to-day considerations.

[gresrun]

While I'm 100% positive the details of operational concerns like this are classified, there are 2 distinct types of submarines today with 2 different objectives:

1) Attack Submarines (e.g. Los Angeles-class & Virginia-class for USN) which usually roam within a designated operations area, surveilling, tracking, and generally keeping tabs on other nations' surface & sub-surface fleet dispositions. These subs typically have multi-week sorties and may intermittently surface for surveillance & comms.

2) Ballistic Missile Submarines aka "Boomers" (e.g. Ohio-class for USN) which are given a strategic area in which to operate and their objective is to remain silent & undetected, waiting for the hopefully-never-coming order to launch their SLBMs. These subs usually have multi-month sorties and often don't surface until the end of their patrol.

[ThinkBeat]

Clearly the Ballistic Missile Submarines surfaces intermittently surface for comms as well? If not, they won't know when to set off their missiles making then not very useful as a deterrent

I have often wondered how close to the surface they need to get.

I would presume retractable antennas could be extended from a sub from a non-trivial depth. Or cable attached to buoys Or something much smarter that I have not thought about yet.

[yencabulator]

There's a couple of different "wake up" signals that can reach deeper into water. Their biggest limitation is very low bandwidth, so an attack sub will emerge (/send up a buoy on a tether) to get an updated tactical map.

https://hackaday.com/2020/07/15/the-many-methods-of-communic...

https://en.wikipedia.org/wiki/Communication_with_submarines

[ethbr0]

Also highlighting the E-6B Mercury (and the upcoming EC-130J), which among other communication options has a 5-mile (!) VLF antenna it deploys vertically in midair (!!) to establish limited-bandwidth communications with submarines.

https://m.youtube.com/watch?v=en-qekZX4ws

But I believe that's only good down to ~60 ft. Anything deeper requires the really long land-based ELF arrays.

[oynqr]

Layman here as well, but there are probably no fuel concerns because the reactors are constantly running anyway.

[thatguy0900]

I wonder what the fuel difference is between fighting the currents and generating electricity while staying in the same area VS just moving around

[jedc]

The main thing is that you need to be moving at least a few knots in order for your control surfaces (rudder, forward planes, stern planes) to work.

[timthelion]

Do military subs have antenna buoys on cables that they can raise up without having to surface?

[nradov]

Yes, modern subs typically carry comms buoys. Some are tethered and allow for two-way communication. Others are transmit only and float up to the surface independently.

[jedc]

antenna buoys on cables? No.

But there are VLF antennas that Ohio-class submarines have to receive low-data-rate comms while submerged.

Any reasonable communications have to be made while at periscope depth. (Which is subtly different than "having to surface"... at PD just the thin mast is out of the water.)