Ohio Class, Ballistic Missile Nuclear Powered Submarine, USS Kentucky SSBN737

Add-onís, things you might / might not include:

A) Recharge access:
When the subís power reaches a pre-defined limit, the large hatch at the back of the missile deck opens, exposing a gold plated jack plug. This opening of the door is controlled by the electronics that is disabled after opening the door to avoid malfunction. This automatic operation is also a quite good indicator to the operator of the subís battery level at all times. After charging has been completed, the door is shut by hand.
I chose to build this feature. Images here.

B) Recharge through the periscopes:
Building the periscopes in metal, you can use them as electric poles when charging. Simply attach the inside ends of the periscopes to the charging circuits. Do not wire them directly to the batteries! If you leave the battery power connected directly to the periscopes during operation, you risk a metallic reaction, especially in salty waters.
There would also be a potential risk of the poles shortening, if the periscopes bents, collapses, or otherwise gets into electric conductive contact. Shortening of the batteries means total loss of sub control, and possible explosion / fire. 
I chose not to build this feature at all.

C) Missile and torpedo firing:
Missile and torpedo firing during operation can be done using the pressurized air onboard. This requires quite a lot of work with valves, but although difficult, Iíve seen it done and described several places on the Internet.
I chose not to build this feature in the initial stage.

D) Periscope, radar and snorkel control:
The periscopes, the snorkel and the radar can be extended and withdrawn using the same method as in automatic car antennas, or similar. It would sure be a nice detail.
I chose not to build this feature in the initial stage.

E) Dive horn:
Using a high mounted water resistant loudspeaker fitted within the hull, you can add a little drama to the sub. When the sub initiates a dive, the speaker could sound the well-known klaxon horn ďDive! Dive!Ē. The electronics could control this quite easy.
I chose not to build this feature in the initial stage.

F) Automatic Rescue Unit:
What happens if your sub gets out of radio range, runs low on power, or mal-functions? How do you reclaim your sub? This unit might help you, except in the event that you loose all pressurized air.
Itís a little closed box mounted, facing down, in the very bottom of the sub. It is closed by a hatch, and although visible from the outside, no one will notice it due to itís location. This compartment remains filled with air during operation, as the air cannot escape. Within this box is a rubber hose. Along with the box is another small electronics box with itís own battery. This control box monitors the operation, and if the sub has been without any contact for a period of time (1 to 2 hours?), it opens a valve and lets air into the hose. This expands, pushes off the hatch, and pulls the sub up to the surface. Also, see option H)
I chose not to build this feature in the initial stage.

G) Onboard compressor:
As an option, you might want to consider adding an onboard compressor, and a snorkel within the tower. Car Accessories shops carry small compressors that you might find use full. This would enable you to fill the air tank while surfaced, but the price is battery power.
If you chose to build this add-on, please pay close attention to the construction of the snorkel. Air is NOT allowed to enter the compressor at any time, so the snorkel needs to have an air lock blocking out all water while the sub dives. It will not look pretty, but it needs to be there, and  this is also the reason I suggests that you place the snorkel as high as possible within the tower. The compressor can be controlled by a simple on/off function on the remote, just like the lights, and should have auto-shut-off when the desired pressure is reached, or it could operate automatically. In both cases, DO build in a security feature that disables the compressor during a dive!
I chose not to build this feature in the initial stage.

H) "Sub sunk here"-buoy:
If you sub should sink, and run out of air or battery, it might be smart to have a buoy released indicating where the sub is. If you cut clear one of the missile hatches (or personnel hatches) and glue it on top of a bright yellow or red watertight plastic cylinder, connect a 9 -> 15 ft. string to the bottom of the buoy and the bottom half of the hull through the bottom of the silo, and then construct a mechanism that releases the buoy when needed, (Use an electrical push-actuator or similar) then you're set to go. The actuator would release the buoy when the power goes, as it pushes as long as power is up, but because it is spring actuated, it falls back in the resting situation. When you are at the surface and turns your sub off the buoy would be released as well, but since you are at the surface, it'll stay in it's assigned silo. The string should be strong enough to endure a moderate pull to the hull, as this might enable you to shake the sub loose from entanglement, or even complete a recovery attempt.  
I chose to build this feature.
Images here.

I) "Towed sonar" antenna:
As you know, the radio range is not all that great in water, and often limits the diving depth to only a fraction of what the design could otherwise endure. To solve this issue, I came up with this idea:
Real subs have a towed sonar that is used to cover the otherwise deaf angles behind the sub's own propeller noise. The towed sonar is basically a microphone, dragged in a long cable way behind the sub. If you build a "towed sonar", and drag it behind the sub in a long wire, and then construct the sonar to ride in the surface, then you can place your antenna in this surface-towed sonar, GREATLY improving max. diving depth. The towed sonar could be constructed so that it can be detached if not used, as the cable will take up too much space within the hull. The only danger is that the cable might get entangled in something (or somebody) during a run.
I chose not to build this feature in the initial stage, but have reserved space for this feature.

J) Thruster:
A thruster could really help you maneuvering the sub. There are two ways to control the thruster:
1) Connect a regulator to the rudder servo using a "Y"-cable, and it will follow the servo movements. This way you would save a channel, but it would mess up if you tried to turn using the rudders during reverse travel, as the thruster would now "thrust" the wrong way.
2) Assign the side to side movement of the throttle stick on the remote to the thrusters regulator alone. This allows you to operate the thruster independently of the rudders, so it could even be used when traveling in reverse. A switch on the remote could then mix the rudders and the thruster, so you can select them to co-work. The actual controller that operates the thruster motor, should be an RS-5 from SubTech. It's a "Reverse-off-on" switch module, that is the wired to a remote channel as described above.
I chose to build this feature (2) after advice regarding the turning radius of the SSBN's.
Images here.

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