The S-80 class submarines, still under construction for the Spanish navy, have been elongated due to technical problems. This model, at 1:120 scale, is of the original project.
The hull was modelled in Fusion 360 using photographs found online and some rather vague lateral and top views found on related publications.
Then, it was 3d printed in grey ABS and acetone-welded together.
The WTC, housing all the control electronics, servos, pumps, and propulsion motor is made out off a convenient water bottle I found at a local shop. All movements are magnetically coupled to the exterior, and two tubes go out through the end cap, one housing the antenna and another to a external balloon, the bladder of the RCABS.
The pumps and solenoid needed for mentioned system are controlled by the circuit below: By means of some op amps and passive components, the pulse width of the PPM signal from the received is turned into a voltage, which then goes to some comparators that in turn control the pumps and solenoid via a couple transistors. It could have been done with a microcontroller, but perhaps this can be simpler.
Here is a summary video of the build:
All files for the 3d printed parts and a couple drawings are available here.