I am somewhat new at the sub building game, but have many ideas!! As I have built a 1/72 scale Gato as well as the ability to launch 4 torpedoe's, here are my thoughts for the 1/72 scale Gato.
1) The torpedo system must be light (weight) so as not to screw up the dynamic balance of the sub and make it somewhat Bow heavy.
2) The torpedo should be light enough to float when the gas is expended.
3) The torpedo should also be dynamically balanced so that it will go "true" in the water as well as being able to travel a respectful distance.
I used 3/64 x .006 Brass tubing for the torpedo exhaust tube that extends to near the fill valve. The reason is so that the torpedo will travel about 35' to 40 '.
The Torpedo fill valve is a modified car valve inserted (screwed) into a machined plastic rod that has an O-ring as a gas seal.
The tail section of the torpedo is a molded plastic section with fins & O-ring seal that spin the torpedo in the water.
The launch tube is extended with torpedo inserted into launch tube some distance out from the Bow of the sub when filling the torpedo with propellant. Then retracted into sub when ready to launch the Gato.
I modified my R/C transmitter to have a push button On/Off switch that sends a signal to a 8 bit shift register that will fire the torpedo's. Position "0" in the start position of the shift register, position 1-4 are the firing positions for each torpedo and position 5 is the reset position for the shift register. The shift register triggers the 4 control valves via a power optical coupler, one torpedo at a time with each press of the transmitter launch switch.
I have attached some pictures and I will be happy to answer any questions you might have.