The installation was composed of torpedo tube (Rohr 5), torpedo compensating tank (Torp Zelle 1), vent pipes, drainage pipes and valves: feeding valve (c2) which connected the installation with boat's low pressure system, control valve (q), cut-off valves (g), equalization valve (h), drain valve (i) and vent valve (k1). Some of these valves were brought together into a control panel (Ausgleicharmatur). The torpedo compensating tank's capacity was greater than torpedo tube capacity and was initially filled with water.

The flooding sequence was as follows:

1. drain valve (i) - open;
2. cut-off valves (g) - open;
3. control valve (q) connecting boat's low pressure system with compensating tank (Torp Zelle 1) and connecting tube with the opened vent;
4. feed valve (c2) was then opened - pressurized air from the boat's low pressure system forced water from the compensating tank to the torpedo tube. When the water started draining from the vent valve (q), the pressurized air was cut off and valve (i) was closed.
5. Finally pressure was equalized by momentary opening of the equalization valve (h), which connected the torpedo tube with the outboard sea water.

Then the torpedo tube muzzle door could be opened and torpedo could be fired. After launching the torpedo and closing the outer door, over one ton of sea water was inside the tube. To drain the tube the following sequence had to be completed:

1. drain valve (i) - open;
2. cut-off valves (g) - open;
3. control valve (q) connecting boat's low pressure system with the tube and compensating tank with the opened vent;
4. feeding valve (c2) was then opened - pressured air from boat's low pressurized system forced sea water from the tube to the compensating tank. When the air started venting from the vent valve (q), the pressurized air was cut off and valve (i) was closed.

Then the compensating tank could be empted into the sea by means of pressurized air. A special interlock prevented simultaneous opening of the muzzle door and drain valve (i).

1. control valve (q)
2. cut-off valves (g)
3. feed valve (c2)

1. Drain valve (i).

Photo 10. Hull valve with pressure equalization valve.

3. Torpedo firing gear

The torpedo firing gear was designed to give the torpedo some starting velocity and activate its engine. The first widely used firing gear was a simple pneumatic system used till the end of World War I. The torpedo was ejected by means of pressurized air, which was pumped into the space between the torpedo and tube breech door. An undesirable result was the appearance of air bubbles on the surface of the water, which betrayed the position of the attacking boat. That's why splashless launching systems were developed. The German design used a steel piston closely matched to the inside diameter of the tube, which was placed behind the loaded torpedo.

Photo 11. Opened torpedo tube with piston inserted on U-570 [2].
 

Photo 12. Aft torpedo tube and piston on U-995.
 

The piston was made from steel, weighed about 35 kg and had two guide wings on its edge which fitted into guide recesses inside the torpedo tube which ran almost the length of the tube up to about 90 cm before the muzzle door. The torpedo tube was strengthened by means of external framing for one meter from the muzzle door in the area where arresting the forward movement of the piston created extra stress.

Photo 13. Interior of the aft torpedo tube on U-995.
 

Drawing 2. External framing of the forward torpedo tubes [6].

Photo 14. External framing of the forward torpedo tube on the U-260 [7].

Photo 15. External framing of the forward torpedo tube on the U-352 [8].

Compressed air was blown into the space between the piston and the closed breech door. The air pushed the piston away with the an average speed of 10 m/s, which in turn pushed the torpedo down the tube. The piston was stopped about one meter from the outer end of the tube where guide recesses inside the torpedo tube ended preventing the air from escaping. Then the tube was automatically vented inside the boat and external water pressure pushed the piston toward the inner breech door. Finally the muzzle door was closed and the tube was drained.

The above sequence was used only for a submerged torpedo launch. When a torpedo was fired during a surfaced attack it didn't work because water pressure at depth of 1-2 m was insufficient to push back the piston. That's why during a surfaced attack the torpedo was fired without using the piston - compressed air was blown into the space between the piston and aft part of the torpedo pushing the torpedo away. The lock coupled with the director valve locked the piston in the tube.

The type of launch (submerged or surfaced) was selected by means of a director valve. The air pressure used for submerged and surfaced launch was respectively 10,5 at and 16 at.

This type of splashless launching gear was simple but required precise manufacturing of the tube and piston. The fit of the piston in the tube had to be tight but with enough clearance to not jam. For that reason the tubes were easily damaged by explosions of aerial bombs and depth charges. The Permanent Order No. 61 issued by BdU [9] described procedure of checking torpedo tubes after each major attack.

The gear consisted of:

• pressurized air flask with a maximum pressure of 30 at and a volume about 0,22 m^3
• director valve (p) selecting type of launch - surfaced/submerged
• vent valve (m) - this valve worked only when submerged launch was selected, in case of a surfaced launch it was blanked by piston
• firing valve (o) - which blows pressurized air from the flask to the tube and which was opened by a smaller, pilot valve, which in turn was opened by the firing rod
• pilot valve (s)
• feed valve (e)
   

When the tube was ready to launch, the firing rod stretched the spring, the pressurized air flask was filled to maximum pressure and the feed valve (e) was closed. The launch was initiated by releasing the firing rod by means of electromagnet (when remote firing from conning tower or bridge) or manual handle. The firing rod moved back and pulled out the torpedo stop bolt, retracted the trigger latch and finally opened the pilot valve (s). Opening the pilot valve caused the firing valve to open and blow pressurized air from the air flask through the director valve into the space between piston and breech door (in case of submerged launch) or between piston and the aft part of torpedo (in the case of surfaced launch). The torpedo was pushed down the tube and the trigger latch activated the torpedo engine. When submerged, the vent valve (m) was opened automatically allowing the venting of the pressurized air into the boat. The piston was moved back by external water pressure and the tube was filled with sea water. After closing the muzzle door the tube could be drained. When surfaced, the pressurized air escaped through the external end of tube.

Drawing 4. Operating principle of the firing valve [10].

Photo 16. Forward torpedo room on U-190 [4].

1. Feed valves (e) of tubes No. 2 and No. 4
2. Tube No. 2 pressurized air flask
3. Tube No. 2 firing valve (o)
4. Tube No. 2 director valve (p)

Photo 17. Forward torpedo room on U-995 [10].

1. Electromagnet allowing remote release of firing rod.
2. Firing rod.
3. Firing rod manual release handle.
4. Firing rod safety catch.
5. Director valve (p).
6. Firing valve (o).
7. The gear locking the piston in the tube during the surfaced launch.
8. Pilot valve (s).

   Photo 18. The gear locking piston during surfaced launch at the torpedo tube No 4.

1. The locking gear coupled with the direction valve. When the valve is set to the "surfaced", the locking bolt is pulled inside the torpedo tube.

Photo 19. The piston from the starboard torpedo tubes of the U-505 in the Chicago Museum of Science and Industry.

1. Top - right guiding wing.
2. Top guiding wing.
3. The hole for the locking bolt.

Photo 20. The vent valve of the torpedo tube No 3.

1. The vent valve (m).

The vent valve was connected via the pilot valve with the air flask. When the air pressure in the flask dropped to about 5 atm, the valve (m) was opened and the tube was vented to the boat's interior.

Drawing 5. Graph of the pressure and piston and torpedo velocity depending on the position in the tube [10].

There were occasions, when the torpedo engine activated inside the tube by accident (Rohrlaufer). To avoid tube damage due to hot exhaust gases (G7a steam torpedoes) or the high temperature of a non-cooled engine the torpedo had to be removed from the tube as fast as possible - by means of pressurized air from the firing flask or in case of lack of air [11] - from high pressure mine discharge installation.

More frequent accidents led BdU to issue a Current Order No. 68 in November 1944, which described maintenance of torpedo tube [13].

4. Mine discharge gear

German torpedo tubes were fitted with additional gear which allowed discharging naval mines. The tube could be loaded with 2 mines of type TMA or TMC or 3 mines of type TMB. Mines were discharged by means of pressurized air from the boat's high pressure air installation.

Drawing 6. Mine discharge gear [3].

Pressurized air was blown in sequence through the pipes (s2), (s1) and (s) pushing away respectively external, middle and internal mine. Switching (selecting which pipe to use) was accomplished by means of selector valve (p1). Master valve (o1) opened the high pressure air supply. A pipe not described on the drawing, passed through the pressure hull and was connected to an actuator, which lifted the mine stop bolt, which locked the most external mine in the tube. If necessary, all mines could be discharged at once by means of pressurized air blown between piston and breech door.

Photo 21. Mine discharge gear [1].

1. Master valve (o1)
2. Control valve (p1)
3. Mine stop bolt shaft lever
4. Rod preventing opening master valve (o1) when mine stop bolt was retracted.
5. Rod preventing releasing firing rod when mine stop bolt was retracted
6. Rod opening valve inflow of pressurized air into the mine stop bolt actuator.

5. Interlocks

Torpedo tubes were fitted with a number of interlocks, which prevented improper operation of the torpedo tubes which endangered the safety of the boat. A common element of these interlocks was an interlocking rod.

- An interlock preventing simultaneous opening of breech and muzzle doors of torpedo tube consisted of an angle plate attached to a rotating ring which locked the breech door, interlocking rod and the operating shaft which opened muzzle door. The operating shaft could be rotated only when it was not blocked by the angle plate - the rotating ring was in position locking the breech door. When the shaft was rotated, it moved the interlocking rod back toward the rear end of the tube by means of the screw gear and locked the angle plate preventing rotation of the locking ring and the opening of breech door.

- The firing rod had to be recocked before opening the torpedo tube breech door. Firing rod recocking caused retraction of the torpedo stop bolt into the tube and lifting of the trigger latch. When fired, the firing rod was moved back towards the rear end of tube entering a special socket on the rotating ring locking the breech door. The ring could be rotated only after recocking the firing rod.

Photo 22. Torpedo tube doors interlocks on U-995.

1.Ring locking breech door in "unlocked" position.
2. Angle plate in "unlocked" position.
3, 5, 11. Socket for firing rod interlock.
4, 10. Angle plate in "locked" position.
6. Muzzle door operating shaft.
7. Firing rod.
8. Disengaged interlocking rod.
9. Engaged interlocking rod.
12. Screw gear rotating the locking ring.
13. Firing rod recocking gear.

- The interlock preventing launching a torpedo when the muzzle door was closed was composed of safety catch locking the firing rod. The safety catch was pulled out while opening muzzle door.

- The tube drain valve was locked in the "closed" position by the feather on the interlocking shaft. When the drain valve was opened, a sliding bolt operated by the link gear from the valve handle, prevented revolving the muzzle door operating shaft.

Photo 23. Drain valve interlocks on U-995 [1].

1. Muzzle door operating shaft
2. Tube drain valve
3. Interlocking rod
4. Gear locking revolving of muzzle door operating shaft
5. Link gear from drain valve handle to sliding bolt

- The mine discharge gear master valve couldn't be opened when the muzzle door was closed - the valve was locked by the feather on interlocking shaft.

- The mine discharge gear master valve couldn't be opened when the mine stop bolt was retracted - the valve was locked by the link gear from the stop bolt operating shaft

- Launching the torpedo when the mine stop bolt retracted was prevented by the link gear connecting the stop bolt with the firing rod.

Photo 24. Mine discharge gear interlocks on U-995 [1].

1. Mine stop bolt operating shaft.
2. Mine discharge gear master valve.
3. Interlocking rod.
4. Mine discharge gear control valve.
5. Link gear locking the firing rod.
6. Link gear to valve supplying pressurized air to mine stop bolt actuator.

6. Torpedo setting gear

The steering gear of a torpedo loaded in the tube had to be set to desired values before launch. These settings were: run depth, speed and gyro angle. In later years of the war, as new maneuvering FAT and LUT torpedoes were introduced, there were additionally straight run length and first turn direction (FAT steering gear) and three additional angles (LUT stearing gear). The settings were adjusted by means of rotating spindles, which were inserted into the torpedo through holes in the tube wall. Speed, depth and settings for FAT steering gear were adjusted manually. These setting gears had to be removed before launch. The most important was the gyro angle setting which defined the torpedo course after leaving the tube. This angle was calculated during attack approach with the Torpedo Data Computer and passed to the gyro angle receivers (T-Schußwinkelempfänger) in the forward and aft torpedo rooms by means of a synchro system. The gyro angle receivers were connected with gyro angle setting gears (GA-Stellzeuge) attached to the tubes by means of rotating shafts. The gyro angle setting gear set the desired gyro angle by means of a rotating spindle inserted into the torpedo. This spindle was lifted by the firing rod when the torpedo was launched. In case of damage to the synchro system, the angle could be set manually by a wheel on the gyro angle receivers. The receiver in the forward torpedo room also received the spread angle for a salvo shot. The receiver passed modified gyro angles to the torpedo tubes selected for the salvo shot.

From June 1944 U-Boats were armed with maneuvering torpedoes with LUT steering gear. These torpedoes required additional setting gear which adjusted additional angles. These angles were set by means of the LUT control panel located in the forward torpedo room, between the torpedo tubes. The control panel was connected with setting gear (GA-Stellzeuge) attached to each torpedo tube by means of a rotating shaft and two flexible rods.

Photo 25. Gyro angle setting gear of aft torpedo tube on U-995.

1. Gyro angle setting gear (GA-Stellzeuge).
2. The shaft connecting gyro angle setting gear with gyro angle receiver (T-Schußwinkelempfänger).
3. Depth setting gear manual handle.
4. Muzzle door opening gear operating shaft.
5. Firing rod.
6. and 7. Mine discharge gear pressurized air pipes.

Photo 26. Gyro angle receiver in aft torpedo room on U-995.

1. Gyro angle receiver (T-Schußwinkelempfänger) - one of the dials shows angle in the range of 0-360°, the other in the range of 0-10°. Each dial has a pointer and rotating "window" with red mark. The pointer shows the value passed from the TDC, the "window" - shows the current value set by means of the hand wheel and passed to the GA-Stellzeuge by means of the rotating shaft.
2. Handle to manually set desired angle.

Photo 27. Gyro angle receiver in forward torpedo room on U-995.

1. Gyro and spread angle receiver (T-Schußwinkelempfänger).
2. Wheel to manually set the gyro angle.
3. Shaft connecting gyro angle receiver (T-Schußwinkelempfänger) with gyro angle setting gear (GA-Stellzeuge) of tube No. 2.
4. Shaft connecting gyro angle receiver (T-Schußwinkelempfänger) with gyro angle setting gear (GA-Stellzeuge) of tube No. 1.
5. LUT control panel.

Unlike the smaller and simpler gyro angle receiver in the aft torpedo room, the forward gyro and spread angle receiver was equipped with the servomotor, which automatically set the gyro angle setting gears attached to the tubes according to the values received from the TDC. In case of servomotor damage, the desired angle could be set manually by means of the hand wheel.

Photo 28. The gyro angle and LUT angles setting gear at the torpedo tube No 4 in the forward torpedo room of the U-995.

1. Gyro angle and LUT angles setting gear.
2. Shafts connecting the gear with the LUT control panel.
3. Shafts connecting the gear with the LUT control panel.
4. Shaft connecting the gear with the gyro angle receiver.
5. Muzzle door opening gear operating shaft.
6. The linkage connected to the firing rod, retracted the setting gear from the tube the instant the launch was initiated.

Drawing 7. Setting gear transmission diagram [14].

Photo 29. Depth setting gear on the aft torpedo tube of U-995.

1. Depth setting gear.
2. Gyro angle and LUT angles setting gear.

 Drawing 8. FAT setting gear - drawing sketched by survivor from U-664 [15].

 Drawing 9. FAT setting gear [16].

7. Type XXIII U-Boats torpedo tubes

Type XXIII U-Boats were fitted with two bow torpedo tubes. Their construction and operation were the same as tubes fitted on previous types of boats, but there were some differences caused by new tactical assumptions and design constraints of type XXIII. First, because of the small size of the boat there was no room for stowage of spare torpedoes - the only torpedoes were the two loaded into the tubes. Because of the small size of the forward torpedo room (total length about 9 m) the tubes were loaded in harbor, from outside, by means of a special device (photo 30). Furthermore, because of the small distance between the tube breech door and the control room - forward torpedo room bulkhead (about 4,5 m) it was only possible to partially draw the torpedo (of total length of about 7 m) from the tube for checks and maintenance.

Photo 30. Loading torpedo tube on type XXIII U-Boat [17].

Drawing 10. Torpedo firing gear on type XXIII U-Boats [18].

An interesting thing is the element described as "Schwallfang" - it was a kind of overlap installed over the outlet of the tube. It was intended to catch air bubbles, which escaped from tube during torpedo launching. The "Overlap" could be later vented inside the boat.

Photo 31. "Schwallfang" of the U-2344 [17].

8. Torpedo tubes and electric torpedoes G7e

To ensure maximum performance of the battery cells powering the electric motor driving the G7e torpedo (that is achiving speed of 30 knots at distance 5000 m for T-II and T-III torpedo and running distance of 7500 m for T-IIIa torpedo), shortly before the launch the cells were heated up to a temperature 30 °C. Electric heaters controlled by a thermostat were embedded in the cells for this purpose. The powering socket was located at the top part of the torpedo hull, in the aft part of the battery compartment. There was a dedicated hole in the top part of the tube, in the area between the gyro angle setting gear and the pressure hull to enable connecting the cable with the powering plug to the torpedo loaded into the tube. This hole was closed with a water-tight plug when not in use. The tubes were not provided with an interlock that would prevent launching torpedo with the connected powering cable.

The G7e torpedo battery consists of two battery trays. Each tray has its own heating device that consists of resistance grids, vulcanized into special heating plates, which lie between the pairs of cells and which heat their transverse sides. The resistance grids of all the heating plates of a battery are connected in series. Their rating is measured in such a way that they pass a heating current of 7 to 8 A across a voltage of 110 to 130 V. The battery heating circuits of the two trays are connected in parallel resulting in a current flow of 14 to 16 A. The two poles of the battery heaters are wired to two pins of a 4-pin socket, located in the top, aft part of battery compartment. Each battery tray contains an opening for the thermostat. The thermostat has a form of contact thermometer - a long glass tube inserted through the tray's top cover into the electrolyte. Inside this glass tube are two wires and a mercury reservoir. When the temperature of the battery reaches the desired limit (30 °C) the mercury column has risen sufficiently to short circuit the two wires. The contact thermometers of two trays are also connected in parallel and two poles are wired to another two pins of 4-pin socket.

Drawing 11. Wiring diagram of torpedo battery heaters [19].

Photo 32. Battery heaters powering socket on the top part of battery compartment of T-V torpedo,

Photo 33. 4-pin battery heaters powering socket [20].

Photo 34. Torpedo battery heating control box (ETO Heizkasten) [21].

Photo 35. Torpedo battery heating control box in the forward torpedo room

Drawing 12. Diagram of battery heaters installation [21].

Drawing 13. Heating configurations of multiple torpedoes [23].

The torpedo tubes were not equipped with any installation that allowed charging the battery of the torpedo while it is in the tube. This was probably not done because there would also have to be provisions for ventilating the battery compartment (because of hydrogen gas generated during the charging process) and that would just further complicate the torpedo tube. The G7e torpedo had to be withdrawn from the tube to recharge its batteries.