SST-720 torpedo
| SST-720 Torpedo | |
|---|---|
 | |
| Type | Heavyweight torpedo |
| Place of origin |  Carthage |
| Service history | |
| In service | 1999–present |
| Used by | Carthage Gensokyo Republic |
| Production history | |
| Designer | RMA Corporation |
| Designed | 1992-1998 |
| Manufacturer | RMA Corporation |
| Unit cost | NSD$3.2 million (FY2015) |
| Produced | 1998-present |
| No. built | 72,650 |
| Variants | Mark 3 Rev. 2 |
| Specifications (Mark 3 Rev. 2) | |
| Weight | 2,150 kg (4,740 lb) |
| Length | 4.85 m (15.9 ft) |
| Diameter | 660 mm (26 in) |
| Effective firing range | 30 km (19 mi) @ 90 kn (170 km/h) 60 km (37 mi) @ 40 kn (74 km/h) |
| Warhead | Aluminised PBX explosive |
| Warhead weight | 290 kg (640 lb) |
Detonation mechanism | Proximity or contact |
| Engine | SHAFT TR190A gas turbine 1,400 kW (1,900 hp) |
| Propellant | Otto fuel II + hydroxyl ammonium perchlorate |
| Maximum depth | Up to 1,000 m (3,300 ft) |
| Speed | Up to 90 kn (170 km/h) |
Guidance system | Active sonar Inertial guidance |
Launch platform | Submarine |
The SST-720 Double Spark is a heavyweight torpedo used by submarines of the Punic Navy. It replaced the previous Type 28 heavyweight torpedo and is designed to counter modern high-speed, deep-diving submarines as well as surface targets. The Double Spark is distinguished by its unusual proportions which are similar to those of the Type 28, being comparatively short for its diameter relative to other torpedoes. This design was selected to optimize length-to-beam ratio to minimize drag and allow more torpedoes to be stored in the existing space aboard submarines.
The SST-720 is used alongside the SST-721 Master Spark and SST-722 Starlight Typhoon as the primary torpedo armament of Carthaginian submarines. It is the most commonly deployed heavyweight torpedo in the Carthaginian inventory and the primary anti-submarine weapon for Punic Navy submarines.
History
The 660-millimeter (26 in) torpedo caliber was first introduced in 1968 aboard the Type-045N attack submarines, designed to cripple and sink European aircraft carriers of the Illustrious-class and even larger follow-on designs. The larger caliber enabled the use of a significantly heavier 400-kilogram (880 lb) warhead, increasing lethality against large, high-value surface targets such as aircraft carriers and large merchant vessels. The Type-045N, Type-046N, and Type-047N submarines carried 660-millimeter (26 in) torpedoes as a supplement for their 530-millimeter (21 in) torpedoes, which were intended for general-purpose anti-submarine and anti-surface roles. The standard heavyweight torpedo at this time was the 530-millimeter (21 in) Type-25, a monopropellant piston-engined design delivering a 270-kilogram (600 lb) warhead.
During the 1970s, a number of conceptual torpedo programs were initiated, testing concepts including supercavitating weapons, alternative form factors, and alternative drive systems. Since the introduction of the 660-millimeter (26 in) caliber, the Admiralty had expressed a desire to unify on a single torpedo caliber, likely the larger 660-millimeter (26 in) caliber but potentially the older 530-millimeter (21 in) caliber if sufficient performance could be achieved in future designs. One of the primary concerns was ammunition storage, as future submarines designed around the 660-millimeter (26 in) caliber could be expected to carry a sufficient number of weapons, but existing designs would be suffer from a sharp reduction in weapon count.
Type-28 torpedo
One effort to address this issue was the Compact Torpedo Program, which explored shorter 660-millimeter (26 in) torpedo designs with similar volume to existing 530-millimeter (21 in) designs. These shorter designs could potentially allow twice as many weapons to be stowed in the same volume as a single large 660-millimeter (26 in) torpedo while retaining performance similar or superior to the older 530-millimeter (21 in) caliber. The design promised superior hydrodynamic efficiency and slightly increased internal volume, providing increased range. The success of the program encouraged the Admiralty to accelerate its plans to unify on the 660-millimeter (26 in) caliber for all future submarine use.
By 1980, the Punic Navy had resolved to switch entirely to 660 millimeters (26 in) as the standard torpedo caliber, retiring the 530-millimeter (21 in) type from use. To address concerns about weapon storage, the new Type-28 torpedo was to be developed out of the Compact Torpedo Program, introducing a new "short" 660-millimeter (26 in) torpedo as a workhorse design while retaining the larger Type-24 and Type-27 for use against larger targets. The Type-28 was essentially an enlarged Type-25, retaining the axial piston engine for its superior deep-diving capability and using a reconfigured warhead and seeker. The Type-28 was introduced in 1982 and became the most common torpedo in the submarine force by the end of the 1980s.
Despite the promise of the design, a number of issues were discovered during service as a result of the Type-28's rapid development and introduction. Noise levels were relatively high due to the use of the axial piston engine, and the torpedoes exhibited poor reliability. Live fire tests conducted in 1987 indicated that the Type-28 functioned as expected less than 50% of the time, and crews expressed low confidence in the weapon. A legislative investigation into the ongoing problems highlighted manufacturing defects and lax quality control standards on the part of the manufacturer but also blamed the Navy establishment for fast-tracking the torpedo into service without an adequate testing regimen. Stricter testing and inspection methods implemented in the early 1990s partially addressed the problems, but confidence in the weapon among sailors remained low.
SST-720 Double Spark
In spite of these issues, the Admiralty remained confident in the concept of the design and in 1994 received legislative approval for a replacement program. The new Next Generation Torpedo would be an entirely new design, with stricter specifications and oversight. The new design retained an internal combustion engine for power but replaced the noisy axial piston engine with a quieter gas turbine, designed to alleviate the pressure issues experienced by conventional gas turbines at depth. A new, more accurate and more modern seeker was also developed. The only component retained from the Type-28 was the warhead, although the design was improved with a new detonator.
First tests of the new torpedo were conducted in 1996, with the torpedo reaching final design approval in late 1998. Low-rate initial production began in 1999 and continued until 2003, a much longer time period than the Type-28, during which time extensive data was collected on torpedo reliability and performance. Modifications based on this data were incorporated into the new Mark 2 standard, which was finally approved for full production in 2004. By 2011, all Type-28 torpedoes had been retired.
Characteristics
Guidance
The SST-720 is equipped for both acoustic and wake-homing guidance modes and includes a fiber-optic tether for wire guidance at extended ranges. In normal operation, the Double Spark is guided within active homing range of its target via wire, relying on the launch platform's more capable sonar. It can also be launched in an autonomous pattern search mode using both acoustic and inertial guidance, searching an assigned volume of space for targets.
The SST-720 Mark 1 was originally equipped with an RMA Corporation 240-element acoustic seeker, but this design proved unreliable in service with a high false alarm rate in littoral environments. The poor performance of this seeker influenced the development of the SST-721 torpedo when the Navy altered program requirement to include the development of a new, alternative seeker for the SST-721 produced by Acheron Technologies. A new software update and minor changes to the hardware were included in the Mark 2 design, but continuing problems with accuracy and reliability resulted in the replacement of the RMA seeker with the Acheron seeker developed for the SST-721 in the Mark 3.
Warhead
The initial models of the Double Spark use the same 290-kilogram (640 lb) aluminised PBX shaped charge warhead as the Type-28, designed to penetrate the reinforced pressure hulls of deep-diving submarines as well as large surface ships. Type-28 warheads were refurbished with a new detonator before being assembled into new torpedoes. Beginning with the Mark 3, the original Type-28 warhead has been replaced by a new warhead with a new PBX formulation for increased yield and greater resistance to shock.
The SST-720 is equipped with an impact fuze, magnetic influence fuze, and may be triggered by active sonar or remote detonation in the event other fuzing methods fail. The SST-720 is also capable of carrying the R-47 nuclear warhead with a 10 kiloton yield.
Propulsion
The Double Spark is prowered by a SHAFT ADS TR190A gas turbine providing 1,400 kilowatts (1,900 hp) of motive power. Turbine propulsion was originally considered for the Type-28 but passed over in favor of an axial piston engine due to concerns about performance at deeper depths and fuel consumption. In spite of these disadvantages, the gas turbine demonstrator offered a reduced acoustic signature, encouraging further research into higher-pressure turbines for better performance at deeper depths. The TR190A gas turbine was specifically designed for high-pressure operation up to 40 megapascals (5,800 psi), allowing operation at depths greater than 900 meters (3,000 ft) without excessive power loss to exhaust back pressure. To increase range and fuel efficiency, an additional tank of hydroxyl ammonium perchlorate was added to ensure a more efficient fuel mixture. The turbine is connected to a pump-jet propulsor for increased efficiency at high speed and protection from debris and obstructions in littoral operations.
The SST-720 is capable of a maximum speed of 90 knots (170 km/h) and can sustain this speed for up to 30 kilometers (19 mi). At a lower speed of 40 knots (74 km/h), the torpedo has a maximum range of 60 kilometers (37 mi). The shorter body of the torpedo and more favorable length-to-diameter ratio are key contributors to the Double Spark's high speed, significantly reducing hydrodynamic drag over a longer torpedo like the SST-721 Master Spark.