Combined gas or gas ( COGOG ) is a propulsion system for ships using gas turbine engines.
22-476: Combined diesel or gas (CODOG) Combined diesel and gas (CODAG) Combined diesel-electric and diesel (CODLAD) Combined diesel–electric and gas (CODLAG) Combined diesel and diesel (CODAD) Combined steam and gas (COSAG) Combined gas or gas (COGOG) Combined gas and gas (COGAG) Combined gas and steam (COGAS) Combined nuclear and steam propulsion (CONAS) Integrated electric propulsion (IEP or IFEP) A high efficiency, low output turbine
44-498: A CAD model of interest plus a large set of parameters to describe a specific thermal environment and the internal temperatures of the platform and thermal properties of the construction materials. The software then solves a set of thermal equations across the boundaries and for electromagnetic propagation in a specified infrared waveband. The primary output is a measure of infrared signature, though usually surface temperatures can be given (since this usually has to be calculated to obtain
66-437: A high volume of very hot exhaust gasses, which can hinder onboard helicopter operations, and also greatly increases a ship's infrared signature making it more conspicuous to enemy sensors and guided weapons. The ducting and filters required take up a considerable amount of space in a ship, and the volume of air being drawn in can exacerbate an internal fire. This was found to be a factor in the loss of HMS Antelope during
88-550: A maximum speed of 32 knots (59 km/h; 37 mph). However, they were beaten into service by the Canadian Iroquois -class destroyers , which were powered by two Pratt & Whitney FT4A2 gas turbines creating 50,000 shaft horsepower (37,000 kW) and two Pratt & Whitney FT12AH3 cruising gas turbines creating 7,400 shp (5,500 kW), giving a maximum speed of 29 knots (54 km/h; 33 mph). The operation of large gas turbines on ships produces
110-417: A non-circular tail pipe (a slit shape) to minimize the exhaust cross-sectional volume and maximize the mixing of hot exhaust with cool ambient air (see Lockheed F-117 Nighthawk). Often, cool air is deliberately injected into the exhaust flow to boost this process (see Ryan AQM-91 Firefly and Northrop B-2 Spirit ). The nozzle shape could be designed to facilitate mixing of the exhaust with ambient air as with
132-495: A significant impact on the results. As such, there are limitations on what can be achieved from modelling the infrared problem, and sometimes experimentation is necessary to achieve accurate knowledge of the nature of an object's physical existence in the infrared wavebands. Infrared stealth is an area of stealth technology aimed at reducing infrared signatures. This reduces a platform's susceptibility to infrared guided weapons and infrared surveillance sensors, and thus increases
154-820: Is more fuel efficient than a bigger turbine running at 50% power. The system is currently used in the 2 ships of the Russian Navy 's Slava -class cruisers , the Japanese Maritime Self-Defense Force 's Hatsuyuki-class destroyers , and the Royal Netherlands Navy Kortenaer -class frigates (on which the Greek Navy Elli -class frigates are based). It was formerly used in the Royal Navy 's Type 42 destroyer and Type 22 frigate , as well as
176-422: Is one diesel engine for cruising speed and one geared gas turbine for high speed dashes. Both are connected to the shaft with clutches ; only one system is driving the ship, in contrast to combined diesel and gas (CODAG) systems that can use the combined power output of both. The advantage of CODOG is a simpler gearing compared to CODAG, but it needs either more powerful or additional gas turbines to achieve
198-408: Is used for cruising speeds with a high output turbine being used for high-speed operations. A clutch allows either turbine to be selected, but there is no gearbox to allow operation of both turbines at once. This has the advantage of not requiring heavy, expensive and potentially unreliable gearboxes. The reason that a smaller turbine is used for cruising is that a small turbine running at 100% power
220-582: The Avro Vulcan bomber and further developed for the Concorde supersonic airliner. Cruising power was provided by two Rolls-Royce Proteus gas turbines, originally designed for turboprop airliners, each rated at 3,250 shaft horsepower (2,420 kW). The Olympus had to be down-rated to 15,000 shaft horsepower (11,000 kW) to keep within the limits of the Exmouth's hull structure. Even before
242-595: The Exmouth trials had started, the Royal Navy had already ordered the first class of vessels to be designed from the start for COGOG propulsion, the Type 21 frigates , in which the Proteus turbines were replaced by a pair of Rolls-Royce Tyne engines. The Tynes were rated at 4,250 shaft horsepower (3,170 kW) each, giving a cruising speed 18 knots (33 km/h; 21 mph), while an Olympus rated at 25,000 shp gave
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#1732802043189264-512: The Royal Canadian Navy 's Iroquois -class destroyer . Having previously pioneered the combined diesel or gas (CODOG) system, in 1968 the Royal Navy converted an old frigate , HMS Exmouth , to COGOG propulsion as a test bed for use in later ships. Because developing a new gas turbine purely for marine use would be very expensive, it was decided to adapt a Rolls-Royce Olympus engine, which had been originally designed for
286-884: The 1982 Falklands War . Many navies have now abandoned pure gas turbine propulsion in favour of combined diesel-electric and gas (CODLAG) systems. Combined diesel or gas Combined diesel or gas ( CODOG ) is a type of propulsion system for ships that need a maximum speed that is considerably faster than their cruise speed, particularly warships like modern frigates or corvettes . Combined diesel or gas (CODOG) Combined diesel and gas (CODAG) Combined diesel-electric and diesel (CODLAD) Combined diesel–electric and gas (CODLAG) Combined diesel and diesel (CODAD) Combined steam and gas (COSAG) Combined gas or gas (COGOG) Combined gas and gas (COGAG) Combined gas and steam (COGAS) Combined nuclear and steam propulsion (CONAS) Integrated electric propulsion (IEP or IFEP) For every propeller shaft there
308-434: The design phase, it is often desirable to employ a computer to predict what the infrared signature will be before fabricating an actual object. Many iterations of this prediction process can be performed in a short time at low cost, whereas use of a measurement range is often time-consuming, expensive and error-prone. A number of software houses have built infrared signature prediction software packages. These generally require
330-494: The infrared signature of an object are the apparent temperature difference at the sensor and the contrast radiant intensity (CRI) definitions. The apparent temperature difference method of defining infrared signature gives the physical temperature difference (e.g. in kelvins ) between the object of interest and the immediate background if the recorded radiance values had been measured from perfect blackbody sources. Problems with this method include differences in radiance across
352-409: The infrared signature of their own assets to threat sensors. In practice this might mean equipping a warship with sensors to detect the exhaust plumes of incoming anti-ship missiles while also having an infrared signature below the detection threshold of the infrared sensor guiding the missile. An exhaust plume contributes a significant infrared signature. One means to reduce IR signature is to have
374-422: The infrared signature prediction) and also visual representations of how the scene may appear to various imaging infrared detectors. Infrared signature prediction models are very difficult to validate except for simple cases because of the difficulty in modelling a complex environment. Both sensitivity analysis of this type of software and experimental measurements has shown that small variations in weather can have
396-471: The object or the immediate background and the finite size of the detector's pixels. The value is a complex function of range, time, aspect, etc. The contrast radiant intensity method of defining infrared signature is to take the difference in average radiance of the object and that of the immediate background and multiply this by the projected area of the object. Again the CRI value will depend on many factors. In
418-410: The object's surface, the background against which it is viewed and the waveband of the detecting sensor. As such there is no all-encompassing definition of infrared signature nor any trivial means of measuring it. For example, the infrared signature of a truck viewed against a field will vary significantly with changing weather, time of day and engine loading. Two fairly successful examples of defining
440-440: The platform's overall survivability. Infrared stealth is particularly applicable to military jets because of the detectable engines and plumes from non-stealth aircraft, but it also applies to military helicopters, warships, land vehicles and dismounted soldiers. A military aim in studying infrared signatures is to understand the likely infrared signature of threats (and develop the equipment required to detect them) and to reduce
462-620: The rectangular nozzles on the Lockheed Martin F-22 Raptor . Sometimes, the jet exhaust is vented above the wing surface to shield it from observers below, as in the Lockheed F-117 Nighthawk , and the unstealthy Fairchild Republic A-10 Thunderbolt II . To achieve infrared stealth , the exhaust gas is cooled to the temperatures where the brightest wavelengths it radiates are absorbed by atmospheric carbon dioxide and water vapor , dramatically reducing
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#1732802043189484-486: The same maximum power output. The disadvantage of CODOG is that the fuel consumption at high speed is poor compared to CODAG. Infrared signature Infrared signature , as used by defense scientists and the military , is the appearance of objects to infrared sensors . An infrared signature depends on many factors, including the shape and size of the object, temperature , and emissivity , reflection of external sources ( earthshine , sunshine , skyshine ) from
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