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German UC-1 class World War I submarine
A submarine is a watercraft that can operate independently underwater, as distinct from a submersible that has only limited underwater capability. The term submarine most commonly refers to large manned autonomous vessels, however historically or more casually, submarine can also refer to medium sized or smaller vessels, (midget submarines, wet subs), Remotely Operated Vehicles or robots. The word submarine was originally an adjective meaning "under the sea", and so consequently other uses such as 'submarine engineering' or 'submarine cable' may not actually refer to submarines at all. Submarine was shortened from the term 'submarine boat'.
Submarines are referred to as "boats" for historical reasons because vessels deployed from a ship are referred to as boats. The first submarines were launched in such a manner. The English term U-Boat for a German submarine comes from the German word for submarine, `U-Boot`, itself an abbreviation for Unterseeboot ('undersea boat').
Although experimental submarines had been built before, submarine design took off during the 19th century. Submarines were first widely used in World War I, and feature in many large navies. Military usage ranges from attacking enemy ships or submarines, aircraft carrier protection, blockade running, ballistic missile submarines as part of a nuclear strike force, reconnaissance and covert insertion of special forces. Civilian uses for submarines include marine science, salvage, exploration and facility inspection/maintenance. Submarines can also be specialised to a function such as search and rescue, or undersea cable repair. Submarines are also used in tourism and for academic research.
Submarines have one of the largest ranges in capabilities of any vessel, ranging from small autonomous or one- or two-man vessels operating for a few hours, to vessels which can remain submerged for 6 months such as the Russian Typhoon class. Submarines can work at greater depths than are survivable or practical for human divers. Modern deep diving submarines are derived from the bathyscaphe, which in turn was an evolution of the diving bell.
Most large submarines comprise a cylindrical body with conical ends and a vertical structure, usually located amidships, which houses communications and sensing devices as well as periscopes. In modern submarines this structure is the "sail" in American usage ("fin" in European usage). A "conning tower" was a feature of earlier designs: a separate pressure hull above the main body of the boat that allowed the use of shorter periscopes. There is a propeller (or pump jet) at the rear and various hydrodynamic control fins as well as ballast tanks. Smaller, deep diving and specialty submarines may deviate significantly from this traditional layout.
[edit] Military usage
Until the development of the homing torpedo in World War Two, the primary role of the diesel/electric submarine was anti-ship warfare, inserting and removing covert agents and military forces, and intelligence-gathering and was generally not used against other submarines (although British developed an anti-submarine submarine in World War I, dubbed the "R1"). The impact-detonated torpedoes of the era were difficult to use against a submarine because they ran a fixed course at a fixed depth and were relatively easy for the small submarines to avoid with three dimensional maneuvers. Submarines were also used in limited roles for artillery support or raids, and rescuing aircrews during large-scale air attacks on islands, where the aircrewmen would be told of safe places to crash-land damaged aircraft so the submarine crew could rescue them.
With the development of the homing torpedo, better sonar systems, and nuclear propulsion, submarines also became able to effectively hunt each other as well as surface ships. The development of submarine-launched nuclear missiles and submarine-launched cruise missiles gave submarines a substantial and long-ranged ability to attack both land and sea targets with a variety of weapons ranging from cluster bombs to nuclear weapons.
Mine laying submarines were developed in the early part of the 20th century. The facility has been used in both World Wars. Such capabilities continue today.
The primary defensive power of a submarine lies in its ability to remain concealed in the depths of the ocean. Modern submarines are built with an emphasis on stealth. Advanced propeller designs, extensive sound-reducing insulation, and special machinery allow a submarine to be as quiet as ambient ocean noise, making them extremely difficult to detect. Such submarines can launch an attack on land targets, surface ships, and other submarines from seemingly nowhere, and require specialized equipment to find and attack in retaliation. Water is an excellent conductor of sound, and submarines have excellent sonars that can detect and track comparatively noisy surface ships from long distances. This allows an attacking sub, at its discretion, to quietly maneuver to and attack from the best possible position at the best possible time.
A concealed military submarine is a real threat and, because of its stealth, it can force an enemy navy to waste resources searching large areas of ocean and protecting all ships against possible attack, while in reality only threatening a small area. This advantage was vividly demonstrated in the 1982 Falklands War when the British SSN HMS Conqueror sank the Argentine cruiser General Belgrano. After the sinking the Argentine Navy realized that they were vulnerable to submarine attack, and that they had no defense from it. Thus the Argentinian surface fleet withdrew to port for the remainder of the war, though an Argentinian submarine remained at sea.
During World War II some military submarines were used as supply vessels for U-boats.
[edit] Anti-submarine net
One of the defenses against submarines is an antisubmarine net that blocks the passage, e.g. at the entrance of a harbor. It can sometimes be lowered to let friendly ships pass. See antisubmarine nets at Pearl Harbor or net laying ship.
[edit] Civil uses
Although the majority of the world's submarines are military ones, there are some civil submarines. They have a variety of uses, including tourism, exploration, oil and gas platform inspections and pipeline surveys.
A semi-civilian use was the adaption of U-boats for cargo carrying during both the First and Second World Wars. Another is that of submarine crew rescue.
[edit] Technology
[edit] Submersion and trimming
Submerged submarine seen from a plane
All surface ships, as well as surfaced submarines, are in a positively buoyant condition, weighing less than the volume of water they would displace if fully submerged. To submerge hydrostatically, a ship must have negative buoyancy, either by increasing its own weight or decreasing displacement of the water. To control their weight, submarines are equipped with ballast tanks, which can be filled with either outside water or pressurized air.
For general submersion or surfacing, submarines use the forward and aft tanks, called Main Ballast Tanks or MBTs, which are opened and completely filled with water to submerge, or filled by pressurized air to surface. Under submerged conditions, MBTs generally always stay flooded, which simplifies their design, so on many submarines these tanks are simply a section of interhull space. For more precise and quick control of depth, submarines use smaller Depth Control Tanks or DCTs, also called hard tanks due to their ability to withstand higher pressure. The amount of water in depth control tanks can be controlled either to reflect changes in outside conditions or change submersion depth. Depth control tanks can be located either near the submarine's center of gravity, or separated along the submarine body to prevent affecting trim.
When submerged, the water pressure on submarine's hull can reach 4 MPa for steel submarines and up to 10 MPa for titanium submarines like Komsomolets, while the pressure inside stays the same. This difference results in hull compression, which decreases displacement. Water density also increases, as the salinity and pressure are higher, but this does not compensate for hull compression, so buoyancy falls with depth. A submerged submarine is in an unstable equilibrium, having a tendency to either fall down to the ocean floor or float up to the surface. Keeping a constant depth requires continual operation of either the depth control tanks or control surfaces.[1]
Submarines in a neutral buoyancy condition are not intrinsically stable in trim. To sustain desired trim, submarines use specialized forward and aft trim tanks. Pumps can move water between these tanks, changing the weight distribution and therefore creating a moment to turn the sub upwards or downwards. A similar system is sometimes used to maintain stability.
Sail of the French nuclear submarine
Casabianca; note the diving planes,
camouflaged masts, periscope, electronic warfare masts, door and windows.
The hydrostatic effect of variable ballast tanks is not the only way to control the submarine underwater. Hydrodynamic maneuvering is done by several surfaces, which can be turned to create corresponding hydrodynamic forces when a submarine moves at sufficient speed. The stern planes, located near the propeller and normally oriented horizontally, serve the same purpose as the trim tanks, controlling the trim, and are commonly used, while other control surfaces may not be present on many submarines. The fairwater planes on the sail and/or bow planes on the main body, both also horizontal, are located closer to the centre of gravity, and are used to control depth with less effect on the trim.
When a submarine performs an emergency surfacing, all depth and trim methods are used simultaneously, together with propelling the boat upwards. Such surfacing is very quick, so the sub may even partially jump out of the water, but it inflicts serious damage on some submarine systems, primarily pipes.
[edit] Submarine hull
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Main article: Submarine hull
[edit] Overview
The
Los Angeles class attack submarine
USS Greeneville in dry dock, showing typical cigar-shaped hull.
Modern submarines are usually cigar-shaped. This design, already visible on very early submarines (see below) is sometimes called a "teardrop hull". It significantly reduces the hydrodynamic drag on the sub when submerged, but decreases the sea-keeping capabilities and increases the drag while surfaced. Since the limitations of the propulsion systems of early military submarines forced them to operate on the surface most of the time, their hull designs were a compromise. Because of the slow submerged speeds of those subs, usually well below 10kt (18 km·h−1), the increased drag for underwater travel was considered acceptable. Only late in World War II, when technology allowed faster and longer submerged operations and increased surveillance by enemy aircraft forced submarines to stay submerged, did hull designs become teardrop shaped again, to reduce drag and noise. On modern military submarines the outer hull is covered with a thick layer of special sound-absorbing rubber, or anechoic plating, to make the submarine quieter.
The human-occupied pressure hulls of extremely deep diving submarines such as DSV Alvin are spherical instead of the more traditional cylinder. This allows for a more even distribution of the stress at the great depths such subs operate at. A titanium frame is usually welded or bolted to the pressure hull to provide attachment points for ballast and trim systems, scientific instrumentation, battery packs, syntactic flotation foam, and lighting.
A raised tower on top of a submarine accommodates the length of the periscope and electronics masts, which can include radio, radar, electronic warfare, and other systems including the snorkel mast. In many early classes of submarines (see history), the Control Room, or "Conn", was located inside this tower, which was known as the "conning tower". Since that time, however, the Conn has been located within the hull of the submarine, and the tower is more commonly called the "sail" today. The Conn should not be confused with the "bridge", which is a small, open platform set into the top of the sail used for visual observation while operating on the surface.
"Bathtubs" are related to conning towers but are only for smaller submarines. A bathtub, in the context of smaller submarines, is a metal cylinder attached to the hull which surrounds the hatch and prevents waves from breaking directly into the cabin. It is needed because submarines on the surface don't have a lot of freeboard, i.e., they lie very low in the water, and were waves to break into the cabin, are in serious danger of sinking.
[edit] Single / double hull
U-995, Type VIIC/41 U-Boat of WWII, showing the typical combination of ship-like non-watertight outer hull with bulky strong hull below
Type XXI U-Boat, late WWII, with pressure hull almost fully enclosed inside the light hull
Modern submarines and submersibles, as well as the oldest ones, often have a single hull. Large submarines generally have an additional hull or hull sections outside. This external hull, which actually forms the shape of submarine, is called the outer hull (casing in the Royal Navy) or light hull, as it does not have to withstand any pressure difference. Inside the outer hull there is a strong hull, or pressure hull, which withstands sea pressure and has normal atmospheric pressure inside.
As early as World War I, it was realized that the optimal shape for withstanding pressure conflicted with the optimal shape for seaworthiness and minimized water resistance, and construction difficulties further complicated the problem. This was solved either by a compromise shape, or by using two hulls; internal for holding pressure, and external for optimal shape. Until the end of World War II, most submarines had an additional partial cover on the top, bow and stern, built of thinner metal, which was flooded when submerged. Germany went further with the Type XXI, the general predecessor of modern submarines, in which the pressure hull was fully enclosed inside the light hull, but optimised for submerged navigation, unlike earlier designs that were optimised for surface operation.
After World War II, approaches split. The Soviet Union changed its designs, basing them on the latest German developments. All post-WWII heavy Soviet and Russian submarines are built with a double hull structure. American and most other Western submarines switched to a primarily single-hull approach. They still have light hull sections in the bow and stern, which house main ballast tanks and provide a hydrodynamically optimized shape, but the main cylindrical hull section has only a single plating layer. However, the double-hull approach is today being considered for future submarines in the United States as a means to improve payload capacity, stealth and operational reach.[2]
[edit] Pressure hull
The pressure hull is generally constructed of thick high-strength steel with a complex structure and high strength reserve, and is separated with watertight bulkheads into several compartments. There are also examples of more than two hulls in a submarine, like the Typhoon class, which has two main pressure hulls and three smaller ones for control room, torpedoes and steering gear, while the missile launch system is located between the main hulls.
The dive depth cannot be increased easily. Simply making the hull thicker increases the weight and requires reduction of the weight of onboard equipment, ultimately resulting in a bathyscaphe. This is affordable for civilian research submersibles, but not military submarines, so their dive depth was always bound by current technology.
WW1 submarines had their hulls built of carbon steel, and could not submerge below 100 meters. During World War Two, high-strength alloyed steel was introduced, allowing for dive depths of up to 200 meters. High-strength alloyed steel is still the main material for submarines today, with 250-400 meters depth limit, which cannot be exceeded on a military submarine without sacrificing other characteristics. To exceed that limit, a few submarines were built with titanium hulls. Titanium is almost as strong as steel, but lighter, and is also not ferromagnetic, which is important for stealth. Titanium submarines were favored by the Soviet Union, which developed specialized high-strength alloys and built an industry capable of producing titanium at an affordable cost. It has produced several types of titanium submarines. Titanium alloys allow a major increase in depth, but other systems need to be redesigned to cope, so test depth was limited to 1000 meters for K-278 Komsomolets, the deepest-diving combat submarine. An Alfa class submarine may have successfully operated at 1300 meters,[3] though continuous operation at such depths would be an excessive stress for many submarine systems. Titanium also does not flex as easily as steel, and may be come brittle over many cycles of diving and surfacing. Despite its benefits, the high cost of titanium construction led to the abandonment of titanium submarine construction as the Cold War ended.
Deep diving civilian submarines have used thick glass pressure hulls.
The task of building a pressure hull is very difficult, as it must withstand pressures up to that of its required diving depth. When the hull is perfectly round in cross-section, the pressure is evenly distributed, and causes only hull compression. If the shape is not perfect, the hull is bent, with several points heavily strained. Inevitable minor deviations are resisted by the stiffener rings, but even a one inch (25 mm) deviation from roundness results in over 30 percent decrease of maximal hydrostatic load and consequently dive depth.[4] The hull must therefore be constructed with very high precision. All hull parts must be welded without defects, and all joints are checked several times using different methods. This contributes to the very high cost of modern submarines. (For example, each Virginia-class attack submarine costs 2.6 billion dollars, over $200,000 per ton of displacement.)
[edit] Propulsion
Originally submarines were human propelled. The first mechanically driven submarine was the 1863 French Plongeur, which used compressed air for propulsion, and anaerobic propulsion was first employed by the Spanish Ictineo II in 1864. Ictineo's engine used a chemical mix containing a peroxide compound to generate heat for steam propulsion while also providing oxygen for the crew. The system was not employed again until 1940 when the German Navy tested a system employing the same principles, the Walter turbine, on the experimental V-80 submarine and later on the naval U-791 submarine.
Until the advent of nuclear marine propulsion, most 20th century submarines used batteries for running underwater and gasoline (petrol) or diesel engines on the surface and to recharge the batteries. Early submarines used gasoline, but this quickly gave way to paraffin, then diesel, because of reduced flammability. Diesel-electric became the standard means of propulsion. The diesel or gasoline engine and the electric motor, separated by clutches, were initially on the same shaft and drove the propeller. This allowed the engine to drive the electric motor as a generator to recharge the batteries and also propel the submarine if required. The clutch between the motor and the engine would be disengaged when the submarine dove so that the motor could be used to turn the propeller. The motor could have more than one armature on the shaft, and these could be electrically coupled in series for slow speed and in parallel for high speed. (These alternative connections were known as "group down" and "group up", respectively.)
The principle was modified for some submarine designs in the 1930s, particularly those of the U.S. Navy and the British U class submarines. The engine was no longer attached to the motor/propeller drive shaft, but drove a separate generator to drive the motors on the surface while recharging the batteries. This diesel-electric propulsion allowed much more flexibility; for example, the submarine could travel slowly while the engines were running at full power to recharge the batteries as quickly as possible, reducing time spent on the surface, or use its snorkel. It was then possible to insulate the noisy diesel engines from the pressure hull, making the submarine quieter.
Other power sources were attempted. Oil-fired steam turbines powered the British "K" class submarines, built during the first World War and in the following years, with the intent of giving them the necessary surface speed to keep up with the British battle fleet. The "K" class subs were not very successful, however. (The design was also over-endowed with hatches, which proved troublesome in service.) German Type XXI submarines attempted the application of hydrogen peroxide to provide long-term, fast air-independent propulsion, but were ultimately built with very large batteries instead.
At the end of the Second World War, the British and Russians experimented with hydrogen peroxide/kerosene (paraffin) engines which could be used both above and below the surface. The results were not encouraging enough for this technique to be adopted at the time, and although the Russians deployed a class of submarines with this engine type (codenamed Quebec by NATO), they were considered unsuccessful. Today several navies use air-independent propulsion. Notably Sweden uses Stirling technology on the Gotland class and Södermanland class series of submarines. The Stirling engine is heated by burning diesel fuel with liquid oxygen stored in cryogenic tanks. A newer development in air-independent propulsion is the use of hydrogen fuel cells, first applied in series on the German Type 212 submarine, with nine 34 kW or two 120-kilowatt cells.
Steam power was resurrected in the 1950s with the advent of the nuclear-powered steam turbine driving a generator. By removing the requirement for atmospheric oxygen, these submarines can remain submerged indefinitely. (Air is recycled and fresh water is distilled from seawater.) These vessels always have a small battery and diesel engine/generator installation for emergency use if the reactors have to be shut down.
Nuclear power is now used in all large submarines, but due to the high cost and large size of nuclear reactors, smaller submarines still use diesel-electric propulsion. The ratio of larger to smaller submarines depends on strategic needs; for instance, the US Navy and the Royal Navy operate only nuclear submarines,[5] which is usually explained by the need for overseas operations. Other major operators rely on a mix of nuclear submarines for strategic purposes and diesel-electric submarines for defensive needs. Most fleets have no nuclear submarines at all, due to the limited availability of nuclear power and submarine technology. Diesel-electric submarines also have a distinct advantage over their nuclear cousins in terms of stealth. Nuclear submarines are always generating noise from the coolant pumps and turbo-machinery needed to operate the reactor, even at low power levels. A conventional submarine operating on its batteries is almost completely silent, the only noise coming from the shaft bearings and flow noise around the hull, all of which stops when the sub hovers in mid water to listen. Commercial submarines usually rely only on batteries, as they are never expected to operate independently of a mother ship.
Toward the end of the 20th century, some submarines, such as the British Vanguard class, began to be fitted with pump-jet propulsors instead of propellers. Although these are heavier, more expensive, and less efficient than a propeller, they are significantly quieter, giving an important tactical advantage.
The magnetohydrodynamic drive, or "caterpillar drive", which has no moving parts was popularized as a submarine propulsion system by the movie version of The Hunt for Red October, written by Tom Clancy, which portrayed it as a virtually silent system.
Although experimental surface ships have been built with this propulsion system, speeds have not been as high as expected. In addition, the drive system can induce bubbles to form, compromising stealth, and the low efficiency leads to very high required reactor powers. These factors make it unlikely to be considered for any military purpose.
[edit] Armament
A sequence of photos showing the decommissioned Australian warship
HMAS Torrens sinking after being used as a target for a submarine-launched torpedo.
The forward torpedo tubes on HMS Ocelot
The success of the submarine is inextricably linked to the development of the torpedo, invented by the English engineer Robert Whitehead in 1866. His invention is essentially the same today as it was 100 years ago. Only with the arrival of self propelled torpedoes could the submarine make the leap from mechanical novelty into a weapon of war. Until the perfection of the guided torpedo, multiple torpedoes of the straight running kind were required to attack a target. With at most 20 to 25 torpedoes stored onboard, the number of attacks that could be made was limited. To increase combat endurance most submarines of the First World War functioned as submersible gunboats, using their deck guns against unarmed targets and diving to escape and engage enemy warships. The importance of guns encouraged the development of the unsuccessful Submarine Cruiser such as the French Surcouf and the Royal navy's X1 and M class submarines. With the arrival of ASW aircraft, guns became more of means of defence than of attack. A more practical method of increasing combat endurance was the external torpedo tube which could only be loaded in port.
The ability of submarines to approach enemy harbors covertly led to their use as minelayers. Minelaying submarines of the First and Second World War were specially built for that purpose. Modern submarine-laid mines, such as the British Mark 6 Sea Urchin, are designed to be deployed by a submarine's torpedo tubes.
After World War II, both the USA and the USSR experimented with submarine launched cruise missiles such as the SSM-N-8 Regulus and P-5 Pyatyorka however with such missiles the submarine had to surface to fire its missiles. Such missiles were the forerunners of modern submarine launched cruise missiles which can be fired from the torpedo tubes of submerged submarines e.g. the US BGM-109 Tomahawk and Russian RPK-2 Viyuga. Ballistic missiles can also be fired from a submarine's torpedo tubes, for example missiles such as the anti-submarine SUBROC, and versions of surface to surface anti-ship missiles such as the Exocet and Harpoon, encapsulated for submarine launch. With internal volume as limited as ever and the desire to carry heavier warloads, the idea of the external launch tube was revived, usually for the encapsulated missiles and such tubes being placed in the space between the internal pressure and outer streamlined hulls.
The strategic mission of the SSM-N-8 and the P-5 were taken up by submarine-launched ballistic missile beginning with the US Navy's Polaris missile, then the Poseidon and Trident missiles.
[edit] Sensors
A submarine will have a range of sensor types that depends on its purpose. Modern military submarines rely almost entirely on an extremely sensitive suite of passive and active sonars to find their prey. Active sonar relies on an audible "ping" to generate echoes revealing objects around the transmitting submarine. Active systems are rarely used, as the transmitting submarine will invariably reveal its own position to its target. Passive sonar is literally a set of extremely sensitive hydrophones set into the submarine's hull or trailed behind said submarine in a towed array, generally several hundred feet long, if not more. The towed array is the mainstay of NATO submarine detection systems, as it reduces the amount of flow noise that is heard by the operators. Hull mounted sonar is employed to back up the towed array, and in confined coastal waters where a towed array could be fouled by sea floor obstacles.
Submarines also carry radar equipment for detection of surface ships and aircraft. Again, sub captains are more likely to use radar detection gear rather than active radar to detect targets, as radar energy can be detected far beyond its own return range, revealing the transmitting submarine's position. Periscopes are hardly ever used except to take position fixes and to verify the identity of a contact.
Civilian submarines, such as Alvin or the Russian Mir submersibles, rely on small active sonar sets and viewing ports to navigate. Light does not penetrate beyond about 300 feet (91 m), so high intensity lights must be carried to illuminate the area around the submersible.
[edit] Navigation
Although early submarines had very little in the way of navigation aids, modern submarines have a variety of navigation systems. Modern military submarines use an inertial guidance system for navigation while submerged, but drift error unavoidably builds up over time. To counter this, the Global Positioning System will occasionally be used to obtain an accurate position. The periscope - a retractable tube with prisms allowing a view to the surface - is only used occasionally in modern submarines, since the range of visibility is short. The Virginia-class submarines and Astute Class submarines have "photonics masts" rather than hull-penetrating optical tube periscopes. These masts must still be hoisted above the surface, and employ electronic sensors for visible light, infrared, laser range-finding, and electromagnetic surveillance.
[edit] Communication
Military submarines have several systems for communicating with distant command centers or other ships. One is the VLF radio, which can reach a submarine either on the surface or submerged up to a fairly shallow depth, usually less than 250 feet (76 m) or so. ELF frequencies can reach a submarine at much greater depths, but has a very low bandwidth and is generally only used to call a submerged sub to a shallower depth where VLF signals can reach. A submarine also has the option of floating a long, buoyant wire to a shallower depth, allowing VLF transmissions to be made by even a deeply submerged boat.
By extending a radio mast, a submarine can also use a "burst transmission" technique. A burst transmission takes only a fraction of a second, minimizing a submarine's risk of detection. To communicate with other submarines, a system known as Gertrude is used. Gertrude is basically a sonar telephone. Voice communication from one submarine is transmitted by low power speakers into the water, where it is detected by passive sonars on the receiving submarine. The range of this system is probably very short, and using it radiates sound into the water, which can be heard by enemy submarines, surface ships and aircraft.
Civilian submarines can use similar, albeit less powerful systems to communicate with support ships or other submersibles in the area.
[edit] Command and control
All submarines need facilities to control their motion. Military submarines also need facilities to operate their sensors and weapons.
[edit] Overview
With nuclear power, submarines can remain submerged for months at a time. Diesel submarines must periodically resurface or snorkel to recharge their batteries. Most modern military submarines are able to generate oxygen for their crew by electrolysis of water. Atmosphere control equipment includes a CO2 scrubber, which uses an amine absorbent to remove the gas from air and diffuse it into waste pumped overboard. A machine that uses a catalyst to convert carbon monoxide into carbon dioxide (removed by the CO2 scrubber) and bonds hydrogen produced from the ship's storage battery with oxygen in the atmosphere to produce water, also found its use. An atmosphere monitoring system samples the air from different areas of the ship for nitrogen, oxygen, hydrogen, R12 and R114 refrigerant, carbon dioxide, carbon monoxide, and others. Poisonous gases are removed
is the intake mass flow rate
represents the nozzle gross thrust
represents the ram drag of the intake.
