Defense Issues

Military and general security

Defining stealth

Posted by picard578 on March 1, 2017

Introduction

Word “stealth” has lately become a catchword used to define the weapon – mostly aircraft – as “superior”, with little or no thought as to what the term actually means. Stealth fighters, stealth bombers, stealth ships… even stealth tanks, the craze is in full swing. But how much do these weapons deserve the label? What is stealth? Is merely having low radar cross section enough – as commonly held – to define the weapon as “stealth”? Is USAF stuck on denial that no military advantage lasts forever, or even on denial that it never understood the true meaning of stealth? Every successful use of stealth aircraft had seen them acting as a support of, and being supported by, an array of nonstealthy aircraft – AWACS, standoff jammers etc. Yet USAF is now aiming for an all-stealth tactical fighter force, even though it will make the force less flexible and arguably less capable as well. How stealthy these aircraft really are, and what are their vulnerabilities?

The term

http://www.dictionary.com/browse/stealth

noun

1.secret, clandestine, or surreptitious procedure.

2.a furtive departure or entrance.

3.Obsolete.

an act of stealing; theft.

the thing stolen; booty.

4.(initial capital letter) Military. a U.S. Air Force project involving a range of technologies, with the purpose of developing aircraft that are difficult to detect by sight, sound, radar, and infrared energy.

adjective

5.surreptitious; secret; not openly acknowledged

Types of stealth

Overview

Taking from the above, “stealth” is a range of measures taken with the aim of remaining unseen. These can be technical or behaviorial. Technical stealth measures themselves can be physical – e.g. ghilly suit that snipers use – or electronic. Since various techniques exist for detecting targets, utilizing various wavelength spectrums, so stealth has to cover multiple detection techniques.

Visual spectrum

Visual stealth refers to reducing the possibility of acquisition by direct visual contact or through visual-spectrum sensors. Three main factors determining visual signature are physical size, camouflage colouring (or lack thereof), and possible visual-spectrum emissions such as smoke. Reflective surfaces have a major impact on visual signature as well.

Radio spectrum

First aspect of radio spectrum stealth is what everyone thinks about when stealth is mentioned, which is object’s radar reflectiveness, measured through the radar cross section (RCS). Basic way to reduce RCS is through shaping, resulting in unique shapes of low-RCS vehicles such as aircraft and ships. However, RCS can also be reduced through materials as well as electronic means. Either way, proper tactical behaviour is required to fully exploit low RCS characteristics, as even just a quick maneuver exposing unfavourable aspect can result in aircraft’s detection. Same goes for ships, albeit they have the advantage in that most threats are always in the same plane as the ship is.

Second aspect is emissiveness of the object itself. These emissions can be various in nature. Active radar, data links, radio communications, etc. all generate electromagnetic emissions. These then can be detected, revealing location of the object (aircraft, ship).

Infrared spectrum

Due to active infrared sensors not being in use any more, this only deals with passive sensors. One of main contributors to infrared signature in all vehicles is the engine, both in terms of the hot exhaust plume as well as the heat signature of the engine itself. Other contributors are sunlight reflection (in particular glint off the canopy), cooling of electronics, as well as (in aircraft) air friction.

Acoustic spectrum

Acoustic stealth is primarily important in naval and ground combat, less so in air combat. In naval combat, density of the medium (water) severely limits usefulness of either visual or infrared sensors. However, it also allows excellent propagation of sound, thus making sound critical for naval units. In ground combat, sound likewise can enable detection when vehicle is behind cover and thus invisible to line-of-sight sensors (optical, IR, radar). In air combat, acoustic stealth is only relevant in ground attack missions, especially close air support, when aircraft fly low enough to be potentially heard by soldiers on the ground. CAS aircraft in particular often fly low enough to utilize terrain for cover.

Basing and logistics signature

This is an aspect of stealth which is oftentimes ignored. However, fighter aircraft spend most of the time on the ground (two-thirds at least), during which they are extremely vulnerable to sneak attacks. Even ships and ground vehicles need place to refit, repair and resupply. Unlike aircraft, they can resupply in the combat zone as well, but that requires even lager logistical chain as well as supply bases. If this infrastructure is taken out, weapons that rely on it are quickly rendered useless – no matter how advanced and/or capable they may be otherwise.

Comparing fighters

Airframe characteristics

Size is obviously important in visual stealth. All other things being equal, larger aircraft will be noticed earlier (camouflage also has major impact). However, size is also important in infrared signature as well. At high altitude, where air is rare and cool, with little moisture (99,8% of atmospheric water is below 45.000 ft), there is little to block infrared radiation from propagating. Rarified atmosphere and in particular lack of moisture at high altitude means that absorption of IR radiation decreases across much of the IR spectrum, opening bands that would normally not be usable. Even cirrus clouds that are the only type of cloud typically present above 8 kilometers are infrared transparent. This creates ideal conditions for propagation of longwave IR emissions, which is especially important since LWIR detectors have good sensitivity against targets that are close to ambient temperatures. Longwave radiation is created in particular by aerodynamic heating of the skin. At Mach 0,8, difference between the aircraft skin and ambient air will be 15-27 degrees Celsius, climbing to 31-44 deg at Mach 1, 130-145 deg at Mach 1,7 and 172 deg C at Mach 2,0. All these differences, even minimal ones, are more than enough for the longwave IRST to detect a fighter-sized target. Another consideration is physical size of a fighter aircraft and, at supersonic speeds, size of the shock cone. Since IRST is still an optical sensor, detection range is limited as much by target size as it is by temperature difference between the fighter and surrounding air. IRST sensor of Dassault Rafale’s OSF can, at 20.000 ft, detect a subsonic fighter-sized target at 80 km from the front and 130 km from the rear. At low altitude, range from the rear is 110 km, which would indicate frontal range of 68 km. F-22 achieves supercruise speed of Mach 1,72-1,75 at 38.000 ft. Assuming similar increase in range between 20.000 and 40.000 ft, OSF should be able to detect the subsonic F-22 at distance of 90-95 km from the front and 145-155 km from the rear. If F-22 is supercruising at Mach 1,70 and 40.000 ft (about the limit of its supercruise performance at that altitude), combination of large shock cone and increase in heat signature increases detection distance three times compared to subsonic performance; thus supercruising F-22 should be detected by OSF at distances of 270-285 km from the front or 435-465 km from the rear. Even at speed of “only” Mach 1, shock cone and temperature increase combined double detection distance compared to when F-22 is subsonic. While F-22 does have IR signature supression measures which may slightly reduce detection range v/s subsonic F-22 compared to the range listed here, these do not affect the shock cone formation at supersonic speeds. Larger aircraft require more powerful engines and are thus also louder, which can be a major factor in low-level penetration of ground air defenses. They also require more basing space, more fuel, and more parts, increasing logistics signature.

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However, reducing speed has its disadvantages. While increasing detection range three times by cruising at Mach 1,7 will increase probability of detection nine times, probability of attack from the rear will decrease as any attackers will have harder time to catch up, and  attacker’s effective missile range will be significantly reduced. It will also reduce fighter’s own weapons range.

Shape likewise is very important for stealth. Most obvious is its impact on radar return (RCS), as careful shaping can redirect radar waves away from the emitter. In fact, shaping outweights any radar-absorbent materials when it comes to RCS reduction. As a result, stealth fighters utilize angled, mostly parallel surfaces to deflect radar waves away from the emitter; curved air duct is also used to block the line of sight to the engine face. But there is a catch: effectiveness of shaping is dependant on the wavelength. If wavelength is greater than the object, or a component of the object, then actual shape of the object or the component does not matter; only the volume matters. Normal fighter radars of 8-12 GHz frequency produce wavelengths of 3,75-2,5 cm, so smooth surfaces quite easily reduce crossection. Only very small shaping elements can cause increase in RCS: even air-to-air missile fins may not do so. But VHF band has wavelength of 1-10 m, and HF of 10-100 m. As wavelength increases there is a tendency for parts of the aircraft and even the entire aircraft to act like one big source. At VHF band frequencies, minor shaping details lose meaning and only the general shape matters. At HF band frequencies, shaping becomes irrelevant and only the volume matters. This effect can happen even at VHF band frequencies with individual components such as stabilizers and tail fins, albeit increase in RCS will obviously be limited due to relatively small size of these components. As an example, Chinese DF-15 ballistic missile has 0,002 m2 RCS in X-band, but 0,6 m2 in VHF band. F-117 achieved success because nobody knew about low-RCS aircraft in service, let alone its specifics such as shape used. But the cat is out of the bag, and that success cannot be repeated again. In 1999. col Zoltan Dani shot down an F-117 using a combination of VHF early warning and L-band tracking radar. His battery also shot down an F-16C earlier. It is sometimes claimed that the F-117 was detected because its bomb bay doors were open, but closer look shows otherwise. Dani’s VHF radar detected a group of four F-117s flying north at 20:40, but Dani did not have the ability to engage using solely comparatively ancient VHF radar. Instead, VHF radar was used to direct the more accurate 9 GHz Low Blow radar (3 cm wavelength, allowing high precision but suboptimal for anti-stealth work) which obtained a lock after third illumination. Lock was obtained at distance of 13 km and altitude of 8 km, and one out of two SA-6 missiles obtained proximity fuze kill. Stealth aircraft that came after F-117 had no such high security; their shape was known from the start of the development process. They also have far lesser focus on stealth, employing very non-stealthy approaches such as supercruise, active sensors, datalinks etc. CIA has even predicted development of VHF radars expected to counter the stealth aircraft.

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Another problem is shaping of internal components. Unless canopy is gold-plated, radar emissions will pass through the glass and reflect off its interior components. Emissions may also pass through the fighter’s radome and reflect off its radar. Radar is essentially a big flat plate, but behind it it typically has more complex shaping oftentimes including several corner reflectors. Antenna itself is typically vertical, albeit some Western fighters have angled antenna. Since radar’s emissions have to be able to get back to the source, radome must be transparent in certain frequencies. But this places a dilemma upon the designer. If radome is transparent to a wide range of frequencies, there is a high possibility of opponent’s radar emissions getting past the RAM coating and reflecting off the radar itself. If only few frequencies are utilized, there is higher probability of radar emissions getting detected even by a less-than-competent RWR/ESM devices.

What is not so obvious is impact of shape on infrared signature. Aircraft with superior aerodynamic shaping will not need as much thrust to achieve same speed as less optimized aircraft, which automatically reduces their infrared signature. However, since aircraft has to maneuver, it may expose less favourable aspect to the sensor, which automatically increases its signature. Shape also should reduce vortex creation as vortices can increase radar signature, especially at high altitude; engine exhaust may produce similar effect. Shaping of the engine nozzle has impact on both radar crossection and IR signature. Round nozzles are good radar reflector and increase side RCS. Rear RCS is also increased due to cavity effect unless a blocker is installed (as it is in both F-22 and F-35). Flat nozzles avoid these shortcomings, and have the added effect of shaping the exhaust in a way that increases its surface to volume ratio, thus improving cooling. They also partly mask the hot spots of the exhaust nozzles. This IR signature reduction effect however is somewhat offset by the thrust loss due to less-than-optimal nozzle shape, necessitating more powerful engines to produce the same net thrust, which increases heating of the airframe around the engine. Round nozzles can also be shaped in a way that reduces IR signature and even EM spikes to an extent. Rafale’s M88 engine has secondary cooling channel around the engine core, complete with the external nozzle. This helps reduce engine IR signature in several ways. First, it places another layer between the hot engine core and the surrounding airframe, thus reducing airframe heating due to the engine operation. Second, cool air is redirected around the engine, cooling down both engine core and its outer shell, and reducing the transfer of heat to the aircraft skin. Third, cooler air exiting the outer nozzle limits the spread of the hot core air, limiting its visibility from the front. Lastly, outer nozzle itself hides the hottest part of the engine plume at least from the front sector, and hides most of it even when aircraft is viewed from the side. Even when the angle is only 45* from the aircraft centre axis, viewed from the rear, about 50% of the hottest part of the engine is still hidden from the view. External nozzles also help reduce EM spikes when combined with a “filler” between the nozzles. Typhoon’s EJ200 does not have as many cooling features, however its higher bypass ratio (0,4 vs 0,3 for M88) means that its exhaust is cooler to start with. Engine itself may still be hotter due to thinner engine walls. And while its exhaust may be cooler, EJ200 has no external nozzle to hide the hottest portion of the exhaust; only a slight reduction in IR signature and RCS is achieved through utilization of serrated nozzle edges. Same goes for the F135, except it is far hotter than either engine. It does however have even higher bypass ratio than EJ200 and additional cooling provided from two small underwing intakes. Shape also has impact on acoustic stealth, with increase in air flow irregularity increasing noise (due to, among other things, more engine power required to overcome increased drag).

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Twin engine nozzles in many fighters have an empty space between them. Due to complexity of radar reflections, this empty space can increase rear-aspect radar cross section. When it comes to infrared signature, things are a bit more complex. Twin engine configuration allows superior area/volume ratio of the emissions, thus improving cooling and reducing IR signature for any given thrust. However, single-engined fighters typically have superior aerodynamics – area ruling in particular – which means that they require less thrust for any given speed. They are also typically smaller. As a result, twin-engined fighter will likely have lower IR signature for given thrust, but single-engined fighter is more likely to have lower overall IR signature. Logistical signature is also increased somewhat because two engines require greater logistical support in terms of replacement parts as well as, typically, fuel.

When it comes to ground attack, acoustic stealth plays a major role. Here, engine type too becomes important. High-bypass turbofans can be far quieter than either low-bypass turbofans or turboprops, and are thus an ideal choice for close air support aircraft. This requires fan tips to, if possible, remain subsonic. However, majority of the noise is due to the engine jet exhaust. This noise is likewise impossible to eliminate, though it can be reduced by reducing engine thrust and thus exhaust velocity. Due to relationship between exhaust velocity and noise, even small reductions in the exhaust velocity can mean large noise reduction. Aerodynamic noise is another problem, being created by airflow over aircraft’s surface. This noise can be reduced by eliminating protrusions and, more effectively, by reducing aircraft’s speed.

Sensory suite and stealth

Sensory suite is another aspect of stealth. This in particular pertains the question of active vs passive sensors. In the simplest terms, difference is like waving around a flashlight in the dark room or using night vision. Reality is more complex, but the same idea stays. Comparison is primarily between radar and IRST, but today, neither sensor is a standalone. Modern radar warners utilizing interferometric approach can achieve directional accuracy better than 1 degree at 100 km (200 km for Rafale’s SPECTRA). Therefore they can be used, even alone, for weapons targeting and launch. This can also be used to adjust the frequency coverage as well, with each fighter or flight covering only a portion of expected enemy radar frequency range (typically X-band), improving the probability of detection against LPI radars. RWR in fact has less clutter problems that radar, since radar is a point target and not reflection off the object, and there are few interfering signals. This means that even if fighter is directly in the path of the radar beam, it will likely detect radar emissions long before radiating fighter detects it. And since even AESA radars have both sidelobe and backlobe, albeit greatly supressed compared to old-type mechanical radars, RWR can detect enemy even if the passive fighter is not in the path of the radar beam, allowing easy surprise attacks. And when a fighter has already been illuminated by a radar beam, side lobes will remain and emit in its direction long after the main beam has moved on. These characteristics mean that RWR/ESM suite has much longer time to analyze and geolocate radar’s position than normally thought. In fact, the only stealth aircraft to be proven in service – F-117 – had no radar, no radar altimeter, no datalinks, and the pilots kept antennas retracted and radios turned off during the missions. Side lobes also have an effect of reducing radar’s performance, as reflections of side lobe signal from the ground creates a lot of clutter. Despite side lobes’ lesser power than the main beam, ground is far closer to the antenna than a distant target and thus can swamp out valid returns.

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IRST likewise has seen improvements in range, angular accuracy and passive rangefinding (kinematic range processing, which can also be used by RWR). Datalinking several aircraft together also can allow very accurate rangefinding with no emissions directed towards the target. This allows completely passive engagement of targets at beyond visual range, with no radar or laser emissions necessary. Overall, any fighter aircraft claiming to be stealth has to have very extensive passive sensors suite. Radar may be used, but only for quick, accurate rangefinding after the target has already been detected and had distance roughly determined by passive sensors. Since radars of fighters in formation have to be tuned to different frequencies, analysis may even reveal number of fighters using radar.

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Impact of weapons

In radar cross-section, main impact of weapons is wether they are internal or external. External weapons carriage results in far more complex surface, as well as major increase in number of radar scattering points. This significantly increases aircraft RCS even when aircraft is carrying missiles; with bombs and drop tanks, increase is even greater. With air-to-air missiles, there are two possible solutions. First solution is conformal / recessed carriage of missiles, as utilized by most fighters since the F-16. This solution eliminates the pylon as well as the space between the missile and missile rail and the aircraft which can significantly increase RCS in certain situation by acting as a corner reflector in addition to its impact due to radar scatter. However, it can only be utilized with a limited number of missiles (two missiles for F-16 and Flanker / Fulcrum series, four missiles for Rafale and Typhoon). Second solution is internal missile carriage as utilized by radar-LO fighters. This solution also limits number of missiles carried but less than conformal carriage does. It also completely eliminates missile impact on radar signature, instead of merely reducing it, but at the cost of increased size, weight and complexity.

In infrared spectrum things are, as usual, less clear. External weapons carriage can significantly increase drag in a classical configuration, thus increasing IR signature due to among other things more engine power required. Impact of conformally carried missiles is far lesser due to lack of pylons, which eliminates interference drag. Even with classical carriage, careful pylon shaping can significantly reduce drag. Internally carried weapons eliminate weapons drag, but have their own penalties. Presence of missile bays means that for the same range and payload, fighter aircraft has to be significantly larger – at the very least it has to be fatter, even if extreme dimensions remain the same (e.g. F-22 vs F-15, F-35 vs F-16). This means increased base drag due to increased frontal area as well as increased wing area (to counter increase in weight), which in turn means more powerful engines and thus higher IR signature. Increased size itself also contributes to IR signature.

Impact of weapons on stealth does not end in missile carriage type. Active radar missiles emit radar signals which warn the enemy of the missile approach early on, allowing more timely reaction as well as jamming the missile. Consequently, in order to take full advantage of fighter’s passive sensor suite, missile itself has to be passive as well. Two main types of passive missiles are infrared and anti-radiation missiles. Infrared guided missiles are more versatile because they do not rely on target aircraft emitting signals during the entire time of flight and are not vulnerable to being decoyed by decoys such as towed decoys or disposable jammers. It is also questionable whether anti-radiation missile can have antenna and the processing power required to detect emissions from modern LPI radars, albeit the task is made less difficult by the fact that fighter radars operate almost exclusively in the X-band.

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When it comes to basing and logistics, internal carriage is disadvantaged because it increases complexity and maintenance requirements. It also increases aircraft weight, reducing the possibility of dirt-strip basing.

Basing and logistics signature

Fighter aircraft are less air “craft” and more air “hoppers” that spend most of their time on the ground, only going into the air for short intervals. At the very most, aircraft will spend about one third of their time in the air. Yet it is on the ground that they are at their most vulnerable. Consequently, ground stealth is crucial for survival, and this – at the very least – means road basing. Such bases could use camouflage nets to protect themselves from visual, radar and IR surveillance, and stay mobile to prevent or at least delay discovery and attack. But in order to do so air force has to be equipped with easily maintained fighters with low logistical requirements – particularly fuel and maintenance personnel must be in low demand. Extremely low RCS is clearly incompatible with this. Radar VLO fighters spend extended amounts of time on the ground due to maintenance requirements of their stealth coatings and weapons bays, and also require specialized air bases for extended deployments. Their larger size also means higher fuel expenditure, thus increasing the logistics trail which is a vulnerability into itself. All of this makes them vulnerable to being attacked on the ground. In 1945., despite nearly total Allied air superiority at the time, German Luftwaffe still managed to launch Operation Bodenplatte: 800 German fighters attacked 17 Allied air fields, destroying or damaging over 500 aircraft. Damage would have been far heavier, but one German flight accidentally ran into USAAF early patrol which warned the air fields of the impending attack. During the entire war, as well as later Korean and Vietnam wars, both ground and air attacks on air bases were a normal fact of life. Today, ballistic or cruise missiles can attack known air fields without placing pilots in danger. Modern fighters also cannot fly from open grass fields, so even if aircraft survive in shelters, submunitions attack can put the base out of operation by destroying air strips. But low RCS fighters require dedicated, immobile and easily found and neutralized air bases to operate; to be stealthy on the ground means placing focus on low maintenance and support requirements, which stealth fighters are incompatible with. Only small, single-engined, conventional fighters can achieve acceptable basing stealth.

Another problem are support assets. Western air forces, especially USAF, rely heavily on airborne support assets such as AWACS and tankers. “Stealth” fighters in particular rely heavily on them because they cannot carry drop tanks while in combat zone, cannot utilize their own radar for fear of being discovered, and also cannot afford outbount communication except the most basic one. Thus they have to rely on AWACS to be their eyes and ears (and mouth), especially if they lack the modern IRST (as F-22 does), and also rely on tankers to extend their endurance in the combat zone. But none of these assets have even the limited stealth of “stealth” fighters. Losing them would put stealth fighters in a precarious position, having no way to refuel and being forced to rely on onboard sensors for situational awareness. Thus stealth fighters have to intercept any attack against said support assets, but this makes them relatively easy to find, largely negating their stealth.

Tactics

But in the end, stealth is a tactical characteristic. And even non-stealthy aircraft can achieve practical invisibility by using terrain for masking. Once aircraft is behind the hill, it can carry as much external ordnance as it can lift. This was done in the World War II, Korea, Vietnam and the Gulf War I. The only signature that matters in this mode of operation is acoustic, since all others are blocked by the terrain (and even sound waves are largely blocked, especially at higher frequencies). It was always considered acceptable – Paveway III laser-guided bombs were designed for low-level utilization. And in cloudy Europe, supporting the ground forces against Warsaw Pact meant flying low since clouds would preclude any high-altitude attacks against ground targets. AWACS and F-15Cs radar were developed to deal with low-flying enemies. In the Desert Storm, once incompetent Iraqi air defenses were destroyed, Coalition withdrew to medium altitude to prevent losses from the impossible-to-find optical AAA and IR MANPADS. Only the A-10 was allowed to go below 15.000 ft due to its extreme survivability. This withdrawal was only made possible by destruction of Iraqi air defences. Aircraft that were hit at low altitude were only even hit while directly attacking heavily defended areas: A-10s were hit when engaging ground forces, which is not surprising considering the amount of ordnance that army division or even brigade can fill the air with. Tornadoes were hit when attacking air fields, the most heavily defended targets, and doing so before the defenses were suppressed. Retreating to higher altitude would not have prevented the losses; reduced them, maybe, but not by much and definetly not enough to be worth reduced effect. Only after radar SAMs were supressed did the high altitude become viable. But survivability was not the function of altitude, it was the function of a mission: airfield attack was the most dangerous one, followed by battlefield interdiction and close air support. The least risky mission was reconnaissance. Aircraft that loitered over the battlefield were at most risk outside actual combat. Most aircraft that were hit were hit by either optical AAA or IR SAMs (MANPADS). Typically multiple SAMs were fired for each hit, even though air crews reported not seeing any weapons combing until the first miss flew past, or even the actual hit. In Allied Force, ban on low-altitude attacks increased danger to air crews by requiring them to revisit targets that they missed because they attacked from too high altitude.

But low altitude flight is a perishable skill. For this reason, even maintenance downtime and operating cost factor into aircraft’s stealth capabilities – aircrew that do not train for low-altitude flight do not get to use it, and thus risk detection. Electronic warfare, SEAD and DEAD are also perishable skills, yet USAF dismantled the “Electronic Combat Triad” (EF-111A, F-4G and EC-130) and, more critically, it closed down the Wild Weasel school as well as the Green Flag exercises, both dedicated to EW and ECM. With that, USAF has lost the capability to penetrate defended airspace, as well as the capability to know it has lost the capability to penetrate the defended airspace.

One problem with low altitude is that it works both ways. If radar can’t see the aircraft, then aircraft can’t see the radar. This can be worked around by having a fighter outside the SAM range drawing attention and using datalink to send information to the strike fighter. Another option is usage of air-launched decoys, which could transmit data to aircraft in the ground shadow (jamming only affects receivers).

Comparing tanks

Physical characteristics

With tanks, having a smaller profile is an obvious advantage. It means that tank is harder to see and hit out in the open, and also easier to hide behind terrain features. However, having low profile can be a disadvantage in hull-down shooting, as very low profile as found on Eastern tanks can limit main gun elevation. Size of turret is another factor, as is gun position on the turret. Large turret obviously makes a tank much easier to spot, especially in hull-down position. Low gun position on the turret, as in Challenger 2, means that the entire turret has to be exposed when hull-down. Much like with fighters, camouflage paint can be used to disguise either tank’s presence or exact contours and speed (dazzle camouflage).

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Polish PL-01 is allegedly stealthy in visual, IR and radar spectrums. While it may be stealthy to radar, this depends on aspect, so aircraft should have no trouble detecting it from at least some angles. It also has “chameleon skin” which allegedly allows it to match the IR signature of the surroundings and even mimic signature of e.g. car. If true, this would prove extremely useful characteristic, but question it posits is how the tank deals with engine heat. Overall, claimed capabilities of PL-01 would make high-altitude fighters useless and would require dedicated, visual range tank killer aircraft such as A-10 to counter.

Sensory suite and stealth

With tanks as with fighter aircraft, active sensors are a disadvantage. Most tanks do not use any active sensors other than a laser range finder, and even laser rangefinder is not used unless absolutely necessary. At short ranges (cca 1.000 m) optical rangefinding is used instead. Russian T-72 used IR illumination combined with passive IR sensor to better detect targets, but this made T-72 itself an easy target for the completely passive IR sensors of Western tanks. For this reason, Western tanks did not use active IR sensors other than in some early generations of Leopard 1 / AMX30 / M60 / Chieftan tanks, and these were replaced as soon as acceptably good passive sensors became available.

Engine type – diesel or turbine

Turbine engine has significant advantage in acoustic signature, being far quieter than typically very loud diesel engine. However, M1’s turbine engine produces 499 degree Celsius exhaust, hot enough to prevent close escort by infantry. For comparison, diesel engine does not get hotter than 220 degrees Celsius, and even that only during acceleration phase. Laws of radiation mean that going from 220 to 500 deg C increases radiation by (500/220)^4 = 26,68 times, which means that detection distance increases 3√26,68 = 2,99 times. This difference is reduced by the fact that diesel engine has far higher soot emissions, thus increasing emissivity compared to the gas turbine, but this does nothing to affect engine compartment temperature. Further, modern diesel engines emit hot fumes from exhausts directed at ground, whereas turbine exhaust is directly at the rear of the tank. Turbine also always runs at maximum RPM for efficiency, while diesel engine is most efficient at low RPM. This is a major issue when idling, as tanks’s engine compartment heats up, and gas turbine heats it up at far quicker rate and higher temperature than diesel engine does.

Basing and logistics signature

Gas turbine is far less efficient than diesel engine, especially when idling. As a result, turbine-engined tanks require far more fuel and thus far larger and more vulnerable logistical trail. Gas turbine also has major issues with FOD, as US M1 tanks demonstrated in both Gulf wars, with many breaking down due to sand particles damaging turbines. Diesel engine will at most suffer a temporary shutdown and require cleaning and filter replacement; gas turbine that eats up sand for some time requires complete replacement.

Comparing ships

Impact of size

Smaller ships are harder to detect visually due to both smaller size of a ship and reduced wake. Dazzle camouflage, designed to confuse the enemy as of ship’s exact size and heading (pioneered in United Kingdom in WWI), is also more effective on smaller ships, as is classical light gray camouflage which is designed to hide the ship in the haze. Navy blue proved effective against aerial reconaissance during World War II, but its effect likewise is limited by ship’s wake. British Western Approaches scheme proved so effective that when HMS Brooke first utilized it, it caused a collision. However, since most ships have very similar (and complex) features, as well as huge size range, size also has major impact on radar signature as well as acoustic and infrared signature. It also impacts magnetic signature, although magnetic signature can be reduced through degaussing as well as construction techniques.

Sensory suite and stealth

Just like with aircraft, using active radar is suicidal. During the Falklands war, HMS Sheffield kept its radar turned on which caused it to be found by Argentine EW 707 aircraft and subsequently sunk. This is especially problematic for ships, far more than for aircraft, because radar is a line-of-sight sensor: anything below the horizon it cannot detect. But due to the nature of radio wave propagation, especially above the surface of the sea, radar emissions can still be detected even from below the radar horizon. This allows radiating ships to be attacked by aircraft from outside their response range.

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During the Vietnam war, a Shrike missile accidentally homed in on USS Worden’s radar emissions, exploding 100 ft above the superstructure and disabling ship’s command center. This could be avoided by using VHF radars, whose long wavelengths make them more-or-less immune to anti-radiation missiles (missile has to have antenna that is at least 1/2 of the radar wavelength; mounting a 50 cm antenna on a missile is obviously impractical). Fact of the matter is that the Vietnamese used these radars with impunity during the Vietnam war, and Serbs also used low-frequency radars during the war in Kosovo. These radars would also provide the advantage of improved performance against low-RCS aircraft. However, even VHF radar’s emissions would still be detectable by dedicated EW platforms (such as the 707 mentioned above) and other ships.

Consequently, IR sensors are highly important for ships as well. However, their short range in humid conditions near sea surface would leave the fleet with limited advance warning. One possible solution is having few ships equipped with frequency-agile VHF radar. Such radars, as noted, would not be vulnerable to anti-radiation missiles and the emitting ship could sail a distance away from the main fleet in the role of a radar picket. This role would be best left to small, fast ships.

Russian submarines, during and after the Cold War, routinely approached US aircraft carriers so closely that they would have assuredly sunk them had actual conflict broken out. Even noisy US nuclear submarines found no difficulty in “sinking” nuclear carriers, with Captain John L. Byron stating that the US carrier’s ASW protection often “resembles the Swiss cheese”, and most US submarine skippers have a photo of an aircraft carrier taken through the periscope from well within the torpedo range.

Impact of propulsion matrix

Two main propulsion types are diesel-electric and nuclear. In terms of stealth, both have pros and cons. In surface ships, main advantage of nuclear propulsion is eliminating need to refuel and possible smoke emission. However, it also means fewer and larger ships, and is problematic in terms of acoustic signature. Another source of noise is propeller. This noise can be reduced by curving the propeller blades so as to reduce cavitation, but that is typically not done due to loss of efficiency. Same technique is used on submarines, where reduced noise is worth the loss of propulsion efficiency. Loaded tip may also be used. Propeller should be shaped to avoid cavitation as collapsing bubbles can be a source of noise. Cavitation can also be controlled by reducing propeller rotation rate, but this has effect on thrust.

Impact of weapons

Most weapons on surface ships are radar-guided. While this is logical due to high humidity and salinity prevalent near the ocean surface, it is also problematic because it means that ships can be relatively easily detected by the enemy, and their missiles jammed or decoyed. Therefore, backup IR systems are required to allow completely passive operation when stealth is required.

Basing and logistics signature

Large ships, and nuclear carriers in particular, require huge and very obvious bases for maintenance and resupply. Smaller ships can use makeshift, concealed bases which are far safer from any attacks. While resupply on sea is possible for both, it causes a significant drop in readiness. Bases themselves are far from secure, being vulnerable to threats such as submarine-launched cruise missiles. When it comes to specifically US bases, Russian spy ships routinely go snooping around them. Even during the Cold War, US Navy Seals assigned to test base security had little trouble in their simulated diversionary attacks against the bases. Had the attacks been real, Soviets would have had good account of US Navy practices, and USN would have lost at least one if not several nuclear submarines. Overall, a repeat of another Pearl Harbour is entirely possible.

Comparing submarines

Impact of size

Smaller submarines are harder to detect due to lower generated noise levels and smaller magnetic signature for same technological level. They can also more easily hide on the bottom, avoiding active sonar sweeps, and are more maneuverable in restricted and often noisy coastal waters. Unlike aircraft, submarines can lie in wait for their targets, and smaller boats are harder to detect among all the irregularities on the sea floor. This should not be discounted even against nuclear-powered ships, as in many cases they will be limited to a certain area by operational requirements. Magnetic signature can also be reduced through degaussing as well as construction techniques.

Sensory suite and stealth

While active sonar is stereotypical for a submarine, it also gives away submarine’s position. For this reason, submarines rely primarily on passive sonar when in area of operations, using databases to identify ships detected through propeller noise.

Impact of propulsion matrix – diesel vs AIP vs nuclear

DE and AIP propulsion allows for very small, very quiet boats that can easily hide “in the bushes”, using underwater terrain features. However, they are limited to no more than three weeks of underwater operations at very slow speeds. Nuclear submarines can operate for months under water, limited only by provisions of food and ammunition. This comes at price of increased acoustic and magnetic signature compared to conventional submarines due to increased size as well as the need to more-or-less constantly run noisy coolant pumps. Infrared signature is also increased due to requirement for cooling the nuclear reactor, albeit this is not a factor unless submarine is at very low depth. Oftentimes greater speed of nuclear boat is no advantage as it simply means that it makes more noise as it goes faster, as observed by the C.O. of HMAS Sheehan. During RIMPAC 2000, two US nuclear submarines were sunk by the HMAS Waller while unable to locate it despite knowing that it was within (very restricted) exercise area. Such situations have happened many times before and since that example; more here. In general, European submariners were far more concerned about colliding with US nuclear submarines (running fast and thus blind) than they were of being “attacked” by either them or by the surface ships. Propeller also has impact on noise, but was adressed in the previous section.

Impact of weapons

Active torpedoes naturally warn the target ship of their approach, but even passive torpedoes are not undetectable due to their speed and the fact that water is very good sound medium.

Basing and logistics signature

While diesel-electric submarines may be able to operate from makeshift bases, nuclear submarines are completely dependant on few obvious large bases, even though they do not need to visit them nearly as often. Why is this troublesome was already explained in the previous section.

Conclusion

Stealth cannot be achieved through technology – best effect is always tactical, and Earth itself provides the best cover. Low-flying A-10 is as stealthy to enemy radars as the F-35 hundred kilometers distant, but is far more useful than the F-35 is. No sensor can see through the hill, and fleeting appearances of an aircraft flying at low level are hard to engage with anything except small arms fire. When below the horizon, or shielded by the terrain, Tu-95 and F-35 are equally impossible to detect; T-95s enormous radar or F-35 just as enormous infrared signature make no difference. Only sound can pass around the terrain features, but A-10’s high-bypass turbofans are deathly quiet. But US military has trouble acknowledging this. Former U.S.N. F-14 Radar Intercept Officer Jerry Burns said in 2000: because “Anyone who says something is wrong gets thrown out of the Navy.”. In that regard at least, US Army, USMC and especially USAF are no different. Low rankers have to tow the party line set by the generals, and generals are bureocrats eyeing contracts with military industry, thus not really caring about actual wartime utility of the weapons they buy. Thus they conflate “capability” with “cost”, being only interested in what is most expensive and not what is actually necessary. US Navy also forbids its submarine crews from acknowledging any kills against nuclear aircraft carriers in exercises; it would not be surprising if USAF turned out to be doing something similar with regards to “stealth” fighters, F-22 in particular. In one particular example, a US Navy submarine commander put six torpedoes into an aircraft carrier and was commended – for reducing the carrier’s efficiency by 2% (for the record, just one torpedo under the keel is a certain mission kill even against a large nuclear carrier, and anything more than that will probably result in sinking).

P.S. A “perfect” stealth fighter should be very small, single-engined, with internal missile carriage, stealth shaping but no (or very low-maintenance) RAM coating, and no radar, relying only on passive sensors which themselves should be shaped with stealth in mind. A “perfect” stealth submarine would be very small, relying only on electrical engine and with no active sonar.

Further reading

Airborne IRST properties and performance

F-35 reality check

US Air Force and stealth – stuck on denial, part 1

US Air Force and stealth – stuck on denial, part 2 – rediscovering low altitude

US Air Force and stealth – stuck on denial, part 3 – low altitude penetration

http://www.cuttingedge.org/news_updates/nz1839.htm

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13 Responses to “Defining stealth”

  1. altandmain said

    Great article.

    One note. Much like how bombers use their radar for terrain mapping, submarines do too with active sonar. It is especially important for where the sea floor is shallow enough that the submarine is in danger of colliding with the sea floor. It is often used under the ice too in the Arctic Ocean.

    • Leum said

      Clearly active sensor have their sense. if they had no sense, only the USA will use them.

      • picard578 said

        They do, mostly because of superior range accuracy (and nominal range, depending on the sensor and conditions). But it is best to not use them unless you absolutely have to.

        • altandmain said

          The problem is that when terrain mapping underwater, you often need very accurate terrain, lest the submarine run aground. Otherwise, it’s uses are more limited.

          If I recall, the detection range of active was over double the range that the active sonar could detect, kind of like radar vs radar warning receivers.

      • Duviel said

        USA makes some stupid acquisition decisions but we still kick everyone’s ass.

        Just Fyi.

  2. Tarjei T. Jensen said

    The F-117 kill and damaging were done by Zoltán Dani’s SA-3 battery.

    https://en.wikipedia.org/wiki/Zolt%C3%A1n_Dani
    https://en.wikipedia.org/wiki/1999_F-117A_shootdown

    I think that there was also an additional a F-16 shot down by a SA-6 battery, but I’m uncertain about when.

  3. Back in the 1980s, “Turbo” was the craze word, like “Stealth” today. Everything was “Turbo” cars, motorcycles, hair dryers, and hamburgers,
    “Stealth” today has about as much mindless relevence, It’s pathatic,

  4. altandmain said

    Turbine engine has significant advantage in acoustic signature, being far quieter than typically very loud diesel engine. However, M1’s turbine engine produces 499 degree Celsius exhaust, hot enough to prevent close escort by infantry. For comparison, diesel engine does not get hotter than 220 degrees Celsius, and even that only during acceleration phase. Laws of radiation mean that going from 220 to 500 deg C increases radiation by (500/220)^4 = 26,68 times, which means that detection distance increases 3√26,68 = 2,99 times. This difference is reduced by the fact that diesel engine has far higher soot emissions, thus increasing emissivity compared to the gas turbine, but this does nothing to affect engine compartment temperature. Further, modern diesel engines emit hot fumes from exhausts directed at ground, whereas turbine exhaust is directly at the rear of the tank. Turbine also always runs at maximum RPM for efficiency, while diesel engine is most efficient at low RPM. This is a major issue when idling, as tanks’s engine compartment heats up, and gas turbine heats it up at far quicker rate and higher temperature than diesel engine does.

    I wonder if we could use what is available in the civilian world to make diesel engines more fuel efficient.

    – Selective non-catalytic reduction
    – Particulate matter filters
    – Exhaust gas recirculation (although only at lower loads)

    Interestingly when the first 2 have been combined, fuel efficiency actually improved, although the cost to buy the engine went up.

  5. Active stealth, a la Rafale, is the future. Rafale is the future, the USAS should just build a super Rafale, under licence.

    Western Strategic Submarines Ballistic Nuclear (SSBN) are extremely stealthy. A few years back, in February 2009, while at great depth, a British Trident SSBN collided with a French built SSBN. Those subs are 150 meters long. The impact happened “at very low speed”. However Triomphant took out a huge chunk of the front of Vanguard, then scrapped all along, exposing plenty of bottles of High Pressure Air. If one of them had exploded, Vanguard would have sunk. It returned to Scotland very slowly.

    Each boat carried 48 thermonuclear warheads. Each ten times Hiroshima (in case of looming nuclear war, they could carry four times more warheads!).

    Hervé Morin, France’s Minister of Defence at the time, said that they “face an extremely simple technological problem, which is that these submarines are not detectable”. They actually lurk in part of the ocean where reflections from current, etc. make them even less detectable.

  6. David said

    How effective do you think that modern Russian ground-to-air defense systems (namely the S-400 and S-500) are likely to be against passive stealth aircraft such as the F-35, and vice versa? (do you think you could possibly do an analysis on this?) Also, do you think that “low probability of intercept” radars, such as those that the F-22 and other “fifth generation” fighters are reputed to have, are as stealthy as advertised?

    • picard578 said

      Air defenses never were very effective, but I suspect supercruise might cause them more problems than stealth, considering VHF radars and the like.

      LPI radars are definetly not as stealthy as advertised, and besides, LPI measures such as frequency hopping had been in use since World War II. LPI radars are by no means a new development.

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