Introduction

For a long time, visual detection was the only type of detection possible. But in World War II, two significant advances appeared: radar, as well as radar- and IR- -guided missiles. Until 1970s, these were defeated through jamming and decoys. Early German attempts at building LO aircraft – Ho-229 – never got anywhere, albeit their RAM paint utilization at snorkels and aircraft was noted. In United States, first attempt at reducing the radar signature of aircraft was on U-2, by utilizing RAM paint, but it was not very successful. First actual stealth aircraft appeared in early 1960s – SR-71 Blackbird, which utilized shaping such as canted surfaces to reduce radar signature. In 1970s a second generation of stealth aircraft appeared with B-1A, and also began a programme of development of VLO aircraft. Result of that was diamond-shaped F-117, to soon be followed by B-2. All these aircraft successfully performed against enemy air defenses, but in the case of B-1A and later aircraft, their performance against air defenses was similar or identical to performance of conventional aircraft they were deployed alongside. Fourth generation of stealth aircraft are F-22, F-35, PAK FA, J-20 and J-31. While still stealth aircraft, they sacrifice stealth characteristics for the sake of better flight characteristics, allowing them to match conventional fighters in terms of maneuverability. However, their stealth requirements make them larger and heavier than comparable conventional aircraft, thus sacrificing kinetic performance for the sake of stealth.^1

Limitations of stealth

Main areas where aircraft’s signature have to be reduced are radar reflection, IR emissions, visual signature and electronic emissions. Radar reflection (RCS) reduction requires careful shaping of the aircraft, so that the return signal becomes weak enough to be lost in the electromagnetic noise.^2 However, a shape that is conductive to low radar signature does not correspond well to aerodynamic configuration optimized for flight performance, even though many problems had been solved through fly-by-wire systems. As a result, low-observable air superiority or multi-role fighters have reduced maneuverability compared to what would have been possible without stealth optimization. RAM coating and internal weapons bays are the main reason for this; RAM coating has to be of certain thickness to work against radar (0,6-1 cm for X-band), and weapons bays increase aircraft cross-section and complexity.

Aircraft shaping and RAM are effective against centimetre band radars. However, radars that work in metre-band area (such as early warning radars, and other air defense search radars) can cause resonance in certain parts of the aircraft – stabilizers, wing leading edges – and thus receive strong radar reflections. Further, modern aircraft have many maintenance panels, doors and access points, which are ideal radar reflectors, which means that extreme tolerances have to be observed during the production. This in turn raises price of both production, and later of logistical support.

Main sources of IR radiation in aircraft are engine emissions and airframe heating during the flight, which produce IR radiation of medium and long wavelengths. These are the same bands as used by modern dual-band IRST systems. Skyward G can pick up targets flying at 300-400 knots solely through skin heating. Skin heating cannot be significantly reduced, and neither can heating of air due to compression (especially noticeable at supersonic speeds). IR signature of engines can be significantly reduced, but only by sacrificing engine performance – flat nozzles lead to thrust loss in 14-17% bracket, meaning that F-22s nominal TWR of 1,35 might be less than 1,16. In F-22 itself, usage of thrust vectoring, also intended to reduce radar signature by allowing more precise control of aircraft during the flight as well as reduced need for aerodynamic control surface movement, has meant increasing the weight of the nozzle, greater maintenance requirements, reduced lifetime, and increased rear aspect RCS.

When it comes to visual detection, stealth aircraft are at disadvantage because they tend to be larger than comparable conventional fighters – even though extreme dimensions may be the same, stealth aircraft’s internal carriage requirement results in “fatter” body of the aircraft. This factor is typically underestimated, despite the proliferation of optical and electrooptical systems in both aircraft and surface units (even with IRST, at high altitude physical size is also a factor in detection, since modern IRSTs can detect temperature gradients of less than 1* Celsius, and can in fact detect fighter aircraft merely through sun glint from the canopy glass).

Radar is still used as a main sensor by air forces despite its tactical disadvantages, because it offers longer theoretical detection range and better bad-weather performance than IRST (even though, in reality, a fighter using radar would be detected first). Stealth aircraft, which have to remain completely passive to prevent being discovered, have to rely on either offboard sensors or on their own passive sensor suites. However, due to sensitivity of uplinks to jamming as well as vulnerability of AWACS to long-range missile systems such as Ks-172 and Meteor, AWACS is not a good solution.

Impact

Appearance of stealth fighters caused a necessity for defining new tactical approaches. In the last few decades of the 20th century, radar had become the main sensor of a fighter aircraft, for both long-range combat with radar guided missiles and for gunlaying in visual-range dogfight. Radar is also the main sensor for control of airspace by ground or air based radar platforms. Appearance of LO aircraft means reduction in radar detection range, which reduces the ability to cover the whole defended area with radar surveillance. This leads to appearance of “passages” that allow strikes against targets deep within the defended territory. Likewise, low-level strike aircraft also find themselves in danger from surprise attacks. However, in either case, stealth should be combined with supercruise to fully exploit its potential.

Stealth aircraft vs conventional aircraft

Stealth aircraft will be primarily used to disable high-value ground systems, through usage of stealth ground attack aircraft such as F-35 and various UCAVs in the future. This way, risk would be reduced for conventional aircraft, such as CAS and conventional strike aircraft. To defend against this, it is possible to analyze enemy forces (tactical and technical capabilities of stealth aircraft, force organization etc.) to determine likely targets and flight corridores. This can then be combined with deployment of sensory systems optimized for detection of stealth aircraft, such as HF and VHF band radars as well as ground and airborne IR systems. Since stealth aircraft can only be stealthy in a single band, combination of sensors optimized for various frequency bands can provide an effective countermeasure.

Gaps can also be covered with fighter aircraft. In that case, there are two options. One is usage of small, fast cruising single-engined aircraft equipped with passive sensors. These can then be combined with other systems or used individually to search for stealth aircraft, since IR signature cannot be significantly reduced. Second one is usage of large aircraft with powerful AESA radars (e.g. F-15, Typhoon, Su-35). These would fly in a widely separated formations, using datalinks to exchange and fuse sensory data. By comparing the data from different aircraft, false returns could be eliminated, significantly lowering the detection treshold and allowing early detection of a stealth aircraft, which would then be attacked by the closest fighter. This is also important since stealth fighters’ RCS is lowest nose-on; from sides and rear it is significantly higher, and even more so if a fighter using radar is not at the same altitude as the stealth aircraft. By using datalinks, such groups could form a flying multistatic radar system, improving the chances of detecting stealth aircraft. However, since twin-engined fighters with large radars tend to be expensive, best option would be combining them with smaller single-engined fighters such as ones described (example here), with twin-engined fighters serving as command aircraft for groups of fighters. In this case, taking out command aircraft would affect effectiveness of a group, but that could be mitigated by using first approach. Also, since stealth aircraft are built around the notion of radar-based air combat, usage of radar jammers, decoys or jammer-decoys would reduce their effectiveness by forcing them to come close to utilize the radar, as it would need to burn through the jamming; this would increase probability of detection. Effective enough countermeasures – such as DRFM jamming which jams active radar missiles and even fighters’ AESA radars – might even force visual-range combat, where stealth fighters are at disadvantage due to their larger size and inferior kinematic performance.^1 While this problem can be relatively easily countered by installing IRST systems, and relying on them for beyond-visual-range combat, this negates and even reverses the advantage that radar stealth offers to LO aircraft.

Combat between stealth fighters

When both sides have stealth fighters, detection in air combat will rely solely on TV and IR systems, in which case missile launches can provide significant cue as to the opponent’s position. When penetrating enemy’s air defense, stealth fighters could launch small decoys such as US TALD in order to force the enemy’s reaction and discover his stealth fighters. Also, air-to-air missiles can only carry small radars, which can only be solved through two possibilities. One is semi-active radar homing, where aircraft’s own radar sends out emissions whose reflection is caught by the missile. However, this necessitates the fighter itself to give away its position by constantly radiating, and also makes it vulnerable to anti-radiation missiles. Further, due to stealth fighters not having “swashplate” radars, they would need to continually close in onto the target, risking their destruction by IR missiles (this happened in AIMVAL/ACEVAL tests, where F-14/F-15 would get destroyed by F-5s; because they had to constantly illuminate the target, F-5 could close in to small distance and launch fire-and-forget IR missile before being destroyed). One way out of the problem is to combine radar and IR seeker in the missile; other possibility is to rely only on IR missile, with guidance being done via fighter-missile data link.

If radar missiles are made useless by stealth, jamming, or combination of two, and neither side has a combination of QWIP IRST and IR BVRAAM (example of that is Rafale’s OSF + MICA IR), both sides will have to engage in close-range maneuvering combat (“dogfight”). In this case, victory comes down to pilot training, numerical balance of forces, and technical factors such as aircraft maneuverability, agility, endurance, usage of HMD and HOBS missiles, and eventually DIRCM. Further, in such a case stealth fighters will only be able to detect each other from short range, as will other units not equipped with IRST. If no air units have IRST, stealth fighters will have problems finding each other and will go after highly visible targets – AWACS, tankers and air bases, which will force enemy stealth fighters to defend them. This will largely negate the advantage of stealth, as it will result in either mutual destruction of support assets, or close-range dogfight. Designers have accounted for this, and all stealth fighters that are actually intended to fight other aircraft are highly maneuverable.

Strategic considerations

Main problem with stealth aicraft is that they are expensive. USAF has a fleet of 186 F-22 fighters, of which only 123 are combat-coded, with additional 20 being backup aircraft inventory machines, and remaining being test and training assets. With availability rate of 63% and sortie rate of one sortie every two days, this means that the entire F-22 fleet can generate 40 sorties per day. Regardless of how good F-22 may be – and that too is up in the air – one aircraft can only be in one place at one time. USAF has only six understrength F-22 squadrons in operational status. While F-15/16 units have 24 primary authorized aircraft and 2 backup inventory jets, F-22 units have 21 PAA aircraft plus 2 BAI machines; the exception is Air National Guard’s sole F-22 squadron at Hickam AFB at Hawaii, which has 18 PAA and 2 BAI aircraft. But even to achieve that, USAF had to cut its test and training force to the bone. Pilots of Air Force Weapons School at Nellis AFB have to share their 13 F-22s with the 53rd Test and Evaluation Group; two squadrons sharing half-a-squadron worth of aircraft between them.

Conclusion

Overall, it is not impossible to create a stealth air superiority fighter, as demonstrated already by F-22, PAK FA and J-20. However, doing so is not worth the money invested, and countries that do invest money in such projects are generally doing so as statements of a prestige. Stealth aircraft are a fad, much like battleships used to be, and wasting money on such ego trip projects may display one’s power and technological prowess, but is ultimately counterproductive. Even United States cannot afford the expenses of development, deployment and support of stealth fighters, but continue to do so at great cost (failling infrastructure included), simply to show off their power, and provide profit for overly influential corporations.

Notes

1^F-15C has similar dimensions to F-22, yet F-22 is 55% heavier (19.700 vs 12.700 kg). F-35 also has similar dimensions to F-16, yet it is 54% heavier (13.199 vs 8.573 kg). Su-35 has significantly larger dimensions than PAK FA (21,9×15,3×5,9 vs 19,8×13,95×4,74 m), yet is 500 kg lighter.

2^This is also why LPI techniques do not work (for long). Only about 1% of the emissions which reach the target aircraft get reflected towards the source. If, by chance, a technique could be discovered to near-perfectly reliably separate radar’s return emissions from the background noise, radars would indeed become almost undetectable to RWR, but such an advancement would also make any RCS reductions in aircraft meaningless as any return signal – no matter how meagre – would be correctly identified as belonging to radar itself.

Further reading

Hrvatski Vojnik, Broj 8., Godina VI, Veljača 1996. (Croatian Soldier, No.8, Year VI., February 1996.)

Airborne IRST properties and performance

Value of stealth aircraft

Stealth – evolution of justification

http://nationalinterest.org/blog/the-buzz/americas-f-22-raptor-stealth-fighter-killer-it-can-be-14088

http://nationalinterest.org/blog/the-buzz/the-us-militarys-top-guns-the-air-have-big-weakness-14065

http://nationalinterest.org/blog/the-buzz/the-us-militarys-top-guns-the-air-have-big-weakness-14065

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