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Stealth fighter characteristics and requirements overview

Posted by Picard578 on August 8, 2018

Introduction

Stealth fighters are spreading, yet very few to none have what is necessary for a stealth fighter to be truly effective. Some are better than others – PAK FA would eat F-35 for lunch and spit out the bones – but all have serious flaws and are, in essence, half-baked experiments in stealth fighter design. So what would an ideal stealth fighter be? (Note: It may not be possible for some countries to build such a fighter – an optimization for operational stealth might fly in the face of other design requirements).

Requirements

Survivability

Main idea behind stealth is to improve survivability. Survivability is one of the most important characteristics of modern weapons systems, yet it is far more complex than generally appreciated. It does not mean merely avoiding getting hit, but rather surviving to carry out the mission – in tactical terms, aircraft that had been forced to withdraw due to lack of fuel or could never take off due to a big hole in tarmac is as “dead” as one that had been shot down.

Lethality

Stealth improves lethality as well. Effectiveness of weapons depends in large part on employment range: it is not the same thing to shoot from 10 or 100 kilometers.

Endurance

In order to actually make use of the above, aircraft needs to be capable of getting to targets and back. Required endurance necessarily depends on mission profile: will the fighter be employed defensively or offensively? Since stealth fighters are by nature offensive weapons, they will require greater endurance than non-stealth fighters; but defensive stealth design is possible to envision as well. Endurance should be measured by combat tasks: e.g. acceleration from subsonic cruise speed to combat cruise speed followed by climb to operational altitude and acceleration to top speed in order to fire missiles. This could be more precisely specified as:

  • Ingress at 15.000 ft

  • Acceleration from Mach 0,9 to 1,2 at 15.000 ft

  • Climb to 30.000 ft and acceleration to Mach 1,7 (assumed dry cruise speed)

  • Cruise at Mach 1,7

  • Maximum deceleration turn from Mach 1,7 to Mach 0,8 at 30.000 ft and maximum power

Training

All the above characteristics depend on how well trained the pilot is. A well-trained pilot will be able to optimize aircraft employment, weapons employment etc., while badly trained pilot will unnecessarily waste weapons and fuel. As Iraqis discovered in 1991., 2003. and 2014., it is no use to give a rifle to a monkey. A well-trained pilot in shit aircraft will always beat a badly-trained pilot in excellent aircraft. Usefulness of BVR combat in recent conflicts was largely due to enemies being incapable of proper defensive action due to lack of training and equipment failures (no MAWS, no RWR). Human factors trump technology, yet this is not sufficiently realized in many circles. One good thing about newer fighters are their extensive networking capabilities, which can significantly improve performance – assuming, of course, networking works against a peer opponent.

Characteristics

Survivability

Ground survivability

No tactics are as useful as cheating, and in air combat, ultimate cheating is destroying the enemy before he has had a chance to get into air in the first place. Even the most “stealthy” fighter aircraft would find its stealth useless to hide it if its air base can be found as easily as most Western bases can. Fighter aircraft spends most of its time – two-thirds at very least – on the ground, and it is precisely there that it is the most vulnerable. Yet most Western fighters are not really optimized for road basing, let alone dirt-strip basing (even if most can fly from roads in extremis, sustained road operations are much more difficult). A proper road-based fighter needs to have low maintenance requirements, low fuel usage, low wing span and ability to operate from dirt-strip and muddy roads. For stealth fighter, this means very resillient radar-absorbent skin (not paint!) as well as robust undercarriage and FOD-resistant engine. Undercarriage should have good shock absorption and large, low-pressure tires, as well as mud and FOD protection. Nose wheel should be mounted behind the air intakes to prevent any FOD damage to the engine.

Further, fighter should have very good STOL, acceleration and climb performance to escape, if necessary, any attack against the air base itself. While not necessarily as important as it was in pre-missile era, altitude is still an advantage, meaning that fighters should reach combat altitude before enemy attackers come into range.

Combat survivability

By its nature, stealth fighter relies on stealth to protect it. What this means is minimizing radar, infrared and visual signatures. Radar signature is minimized by obvious means: proper airframe shaping, internal weapons carriage, hidden engine front and application of radar absorbent materials. These measures are especially effective against fire-control X-band radars, but are less effective as frequency decreases. VHF radar has significantly improved performance against stealth fighters, while HF radar can ignore stealth alltogether. However, neither can be employed on fighters or missiles.

Infrared signature can be minimized by optimizing both design and performance characteristics. Supercruise capability is especially important here, as it allows supersonic flight without using extremely detectable afterburner. Engine should have an additional cooling channel and external nozzle compared to “normal” fighter engines. Bypass ratio is an opern question: high bypass ratio would reduce engine IR signature at subsonic speed, but low bypass ratio would improve cruise performance and reduce engine power necessary for any given cruise speed. For an air superiority fighter, low bypass ratio engine should be chosen.

Visual signature is minimized via small size, which also helps reduce infrared signature. Another factors are camouflage paint and especially smokeless engine.

Electromagnetic signature has two aspects: incoming emissions (radar) and outgoing emissions. Enemy radar is defeated primarily via shaping for minimum radar cross section. This means a smooth shape with carefully controlled reflections, no corner reflectors, or any random protrusions. Consequently, weapons and sensors must be carried internally: hence faceted IRST/EOS housing on F-35. Radar cross section (RCS) should be based on in-air measurements, as different aspects, air conditions, condensation trails and engine emissions can affect RCS compared to ground-based model measurements. This is just as important for assessing fighter’s infrared signature, if not even more so. Measurement should utilize radars of different frequencies as well, as different aspects of radar signature (shaping, contrails etc.) have different impact on different frequencies, and shaping grows less effective as frequencies increase, being much less effective against VHF radars and irrelevant against HF over-the-horizon radars. Second aspect is emissions control. Fighter itself must minimize or eliminate all outgoing electromagnetic emissions. This means minimal to no radar usage, directional outgoing data links (if any), and either offboard or directional electronic countermeasures. Radar is especially important as it is by far the most powerful source of electromagnetic emission on the aircraft.

However, stealth fighter cannot rely on stealth to always protect it. It may come up against enemies with good IR sensors, be forced to defend a (relatively) stationary asset, or be caught in a position where range is too close. It might run out of BVR missiles and be forced to enter a dogfight. As such, it needs to have backup options: good self-defense suite, good maneuverability and cruise capability.

In order to avoid being surprised, fighter should have 360* coverage with most important sensors – RWR, LWR and MAWS. Radar will naturally be positioned forward, and should be capable of being used in active and passive modes both – with latter itself having options for picking up reflections by radar of an emitting friendly fighter, or else picking up enemy radar emissions – a giant RWR, essentially. IRST will also be forward-oriented, but IR MAWS should be configured so as to allow it being used as a short-ranged IRST. This will allow pilot to “see through” the airframe. Fighter should also be capable of cruising at speeds of Mach 1,5 to 1,8 for at least 20 to 30 minutes in combat area. This however may be problematic to achieve in a stealth design, but 15 minutes cruise should be absolute minimum. For this reason, a turbojet engine may have to be considered.

Electronic countermeasures will have easier time due to stealth fighter requiring less powerful signals to spoof enemy targeting regardless of the range. However, infrared missiles will present a significant threat as IR signature cannot be significantly suppressed. Enemy radar missiles may be jammed by AESA jammers alternating between two fighters in a “blinking” manner, forcing the missile in a home-on-jam mode to to alternate between two targets, expending fuel and energy.

In maneuverability, stealth fighter will likely be at disadvantage due to its very nature: need to carry weapons internally. This means that it will be larger and heavier than an enemy with comparable normal payload. F-22 can carry eight missiles internally; Gripen E, with similar weapons load, is less than half the empty (or combat, for that matter) weight. Stealth fighter might gain some advantage due to having no interference drag from carrying weapons internally, but simple conformal carriage can eliminate most of these advantages, even when ignoring that smaller fighter will likely have maneuvering advantage – less impact from weapons carriage matters much less once one figures in the fact that baseline / starting point is not the same. However, maneuverability of a stealth fighter can still be improved (kept competitive) by ensuring low wing loading, high thrust-to-weight ratio, good transient characteristics (control response) and small size. Transient characteristics in particular can be improved by including close-coupled canards in the design. Since transient maneuverability is the most important aspect of maneuverability, canards should be included. Canards can be high (above the wing) or coplanar with the wing. In either case, some stealth will be sacrificed, but maneuverability benefits should be significant. Sustained turn performance is comparatively irrelevant, but for a stealth fighter meant to fight at beyond visual range, acceleration and climb performances are crucial.

Lethality

Beyond visual range missiles achieved good performance in recent wars. To fight at beyond visual range, fighter must be capable of identifying its targets at beyond visual range as well. While in ideal conditions this can be done via various mechanisms such as radar NCTR, various factors such as interference and jamming, unavailability of AWACS etc. can render radar ID too unreliable. That BVR missiles were used in the first place was due to presence of radar NCTR, persistent AWACS availability, which when combined with incompetent enemies led to excellent effectiveness. In more adverse conditions, radar-guided beyond-visual-range missiles cannot be relied on. This problem can be mitigated in two ways.

First, stealth fighter should be equipped with IR guided BVR missiles, such as French MICA IR. Combined with onboard IRST, it will allow both identification and engagement of unccoperative targets in ECM-heavy environments. Aside from being much more ECM-resistant, IR missiles have inherently greater lethality than radar-guided missiles. Meanwhile imaging IRST is the only reliable means of identification in ECM-heavy environment, unless both sides leave IFF turned on (which they may do if two sides utilize same aircraft types). Radar imaging cannot be used if radar is jammed. It is also the only way of ensuring reliable surprise due to being a passive sensors: radar is likely to give away fighter’s position, unless data is being fed from an offboard platform (possibility of which is questionable) or the enemy does not have a good radar warning receiver. Against Third World air forces, AESA radar can be left safely on. Radar itself should be capable of functioning as RWR and using that data (plus data from IRST) to optimize low-energy emissions for fire control purposes; in essence, radar becomes a rangefinding system.

Second, stealth fighter should have good cruise performance – that is, cruise speed and endurance. Cruise, not maximum, speed will allow it to dictate engagement terms and potentially catch enemies unaware (most fighters do not have sensors covering the rear aspect with the exception of various warning devices). It will thus be capable of choosing when, how and whether to engage. High cruise speed and endurance will also serve to extend its missile range and reduce enemy missile range from rear-quarter attacks – area where all fighters, but especially stealth ones, are relatively most vulnerable from.

Further, narrow-beam two-way datalinks should be ensured. While they still risk giving away fighter’s location, data links should allow it to utilize offboard sensory data for engaging targets. In theory, such a system could be utilized to allow some degree of a completely passive rangefinding. Datalinks should be utilized to share sensory feeds between fighters and from AWACS, as well as for communication. They should not be utilized for command; instead, pilots and flight leaders in particular should be left maximum freedom of action based on mission goals and situation overview provided via datalink.

If the enemy has not been shot down at BVR, and just letting him go is not an option, stealth fighter may need to close to visual range. As noted above, stealth fighter will be at disadvantage in maneuverability. This means that, assuming it has any advantage at all, stealth fighter will have advantage in energy fight: carrying out what are basically “hit and run” attacks instead of engaging in a turning fight. This however is extremely risky in a modern battlefield, as a well-placed IR missile shot can still force it into a turning fight where it will be at a disadvantage. Energy fight will also require significant fuel reserve. Combined with supercruise requirement, this leads to 30% fuel fraction being absolute minimum, 35% a possibly adequate value, and 40% to 45% to be achieved if possible. However, due to design limitations of a stealth fighter, anything above 30% fuel fraction may prove unrealistic for an air superiority design. Meanwhile E-M requirement means high thrust-to-weight ratio.

Stealth fighter should not be too expensive – losses happen, and numbers do matter. Peacetime fighter fleet should include enough extra (reserve) fighters that pilots do not lose out on flight hours due to repairs, maintenance or accidents. This should also include a number of “spare parts” fighters, to be cannibalized in case that spare parts are not delivered in time. Ideally, each pilot should have two fighters, so as to spread wear over two airframes and allow aircraft to undergo proper maintenance. As stealth itself causes additional costs compared to conventional designs, fighter should be as small as possible.

Generally, a fighter in good position to shoot will achieve a kill – this was proven in wars from World War I to Gulf Wars. However, proliferation of missile warners may put that into question: importance of surprise in achieving kills was based on the fact that it was generally too late for a surprised target to do anything once “woken up” (typically by being shot at). In more than a few cases in modern wars – such as the case of Yugoslav MiGs in 1999 – enemy pilots only noticed they were under attack once missile detonated close to their fighters – or flew harmlessly past the canopy. But introduction of MAWS, especially IR MAWS, means that surprise becomes impossible unless one goes for a gun kill (assuming, of course, that MAWS is not configured to warn for fighters, and since modern IR MAWS is basically a high-resolution IR camera, even that will likely not be a surprise). If MAWS notices incoming missile early enough, pilot has time for evasive action. Therefore, visual-range performance of a stealth fighter is still crucial for its overall air combat performance. BVR missiles will still be important for scoring against Third World air forces, as well as for opening up for a closer, more lethal engagement. This means that MAWS IR cameras should be capable of feeding image as well as targeting and ID data to pilot’s HUD or HMD. This will make job of keeping track of targets in both BVR and dogfight much easier.

Training and tactics

Training should be optimized for operations in fours and pairs. Any larger formations harm fighter’s lethality, be it at within visual range or beyond visual range – larger number of smaller formations has advantage over smaller number of larger formations, even if larger formations have overall greater number of fighters. Large engagements in general should be avoided in order to achieve maximum kill-loss ratio. Fighter itself should be highly reliable and easy to maintain in order to facilitate “live” training.

Training should be literally “ground up”. Stealth starts and ends on the ground – a flaming wreck in a blown-up air base is not particularly stealthy, or particularly useful. Any proper stealth fighter should be designed to operate from hidden air bases. This means easy maintenance, low logistical requirements, small size and road basing capability at minimum. Each fighter should come with a fuel truck or two, a spare parts truck, ammunition truck, lightweight mobile maintenance and repair equipment. Additional equipment shoud include enough camouflage netting – effective in visual, radar and IR spectrums – to hide fighter as well as its entire support apparatus. A fighter without that is not a proper stealth fighter. Currently, Gripen C is far more stealthy than F-35 – just in different area, but people who focus only on in-flight stealth cannot understand that.

Conclusion

Ideal stealth fighter would be, in essence, Stealth!Gripen – certainly not an overweight monster like F-35. However, for maximum effectiveness, it should still be supported by non-stealth fighters, exploiting “cracks” these fighters create. Some of these non-stealth fighters could be Western versions of Flankers – large, twin-engined, two-seat aircraft with huge radar and capable of dirt strip operations. These could then act as command fighters.

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6 Responses to “Stealth fighter characteristics and requirements overview”

  1. altandmain said

    The logical answer to this is to not invest so much in stealth aircraft. There are huge trade-offs in terms of cost, reliability, aerodynamic performance, and so on.

    Focus on developing passive sensors rather than defeating active radar.

    This is becoming like chemical warfare in WW1 – a dead end.

    Like

    • Picard578 said

      Certainly seems that way. Also, problem with most modern fighter designs is that they are ignoring huge parts of the kill chain, basically everything not related to fighter in flight.

      Like

      • altandmain said

        Basically the best we can come up with is a “stealthy FLX” for lack of a better term.

        I think that we have reached a point where it is not worth it. Too much other trade-offs. I mean you would want your rough field FLX deployed in concealed areas anyways.

        If it is deployed, it would only be viable in smaller numbers to hit high value targets (ex: enemy AWACs), and supported by a mostly FLX fleet.

        Like

        • Picard578 said

          Indeed, although something the size of PAK FA and rough-field capable may or may not work as a command fighter, AWACS killer and deep strike aircraft.

          Like

        • DLondo said

          What a stupid article. The US is on its 4th generation of.stealth aircraft, the Russians have acknowledged failure and so have the Chinese, both indicating that they will not have the required tech until at least the mid 20s. Price? You can either afford to be in the game or not. Gripen? What nonsense, what has the Gripen to do with stealth, it doesn’t pretend to be stealthy just cheap.

          Like

        • Picard578 said

          Stealth is not limited to stealth in the air. Gripen has stealthy concept of operations: utilization of hidden road bases to avoid enemy bombardment of air fields. Neither F-22 or F-35 have that.

          Like

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