Technology Explained

The importance of Stealth in the tactical scenario of the late eighties cannot be understated. The Russians are finally closing the technology gap with the mass deployment of the lookdown/ shootdown capable MiG-29 Fulcrum and Su-27 Flanker, both serious threats to Allied tactical aircraft. While Russian SAMs did not perform well either in the Bekaa Valley air battles of 1982 or the Libyan air strikes of 1986, the USSR has confronted its difficulties with a proven strategy of vastly increasing concentration. Supplementing existing assets with deployments of the SA-10 Grumble, SA-11 Gadfly and new SA-13 Gopher is alarming as types such as the SA-11 which now carry illuminators on each vehicle and more rounds per vehicle.

While successfully jamming larger numbers of newer radars is difficult, the increased number of rounds salvoed by these systems per engagement must increase their overall kill probability. As NATO doesn’t possess the depth of expendable buffer territory which the Russians have, it relies heavily on Tacair and cannot withstand sustained high loss rates.

The option of deploying newer and more capable jamming equipment has penalties. Because it is threat specific it must be constantly updated, which adds to the high purchase price. Because of its complexity aircraft reliability suffers and large spares stocks must be kept to allow field deployments, which adds to manpower and hardware costs. To successfully jam multiple threats the system must be complex, which inflicts weight penalties. While Anti Radiation Missiles (ARMs) are most effective, they are costly and eat into payload, most noticeably in small tactical aircraft. At some point the cost of these systems becomes unmanageable and other means must be sought. Stealth is an option which offers gains in availability/dollar while coping with a broad spectrum of threats. The following will illustrate this quite clearly.

The Infra-Red Signature of an Aircraft

Infra-red (JR) radiation is associated with heat. All aircraft radiate JR, some is reflected and reradiated sunlight while the dominant source of IA is the powerplant. Jet engines release large amounts of waste heat, most of which is concentrated in the exhaust plume and hot end of the engine, while some is also conducted through the airframe. Engine JR emissions fall into two areas, the tailpipe and the nozzle. On dry thrust the tailpipe will directly radiate intense JR (modelled as Lambertian source – Auth. ) due to a turbine Exhaust Gas Temperature (EGT) of about 1000°C in turbojets and 1300°C in newer turbofan cores.

This radiation is most intense when viewed from inside a solid angle (cone) centred on the engine’s axis. Once the depth of the tailpipe is out of the Field-Of-View (FOV) of the observer, most of the emissions are confined to the outside of the nozzle which heats to hundreds of degrees Centigrade and the exhaust plume. Airframe heating occurs through the conduction of engine heat through structure and the dumping of waste heat from internal systems and avionics. Skin friction can also be significant at high speeds.

Infra-Red Signature Suppression

As combat aircraft require high agility, the need for increased thrust makes it impossible to substantially reduce their heat output. However measures can be taken to reduce detectability. One technique is to shift JR emissions into those wavelength bands where atmospheric carbon dioxide and water vapour rapidly absorb IA, which results in a major reduction of the distance at which an IA source such as a tailpipe can be detected. To implement this, avoiding afterburners is a good first step. Ideally, all waste heat from systems would then be dumped into the exhaust flow, which itself would be mixed with cool air before leaving the exhaust nozzle. Cooling the exhaust is useful as it reduces the temperature of the plume and heating of the nozzle and rear fuselage structure, but understandably this has major implications for powerplant efficiency and performance, these are more apparent at a system level.

While radiation from the structure and plume can be suppressed straightforwardly, concealing direct radiation from the depth of the tailpipe/turbine has penalties in weight and complexity. Fortunately, other measures exist to reduce the probability of detection. Rectangular exhaust nozzles may be used to reshape the lobe of the exhaust radiation pattern (the shape of the volume of space behind the exhaust with some nominal JR emission intensity level), squeezing the ellipsoidal pattern of a circular nozzle into a flattish beaver tail shape. To detect the emissions a threat must be astern and at a close altitude, while the range of altitudes over which detection can be made does widen with increasing distance, the intensity of the IR emissions drops rapidly with distance and this hampers detection.

This strategy (incidentally used on the A-10A) makes it difficult for a ground-based IA Search & Track system to track the tailpipe emissions while also forcing interceptors to approach at the same altitude from astern to get a good IA missile lock. Furthermore, unless the threat aircraft is dead astern, a sharp defensive turn will shift the tailpipe radiation pattern lobe out of the plane of the threat and may break lock.

Another variation on this topic is the use of horizontal louvres in the nozzle. Louvres will further flatten the radiation pattern (for the technically minded the ratio of louvre depth to louvre spacing must be greater than the ratio of nozzle plane of emission depth to nozzle height) but may require cooling and this must be balanced against the gain offered.

Other than such active measures, paints with low IA reflectance will usefully reduce the effect of sunlight illuminating the airframe.

Electromagnetic Emissions

Electromagnetic emissions other than IR, such as radar, IFF, radar altimeters, Doppler nav and communication systems can and will betray the bearing, identity and often intent of a penetrating aircraft. Suppression of this class of emissions is easy to implement in comparison with infra-red. Conceptually it means using passive sensors and not transmitting anything. If transmission is required, then Low Probability of Intercept (LPI) techniques must be applied. These fall into several categories. One aspect of LPI is to carefully manage transmitter power to limit detection range. Another aspect of LPI is the use of very short wavelengths and tight beam transmission to reduce the volume of space where the transmission may be detected. A very powerful technique is pseudo-noise coding which makes transmissions appear like random electrical noise.

In this context classical penetration aids such as Terrain Following Radar can be a problem, as a penetrator climbs over a hilltop sweeping its TFR beam up and down it advertises itself for many miles. Jammers can also cause problems, while the victim radar may be deceived a separate listening device will not and may identify the aircraft and its position.

While the above emissions are the most-significant, electrical noise and interference from an aircraft’s onboard systems could conceivably cause difficulties, particularly in a non-metallic airframe. Computers and digital electronics in general generate a lot of interference, therefore careful screening and the use of optical fibres for data transmission between subsystems may become necessary.

Radar Cross-Section

Radar Cross-Section (RCS) is a measure of the power returned in a radar echo relative to the power impinging on the illuminated target. RCS depends strongly upon the wavelength (which decreases with increasing frequency) of the radar and also upon the shape and skin material used on the target. The dependence of RCS upon frequency is considerable and a cause of grief to radar designers and vehicle designers alike.

Where the wavelength is much greater than the target the return is essentially proportional to the size of the target, the effect is called Rayleigh scattering. For this reason Over the Horizon (0TH) radars such as Jindalee are substantially immune to RCS reduction techniques. Increasing the frequency of the radar to the region where its wavelength is comparable in size to the target, the return becomes extremely sensitive to aspect and’ will fluctuate dramatically with small changes in viewing angle. This region is termed the resonance region. Beyond this point, as the wavelength becomes smaller than the target, the sensitivity to aspect falls off. This is designated as the optics region.

The dependence of RCS upon shape is also significant, while the above applies. The best radar reflectors are trihedral corners, dihedral corners between flat surfaces or flat surfaces and cylinders and flat surfaces themselves. II- of these shapes are common on modern aircraft and all exhIbIt an increasing RCS with frequency (proportional to P). Cylindric! and spherical shapes are lesser but significant reflectors, while straight edges are somewhat weaker reflectors. Nevertheless, straight edges are a known cause of strong reflections from many supersonic aircraft. The weakest reflectors are curved edges and gentle curves on flattish surfaces, most of which become weaker with increasing frequency.

It is for these reasons that modern combat aircraft, essentially composite shapes, exhibit such a great variation of RCS with wavelength and aspect. Other major contributors to an aircraft’s RCS must be considered though. Holes, inlets, scoops, bays and, in particular, cockpits and radar/ECM bays, radar transparent canopies or radomes, are all significant reflectors. External stores fall into the same category. All of these may reflect strongly and from some aspects even dominate the RCS.

Engine inlets and tailpipes are an extreme instance of this category, reflecting but also often allowing incoming waves to impinge on rotating fan/compressors or turbines. These reflect and echo which is modulated often well enough to identify the type of engine (an early bug with the F015A’s APG-63 radar set was its sensitivity to this effect, jet engine modulation, which resulted in multiple target images. Newer radars use it for target recognition).

Another interesting aspect of RCS is the surface (travelling) wave effect, where a wave impinging on a flattish surface, eg. wing, from a shallow angle travels along it until it hits a discontinuity, eg. control surface or leading/trailing edge, from which it bounces off possibly in the direction of the illuminating radar. In general, any aircraft can be seen as a large group of individual radar reflectors, each of which represents some facet of the aircraft’s shape or detail features and all of which depend in size upon the aspect of the aircraft and the wavelength of the illuminating radar.

RCS Reduction

RCS reduction is not a trivial task and is certainly not practical for existing designs. Of the strategies available in designing a new aircraft, the simplest is to identify and eliminate ‘flare spots’, individual contributors to the RCS, placing emphasis on those aspects and radar wavelengths relevant to the mission profile. For most combat aircraft frontal and to some degree aft aspects are most important as these are presented to a threat with a relatively low line of sight (LOS) angular rate during attack and egress. This will quickly lead designers to smooth, flattish airframes with nose chines and wing/body blending to remove all corners and as many edges as possible. Inlets will be on the top of the airframe to shield them from ground-based radar. This strategy will suppress echoes from major airframe features but as these are reduced, the RCS of other features will become dominant.

The next phase involves conductive plating (eg. gold as on EA-68) of the canopy, fitting tuned (wavelength selective) radomes/panels over the radar bay (if used) and using shared apertures for the Radar Homing And Warning (RHAW) and jammers while ensuring that these do not reflect at wavelengths outside of their range of use. Engine inlets acquire $-bends or even baffles to conceal the compressor face. Airflow/pressure sensors are redesigned for low RCS. Rivets/fasteners and latches must be very flush and of materials with similar electrical behaviour to the skin. Panel edges, control surfaces, flaps and bay door edges must fit tightly and must not expose edges under load or use.

The next phase involves the application of Radar Absorbent Materials (RAM) and Radar Absorbent Structure (RAS). RAM and RAS will absorb an impinging electromagnetic wave and dissipate it as heat. The ideal RAM/RAS would be indistinguishable from free space to an impinging wave of any wavelength, which would be absorbed completely without any reflection.

Real world RAM/RAS tends to approach the ideal only in a narrow frequency band and often must be inches thick to be effective.

RAM/RAS is a very broad subject, the two basic categories are broadband and resonant, the latter absorbent to some narrow band of frequencies. There are several families of RAM; dielectric absorbers such as Jaumann absorbers and graded dielectric RAM, magnetic RAM such as carbonyl iron or ferrites and the newer circuit analogue (CA) RAM, which is made of multiple layers or dielectric (eg. plastic) material with geometrical patterns (eg. Jerusalem crosses) of electrically conductive material deposited on them.

Each family of RAM has its strengths, magnetic RAM at lower frequencies, and dielectric RAM in the upper bands. An area where the Americans have made progress (and kept most of it well classified) is hybrid RAM combining layers of several types of RAM to minimise reflection over as wide a frequency band as possible. These materials often fall into the class of RAS, which is highly desirable as most RAM has appalling mechanical properties, particularly for airframe design. Good structural materials such as Aluminium are excellent radar reflectors, even carbon fibre and boron epoxy can be fairly reflective. A typical RAS would be a honeycomb using CA RAM sheets for high band absorption and magnetic RAM for lower bands.

Some measure of RAM performance may be gained from a description of a graded dielectric RAM in ‘Radar Cross-Section’ (Knott EF, Artech House, 1985) which displayed roughly sixtyfold attenuation of an impinging wave (compared to zero for a reflector) in the I-band which peaked at better than thousandfold attenuation in the middle of the J-band.

The RAM designer relies heavily on computer-aided design techniques to model the performance of his material and tailor it to a mission. Airframe RCS analysis as a whole relies heavily on software/computer modelling, Northrop’s GENSCAT, MISCAT and HELISCAT being typical tools for the job.

The designer of a Stealth aircraft has therefore a broad range of options available.

RCS Reduction and Defence Penetration

Radar based air defence systems such as Russia’s RVO employ a broad range of radar and weapon systems. Most longrange surveillance radars operate in the lower bands, while SAM acquisition and guidance radars tend to the G, H, I and J-bands with SAM/MM seekers and Al radars in the I/J-bands (smaller antenna size constraint). While reducing RCS will in general reduce detection and tracking range, important tradeoffs must be made between aerodynamic performance, RCS in the given bands and jammer capability for any given scenario.

A typical PVO response to a penetrator, for instance, would initially involve detection by long-range low band radar. Such systems are seldom accurate, therefore an interceptor is despatched into the immediate area to acquire the target on its Airborne Intercept (Al) radar and carry out an attack with AAMs. If this fails the penetrator will have to pass through a SAM umbrella before and often over the target area, the SAM acquisition radars would be alerted to the approaching aircraft. If an Airborne Early Warning platform is involved it would support interceptors and possibly SAMs by data linking target information acquired with its radar, probably operating in the E/F bands.

To evade detection the penetrator must therefore defeat radars covering virtually all bands while to evade terminal threats it must defeat radars predominantly in the upper bands. Two aspects of defeating these radars must be considered, detection range and burnthrough. Detection range of a target is proportional to the fourth root of target RCS. Therefore a tenfold reduction in RCS reduces detection range to about 56%, a hundredfold reduction of RCS reduces detection range to about 32%, a thousandfold reduction to about 18% and so on. . . While this isn’t a dramatic gain it must be pointed out that the volume of airspace covered by a radar is proportional to (within limits) the cube of detection range, therefore 56% of the detection range translates to about 18% of the original volume of airspace.

Burn through range is the distance at which a radar can successfully detect a target which has been jamming it, at this point the echo has become sufficiently stronger than the jammer’s emissions. What is most significant is that burnthrough range is proportional to the square root of target RCS. Therefore a tenfold reduction in RCS reduces burnthrough range to about 32%, a hundredfold reduction to 10%, a thousandfold reduction to about 3. 2% and so on. . .

A penetrator with a hundredfold reduced frontal RCS compared to a conventional aircraft approaching a radar will be detected at about one third of the range and will be able to successfully jam, assuming the same jammer, over 9/1O of the distance over which a conventional aircraft could be successfully tracked. This becomes understandably significant.

The designer of a stealthy penetrator must weigh the importance of the various threat systems against the cost of his design. Low band (eg. UHF) radars can be defeated relatively cheaply by terrain masking when surface-based, if these are carried on an AEW aircraft the size limit of the antenna rotodome will almost certainly result in large antenna sidelobes which ease jamming while this family of systems is not renowned for spectacular lookdown performance. Higher bands such as G/H and particularly I/J are associated with threats such as fighters and SAMs and therefore RCS reduction offers a large payoff. This is enhanced by the dependence of most IR guided weapons upon radar for target acquisition and tracking, defeating the fire control system will often prevent a successful launch of an IR SAM or AAM.

Ideally a stealthy penetrator would employ standoff fire & forget weapons with a range greater than or equal to the detection range by the target’s defences, or at least the burn-through range. The former instance offers the advantage of surprise if the weapon is not detected. Combining these two families of systems defeats the air defence system as it cannot inflict any attrition by means other than chance.

Given the high cost per airframe of stealthy aircraft the highest payoff is in reconnaissance/electronic reconnaissance, defence suppression and strikes on high value heavily defended targets. A very useful application is the use of stealthy aircraft to clear corridors through SAM belts allowing conventional aircraft to pour through.

A Stealthy Tactical Fighter

The high payoff in using stealthy aircraft for the strike/recce family of roles reflects in the USAF’s CSIRS Stealth fighter programme and the USN’s ATA strike aircraft programme being the first operational stealth aircraft to be deployed or planned for deployment by these services. This class of mission would typically involve penetrating hostile battlefields and static defences up to several hundred nautical miles deep, preferably while not being detected.

It is not difficult to envisage a 25,000lb empty weight class aircraft with two low bypass ratio fans and internal capacity for 2,000 to 4,000lb of disposable payload (ie. a Mk. 84 2000Ib laser guided bomb fits roughly into a 2’x2’x16′ volume folded).

The choice of powerplant would be interesting, while a turbofan has better specific fuel consumption (SFC) and a cooler exhaust plume than a turbojet, its thrust falls off more quickly at higher speeds and reheat is thus required. Afterburning is to be avoided in a stealthy design, while afterburners also add about 30% to the mass of an engine. Fans also tend to larger diameters which make them more difficult to package into a low profile design. For an acceptable combat thrust/weight ratio, given about 10,000 to 12,000lb internal fuel, the aircraft would require about 15,000lb of thrust in military (dry) power.

The airframe would almost certainly employ a double delta planform (cf the F-16F), with Chinese and extensive wing/body blending. The choice of curved or straight chine and wing leading/trailing edges will depend on the designers’ preference for strong glint (echoes) in some directions versus weaker glint over a broader range of angles and of course aerodynamic performance. Vertical stabilizers if used would be canted inward with rounding of edges subject to the above tradeoff.

The crew, presumably a pilot and Electronic Warfare Officer/ Weapon Systems Officer, would sit in tandem as this layout blends into the forward fuselage geometry more easily and offers more instrument panel area per frontal fuselage cross-section. The powerplants could be blended into the wing (cf SR-71/YF-12) or clustered in the aft fuselage, subject to weapon bay placement. This decision would depend upon many tradeoffs in drag, quality of inlet airflow vs AoA and frontal RCS. The engines would employ rectangular nozzles, possibly with louvres, cooling air being drawn from the inlet flow.

Most of the airframe structure would employ carbon fibre particularly to offset the weight penalty of using hybrid RAS skins, doors, panels and RAM in non-structural areas. All fuel would be carried in the wing/body volume. A digital active fly-by-wire flight control system would be used to provide both acceptable handling and enveloped limiting to offset the penalties arising from the above tradeoffs, particularly AoA restrictions and yaw/pitch stability.

The aircraft would carry a sophisticated RHAW and recorder in the class of the APR-38/45 (cf F-4G Wild Weasel) which would be a critical tool in most of its roles. A navigation Flir and raster scan wide angle HUD system (cf F-16/F-15E Lantirn) complemented with narrow FOV Flir and Low Light Television would be useful, together with a long-range TV telescope such as TISEO. A facility to slave the optical sensors’ LOS to RHAW-designated targets would also be a desirable feature for recce and electronic recce, thus alleviating one of the classical problems in electronic intelligence gathering (ie. what signal belongs to what device).

For precision strike, standoff weapons such as SLAM (land attack Harpoon) and an IR Maverick derivative would be used, for defence suppression Maverick and ARMS would be carried. A self defensive weapon could be a later Amraam with passive homing and home-on-jam modes. All weapons would be carried internally and ejected very quickly to minimise bay open time.

Flying mainly night missions an overall dark low IR reflectance paint would be used. The aircraft would ideally be optimised for a medium to low level high subsonic mission profile, taking advantage of terrain if and when appropriate.

The USAF SCIRS Stealth Fighter

The Covert Survivable In Weather Reconnaissance/Strike fighter is a ‘black’ programme and has never been officially acknowledged, therefore of what is published in the open literature, much is in conjecture and some may be US DoD disinformation. A reasonable amount of what has been published in credible and amounts to the following.

In 1973 the USAF and DARPA initiated the Have Blue programme aimed at low observable technology and by the end of the year issued an RFP for the experimental Stealth Tactical (XST) technology demonstrator. The XST was characterised by low frontal and beam RCS, the use of RAM and cooled low IR exhausts.

In 1976 Lockheed won the contract for five prototypes. The Lockheed XST first flew in 1977, a 35ft subsonic airframe with two unreheated J-85 turbojets. The aircraft has been reported as having a double delta planform with chines and wing/body blending much like the SR-71. Although two prototypes crashed, tests were successful and led to a 1981 contract for the full scale CSIRS, often incorrectly designated the F-19A.

The CSIRS itself first flew in 1982 and is reported to be sized much like an F-18, 55ft long with 35ft span and a gross weight of around 30,000lb. Several sources have reported the aircraft as being powered by a pair of modified GE F404 turbofans developing about 12,000lb of thrust each. A Lear Siegler digital flight control system was attributed by one source. Most of the air- craft’s structure has been described as composite, using carbon fibre and boron epoxy composite. Extensive use is apparently made of RAM, particularly magnetic absorbers. Most sources agree that the aircraft is tasked with defence suppression and recce.

The USAF ATB Stealth Bomber

The existence of the Advanced Technology Bomber is somewhat less a matter of conjecture, thanks to the indiscretion of the Carter Administration under fire over the B-1 cancellation. On the other hand less information is available on the programme, though this may reflect its lesser maturity.

RFPs for the ATB were issued in 1981, with a Northrop/Boeing team winning a full scale development programme contract with a long-term outlook for 132 aircraft in a US$30 billion programme. Most sources agree that the aircraft is a flying wing powered by four modified GE F-101 fans, some suggest the designation.

B-2A. Gross weight is reported about 400,000lb, with a maximum 40,000lb weapon load, carried internally. The Common Strategic Rotary Launcher has been designated for use in the ATB and a secure plant at Pico Rivera, near Los Angeles, has been set up by Northrop for work on the programme.

On the basis of Northrop’s known involvement in ECM systems, RCS analysis and flying wing design and Boeing’s well proven track record in the long-range bomber design and offensive and defensive avionic management systems design, this is quite credible.

The ATB will almost certainly penetrate at high subsonic speeds and medium altitudes, using cloud cover to foil detection by thermal imaging satellites (cf Teal Ruby) and ground based sensors. If threats such as OTH radar are encountered, it may descend to low level and exploit clutter and ECM to defeat the threat. Targets will be attacked mainly with standoff weapons such as the stealthy Advanced Cruise Missile (ACM), currently in development. The ATB will certainly exploit extensive RAS, RAM and most of its vitals will be buried deep inside.

JTACMS and ATA

While the CSIRS, ATB and ACM have attracted most of the attention, two other major stealth programs no less significant are under way. One of these is the USAF/Army Joint Tactical Cruise Missile System (JTACMS) which is a stealthy precision deep strike weapon for hitting battlefield and follow-on forces. JTACMS is heavily classified, though we believe it is expected to be able to deliver a wide range of munitions. Interestingly it is an outgrowth of a Northrop proposal, the NV-150 with a Williams fan engine, satellite/ring laser gyro nav and composite airframe (see TE Sept 84 AA p54), which displaced the contending ballistic T-16 and T-22 weapons.

The second programme is the USN’s Advanced Tactical Aircraft (ATA), a projected stealth technology all weather deep strike replacement for the A-6F Intruder. This programme is also classified, but given current USN thinking almost certainly a high subsonic 55,000lb gross weight class aircraft with a 1000+nm unrefuelled strike radius. A derivative of the GE F404 turbofan has been selected for the aircraft. The ATA programme is currently in the demonstration/validation phase, with Grumman/- Northrop and McDonnell Douglas/General Dynamics developing competing designs for the full scale development programme. Significantly in mid 1986 the US DoD directed the USAF to consider adapting the ATA to fulfil the role of an F-111/F-15E replacement with an IOC in the late nineties.

Over the last half decade stealth technology has transitioned from a development concept to a credible tool for the projection of tactical and strategic air power. It promises significant gains in survivability and combat effectiveness which may well offset the additional acquisition costs.

Given the reluctance of Australian governments to make a long-term commitment to supporting the F-111 strike force, one cannot help but think of what options would be sought to supplant this aircraft. In the longer term Stealth may be the only option.