Of Wargames, JSFs and Baby Seals (II)

“Be modest about what military force can accomplish, and what technology can accomplish. The advances in precision, sensor, information and satellite technology have led to extraordinary gains in what the U.S. military can do…But also never neglect the psychological, cultural, political, and human dimensions of warfare, which is inevitably tragic, inefficient, and uncertain. Be skeptical of systems analysis, computer models, game theories, or doctrines that suggest otherwise. Look askance at idealized, triumphalist, or ethnocentric notions of future conflict that aspire to upend the immutable principles of war…” – Robert M. Gates 29 Sept 2008.

Recap

To summarize – we are on the second of a series of posts that look into a recent RAND study on the future of air superiority in general, and as part of the exercise, address some of the open press reporting on the alleged shortcomings of the JSF as extracted from a slide in backup. The context for examination is within three areas that the US has held superiority, primarily in the post-Cold War period, namely (1) secure forward bases for operations; (2) stealth; and (3) BVR capabilities as balanced against a surfeit of numbers. This post will be part II.

Will stealth work as advertised?

Stealth, or more properly “low observable” (LO) is an attribute ascribed to a platform or tactics and techniques that reduce the observability of that platform to a defender and is usually considered to be of a passive nature, as opposed to active measures like chaff, IR decoys and jamming. Note that its purpose is to reduce observability, not render the platform invisible.

LO techniques have been around since the First World War. Since the primary means of detecting aircraft was the Mark 1/mod 0 eyeball, LO techniques included the use of camouflage patterns on upper surfaces to make the aircraft appear to blend in with the ground below and sky colored to blend in with the sky when observed from below. This technique is still employed today as most tactical aircraft carry are painted a low-visibility, non-specular (non-reflective) grey. An additional technique stemming from WWI, night operations, is still used today for some of the same reasons.

It wasn’t until WW2, however, that the non-visible spectrum came into use against aircraft that measures were sought to minimize the advantages those systems brought to bear. The usefulness of such means, primarily radar, was borne out in the employment of the CHAIN HOME radar system which provided not only early warning of approaching German formations, but also in combination with the fighter direction centers, proved key in massing the limited resources of the RAF against those same raids, acting as force multiplier. As the war drew on, the now familiar cat-and-mouse game of measure/counter-measure/counter-counter measure developed in the European and Pacific theaters. Chaff, jammers, deceptive repeaters were met by changes in frequencies, exploitation of other platform vulnerabilities (e.g., the use of ESM to detect Allied use of the H2S navigation radar and target the bomber streams accordingly). With the advent of the Cold War, and its regional hot war iterations (Vietnam, Middle East) and the introduction of the surface-to-air missile tied into an integrated air defense system, or IADS, it seemed that the tyranny of the cycle that demanded newer radar warning receivers and deception aids would continue to escalate in cost while demanding ever shorter development and production cycles. Still, even with improved onboard systems, off-board support was demanded in increasingly large numbers for both Navy and Air Force aircraft. For example, a typical Linebacker strike package going “downtown” during the Vietnam War demanded no less than 60-80% of the 52 aircraft composition providing strike support in the form of MiGCAP, ECM, chaff and Iron Hand packages. Even with this support, losses were high as North Vietnamese tactics changed to take advantage of other vulnerabilities in platforms or US tactics. Israel found the same in its efforts during the opening stages of the 1973 Yom Kippur War when it found 10% of its air force destroyed in a single day due to changes in Arab forces equipment (including the nasty surprise of very capable mobile SAMs, the SA-6, deployed with advancing armor units) and tactics.

Enter LO

Application of LO in the electromagnetic realm saw a few tangential attempts as early as 1940, but the science and technology of the time would not yet support such efforts. To be sure, it was thought that non-metallic surfaces would substantially reduce radar returns – what was less clearly understood were the subtleties and vulnerabilities of what lay beneath the skin that would also contribute to the signature. There were some desultory investigations post-war and the U-2 and SR-71 both attempted a degree of signature reduction, the former with a ferrite-impregnated paint (giving the U-2 its now characteristic black color) and the latter with radar absorptive material in wedges along its wing line as well as utilization of a blended wing/body.

Still, it wasn’t until the review of the work of a Russian radar scientist, Pyotr Ufimstev and his re-discovery of the how Maxwell’s equations may be applied to predict how electromagnetic waves would be reflected by certain shapes. This was an important breakthrough because of the follow-on ability to design an all aspect LO aircraft and gain an understanding of its specular behavior before actually cutting metal and resorting to a prolonged trial and error process. Engineers took these equations and applied the principles of optics to predict the what form the scattered field of EM would look like when an aircraft was illuminated by radar waves at various frequencies.

Reflections

Through the mirror of my mind
Time after time
I see reflections of you and me

Reflections of
The way life used to be
Reflections of
The love you took from me
– Diana Ross, Reflections 1965

The challenge in designing an LO or VLO aircraft is reducing the radar cross section by reducing reflections. The radar cross section is defined as the area the radar “sees” in the form of reflected energy from the target and, is normally larger than the object itself. There are several elements to the reflected energy that constitute the RCS (which is measured in either square meters or decibels per square meter (dBSM)), the principal ones being:

•  Specular reflection: Specular reflection constitutes the majority of the returned radar energy so efforts here typically lend the greatest return (in reducing RCS). The primary determinant in this actor is the shape of the airframe and its major subcomponents. Vertical tail surfaces, large, open intakes – those and other features act like mirrors, reflecting the signal at an angle that corresponds to the direction it arrived. With due regard, this property can be turned to an advantage if the reflecting surface is angled to deflect the majority of the returning signal away from the receiving antenna. This is exactly what lay at the core of the F-117’s design and why Ufimstev’s work was so important in its design.
•  Diffraction: As specular reflection is reduced, other forms come into play. Diffraction occurs when an edge or corner is encountered. This is especially problematic when the object can contain and magnify the reflections – like an inlet duct, for example. A corollary is the reflection that causes a cat’s eye to appear to glow in the dark. A small ray of light enters the cavity of the eyeball and is bounced around the curved walls of the eye’s cavity, producing a flash or glow. As the airframe is adjusted to reduce specular reflection, care must also be given to ensure greater instances of diffraction don’t occur. Again, using the F-117 as an example, one way of controlling diffraction that originated in the inlet duct was to screen it with a mesh that permitted airflow, but would not permit transmitted energy into the inlet. Other applications would include curved inlets to the engine such that reflected energy is directed inward towards dispersion instead of being given a direct path to and from the engine.
•  Traveling waves: When an object is swept with energy, it transmits that energy along its surface as long as there is continuity in conductivity and surface. Where discontinuity is encountered (panel joints, access doors, fasteners) those waves tend to scatter, providing another opportunity for signal return to the receiver. Traveling waves can be managed either through absorption (requiring increasingly thick materials) or deflected. One example given in the latter case is a wing leading edge. The angle of the sweep can aid in deflecting the energy from returning to the source. Of course in so doing there are tradeoffs that may devolve to lessened maneuverability or economy of cruise.

RAM

What about RAM (Radar Absorbing Materials)? RAM has its place – but just like the above its use mandates tradeoffs. For example, in WWII the Germans came to use a carbon sandwiched rubber coating to cover the snorkels on their U-boats to absorb the radar used by Allied ASW aircraft. The employment of radar on ship- and shore-based fixed wing and lighter than air aircraft had an immediate and devastating impact on U-boat operations in the Atlantic, so in combination with other passive measures (like ESM gear tuned to look for the distinctive signal), they also tried using RAM. While it proved successful in lab tests, in sea trials and real world application it was significantly less so, due in no small part to its exposure to the elements (salt water especially). As the experience grows in working with the RAM on 1st and 2nd generation aircraft (and it imposes a significant demand signal on the maintainers), more durable solutions will be sought in materials development and application. In the meantime, RAM falls in line behind the other measures listed above insofar as LO/VLO aircraft design is concerned.

Fuzzballs, Pacmen and Bowties

No, not these are not the elements of a nightmare (unless you’re on the wrong side of the fight), but rather descriptions of the signature of your normal or LO/VLO aircraft. Fuzzball, Pacman and Bowtie are simplified symbols for basic patterns of radar signatures reduction at all frequencies and in turn, are used in mission planning.

The Fuzzball signature displays a reduction in signature from all aspects, all angles. It is the ideal signature reduction (short of total invisibility) and therefore one that probably wouldn’t be flyable unless it was a balloon. With this reduction, an LO/VLO could approach a target from any direction in assurance that it would not trip any defensive wires.

Reality, though, is something else and hence the Bowtie and Pacman shapes. Both represent a compromise of some sort to the LO/VLO application. The Bowtie reduces signature over vulnerable areas in the front and rear of the aircraft, giving a distinctive pinch in the middle, while the Pacman signature reduces frontal aspect. These are commonly found in conventional aircraft that have had LO/VLO measures applied – redesign of key structural elements, application of RAM, etc. The F/A-18E/F is a good example with redesigned inlets that seek to reduce its frontal signature.

The importance of this knowledge comes in mission planning. Knowing where and to what degree one’s vulnerable spots are helps map out an approach to a target area or needs particular care when facing opposing aircraft.

Finally, as previously noted, since RCS will vary with wavelength one needs to be careful in noting what the particular IADS element is that is to be avoided as well as its geographic position. All this brings us to the original question re. Stealth – will it work? And the answer is, it depends.

In the realm of radar, it depends on the type (band) of radar encountered. The current generation’s capability is optimized against X-band tracking and guidance fire control systems. Less well understood will be its capability against the likes of much lower band radars, especially those deployed in the VHF band. This is an issue because the waveforms are large enough in the lower frequencies to overcome many of the LO measures deployed. In times past, this was an acceptable situation because the systems then, while possibly detecting an LO aircraft, did not have a fire control-level of accuracy (ask an E-2 NFO sometime about radar bananas…). Primarily it is because of the design of low frequency antennas and the distance of the object from the radiating source, it would not be unusual to get a return that would measure out at a couple of miles in azimuth and range. Yes – there is “something” there, but absent a fire control system, there isn’t anything kinetically that can be done about it.

That is, until the advent of an AESA variant of VHF radar (1L119 Nebo SVU ), which the Russians are deploying to support their S-300 SAM systems.

Why VHF radar? Recall the relationship between LO/VLO and radar waves – the smaller the wavelength (higher frequency) the “easier” it is to develop/deploy LO/VLO countermeasures. Go in the other direction, however, and eventually just the sheer size of the aircraft will enable detection by the radar. The following image (via www.AUSAirpower.net) is germane –

For instance, let us consider the F-35 JSF in the 2 metre band favoured by Russian VHF radar designers. From a planform shaping perspective, it is immediately apparent that the nose, inlets, nozzle and junctions between fuselage, wing and stabs will present as Raleigh regime scattering centres, since the shaping features are smaller than a wavelength. Most of the straight edges are 1.5 to two wavelengths in size, putting them firmly in the resonance regime of scattering. Size simply precludes the possibility that this airframe can neatly reflect impinging 2 metre band radiation away in a well controlled fashion.

The only viable mechanism for reducing the VHF band signature is therefore in materials, especially materials which can strongly attenuate the induced electrical currents in the skins and leading edges. The physics of the skin effect show that the skin depth is minimised by materials which have strong magnetic properties. The unclassified literature is replete with magnetic absorber materials which have reasonable attenuation performance at VHF band, but are very dense, and materials which require significant depth to be effective if lightweight. The problem the JSF has is that it cannot easily carry many hundreds of pounds of low band absorber materials in an airframe with borderline aerodynamic performance. Some technologies, such as laminated photonic surface structures might be viable for skins, but the experimental work shows best effect in the decimetric and centimetric bands. Thickness again becomes an issue.

The reality is that in conventional decimetric to centimetric radar band low observable design, shaping accounts for the first 10 to 100 fold reduction in signature, and materials are used to gain the remainder of the signature reduction effect. In the VHF band shaping in fighter sized aircraft is largely ineffective, requiring absorbent materials with 10 to 100 fold better performance than materials currently in use. In the world of materials, getting twice the performance out of a new material is considered good, getting fivefold performance exceptional, and getting 100 fold better performance requires some fundamental breakthrough in physics.

In the case of the 1L119 Nebo SVU as a VHF band acquisition and tracking radar, design elements of the antenna along with digital moving target indicators and probable space time adaptive processing, all within the realm of computational power using commercial off-the shelf (COTS) equipment brings into the realm of the possible the use of VHF-band radar for acquisition and tracking of LO/VLO aircraft and countering with late-generation SAMs. An illustration of just such a CONOP (also from AUS Airpower.com) is seen here:

Note how the organic radar (NATO code-named FLAPLID) for the S-300 is used to provide guidance to the in-flight missile while the networked VHF radar provides a tracking feed.

Additionally, recall that LO isn’t just versus radar, but needs to incorporate IR as well. While earlier methods of shielding exhaust and other hot points are generally effective against shortwave IR, long wave IR is another matter. When incorporated in an IRS&T (Infra-Red Search & Track) system, and tied into a helmet mounted site, it will pick up the heat generated by the resistance to the aircraft through the air, especially at super cruise. Such a system is already deployed with the Su-27/-30/-35 family of fighters which are employed by China, India, Venezuela and others, in addition to Russia.

There is much more to this discussion than can be recounted here. The short answer to the question “Will Stealth Work” is still a “yes” – but unlike the early days when the F-117 was first employed with general impunity, the answer now is a qualified “yes.” And just as the earlier cycle of measure/counter-measure/counter-counter measure engaged a spiraling cost in treasure and effort in the active radar countermeasures field, so it appears we are now embarked on a similar path in the LO/VLO realm. Seventeen years ago saw the first large scale employment of combat operations with LO aircraft – what will the next seven bestow?

Part III will address the issue of BVR operations.