Posted: 5/22/2006 3:43:38 AM EDT
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Excellent article on 'stealth' technology… ANdy  Hidden problems of stealth Releasing high level stealth remains a concern for the JSF programme. One solution is to make all the JSFs equal, but some more equal than others, says Bill Sweetman Negotiations leading to almost final international partnership agreements in the Joint Strike Fighter (JSF) programme are in the home straight. Some of the trickiest issues and the least discussed revolve around the fighter's stealth characteristics. When the UK and other customers talk about their need for a sovereign capability to maintain their own aircraft, the fighter's lowobservable (LO) systems are integral to that discussion because they will require regular maintenance and inspection. Access to software hosted on the aircraft is also related to stealth, because there are softwaredriven features of JSF F35 multirole fighter aircraft (and any other LO aircraft) that are critical to exploiting stealth technology in combat. Another key question is whether all JSFs will have the same stealth characteristics. JSF programme officials have never given a clear answer to this question and in fact it is not entirely up to them to answer it. Most recently, Lockheed Martin Vice President (JSF) Tom Burbage disclosed that export JSFs apart from UK aircraft would be designed to national specific operational requirements documents (ORD5) that would be different from the Joint ORD (JORD) signed by the US services and the UK. He also said that the key performance parameters embodied in the national ORDs, which include LO characteristics, would reflect US national disclosure policy. While there is no JSF export 'variant' as such, it is apparent from a review of open sources that it would be practical to deliver JSFs in different configurations with different signatures to match national ORDs, protecting US and UK aircraft from any compromise of their own LO systems. At the root of all these discussions is the question of how stealth works and what LO systems need in terms of maintenance. Although the details of stealth technology are usually considered secret, enough information has been declassified over the years to put together a relatively clear picture of how the F35 family achieves its low observable targets. Stealth is not like the mythical ring of invisibility in JRR Tolkien's Lord of the Rings: it is not a single feature that confers invisibility. It is a blend of things on the aircraft that are obviously technologies that are generic and quite widely used and understood; components which are unique and exotic; and some features that are not visible on the aircraft at all but are nevertheless crucial to making the LO system work. Stealth pioneer Denys Overholser once remarked that the four most important aspects of stealth design were "shape, shape, shape and materials" and given a basic understanding of the laws of electromagnetic scattering it is relatively easy to understand why stealth aircraft (and stealthy ships) are shaped the way they are. Right angles, flat surfaces normal to the incident angle of a likely radar signal, and rounded corners are avoided. The F35's outer mould line can be summed up as upper and lower segments made of canted planes and upward or downwardfacing curves, meeting at a continuous sharp chine line. Essentially, the LO shaping used on the Lockheed Martin F22 and F35 is a refined version of that used on the Lockheed Martin F 117 stealth fighter, with the addition of rounded corners and curved surfaces. The differences reflect greater computer power that gives designers more confidence in their ability to predict scattering from curved surfaces. Lockheed Martin proposed using some curvature on the F1l7, but the urgency of the requirement and perceived technical risk led to the retention of the proven faceted shape of Have Blue. 'Hotspots' in the shape corners, door edges, seams and hingelines, for instance are treated with radarabsorbent material (RAM). RAM is simple in principle, comprising an active ingredient, such as carbon or ferrite materials, which absorb radio frequency energy and convert it into heat in a paintlike matrix. Some USdeveloped RAM has been widely exported as part of the Have Glass radar crosssection (RCS)reduction kit applied to Lockheed Martin F16 multirole fighters, which includes a spray on coating in the inlet duct and RAM applied to the radar bulkhead, and effective RAM has been widely developed and produced outside the US. Recent developments in RAM are aimed at making application easier and reducing maintenance requirements, rather than improving performance. On the JSF, the goal is to develop RAM coatings, which will last the life of the aircraft and will require only visual inspection. Damage can be compared with sample images on a portable computer and repaired quickly if necessary. This capability is being developed through a comprehensive 'effects of defects' programme using flight test aircraft (to assess the durability of new materials) and a full scale airframe. The common sense application of shaping and simple RAM can certainly reduce RCS and there has never been any suggestion that the JSF's shape will be altered for export. However, there is another level of technology needed to achieve the kind of survivability desired for the F22 and F35. The chine and hard edge around the aircraft would have no RCS contribution if they were infinitely sharp, but they are not. Unable to eliminate the echoes from the edges, the designers of the F117's precursor, Have Blue, hit on the idea of focusing them as sharply as possible (that is, by making the edges long) and pointing the resulting spikes in a few directions and away from the crucial headon aspect. On the production F117, this discipline of edge alignment extended to doors and other apertures; on the F22 and F35,it covers manufacturing joints as well. However, this does not make energy go away. In fact, it focuses it. The principle of edge alignment demands that the focused 'spike' reflection from the leading edge and trailing edge be suppressed as much as possible, to reduce the risk that it will cue defences to the presence of a target. The spikes are the dominant part of the signature and therefore the most important to protect from early warning radars, which may operate in lower frequency bands than other radars. The result is that the edges need to be treated with a broadband, low-frequency, high performance absorber. This is not easy because the effectiveness of RAM is proportional to its physical depth compared with the wavelength of the radar. Solid RAM coatings cover a frequency range of about 20:1. This is enough to address air-to-air and surface-to-air missile radars (from the L-band up to the Ku-band) but more elaborate schemes are used to cover the full radar spectrum. The task is complicated by the fact that different radars affect the target in different ways. According to a Lockheed Martin scientist, Dr Vaughn Cable, speaking during a brief period of openness in 1992: "High frequencies have the same effect that you see when you shine a flashlight down the street and see a cat's eyes flash. At low frequencies, we consider radar as hammering the target and leaving the target in a mode that rings. It's a resonance effect." Around the same time, F- 117 Chief Engineer Alan Brown compared a typical wide-band radar absorbent structure (RAS), used on the edges of a stealth aircraft, to "a stereo system, with a tweeter and a woofer". The tweeter is a high-frequency ferromagnetic RAM coating, applied over a resistive layer that reflects higher frequencies but allows low-frequency signals to pass through. Beneath this resistive layer is the low-frequency woofer: a glass -fibre honeycomb core, treated from front to back with a steadily increasing amount of resistant material. Brown called it "an electromagnetic shock absorber. It's very soft in front, but we still absorb pretty much all the energy inside, because we don't want the energy to hit the vertical front face of the structure". Lockheed Martin's ultra-secure Skunk Works is still the sole source for edge materials on the F-22 and F-35. Added late in the production stage, they could very easily be made to differ between one country's aircraft and another. Another feature on any LO aircraft is associated with edge alignment. Even with high-performance broadband RAS on the structure, there is still an RCS spike normal to the leading edge and it should not be allowed to dwell on the hostile radar. This can be controlled by manoeuvering the aircraft. The need to do this was first recognised by the Red Team that investigated stealth in the early 1980s. On the F-1l7, the problem was addressed by developing a ground based mission planner (nicknamed Elvira); the aircraft flies a predefined course based on the known electronic order of battle and does not deviate from it. The B-2 was designed with an onboard electronic surveillance measures (ESM) and threat-avoidance system; the results would have been amusing had they not been so expensive, the technology of the day not being up to the job. Both the F-22 and F-35 have an onboard, real time mission planning capability. The key to this is onboard software that calculates the detection range of any known radar against the fighter, at any aspect angle. The sensitivity of the data contained in this software cannot be overestimated because it tells an adversary exactly what is needed in order to detect an F-35. This is one reason for the US insistence on source-code protection. However, if the actual signatures of an export aircraft are (or may be) different, the data would not be useful for the detection of US/UK aircraft. Building less-stealthy JSFs for export has been considered. One source close to the JSF programme has said that the US Department of Defense's theatre commanders-in-chief have advocated a single JSF configuration, because multiple levels of LO would complicate joint operations. The LO community itself- covering JSF, F-22 and other programmes - opposed this view, fearing technology compromise. "The commanders in the field want coalition allies to have highly capable aircraft. It's the lower level people who don't want to share the technology," the source said. Further complicating this issue is that the rules on the export of stealth technologies, as well as of dual-use technologies that are important to stealth, are not made by the JSF programme office but by senior Pentagon leaders who define disclosure policy with the help of the Low Observables Executive Committee (LOEXCOM). The EXCOM includes representatives from the services, intelligence agencies and all major stealth programmes, including 'black' or unacknowledged programmes. Restricting access to sensitive technologies on board the aircraft is another way to protect the secrets of LO - one that may be either an alternative or a complement to a less-stealthy configuration. One programme source has suggested that export customers would not necessarily need the ability to repair complex LO materials. The JSF programme goal is to have two-level maintenance - flight-line and depot - and to make LO systems so durable that they seldom require maintenance unless they suffer impact damage. "Now, whether you're going to export the capability of repairing a leading edge, I don't know," the source said. "You might ship it back and change itforaspare part." However, such a restriction would clearly violate the principle of sovereign maintenance and support as defined by the UK, which includes the ability to diagnose and fix problems with the LO systems. It will be more of a problem if the JSF needs more frequent or more detailed LO diagnostics than the programme's goal implies. One goal on JSF is to reduce as far as possible the need to check the performance of the LO systems. The US Air Force has been trying since the mid- 1990s to develop a portable, largely automated ground-based Common LO Verification System (CLOVerS), but the effort has fallen behind schedule: a development contract was awarded to Boeing in 1999, but the low-rate production decision has just been deferred, once again, to mid-2007 and a full-rate production goahead is not expected before 2009. For JSF, the target is that most inspection should be visual; failing that, very simple radio frequency tools can be used and whole-body ground-based imaging of the kind that CLOVerS was designed to perform should be rare. Actual flights over an instrumented test range should be precluded or minimised according to a 2003 briefing. Even if all goes to plan, according to the same briefing document, very-low-maintenance stealth will not be available at the start of the programme. The F-35 will start out dependent on whole-body imaging and be gradually weaned off it as experience builds. How onerous these restrictions would be in practice would depend on how frequently LO materials needed to be inspected and repaired. While the JSF goal for this is "hardly ever", that is not, so far, the experience with stealth aircraft. Bill Sweetman is IDR Technology andAerospace Editor, based in Minnesota. www.janes.com |
Exactly! Simple solution to this: No speeky inglish no getty plane! Don’t export our best technology except to our best allies. The rest of the EC, NATO, etc. are so unreliable if it comes to a fight why bother with them! |