Here are some partial answers to the questions we had regarding the method in which, particularly Boeing, where going to reduce the EM signature of theyre production JSF models:
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Aviation Week reports on JSF stealth engine
20 March 2001
A report in the March 19 issue of Aviation Week & Space Technology reveals the secrets of new technologies created to eliminate radar reflections from air inlets and engines of the Joint Strike Fighter.
“In a clever use of technology (a technique considered a proprietary secret by the two companies), Boeing and Pratt & Whitney worked together to add stealth to the inlet guide vanes to mask the fan blades behind them. The inlet vanes are variable and open to provide maximum air to the engine in vertical flight, but close to minimize radar reflections during flight at operational altitudes,” reports AW&ST.
The Boeing JSF’s intake radar blocker is built as part of the face of the engine with a bullet-like centrepiece surrounded by angled, radiating vanes. In parallel, the US has developed infrared and radar suppression devices for jet exhausts that have been flying on US stealth aircraft for a number of years. These two types of blockers are generally used in conjunction with one another, and the latter has become increasingly sophisticated as researchers find better ways to deal with an environment of extreme exhaust heat.
Knowledge of the emerging JSF technology also provides insight into the broader evolution of cheaper, reliable, low-maintenance stealth and critical low-observability improvements in other programmes such as the Air Force’s F-22 Raptor and Navy’s F/A-18E/F Super Hornet.
The Joint Strike Fighter competition has yielded secrets about the stark differences in the methods contractors have used to manipulate stealth technologies, in particular to eliminate radar reflections from air inlets and engines.
According to the article, “The JSF competition is a good primer for how technology and tactics can be employed to keep radar from reflecting out of engine, exhaust and weapons bay cavities. The front of Boeing’s JSF engine, for example, is only a few feet inside the air intake. To avoid radar reflections, the engine face has been hidden by special inlet guide vanes that have been treated with radar-absorbing materials (RAM) and shaped to cause radar beams to make multiple bounces, including onto the air-duct walls. There, radio-frequency energy is trapped by RAM or bounced from interior surfaces, each time being greatly attenuated. One way or another, the radar energy becomes too weak to constitute a dangerous reflection.
“By contrast, Lockheed Martin and McDonnell Douglas had both developed serpentine air ducts leading to the engine to avoid such reflections. The ducts coil horizontally and vertically on the way to the engine to avoid a line-of-sight path for radar. Once into the ducts, most radar beams are directed onto surfaces made of, or coated with, RAM. Radar specialists say that after a couple of bounces, there’s virtually no radar energy left for a dangerous reflection.”
Advocates of the Boeing design say new technology makes the short inlet a better bet. “The issue is purely one of how much distance is involved in dealing with the (radar) energy,” said an aerospace industry official with long experience in the JSF competition. “While the longer inlets are generally easier to model (and build), they consume a lot of internal volume in the aircraft and often produce aerodynamic or maintenance challenges.”
“Boeing’s JSF demonstrator is designed for direct thrust from the engine to provide its STOVL capability,” the article says. The engineering demands of the system required the engine to be much farther forward in the fuselage, allowing only enough room to hide the upper half of the engine face. Instead, Boeing is using a radar blocker built ahead of the engine’s face, similar to that developed for the company’s F/A-18E/F. The Super Hornet design differs in that it combines slightly curved inlets with a blocking device on the engine face.
“With the distinct differences between the two JSF designs, Boeing and Lockheed Martin are offering the Pentagon a lot of clear-cut choices in STOVL systems, inlet designs, weapon bay positioning and sensor arrays, aerospace industry officials agree,” according to AW&ST.
Boeing and Lockheed Martin are competing to build the JSF for the US Air Force, Navy and Marine Corps and the Royal Navy and Royal Air Force. A down-select decision is expected later this year, if the programme survives the Bush Administration’s ongoing ‘root and branch’ review of defence programmes.
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So the answer seems to be some form of EM waveguide in the inlet to reflect the incoming signals into infinity. My first thought is what happens if these vanes, accidentally, get bent by some poor mech while he’s inspecting the turbine blades?.
Or what happens if you get a minor FOD ingestion while taxying (carrier decks are great places to find all sorts of little bits of crud!) does that scrub that aircraft immediately?
Sounds a bit fragile to me, anyone any thoughts?
Steve
Everyone needs something to believe in – I believe I shall have another beer.
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March 20, 2001 at 4:49 pm