Aviation Week & Space Technology 03/19/2001
Stealth Engine Advances
Revealed in JSF Designs
DAVID A. FULGHUM/ORLANDO, FLA. and WASHINGTON
Airframers and engine makers improve classified low-observable propulsion technology for Joint Strike Fighter
The Joint Strike Fighter program still has secrets to give up. The edgy atmosphere of the sharpening competition is helping uncover some striking differences in the methods contractors have used to manipulate stealth technologies--in particular to eliminate radar reflections from air inlets and engines.
Once it became obvious that the competition would drag on at least another six months (until the new fiscal year), designers increasingly worried that details of their competition-sensitive technology would leak out. Their worries appear well founded. Aviation Week & Space Technology has uncovered considerable data about signature, sensor packages, weapons and weapons-bay designs and, now, innovations to engines and inlets.
Knowledge of JSF improvements also provides insight into the broader evolution of cheaper, reliable, low-maintenance stealth and critical low-observability improvements in other programs such as the Air Force's F-22 Raptor and Navy's F/A-18E/F Super Hornet.
When Boeing revealed its full-scale JSF mockup at England's Farnborough air show last summer, most observers were shocked to see what appeared to be the engine face placed a few feet behind the opening of the air intake. One of the basic rules of stealth design is that you find a way to keep radar beams from striking the rotating parts of an engine. Engine faces traditionally produce large, sometimes amplified, and distinct radar reflections that can be analyzed to identify the engine and aircraft.
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.
Techniques to hide engines from radar have evolved in the last 25 years. Engineers placed a grill at the front of the Lockheed F-117 inlet to keep radar waves out and bounce them away from the source (above).
By contrast, Lockheed Martin and McDonnell Douglas (before the latter company was eliminated from the competition) relied more on serpentine air ducts leading to the engine to avoid such reflections. The ducts coil horizontally and vertically 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, radar absorbing materials (RAM). Radar specialists say that after a couple of bounces, there's virtually no radar energy left for a dangerous reflection.
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 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.
"Radar blockers" or "stealth intake devices"have been developed for Boeing's F/A-18E/F and F-22 and are even being improved on the former aircraft. The difference is that the blocker is a separate device on Super Hornet, while it is an integral part of the engine, at least in the Boeing version of the JSF. The F-22 and Super Hornet use a combination of curved inlets and radar blocker technologies.
The first-generation SR-71 used huge inlet spikes to control radar reflections. The second-generation F-117 uses a more primitive grid device over the inlet as a radar blocker. A finer mesh screen was used on Northrop's Have Blue proposal, which would have choked air flow and limited top speed to about Mach 0.65. (AW&ST Feb. 10, 1992, p. 23). These earlier designs were abandoned in response to the demand for supersonic strike aircraft and cheaper, more robust stealth. Keeping radar beams out of the engine is a particular concern for aircraft with a single, large engine inlet.
More recently, McDonnell Douglas added fan-shaped blocker vanes in the inlet of the F/A-18E/F. In the latest implementation, the blocker is an integral part of the Boeing X-32 engine design. Tony Landis photos.
The F-117's inlet screens, when aligned with the rest of the aircraft's external faceting, help create uniformly conducting electrical surfaces that allow radar waves to flow around the stealth aircraft and exit from its aft-most point. Some stealth specialists worry that these signals, emitted to the rear of the aircraft, could provide the basis for a counter-stealth defense system.
The Boeing JSF's intake radar blocker is built as part of the face of the engine with a bullet-like centerpiece surrounded by angled, radiating vanes. In parallel, the U.S. has developed infrared and radar suppression devices for jet exhausts and these have been flying on 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.
"We've been using blockers in aircraft exhausts for many years," said a senior aerospace official. "It doesn't significantly affect the engine's airflow [which translates to power] through the exhaust, but [when used in an inlet, a blocker] has the potential to restrict airflow into the engine."
Some stealth specialists say the loss in engine efficiency when using a blocker would be limited to only a few percent, and may be offset by the greater efficiency of a single large engine inlet (Boeing's option) compared to two smaller inlets (Lockheed Martin's design). Others say the effects of a blocker inside an inlet are more pernicious.
"It's physically easier and more robust to build a long, serpentine duct and hide the [engine face], compared to the difficulties of putting in a [blocker] device," said a second stealth specialist with insight into the JSF program. "You've added something else that scatters [radar energy]. You also have to account for the demand on power and subsystems. For example, you have to deice the [blocker] element.
"[Total engine efficiency] depends on the design of the device, the duct, the lips and how the pressure recovery and bleed systems are operating," the second specialist said. "It's fair to say there will be a performance loss when there isn't a nice, shallow, smooth duct. Finally, having something out there that can be hit by a bird or runway debris is not good [for maintaining the stealth signature]."
Lockheed Martin's JSF design has room for the long, spiraling duct because the engine is located well aft in the aircraft. A shaft transfers power from the engine to a lift fan located just behind the cockpit to permit short takeoff and vertical landings (STOVL).
However, Boeing's JSF demonstrator is designed for direct thrust from the engine to provide its STOVL capability. 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 into the engine's face. The Super Hornet design differs in that it combines slightly curved inlets with a blocking device ahead of and separate from the engine face.
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."
Stealth specialists agree that the choice of longer serpentine ducts versus larger radar blockers is a tradeoff between stealth, cost and aerodynamic performance. In smaller aircraft, the serpentine ducts tend to integrate better "than a big, fat single inlet," said a Northrop Grumman official. But when a larger aircraft is involved, it sometimes becomes more efficient to rely on a larger blocker, he said. There is also the issue of price.
"Anytime you have a [large, complex inlet] front frame, it's more expensive from the aspect of construction and integration costs," the official said. "I know the front frame of the F/A-18 represents a significant development cost. Certainly the inlets on Pegasus [a new unmanned combat air vehicle demonstrator] are one of the most challenging aspects of the aircraft's integration."
It is known that the radar-blocking devices have helped reduce the F/A-18E/F Super Hornet's radar cross section (RCS) to an unprecedented low for non-stealthy aircraft--around 0 dBsm., the equivalent of about a 3-ft.-dia. aluminum ball. That is far smaller than other aircraft that did not start out as stealth designs. By comparison, a human has an RCS of about -10 dBsm.; the JSF, designed from the start for low observability, is to have a stealth signature of -30 dBsm. (about the reflection from a golf ball); and the B-2/F-22 are pegged at -40 dBsm. (about the size of a marble).
Other low-observability initiatives also involve stealth blockers at the rear of the engine to redirect and absorb radar signals that make it into the exhaust cavity from behind the aircraft. Earlier efforts included putting cylindrical radar blockers in the path of the exhaust or creating dog-leg exhaust to avoid the cost of developing and integrating expensive radar blocking devices.
THE F-117, F-22, B-2 and now-canceled DarkStar unmanned aerial vehicles all employed a combination of radar and infrared suppressing technologies in their exhaust designs--including both reflecting and absorbing materials, a long-time Pentagon radar specialist said. In the past, a cylindrical blocking device was placed in the exhaust cavity, but scientists are looking at modifying engine exit cone supports or flow straighteners to duplicate the radar-dampening technology on the engine face. The problem with radar blockers in the exhaust is that they must either be cooled or, if allowed to heat up, they must not be visible from outside the aircraft. This is a problem that Lockheed Martin researchers say they have solved with the afterburning F-22.
Hiding hot elements is critical because newer anti-aircraft missile systems are relying more on infrared and optical sensors than radar to find their targets, according to Pentagon intelligence studies. Such sensor systems foil counter-defensive systems like anti-radiation missiles that home in on radar emissions. Optically guided anti-aircraft missiles may have been involved in the shooting down of an F-117 in Serbia during the 1999 Kosovo air campaign.
A hot, glowing mass of metal with direct line-of-sight to the outside of the aircraft would be an obvious target for such air defense weapons. Some stealth designs, such as the F-117, have a dogleg in the exhaust system that avoids the line-of-sight problem. But what the Pentagon wants are simple, inexpensive and rugged radar and infrared blockers that, along with RAM and reflective coatings, are easy to maintain in a very tough, hot-exhaust environment.
Yet another JSF stealth issue involves how long weapons bay doors are open. There are two options, according to Frank Statkus, Boeing vice president and program manager for JSF. Normally, it takes 1-3 sec. for the lower weapons bay door to open, extend a 5-in. spoiler to deflect the slipstream, fire an air-to-air missile [Amraam or AIM-9X] and close. An ejector punches the missile away from the aircraft to ensure a quick separation. That limits the time a radar receiver can detect a reflection from the open cavity.
For operations involving the use of air-to-ground weapons it takes 6-8 sec. Both doors open, a "swing arm" extends with the weapon and then it is launched. A tactic envisioned for the Boeing version of the aircraft--which has side-mounted weapons bays--is to shoot from the bay that is on the side opposite the enemy radar, thereby avoiding any momentary radar reflection when the bay is open. Weapons that must acquire a target before launch would require the doors to stay open longer and have line-of-sight to the target, but such weapons aren't initially planned for use from the JSF, Statkus said. Lockheed Martin offers weapons bays that open downward.
TECHNOLOGY THAT WAS USED to cut the radar reflectivity of Boeing F/A-18E/Fs--non-reflective pylons and RAM-coated weapons--also will be applied to "dirty" versions of the JSF that carry weapons externally.
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, weapons bays positioning and sensor arrays, aerospace industry officials agree. It also may explain some of the Pentagon's predilection to keep the competition going far longer than anticipated."
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