How "stealth" is achieved on F-117A
There are four key components to the F-117A's "stealth" suite: RAM (radar absorbent material) coating, internal radar-absorbent construction, external LO geometry, and IR emissions control.
The basic principle behind RAM coating is this: the coating contains carbonyl iron ferrite (special paint using this material is known as "iron ball" paint). When a radar wave encounters this coating, it creates a magnetic field within the metallic elements of the coating. The field has alternating polarity and dissipates the energy of a radar signal. A significant portion of radar energy is converted into heat. Such RAM coatings can be manufactured in the form of neoprene-like tiles, with application of various ferric compounds in the synthetic polymer matrix. Early versions of the F-117A employed metal-backed RAM tiles. The tiles were cut to shape and bonded directly to the aircraft's metal structure. Gaps between the tiles were filled with RAM paint. Some of the gaps between the tiles were sealed temporarily only for the duration of the mission. Current models of the F-117A are using RAM paint applied directly to the aircraft's body. The paint is applied employing robotics because the solvent used in the process is highly toxic.
Interesting incidents were observed by F-117A maintenance crews during the Gulf War. Here is a short description from At the Controls: F-117A Stealth Fighter, by Jon Lake: "The effectiveness of F-117A's RAM skin was demonstrated in an unusual manner during the Gulf War, when groundcrews started finding dead bats around the tails of hangared aircraft. The unfortunate creatures had clearly flown "full tilt" into the Black Jet's tailfins, which their high frequency 'sonar' had been unable to detect."
The story of "dead bats" in fact has nothing to do with the F-117A's "stealthy" properties. Bats use ultrasonic signals for echolocation: these are mechanical compression waves not electromagnetic waves, as in case with radars, and have certainly nothing to do with the radar absorbent paint or any geometrical properties of the F-117A. The ultrasonic signals emitted by bats are narrow and highly directional and will reflect from most surfaces, RAM or no RAM. To explain the "dead bats" phenomenon we only need to remember that the F-117As use highly toxic paint and that the aircraft were stored in hot hangars with restricted ventilation. If the maintenance crews have spent as much time in these hangars as bats did, the bodies of bats would not have been the only dead bodies found around F-117As.
Internal Radar-Absorbent Construction
Below the F-117A's skin one would find a special radar-absorbent structure, known as re-entrant triangles. The geometry of this structure "traps" the electromagnetic energy and gradually dissipates it by bouncing it off internal faces. Such radar-absorbent structure in the US was first used on SR-71.
External LO geometry
In 1966 a well-known Soviet mathematician, Pyotr Ufimtsev, published a paper in which he described mathematical methods to predict RCS of two-dimensional objects. Ufimtsev's work was directly used by Lockheed's mathematician, Denys Overholser, to develop computer software known as "Echo 1", which could calculate the RCS of an aircraft constructed of flat panels. This program was used to find the optimum geometry to minimize an aircraft's RCS. The resulting structure became known as "Hopeless Diamond", which lies in the basis of F-117A's external construction. Simply put, the flat, angled external panels of F-117A are designed to reflect radar waves in all directions but the direction of the radar's receiving antenna. This means that to effectively track F-117A one would have to use multiple radars (or, at least, multiple receivers.) Needless to say that the geometrical requirements for minimizing the RCS established by "Echo 1" program had little place for considerations of aerodynamics. The resulting aircraft had the aerodynamics of a flying coffin (some of the modern anti-stealth radars take advantage of F-117A's poor aerodynamics and target the aircraft by detecting the considerable trail of turbulent air left by the aircraft's boxy airframe.)
To answer one simple question: is the F-117A invisible to radars? No, but it is difficult to detect. Theoretically, the F-117A or any other "stealth" aircraft can be detected by a radar operating at any reasonable frequency with sufficient power output and conveniently placed receivers. However, in real life, power output of radars is limited and most radars have only one receiving antenna. This means that reliable detection of F-117As would require considerable modifications to the technology of today's radars and methods of detection. To find out more about theoretical and practical aspects of anti-stealth radars click here.
One of the volnurabilities of the F-117 are its bomb bay doors: when these are opened to release weapons, the aircraft's RCS increases dramatically, making it an easy target for any radar. The bomb bay doors of F-117 are of relatively primitive construction and open to a vertical position, they cause the aircraft's RCS to jump to a level of any conventional aircraft of similar size. It is believed that F-117's bomb bay doors are linked to RHAWS (Radar Homing And Warning System), which prevents doors from opening if the aircraft is being tracked by an enemy radar.
IR emissions control
The only technnique used to reduce IR signature of the F-117 is efficient distribution of jet exhaust by using specially-designed jet exhaust nozzles. The Night Hawk uses broad, flat exhausts to evenly spread jet efflux along a large surface area. This allows to achieve more efficient cooling of the exhaust gases and to avoid any "hot spots". The elongated engine nozzles, however, have a negative impact on the power output and efficiency of the engines.
The Night Hawk's targeting sensor suite consists of two IRADS (Infra-Red Acquisition and Detection System) turrets located above and below the aircraft's nose, containing FLIR (Forward-Looking Infra-Red) and DLIR sensors, respectively. Each of two turrets have three apertures: for FLIR, laser and TV cameras. The turrets themselves are derived from those used by Rockwell OV-10D Bronco NOGS (Night Observation/Gunship System) aircraft. The F-117A's sensors can acquire targets from about 10-15 miles. Each IRADS turret contains a laser designator, which goes on and off periodically to determine the distance to the target. The F-117A relies on its own laser designators for targeting.The target is first detected by upper turret containing FLIR sensor, which leads the target to the point when it's about to disappear under the aircraft. At that point the tracking function is transferred to the bottom turret, containing the DLIR sensor. This transition process is not entirely smooth, as there believed to be a small gap between the coverage areas of the two turrets.
The Night Hawk is compatible with a small selection of precision-guided munitions. The main weapon of the F-117A is the GBU-10 Paveway II LGB (laser-guided bomb). This weapon is based on the 2,000lb Mk 84 iron bomb and has a gimballed seeker, with full deflection guidance. This type of guidance is quite old and ineffective because the bomb's control surfaces can move only between two main positions: neutral and full deflection. This causes the bomb to bounce within its guidance corridor, loosing momentum and energy. This makes the bomb less accurate and more vulnerable to air defense systems.
A more advanced weapon of F-117A is the 2,000lb-class GBU-27B LGB. This bomb is a hybrid of the GBU-24 Paveway III LGB, designed by Texas Instruments, and 500lb-class GBU-12. Basically, a small tail of GBU-12 was slapped on the larger GBU-24 to make the bomb fit into the F-117A's short and narrow bomb bay. The GBU-27B and the GBU-27A/B (which uses the 4340 steel alloy-cased, tail-fused BLU-109B penetrator) have a distinct advantage over the older GBU-10: the new bombs use proportional guidance system, which causes the bomb's control surfaces to move just enough to maintain the correct trajectory. This results in lower losses of speed and energy, increases the bomb's accuracy and decreases its vulnerability to air defenses.
Other weapons, occasionally carried by the Night Hawk, include 500lb-class GBU-12 Paveway II bombs, nuclear B.61 bombs, and certain types of unguided CBUs. The F-117A was also tested with AIM-9 Sidewinder AAMs and AGM-65 Maverick AGMs.To summarize the F-117A's attack capability: the aircraft relies on optical targeting and its effectiveness, as experience in Yugoslavia showed, can be severely undermined by bad weather. The aircraft's maximum weapons-carrying capacity of two bombs makes it a decent diversionary tool but a less-then effective bomber in medium- to large-scale armed conflicts.