Most mysterious of the new projects currently on US drawing boards is the Stealth Fighter currently being developed by Lockheed. This may be the project referred to as CSIRS (Covert Survivable In-Weather Reconnaissance Strike Aircraft). The US Defense Advanced Research Projects Agency (DARPA) is known to be involved in a project known as XST (Experimental Stealth Tactical), perhaps an alternative designation for the same aircraft.
Stealth involves not a single technology, but a blending of several established techniques. Careful shaping of an airframe can greatly reduce the amount of radar energy it reflects, while the use of radar-absorbing material can cause still greater reduction in the radar "signature". Stealth designs avoid flat surfaces and sharp angles which can serve as radar reflectors, smooth shapes being used with the aim of deflecting rather than reflecting the incoming radar energy. Some evidence of this design technique may be seen in the shape of the Rockwell B-1 bomber. Specific airframe features which require careful shaping and the use of radar-absorbing material include air intakes and the leading edges of all aerodynamic surfaces.
The use of advanced materials such as carbon or even more exotic composites can also reduce radar cross-section. The degree to which such materials are used in US Stealth projects may be gauged by the tight-lipped response to any enquiries relating to the electrical properties of composites.
A typical jet fighter such as the F-4 has a radar cross-section of around 2sq m, but in the new Stealth designs this is reduced by a factor of 100 or more. Some reports even talk of radar cross sections of less than one millionth of a square metre.
The techniques described so far are purely passive, but Stealth aircraft will also rely on the use of deceptive electronic countermeasures. Given the fact that the aircraft returns only a minute radar echo, the ECM designer can proceed to mask this by means of false target returns intended to mislead radars as to the size, nature, position and course of the aircraft.
Stealth technology is highly classified by the US Government, and the companies involved with such work maintain a rigid level of security. Early "leaks" of information were ruthlessly suppressed on the orders of the US Government, and in some plants Stealth-related work is carried out in areas of the facility off-limits to most employees.
Several aircraft have been test flown, mostly from a classified site in Nevada. These are thought to be technology demonstrators rather than prototype fighters, and to be in the 20,000lb (10,000kg) class. Since flights started in 1979 several have crashed. Details of the aircraft are fragmentary, but one design is understood to resemble the lifting bodies tested by NASA in the late 1960s.
Designations reported for "stealth fighter" projects include F-19 and at least one YO-series number.
Possible features of an operational Stealth Fighter could include:
- large-scale wing-body blending;
- a wing of delta planform perhaps incorporating buried engines;
- rounded leading edges on all aerodynamic surfaces;
- absence of a conventional vertical fin in favour of winglets or a vee-tail;
- cranked inlet ducts which take the incoming air through a zig-zag route to the engine;
- jetpipes fitted with extensive infra-red suppression devices;
- a radar using spread-spectrum techniques to make interception of its emissions unlikely;
- millimetre-wave radio transceivers for short-range air-to-air communications;
- frequency-hopping radio transceivers for long-range communications;
- advanced electro-optical sensors for long-range target identification and tracking.
Every effort will be made to reduce the distinctive radio/radar "signature" of the aircraft. Conventional radio and radar transmissions effectively yell "Here I am!" to modern ESM sensors and data processing systems, so the Stealth Fighter will either use passive sensors which do not involve transmission, or rely on spread-spectrum techniques to "bury" the transmission among the normal background of radio/radar signals. The use of low-power millimetre-wave transmissions for communications between aircraft in a formation relies upon the fact that highly-directional but compact "smart" antennas may be used accurately to beam the radio energy in the direction of the intended recipient, while the high level of signal attenuation associated with such frequencies (due to absorption by atmospheric water vapour) will make it difficult for hostile eavesdroppers not directly on the line-of-sight of the transmission to detect and intercept the signal.