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Aviation History
1981
1981 - 3327.PDF
Northrop to build Stealth bomber, Lockheed building Stealth fighter NORTHROP has been selected to build the United States Air Force Ad vanced Technology Bomber (ATB). Contracts worth $7,300 million for ATB development have been issued to Northrop, who will build the full- scale prototype in conjunction with Boeing and Vought. Several aircraft employing Stealth technology have been test-flying for some four years as technology demon strators. Boeing, Grumman, Lockheed, Northrop, and Vought have all been involved in the programme. Lockheed, using experience gained on the SR-71/ A-ll programme, is now building 20 Stealth reconnaissance/strike aircraft called Covert Survivable In-Weather Reconnaissance/Strike (CSIRS) with funds drawn from the Advanced Tac tical Fighter project. The 20 CSIRSs will be in service in two years. Other programmes which will benefit from Stealth technology are RPVs and air- launched cruise missiles. The Northrop ATB is a flying-wing design and the USAF is planning full- scale production of the ATB for 1988- 89, as soon as currently-planned pro duction of 100 Rockwell B-lBs is completed. The ATB will be tested extensively before B-1B production ends or funds are committed to ATB production. The B-1B thus functions both as an interim strategic penetra tion aircraft and as an insurance against the failure of the Stealth con cept. Assuming that the ATB is success ful, it could be operational by 1991. About 100 ATBs will then replace the B-1B in the penetration role, and the Rockwell aircraft will then be as signed fully as stand-off cruise-missile launchers. Stealth concept Stealth technology is a complex syn thesis of many techniques. The Stealth programme has been run by the Defence Advanced Research Pro jects Agency (Darpa) since mid-1977, when Darpa funding was increased tenfold on its high-priority research into the reduction of airframe radar cross-sections. By adopting a blended design with minimal radar corner- reflectors such as box-shaped intakes, the basic radar cross-section can be reduced. If this airframe is made of, and skinned with, low-reflectivity materials such as carbonfibre, the radar signature is further reduced. By applying a radar-ablative coating, the electromagnetic waves of a transmit ting radar are absorbed more, leaving a much weaker or attenuated return signal for the radar's receiver to pick up. The radar energy absorbed is dis sipated as heat. Reducing a radar target's reflected energy by 12dB cuts the minimum detection range by 50 per cent, a 20dB attenuation reduces detection range by two-thirds, and so on. Derivatives of PTFE, used widely as an electromagnetic insulator in co axial cables for transmission of radio frequency signals, could be used as a base for lightweight ablative coatings. Purely passive detection and avoid ance of hostile radars will be part of the operational philosophy for Stealth aircraft. The UK-based Ferranti com pany has developed a system called IMP, which plots hostile radar en velopes via ultra-sensitive, all- hemisphere receivers and, with a crew- interpreted computer-generated dis play, allows the aircraft to pick its way around threat radars, thereby further reducing the chances of detec tion. Northrop ATB Details of the ATB are classified, but it is reasonable to conclude that the aircraft will be smaller than the B-1B. The ATB will be optimised to pene trate Soviet defences and to deliver a smaller warload than the B-l's, con sisting of free-fall high-accuracy weapons. The ATB will not need to penetrate at low-level for radar avoid ance, at medium or high altitude, it will be more aerodynamically efficient than an aircraft at sea level and will need less fuel for an equivalent-range mission flown at low-level. The lower load and manoeuvring design limits allow ATB's structure to be lighter. Flying-wing designs offer a higher payload fraction than "conventional" aircraft because the weight of the air craft is distributed evenly over the entire span; bending moments in the outer wing are far lower than in con ventional wing/fuselage designs, where the fuselage weight creates a bending moment. A flying-wing struc ture is therefore lighter. The engines are buried within the centre section. In the late 1940s Northrop demonstra ted stability and control on the B-35 and B-49 flying wings using very small fixed vertical surfaces. Using modern computer-controlled relaxed-stability techniques, it might be possible to re duce vertical surfaces considerably, using vanes on the trailing edge for yaw stability and control. Lockheed (CSIRS) Lockheed's lead in Stealth technology is derived from the A-11/YF-12A/SR 71 series, together with the SR- 71-launched, supersonic, low-radar signature drone. The Lockheed GTD- 21, of which at least 38 were built, was carried semi-recessed beneath the launch aircraft. Some were lost over China and North Vietnam, but at least 17 GTD-21s were counted at the Davis- Monthan storage site in 1977. As only 16 were present in late 1980, one may have been used in the initial stages of Stealth fighter development flying, Darpa's Have Blue programme. Reports indicate that the Stealth- technology fighter is about the same size or slightly smaller overall than the McDonnell Douglas F-18 Hornet, with canted-in fins and a single en gine. The extensively-blended delta planform employs lifting body aero dynamics. The intake is mounted above and behind the cockpit. The high angle-of-attack intake starvation previously associated with this posi tion is cured by intense vortex genera tion from the inboard leading edge at high AOA. These dense vortices en sure intake feed. Engine exhaust is probably mixed with bypass air to cool it before exiting through a two- dimensional "venetian-blind" exhaust vent mounted towards the rear of the wing between the fins. This arrange ment will give a very low IR signa ture. Weapons will be carried intern ally to alleviate radar corner-reflec tions. Left Northrop's 1947 flying-wing bomber, the YB-49. Right A (979 Boeing study for an advanced manned penetrator
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