North American A-5 Vigilante - Design and Development

Design and Development

In 1953, North American Aviation began a private study for a carrier-based, long-range, all-weather strike bomber, capable of delivering nuclear weapons at supersonic speeds. This proposal, the NAGPAW (North American General Purpose Attack Weapon) concept, was accepted by the United States Navy, with some revisions, in 1955. A contract was awarded on 29 August 1956. Its first flight occurred two years later on 31 August 1958 in Columbus, Ohio.

At the time of its introduction, the Vigilante was one of the largest and by far the most complex aircraft to operate from a United States Navy aircraft carrier. It had a high-mounted swept wing with a boundary-layer control system (blown flaps) to improve low-speed lift. There were no ailerons. Roll control was provided by spoilers in conjunction with differential deflection of the all-moving tail surfaces. Use of aluminum-lithium alloy for wing skins and titanium for critical structures were also unusual. The A-5 had two widely-spaced General Electric J79 turbojet engines (the same as used on the McDonnell Douglas F-4 Phantom II fighter), and a single large all-moving vertical stabilizer. Preliminary design studies had employed twin vertical fin/rudders. The wings, vertical stabilizer and the nose radome folded for carrier stowage. The Vigilante had a crew of two seated in tandem, a pilot and a bombardier-navigator (BN)—reconnaissance/attack navigator (RAN) on later recon versions— in individual ejection seats.

Despite being designated by the US Navy as a "heavy", the A-5 was surprisingly agile for such a large aircraft, without the drag of bombs or missiles, even escorting fighters found that the clean airframe and powerful engines made the Vigilante very fast at high and low altitudes. However, its high approach speed and high angle of attack in the landing configuration made returning to the aircraft carrier a challenge for inexperienced or unwary pilots.

The Vigilante had advanced and complex electronics when it first entered service. It had one of the first fly-by-wire systems of an operational aircraft (with mechanical/hydraulic backup) and a computerized AN/ASB-12 nav/attack system incorporating a head-up display (Pilot's Projected Display Indicator (PPDI), one of the first), multi-mode radar, Radar-Equipped Inertial Navigation System (REINS, based on technologies developed for the Navaho missile), closed-circuit television camera under the nose, and an early digital computer known as VERDAN (Versatile Digital Analyzer) to run it all.

Given its original design as a carrier-based, supersonic, nuclear heavy attack aircraft, the Vigilante's main armament was carried in a novel "linear bomb bay" between the engines in the rear fuselage, which provided for positive separation of the bomb from the aircraft at supersonic speeds. The single nuclear weapon, commonly the Mk 28 bomb, was attached to two disposable fuel tanks in the cylindrical bay in an assembly known as the "stores train". A set of extendable fins was attached to the aft end of the most rearward fuel tank. These fuel tanks were to be emptied during flight to the target and then jettisoned with the bomb by an explosive drogue gun. The stores train was propelled rearward at about 50 feet per second (30 knots) relative to the aircraft, not at the aircraft's forward speed as stated in some references. It therefore followed a typical ballistic arc rather than "falling straight down."

In practice, the system was not reliable and no live weapons were ever carried in the linear bomb bay. In the RA-5C configuration, the bay was used solely for fuel. On three occasions the shock of the catapult launch caused the fuel cans to eject onto the deck resulting in one aircraft loss.

The Vigilante originally had two wing pylons, intended primarily for drop tanks. The second Vigilante variant, the A3J-2 (A-5B), incorporated internal tanks for an additional 460 gallons of fuel (which added a pronounced dorsal "hump") along with two additional wing hardpoints, for a total of four. In practice the hardpoints were rarely used. Other improvements included blown flaps on the leading edge of the wing and sturdier landing gear.

The reconnaissance version of the Vigilante, the RA-5C, had slightly greater wing area and added a long canoe-shaped fairing under the fuselage for a multi-sensor reconnaissance pack. This added an APD-7 side-looking airborne radar (SLAR), AAS-21 infrared linescanner, and camera packs, as well as improved ECM. An AN/ALQ-61 electronic intelligence system could also be carried. The RA-5C retained the AN/ASB-12 bombing system, and could, in theory, carry weapons, although it never did in service. Later-build RA-5Cs had more powerful J79-10 engines with afterburning thrust of 17,900 lbf (80 kN). The reconnaissance Vigilante weighed almost five tons more than the strike version with almost the same thrust and an only modestly enlarged wing. These changes cost it acceleration and climb rate, though it remained fast in level flight.