Grumman A-6 Intruder
The Grumman A-6 Intruder is a twinjet, all-weather subsonic attack aircraft developed and manufactured by American aircraft company Grumman Aerospace. It was formerly operated by the U.S. Navy and U.S. Marine Corps.
The A-6 was designed in response to a 1957 requirement issued by the Bureau of Aeronautics for an all-weather attack aircraft for Navy long-range interdiction missions and with short takeoff and landing capability for Marine close air support. It was to replace the piston-engined Douglas A-1 Skyraider. The requirement allowed either single or twin-engined aircraft, as well as either turbojet or turboprop-based engines. The winning proposal from Grumman was powered by a pair of Pratt & Whitney J52 turbojet engines. The A-6 was the first U.S. Navy aircraft to have an integrated airframe and weapons system. Operated by a crew of two in a side-by-side seating configuration, the workload was divided between the pilot and weapons officer. In addition to conventional munitions, it could also carry nuclear weapons, which would be delivered using toss bombing techniques.
On 19 April 1960, the first prototype made its maiden flight; the type was introduced to squadron service during February 1963. The A-6 was operated by both the U.S. Navy and U.S. Marine Corps as their principal all-weather/night attack aircraft between 1963 and 1997, during which time multiple variants were developed and introduced. One derivative of the type was the EA-6B Prowler, a specialized electronic warfare aircraft. Another was the KA-6D, a dedicated aerial refueling tanker. The definitive attack version of the aircraft, which was furnished with vastly upgraded navigation and attack systems, was the A-6E. While the development of further variants, such as the A-6F, were explored, they ultimately did not come to fruition.
The A-6 saw active combat across multiple conflicts. Its combat debut was the Vietnam War, in which the type operated from both carriers and shore facilities. The type proved vulnerable to conventional ground fire and ground-based anti-aircraft measures, which brought down 56 A-6s. In the 1980s, both the Multinational Force in Lebanon and Operation El Dorado Canyon made use of the type. During the Gulf War, a combination of U.S. Navy and U.S. Marine Corps A-6s conducted in excess of 4,700 combat sorties against a variety of Iraqi ground-based targets. During the 1990s, the A-6 was intended to be superseded by the McDonnell Douglas A-12 Avenger II, but this program was ultimately canceled due to cost overruns. Thus, when the A-6E was scheduled for retirement, its precision strike mission was initially taken over by the Grumman F-14 Tomcat equipped with a LANTIRN pod, and later passed on to the Boeing F/A-18E/F Super Hornet.
Development
Background
As a result of the fair-weather limitation of the propeller-driven A-1 Skyraider in the Korean War and the advent of turbine engines, the United States Navy issued preliminary requirements in 1955 for an all-weather carrier-based attack aircraft. The U.S. Navy published an Operational Requirements Document for it in October 1956. It released a Request For Proposals in February 1957. The RFP called for a 'close air support attack bomber capable of hitting the enemy at any time'. The specification was shaped by the service's Korean War experiences, during which air support had been frequently unavailable unless fair weather conditions were present.In response to the RFP, a total of eleven design proposals were submitted by eight different companies, including Bell, Boeing, Douglas, Grumman, Lockheed, Martin, North American, and Vought. Grumman's submission was internally designated as the Type G-128. Following evaluation of the bids, the U.S. Navy announced the selection of Grumman on 2 January 1958. The company was awarded a contract for the development of their submission, which had been re-designated A2F-1, in February 1958.
Grumman's design team was led by Robert Nafis and Lawrence Mead, Jr. Mead later played a lead role in the design of the Lunar Excursion Module and the Grumman F-14 Tomcat. The team was spread between two sites, the company's manufacturing plant at Bethpage, New York, and the testing facilities at Naval Weapons Industrial Reserve Plant, Calverton in Riverhead, New York. During September 1959, the design was approved by the Mock-Up Review Board.
The A2F-1 design incorporated several cutting-edge features for the era. In the early 1960s, it was novel for a fighter-sized aircraft to have sophisticated avionics that used multiple computers. This design experience was taken into consideration by NASA in their November 1962 decision to choose Grumman over other companies like General Dynamics-Convair to build the Lunar Excursion Module, which was a small-sized spacecraft with two onboard computers.
Test program
The first prototype YA2F-1, lacking radar and the navigational and attack avionics, made its first flight on 19 April 1960, with the second prototype flying on 28 July 1960.The test program required to develop the aircraft was prolonged. The very advanced navigation and attack equipment required substantial development, and changes had to be made to correct aerodynamic deficiencies and remove unwanted features. Extending the air brakes, which were mounted on the rear fuselage, changed the downwash at the horizontal tailplane which overloaded its actuator so the tailplane was moved rearwards by. Later evaluation of the aircraft showed that the airbrakes were not effective enough in controlling the speed of the aircraft and they were moved to the wing-tips. Early production aircraft were fitted with both the fuselage and wingtip air brakes, although the fuselage-mounted ones were soon disabled, and were removed from later aircraft. The trailing edge of each wing-tip split to form a much more effective speed-brake which projected above and below the wing when extended.
The rudder needed a wider chord at its base to give greater exposed area to assist spin recovery.
A major difference between the first six production aircraft and subsequent aircraft were the jet nozzles; the close-air support role sought by the U.S. Marine Corps required STOL performance to operate from forward airstrips. Jet deflection using tilting tailpipes was proposed. The performance benefits from varying the angle were not worthwhile, whether operating from short strips or carriers, thus they were fixed at a seven degree downward angle.
Further development
During February 1963, the A-6 was introduced to service with the US Navy; at this point, the type was, according to Gunston and Gilchrist, "the first genuinely all-weather attack bomber in history". However, early operating experiences found the aircraft to be imposing very high maintenance demands, particularly in the Asian theatre of operations, and serviceability figures were also low. In response, the Naval Avionics Lab launched a substantial and lengthy program to improve both the reliability and performance of the A-6's avionics suite. The successful performance of the A-6 in operations following these improvements ended proposals to produce follow-on models that featured downgraded avionics.Various specialized variants of the A-6 were developed, often in response to urgent military requirements raised during the Vietnam War. The A-6C, a dedicated interdictor, was one such model, as was the KA-6D, a buddy store-equipped aerial refueling tanker. Perhaps the most complex variant was the EA-6B Prowler, a specialized electronic warfare derivative. The last variant to be produced was the A-6E, first introduced in 1972; it features extensive avionics improvements, including the new APQ-148 multimode radar, along with minor airframe refinements. The last A-6E was delivered in 1992.
In the early 1980s, the Navy anticipated that an Advanced Carrier-Borne Multirole Fighter would eventually be the long term high-end replacement for both the F-14 and A-6, although this program was short-lived and was succeeded by separate efforts for fleet air defense and deep strike/attack; the latter became the Advanced Tactical Aircraft program resulting in the development of the McDonnell Douglas A-12 Avenger II. Meanwhile, a further model, designated A-6F, was being planned. Intended to feature the General Electric F404 turbofan engine, as well as various avionics and airframe improvements, this variant was cancelled under the presumption that the A-12 would be entering production before long. Instead, a life-extension program involving the re-winging of existing A-6E aircraft was undertaken; initially a metal wing had been used before a graphite-epoxy composite wing was developed during the late 1980s. Other improvements were introduced to the fleet around this time, including GPS receivers, new computers and radar sets, more efficient J-52-409 engines, as well as increased compatibility with various additional missiles.
Design
The Grumman A-6 Intruder is a two-seat twin-engined monoplane, equipped to perform carrier-based attack missions regardless of prevailing weather or light conditions. The cockpit used an unusual double pane windscreen and side-by-side seating arrangement in which the pilot sat in the left seat, while the bombardier/navigator sat to the right and slightly below to give the pilot an adequate view on that side. In addition to a radar display for the BN, a unique instrumentation feature for the pilot was a cathode ray tube screen that was known as the Vertical Display Indicator. This display provided a synthetic representation of the world in front of the aircraft, along with steering cues provided by the BN, enabling head-down navigation and attack at night and in all weather conditions.The A-6's wing was relatively efficient at subsonic speeds, particularly when compared to supersonic fighters such as the McDonnell Douglas F-4 Phantom II, which are also limited to subsonic speeds when carrying a payload of bombs. The wing was also designed to provide a favorable level of maneuverability even while carrying a sizable bomb load. A very similar wing would be put on pivots on Grumman's later supersonic swing-wing Grumman F-14 Tomcat, as well as similar landing gear.
For its day, the Intruder had sophisticated avionics, with a high degree of integration. To aid in identifying and isolating equipment malfunctions, the aircraft was provided with automatic diagnostic systems, some of the earliest computer-based analytic equipment developed for aircraft. These were known as Basic Automated Checkout Equipment, or BACE. There were two levels, known as "Line BACE" to identify specific malfunctioning systems in the aircraft, while in the hangar or on the flight line; and "Shop BACE", to exercise and analyze individual malfunctioning systems in the maintenance shop. This equipment was manufactured by Litton Industries. Together, the BACE systems greatly reduced the Maintenance Man-Hours per Flight Hour, a key index of the cost and effort needed to keep military aircraft operating.
The Intruder was equipped to carry nuclear weapons which would have been delivered using semi-automated toss bombing.