FAC and Combat Dragon
Stephen “Mach None” Ruby
The growing American military involvement in Vietnam in the early 1960s led to strong interest in counter-insurgency (COIN) aircraft. In late 1962, the U.S. Air Force’s Special Air Warfare Center at Eglin Air Force Base’s Hurlburt Field in Florida evaluated two T-37Cs for the role.
The Air Force found the T-37 promising, but wanted an improved version of the aircraft that could carry a much larger payload, and had much greater endurance and better short-field performance. This meant a heavier aircraft with more powerful engines. In 1963, the Air Force awarded a contract to Cessna for two prototype YAT-37D aircraft: T-37s with modifications that included: Stronger wings. Three stores pylons on each wing. Larger wingtip fuel tanks of 360 litre (95 US gallons) capacity.
A General Electric GAU-2B/A 7.62 mm “Minigun” Gatling-style machine gun, with a rate of fire of 3,000 rounds/minute and 1,500 rounds of ammunition. The weapon was fitted in the right side of the aircraft’s nose behind a large, convenient access panel. A gunsight and gun camera were also fitted.
Better avionics for battlefield communications, navigation, and targeting.
Tougher landing gear for rough-field operation.
These changes meant a drastic increase in aircraft weight and the aircraft now had to carry a significant payload as well. Cessna, therefore, doubled the engine power by replacing the two Continental J-69 engines with General Electric J85-J2/5 turbojet engines with 10.7 kN (2,400 lbf) thrust each.
The first YAT-37D flew in October 1964, followed a year later by the second prototype. The second prototype had four stores pylons under each wing, rather than three, and the first prototype was upgraded to this configuration as well.
Test results were good, but USAF interest in counter-insurgency (COIN) aircraft had faded for the moment. The program went into limbo for a time, with the second prototype “put out to pasture” at the National Museum of the United States Air Force at Wright-Patterson Air Force Base in Ohio.
The war in Southeast Asia, however, continued to escalate. Losses of Douglas A-1 Skyraider close-support aircraft in US and South Vietnamese service proved greater than anticipated and USAF interest in COIN aircraft was revived. The YAT-37D seemed like a promising candidate for the job, but the Air Force felt that the only way to be sure was to evaluate the aircraft in combat.
As a result, the USAF issued a contract to Cessna for a pre-production batch of 39 YAT-37Ds, with a few minor changes relative to the prototypes, to be rebuilt from existing T-37Bs. These aircraft were initially designated AT-37D, but the designation was quickly changed to A-37A. The second prototype YAT-37D was pulled out of the Air Force Museum and upgraded to A-37A standards as part of the test program.
The A-37A had a gross takeoff weight of 5,440 kg (12,000 lb), of which 1230 kg (2,700 lb) was ordnance. The A-37A retained the dual controls of its T-37B ancestor, allowing it to be used as an operational trainer.
In combat “forward air control (FAC)” operations, the second seat was occupied by an observer. Only one crewman normally flew in the aircraft for close support missions, permitting a slight increase in ordnance.
Operational History and Viet Nam
In August 1967, 25 A-37As were sent to Vietnam under the “Combat Dragon” evaluation program, and flew from Bien Hoa Air Base on USAF “air commando” missions, including close air support, helicopter escort, FAC, and night interdiction. Combat loads included high-explosive bombs, cluster munition dispensers, unguided rocket packs, napalm tanks, and the SUU-11/A Minigun pod. For most missions, the aircraft also carried two additional external fuel tanks on the inner stores pylons.
During this period the A-37As flew thousands of sorties. None were lost to enemy fire, although two were wrecked in landing accidents. The A-37A was formally named the “Dragonfly”, but most pilots called it the “Super Tweet.” The Combat Dragon program was successful, but unsurprisingly the combat evaluation revealed some of the deficiencies of the A-37A. The most noticeable problem was that the aircraft lacked range and endurance. Other concerns were heavy control response during attack runs (the flight controls were not power-boosted) and the vulnerability of the aircraft’s non-redundant flight control system.
An OA-37 Dragonfly aircraft armed with bombs over Edwards AFB, California
The USAF signed a contract with Cessna in early 1967 for an improved Super Tweet, designated the “A-37B”. The initial order was for 57 aircraft, but this was quickly increased to 127. The A-37Bs were primarily intended to be supplied to the South Vietnamese Air Force (VNAF) as replacements for their Skyraiders. The A-37B prototype was rolled out in September 1967, with deliveries to the South Vietnamese beginning in 1968.
The A-37Bs were all newly built airframes. These were stronger than those of the A-37A, capable of pulling 6 g instead of 5, and were built to have a longer fatigue life of 4,000 hours. Field experience would demonstrate that 7,000 hours between overhauls could be tolerated.
The A-37B weighed almost twice as much as the T-37C. A remarkable fraction of the loaded weight, 2.67 tonnes (5,880 lb), could be external stores. In practice, the A-37B usually operated with at least two and sometimes four underwing fuel tanks to improve combat endurance.
To get this increased weight off the ground, the A-37B was fitted with General Electric J85-GE-17A engines, providing 12.7 kN (2,850 lbf) thrust each. These engines were canted slightly outward and downward to improve single-engine handling. Air commando pilots in Vietnam operating the A-37A had found single-engine cruise an effective means of improving their flight endurance.
Modifications were made to control surfaces to improve handling. To improve aircraft and crew survivability, the A-37B was fitted with redundant elevator control runs that were placed as far apart as possible. The ejection seats were armored, the cockpit was lined with nylon flak curtains, and foam-filled self-sealing fuel tanks were installed.
The A-37 excelled at close air support. Its straight wings allowed it to engage targets 100 miles per hour slower than swept-wing fighters. The slower speed improved bombing accuracy, enabling pilots to achieve an average accuracy of 45 feet.
The A-37B added a refueling probe to the nose, leading to pipes wrapped around the lower lip of the canopy, for probe-and-drogue aerial refueling. This was an unusual fit for USAF aircraft, which traditionally are configured for boom refueling. Other improvements included updated avionics, a redesigned instrument panel to make the aircraft easier to fly from either seat, an automatic engine inlet de-icing system, and revised landing gear. Like its predecessors, the A-37B was not pressurized.
The A-37 required a relatively low amount of maintenance compared to contemporary fighters—only two hours of maintenance for each hour of flight time. This was partially due to multiple access panels in strategic locations.
The 20mm GPU-2/A and AMD 30mm cannon pods were tested with favorable results on the A-37B, but reports indicate that such pods were either seldom or never used in operation.
A total of 577 A-37Bs were built, with 254 delivered to the South Vietnamese Air Force. At war’s end, the A-37 had flown over 160,000 combat sorties with only 22 USAF losses, approximately 187 A-37Bs were in South Vietnamese service when the country fell.
By, Stephen Ruby, aviation writer © December 2012
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