The Northrop F-89 Scorpion was another early all weather fighter/interceptor that first saw service in 1949. It's two afterburning engines produced 7,200 pounds of thrust each. This gave the 42,000 pound aircraft a maximum speed of 635 mph.

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Northrop F-89 Scorpion

By Joe Baugher

On March 23, 1945, the USAAF announced a competition for the successor to the Northrop P-61 Black Widow night fighter. It was to be an all-weather fighter-bomber capable of guarding the northernmost frontiers of the United States, especially Alaska. Initially, a piston-engined design was considered, but in December 1945 the Army changed the requirement to stipulate that only a jet-powered aircraft would be acceptable.

Bell, Convair, Douglas, Goodyear, Curtiss-Wright, and Northrop all submitted proposals for the competition. The Convair entry was a radical, delta-winged design which was eventually to emerge several years later as the F-102. The Douglas entry was a denavalized adaptation of the XF3D-1 Skyknight carrier-based all-weather fighter. The Curtiss XP-87 Blackhawk was a large, four-jet aircraft carrying a pilot and radar operator seated side-by-side. Initially, the USAAF seemed to favor the Curtiss design, if for no other reason than the fact that the Curtiss-Wright company would probably close down if it did not land the contract. Two prototypes of the Curtiss design were ordered under the designation XP-87.

However, the USAAF also thought highly of the Northrop proposal, which was given the designation N-24 by the company. The N-24 project called for a cantilever, mid-wing monoplane with a long, slim fuselage. An unswept laminar-flow wing was adopted to ensure good low-speed stability, important for an aircraft called upon to make frequent landings in bad visibility. Traditional ailerons and flaps were fitted to the wing trailing edge, and provisions were made for wingtip-mounted drop tanks. The aircraft was to be powered by a pair of Allison J35 afterburning turbojets, mounted one on either side of the belly of the fuselage just underneath the wings.

The twin air intakes were mounted flush in front, each intake exactly in line with its engine nacelle. The pressurized cockpit seated the pilot and radar operator in tandem ejector seats underneath a large rearward-sliding bubble canopy. The horizontal tailplane was mounted halfway up on the vertical tail, well out of the way of the engine exhaust. The nosegear was of a very short, twin-wheel design. The main retractable undercarriage had larger than usual wheels, giving the prototype a rather unusual appearance when sitting on the ground.

Since the Northrop aircraft was to be an all-weather fighter, an airborne interception radar was to be installed in the nose. The armament installation was to have been a quartet of 20-mm M-24 cannon carried in a nose-mounted turret. Two different turret designs were to have been considered. One was a Martin design which allowed the guns to be stowed out of sight inside the nose when not needed. When needed, the guns would be elevated and moved forward into firing position. The Martin turret could rotate 360 degrees and the guns could elevate 105 degrees. The other turret design was a Northrop project, which was similar in concept to the Martin design but had only a 30-degree cone of fire.

The USAAF looked favorably upon the Northrop proposal and issued a development contract on May 3, 1946. Two prototypes were ordered under the designation XP-89 in December of 1946. Serials were 46-678 and 679.

In the meantime, the Cold War between the West and the Soviet Union was beginning to get underway. It was generally conceded that the acquisition by the Soviet Union of nuclear weapons was only a matter of time. The appearance of the Tupolev Tu 4 (a copy of the B-29) at the 1947 Tushino air display was a shock to American intelligence, since the USSR would now have a means to deliver nuclear bombs onto continental US targets. There were no all-weather jet-powered interceptors yet available to counter this threat. The US was extremely worried about an all-weather fighter "gap" opening up, leaving the continental US defenseless for several years against an onslaught of nuclear-armed Soviet bombers.

Something needed to get to the squadrons right away. Since neither the Curtiss-Wright nor the Northrop designs promised to be immediately available in quantity for several more years, the USAF approached Lockheed in March of 1948 and asked them to see if the TF-80C two-seat trainer could be adapted as an all-weather fighter, with first deliveries to operational squadrons being made before the end of 1949. Lockheed quickly came up with a design which was eventually to emerge as the F-94 Starfire.

The Northrop XP-89 (46-678) rolled out of the factory in early June of 1948. The XP-89 was powered by a pair of Allison J35-A-9/-15 engines of 4000 lb.st. each Neither the Martin nor the Northrop turrets were ready, so the prototype carried no armament. For rollout, the XP-89 did not carry its jettisonable 600-gallon wingtip tanks. It was painted gloss black overall.

Even before the new Northrop fighter made its first flight, on June 11, 1948, the P-for-pursuit designation was replaced by the F-for-fighter designation, and the XP-89 became the XF-89.

Following a number of taxi and brake tests at Northrop Field, the XP-89 was disassembled and trucked out to Muroc Dry Lake (later Edwards AFB). The XP-89 made its maiden flight on August 16, 1948, with test pilot Fred Bretcher at the controls. Flight test results were generally positive, but the aircraft proved to be seriously underpowered. For the first 32 flights, conventional ailerons were fitted, but on February 1, 1949 a new series of trials began with Northrop-invented "decelerons", which was a split surface that could be operated in one piece as a conventional aileron but which could be opened up to serve also as an airbrake. This feature was made standard on all subsequent F-89s.

Even though the flight test crews were enthusiastic about the XF-89, the USAF ordered that a flyoff take place between the XF-89, the Curtiss XF-87, and the Navy's Douglas XF3D-1 Skyknight. The Curtiss XF-87 with its side-by-side seating arrangement was judged to have the best cockpit arrangement, with the XF3D-1 coming in second. The tandem seating arrangement in the XF-89 made communication between pilot and radar operator difficult. Ease of maintenance was found to be the best in the XF3D-1, with the XP-87 coming in second. However, the evaluation team judged the XF-89 as being the superior fighter and having the best development potential.

Since the Northrop XF-89 was judged as having the superior potential as a fighter, on October 10, 1948, the USAF officially cancelled the Curtiss XF-87 project. The failure of the XF-87 to win any production orders was the end of the line for the Aeroplane Division of Curtiss-Wright. Shortly thereafter, the Aeroplane Division of Curtiss-Wright declared bankruptcy, sold all of its assets to North American, and closed its doors forever.

Even though the XF3D-1 did not succeed in obtaining any USAF orders, it nevertheless did receive orders from the US Navy and the US Marine Corps, and went on to serve as both a land-based and carrier-based interceptor fighter for many years.

In January of 1949, President Harry Truman authorized the Air Force to make an initial purchase of 48 F-89As.

In March of 1949, the name "Scorpion" was officially applied to the XF-89, the suggestion being originally made by ground crews at Edwards who thought that the parked plane with its upward-curving rear fuselage and its high tail looked a lot like the dangerous creature with the deadly stinger in its tail.

On July 14, 1949, the USAF made the order of 48 production F-89A aircraft official. Serials 49-2431/2478 were assigned. An additional 27 aircraft were added to the contract in September.

The second prototype (46-679) made its maiden flight on November 15, 1949. Modifications made when the airframe was almost 90 percent complete led to a change in designation to YF-89, as it was envisaged as being a test vehicle for the production F-89A fighter. The XF-89 had been painted black, but the YF-89 was finished in natural metal overall.

The XF-89 had been unarmed, pending the availability of the nose turret. However, the nose-mounted turret was eventually abandoned as being too complicated, and a more conventional armament of six forward-firing 20-mm Mk 24 cannon was chosen for the Scorpion.

Since the USAF wanted the Scorpion in service right away, production of the F-89A got underway immediately, even before testing of the prototypes was completed. This commitment to production proved to be premature. On February 22, 1950, the XF-89 prototype crashed while making its 102nd flight. The cause of the crash was later found to be a failure of the horizontal stabilizer due to excessive flutter. The YF-89 was grounded for changes, and production of the F-89A was halted.

As a result of the grounding, the YF-89 was extensively modified. The nose was completely redesigned. It was reconfigured to be more tapered and was increased in length by three feet. An AN/ARC-33 radar set was fitted in the nose, along with a Hughes E-1 fire control system. The engines were Allison J35-A-21s, rated at 5200 lb.st dry and 6800 lb.st with afterburners. The engine air intakes were redesigned to include external boundary layer bleed ramps and auxiliary pop-in doors were added to the nacelle sides to allow additional air to be supplied to the engine during ground runups.

The engineers believed that pulsating exhaust gases from the engine were responsible for the tail flutter problems that had wrecked the XF-89, and the engine exhaust area was redesigned by adding deflector plates to the fuselage to direct the exhaust away from the tail. The pitot tube was moved from the vertical tail and installed in the nose. The jettisonable 300-gallon wingtip tanks of the XF-89 were replaced by permanently- attached more-streamlined 300 gallon tanks.

The heavily-modified YF-89 was redesignated YF-89A (Model N-49), and made its first flight on June 27, 1950, and the Scorpion flight test program was resumed.

The first production version of the Scorpion was the F-89A (Model N-35). Forty-eight F-89As had been ordered on July 14, 1949. Production of the F-89A got underway even while the XF-89 and YF-89 were still under test, but the crash of the XF-89 prototype on February 22, 1950 brought the whole program to a halt while the problems were being fixed.

At the time of the XF-89 accident, three F-89As were nearing completion. It was decided that these three machines, acting in conjunction with the YF-89A, would be the test force used to wring out the problems with the Scorpion. The first production F-89A was accepted by the USAF on September 28, 1950, followed by the second and third examples a few weeks later.

The F-89A could be distinguished from the YF-89A by the mounting of an armament of six 20-mm T-31 (M-24) cannon in the nose with 200 rpg. Underwing pylons were provided which could carry sixteen rockets or up to 3200 pounds of bombs. An AN/APG-33 radar was fitted in the nose.

The fix for the tail flutter problem was found to be the fitting of a series of external mass balance horns attached to the hinge area of the horizontal stabilizer/elevator. The configuration of the underfuselage exhaust deflector fairings was changed, and a number of different fairing designs were tried until a final design was adopted. The lines of the rear fuselage behind the jet pipes were altered to overcome some of the excessive turbulence that had been encountered by the YF-89A at high speeds. With these changes, the fail flutter problem was finally believed to be cured, and in January of 1951 production of the F-89A was resumed.

In the event, only eight F-89As were actually built. Subsequent machines in the original order of 48 had enough changes so that they were designated F-89B or F-89C.

The engines of the F-89A were originally a pair of Allison J35-A-21s. After a rash of engine failures on early F-89As, these aircraft were reengined with Allison J35-A-21A turbojets, rated at 5100 lb.st.dry and 6800 lb.st with afterburning. These engines had an engine oil scavenging system and different kinds of afterburner eyelids, which gave smoother afterburner control.

The external mass balances on the early F-89As were eventually replaced by elevators with internal mass balance which were fitted retroactively after having been developed for the F-89C.

Several obsolete F-89As were later modified into radio-controlled target planes under the designation DF-89A.

49-2438 was used by General Electric for experimental work under the designation JF-89A.

After only eight F-89As had been built, production shifted to the F-89B (Model N-35). It differed from the A only in various items of internal equipment. It carried a Lear F-5 autopilot, an instrument landing system (ILS), and a Sperry Zero-Reader (which combined the features of artificial horizon, directional gyro, magnetic compass, and altimeter). The armament and the radar were unchanged.

The first production F-89B was accepted during February of 1951, and entered service with the 84th Fighter Interceptor Squadroan, 78th Fighter Interceptor Group based at Hamilton AFB, California during June of 1951.

After a rash of engine failures on early F-89Bs, the aircraft were reengined with Allison J35-A-21A turbojets, rated at 5100 lb.st.dry and 6800 lb.st with afterburning. These engines had an engine oil scavenging system and different kinds of afterburner eyelids, which gave smoother afterburner control.

All F-89Bs had external mass-balanced elevators which were adopted to overcome a sever flutter caused by the jet exhaust. However, they were fitted retroactively with elevators having internal mass balance after this feature had been developed for the F-89C.

The service of the F-89B with the USAF was quite brief. By 1954, all F-89Bs had been transferred to the Air National Guard. The first Guard unit to re-equip with the F-89B was the 176th Fighter Interceptor Squadron of the 128th Fighter Interceptor Wing of the Wisconsin ANG.

Several obsolete F-89Bs were later modified as DF-89B radio-controlled drones.

The F-89C (Model N-35) was the first major production version of the Scorpion. The first F-89C flew on September 18, 1951. It incorporated most of the changes that had been made to the F-89A/B small-scale production run in an attempt to make the Scorpion a fully operational and safe aircraft.

Some of the changes incorporated in the F-89C were internal. A fuel purging system was added which helped alleviate the danger of fuel vapor explosions. The wingtip fuel tanks had dump valves installed which allowed them to be emptied in flight. The cockpit air conditioning and pressurization system were upgraded. A Lear vertical gyro was added to provide artificial horizon information to the autopilot.

Other changes incorporated in the F-89C were external and hence more obviously recognizable. After production of the first 40 F-89Cs, the external mass balance horns on the horizontal stabilizer/elevator were deleted and replaced by a strengthened horizontal tail with internal mass balances. This feature was retrofitted to all previous Scorpions.

During January of 1952, the 74th Fighter Interceptor Squadron based at Presque Isle AFB in Maine had been declared operational with the F-89C. Other units soon followed.

Even though the Scorpion was rapidly entering squadron service all throughout 1952, the interceptor continued to be plagued by engine failures. The initial production blocks (1 through 20) of the F-89C were powered by a pair of J35-A-21 engines. In service, these engines were unreliable and subject to frequent failures. The problem was solved as in the case of the F-89A and B by retrofitting these early F-89Cs with the improved and more reliable J35-A-21A engine. Beginning with the F-89C-25-NO, the engine was changed yet again to the Allison J35-A-33 jet rated at 5400 lb.st. dry and 7400 lb.st. with afterburner.

Finally, production blocks -35 and -40 were fitted with the Allison J35-A-33A, rated at 5600 lb.st. dry and 7400 lb.st with afterburner. The -33A engine not only had more power, it also had a redesigned inlet, deicing equipment, inlet guide vanes, and redesigned forward engine mounts. The F-89's engines, being mounted quite low on the fuselage, had a tendency to scoop up runway debris into their intakes. To cure this problem, retractable inlet screens were added to the -33A engine.

Other problems with the Scorpion turned out to be much more serious. During 1952, several F-89Cs crashed due to wing structural failures. No less than six aircraft were lost in the spring and early summer of 1952, one spectacular crash taking place in front of thousands of spectators at the International Aviation Exposition at Detroit. The entire Scorpion fleet had to be grounded until the cause was found.

After an exhaustive series of flight tests, the problem was finally traced to a previously unknown effect, known as aero-elasticity. This effect caused the wing to twist at the tip during high-G maneuvers, exerting excessive strain on the wing attachment points and causing them to fail. The large wingtip fuel tanks were found to be a significant factor in exerting this twisting moment. A total of 194 F-89A, B, and C aircraft were shipped back to Northrop where they were fitted with stronger wings with forged steel attachment points. At the same time, a small fin was added to the outboard rear of each wingtip tank, which reduced the aerodynamic forces on the tank which caused it to flex and twist during maneuvering.

Before the F-89C fleet had been fully retrofitted with the new stronger wing, a total of fourteen months had passed and it was not until 1954 that the Scorpion force reached its intended level. As fast as the F-89Cs could be modified, they were flown directly to their operational units. With the modified wing and the improved engines, the Scorpion became one of the safest and most reliable combat aircraft in the USAF inventory. The 74th Fighter Interceptor Squadron, following its transfer to Thule, Greenland in August of 1954 completed a full year of service with the type without a single accident of any kind.

The F-89C served with the 27th, 74th, and 433rd Fighter Interceptor Squadrons from 1952 onwards. The 57th FIS, based in Iceland, the 65th and 66th FIS based in Alaska, and the stateside 438th FIS took delivery of the F-89C in 1953.

The F-89C was phased out of active USAF service during 1954, when later Scorpion models became available. F-89Cs were then transferred to the Air National Guard, equipping some seven units in northern states. ANG squadrons operated the F-89C until well into the 1960s until they were finally phased out of service.

Specification of the F-89C:

Two Allison J33-A-33A, 5600 lb.st. dry, 7400 lb.st. with afterburner. Maximum speed: 650 mph at sea level, 562 mph at 40,000 feet. Initial climb rate 12,300 feet per minute. Service ceiling 50,500 feet. Maximum range 905 miles. Dimensions: wingspan 56 feet 0 inches, length 53 feet 5 inches, height 17 feet 6 inches, wing area 606 square feet. Weights 24,570 pounds empty, 33,100 pounds combat, 37,348 pounds gross. Armed with six 20-mm cannon in nose. Underwing racks could carry 16 five-inch rockets or 3200 pounds of bombs.

Sources:

United States Military Aircraft Since 1909, Gordon Swanborough and Peter M. Bowers, Smithsonian, 1989.

F-89 Scorpion in Action, Aircraft Number 104, Larry Davis and Dave Menard, Squadron/Signal Publications, 1990.

Fighters of the United States Air Force, Robert F. Dorr and David Donald, Temple Press Aerospace, 1990.

The American Fighter, Enzo Angelucci and Peter Bowers, Orion, 1987.

American Combat Planes, Third Enlarged Edition, Ray Wagner, Doubleday, 1982.

Joe Baugher

AT&T Bell Laboratories

2000 North Naperville Road

Naperville, Illinois 60566-7033