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The Northrop F-89 Scorpion

* The Northrop F-89 Scorpion all-weather interceptor was nothing great for looks, and never fired a shot in anger. However, for much of the 1950s it was the backbone of the US Air Force's defense of the United States, and was produced in quantity. This document provides a history and description of the F-89 Scorpion.

Northrop F-89J Scorpion

* In December 1945, the US Army Air Forces (USAAF) issued a request for a jet-propelled night fighter to replace the Northrop P-61 Black Widow. Eight aircraft manufacturers responded the request, with the USAAF selecting two finalists: the Curtiss "XP-87 Blackhawk" and the "Northrop N-24". The USAAF also evaluated the Douglas "XF3D Skyknight" for the requirement.

Curtiss received a contract for a small lot of Nighthawks, though as it turned out the contract would be canceled in 1948 after the flight of a single prototype; the Skyknight was also given the thumb's down, though the Navy and Marines would make good use of it into the Vietnam War. The USAAF settled on the Northrop N-24, signing a development contract with Northrop in the spring of 1946, leading to the manufacture of two prototypes. The first prototype, the "XP-89", flew on 16 August 1948 from Muroc Air Force Base, now Edwards AFB, with Northrop test pilot Fred Belcher at the controls. The XP-89 was painted all black and had a high tail, and so it was named the "Scorpion".

The Scorpion was a hefty aircraft, with a straight mid-mounted wing with a narrow height to chord ratio. The tailplane was mounted halfway up the swept tailplane. Control surface arrangement was conventional, including ailerons, double slotted flaps, elevators, and rudder. The Scorpion had distinctive main gear with large but narrow tires on a wide track that retracted into the thin wing, the gear hinging inward towards the fuselage. The prototype was powered by two Allison J35-A-13 axial-flow turbojets, each offering 16.7 kN (1,700 kgp / 3,750 lbf) thrust. That was too little thrust for too much aircraft.

The pilot and radar operator sat in tandem in a pressurized cockpit, under a canopy that slid back to open. Removeable wingtip fuel tanks with a capacity of 1,158 liters (300 US gallons) were added early in 1949. An airborne-intercept (AI) radar and cannons were to be fitted in the nose, though they were not installed in the prototype. After its initial series of flights, the prototype was fitted with "decelerons", which were ailerons split to open up above and below the wings and act as dive brakes.

* The second prototype, designated "XF-89A", first flew on 15 November 1949. It was substantially redesigned, much closer to production spec, with a more pointed nose and uprated Allison J35-A-21 engines with afterburners, offering 21.8 kN (2,220 kgp / 4,900 lbf) thrust dry and 30.2 kN (3,080 kgp / 6,800 lbf) thrust in afterburner. The engine nacelles were substantially redesigned, with auxiliary inlet doors on the lower front side to provide more airflow for takeoff and so on, and the wingtip tanks were permanently mounted.

The XF-89A wasn't armed, though it did have AN/ARC-33 radar in the nose, coupled to a Hughes E-1 fire control system (FCS). It featured a natural metal finish, instead of the black finish of the first prototype, which crashed on 22 February 1950 due to a tail flutter problem, one of the two aircrew being killed. The crash led to serious doubts about the viability of the Scorpion program, but Northrop proved able to resolve problems, and so the F-89 survived. The tail flutter issue was resolved initially by the modification of the engine exhausts -- the exhaust flow having set up instabilities over the tailplane -- and the addition of external "mass balances" on the elevator to damp out oscillations.

* The USAF (the Army Air Forces became the Air Force in 1947) was pleased with the Scorpion and ordered a batch of 48 aircraft in July 1949. The first of this batch, the "YF-89A", flew on 25 September 1950, leading to the production "F-89A". The F-89A was much like the XF-89A, even retaining the Allison J35-A-21 engines. The major difference was that the F-89A was armed, featuring six T-31 20-millimeter cannon in the nose with 200 rounds per gun, the cannons being staggered to permit ammunition feed clearance. The YF-89A also had underwing provision for bombs or up to 16 unguided rockets -- the Air Force envisioned the Scorpion as having a secondary attack role, though it would never be assigned to the attack mission in practice. The F-89A was left in natural metal finish, except for anti-glare black patterns where needed.

F-89As began to flow to US Air Defense Command and Alaska Air Command units in 1952, though the Scorpion demonstrated an alarming accident rate at first. This problem was not exclusive to the F-89 at the time; the Air Force had been rushing new high-performance aircraft into production, resulting in many technical glitches and pilot training problems, with lethal results. There were crashes at least once a week in that time frame, with scores of aircrew killed.

Only eight F-89As were actually delivered, the aircraft basically amounting to an evaluation variant. They would end up in test and trials, some ultimately being converted to target drones with the designation of "DF-89A". Production moved on to the "F-89B", which featured minor avionics systems updates, such as a Lear-Siegler autopilot. 40 F-89Bs were built, with initial deliveries in 1951, and most F-89As were rebuilt to F-89B standards. They were withdrawn from frontline service in 1954, to serve with the US Air National Guard (USANG) for a few years, though the F-89Bs were never very satisfactory.

* The "F-89C" was the first Scorpion variant really suitable for operational service, with 164 built. The F-89C featured a balanced elevator that got rid of the cluttered external mass balances and many other small tweaks -- later F-89C production including engine inlet screens that were retracted along with the landing gear. The screens were added because the Scorpion's low-mounted engines had such an alarming tendency to ingest rocks and other debris kicked up by the nosewheel that the aircraft was sometimes referred to as the "F-89 Flying Hoover Vacuum Cleaner".

The first 34 F-89Cs retained the J35-A-21 engine, while the next 30 had the similar but much more reliable -21A engine. However, the next 45 F-89Cs built had J35-A-33 engines, with 24 kN (2,450 kgp / 5,400 lbf) dry thrust and 33 kN (3,360 kgp / 7,400 lbf) thrust in afterburner, while the last 55 built were fitted with the -33A, with 24.9 kN (2,540 kgp / 5,600 lbf) dry thrust but the same afterburning thrust as the -33. Armament and radar remained unchanged from the F-89B.

In 1952, six Scorpions, mostly F-89Cs, crashed -- one doing so in front of a crowd at an airshow -- and the fleet was grounded. The problem was traced to a wing design fault, with all Scorpions in service then rotated through a modification program to fit stronger wings, as well as a fin on the end of each wingtip tank to reduce stress on the wing. Once the wings were fixed, the Scorpion's accident rate fell dramatically and the type would then acquire a good flight safety record. By the time the F-89Cs were withdrawn from frontline service in 1954, all had been re-engined with J35-A-47 engines, the thrust levels being similar to those of the -33A but with many technical refinements. The F-89Cs served faithfully with the USANG into the 1960s.

* It should be noted before continuing that trials performed by these early Scorpions included some unusual armament configurations. One F-89A was fitted with an odd nose turret with four aimable 20-millimeter cannon. Under Project GUN-VAL, two F-89Cs were fitted with four 30-millimeter Oerlikon cannon each, and one F-89C was fitted with twin rocket guns that fired Mighty Mouse rockets, each launcher being fed by a magazine with 25 rockets. The rockets were fired by an explosive charge and then ignited their rocket engine after clearing the aircraft.

* The reason for the withdrawal of the F-89B/C variants in 1954 was the introduction of the "F-89D" variant, the definitive Scorpion. The initial YF-89D prototype, a modified F-89B, had performed its first flight on 23 October 1951. The guns were deleted, the interceptor being armed with 104 Mighty Mouse 70-millimeter (2.75-inch) unguided folding-fin air rockets (FFARs), carried in the front of the wingtip fuel tanks, each tank with a capacity of 1,166 liters (308 US gallons).

The rockets were directed by the new Hughes E-6 FCS system and its AN/APG-40 radar, the E-6 automatically directing the Scorpion towards its target to an optimum firing position. The E-6 in principle permitted "collision-course intercepts", with the Scorpion approaching the target from its forward quadrant and firing rockets to destroy it as the paths of the two aircraft intersected. In a more traditional "tail chase" attack, the interceptor would line up behind the target for shots, but in a collision-course intercept there was effectively only one chance to get it right. That was why the unguided rockets were used: although they were notoriously inaccurate, it seemed likely that if a salvo was launched against a target the size of a bomber, one rocket would hit, and that would be enough to take the target down. Rockets could be launched in one, two, or three salvos, either manually or under the automatic direction of the fire-control system. Vents around the rear of the rocket pack diverted the exhaust from the fuel tank behind it; launching the rockets was spectacular to watch.

The F-89D also featured additional fuel tankage in the nose, replacing the guns, and could carry a 760-liter (200 US gallon) drop tank under each wing. A total of 682 F-89Ds was built, initially with Allison J35-A-33A engines, with later production and upgrades moving through improved engine variants up to the J35-A-47.


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