Background The origins of the Scorpion can be traced back to a
United States Army Air Forces (USAAF) Air Technical Service Command specification ("Military Characteristics for All-Weather Fighting Aircraft") for a
night fighter to replace the
Northrop P-61 Black Widow. The preliminary specification, issued to aircraft manufacturers on 28 August 1945, required two engines and an armament of six guns, either machine guns or
autocannons. The revised specification was issued on 23 November; it did not specify jet propulsion, but the desired maximum speed of was challenging to meet via alternative means. The aircraft was to be armed with aerial rockets stored internally and six guns split between two flexible mounts, four guns forward and two in the rear. Each mount had to be capable of 15° of movement from the aircraft's longitudinal axis; each mount's guns were to be automatically controlled by radar. For ground attack, it had to be capable of carrying bombs and to be able to carry a minimum of eight rockets externally. Further requirements included the ability to ascend to 35,000 feet within 12 minutes and a mission radius of 600 nautical miles. Proposals were submitted by six aircraft companies:
Bell Aircraft,
Consolidated-Vultee,
Douglas Aircraft,
Goodyear,
Northrop and
Curtiss-Wright. To reduce
drag, the two
Allison J35 turbojet engines were buried in the lower
fuselage, directly behind their air intakes, and they exhausted underneath the rear fuselage. The
horizontal stabilizer was mounted just above the junction of the vertical stabilizer with the fuselage and had some
dihedral.
Contract and redesign On 13 June 1946, an initial $4 million contract for two aircraft, designated
XP-89, along with a full-scale mock-up, was approved. The requested alterations to the design were formalized in a series of change orders issued to Northrop. Further changes included the position of the horizontal stabilizer also proved to be unsatisfactory, as it was affected by the engine exhaust, and it would be "blanked-out" by airflow from the wing at high
angles of attack. It was moved halfway up the tail, but its position flush with the leading edge of the vertical stabilizer proved to cause extra drag through turbulence and reduced the effectiveness of the
elevators and
rudder. Moving the horizontal stabilizer forward solved the problem. Another major change occurred when USAAF revised its specification to delete the rear gun installation on 8 October. Another mock-up inspection was held on 17 December, and the inspectors suggested only minor changes, though the fuselage fuel tanks were still above the engines. Northrop's efforts to protect the fuel tanks were considered sufficient, as the only alternative was redesigning the entire aircraft. The XP-89 had a thin, straight, mid-mounted wing and a crew of two seated in
tandem. The slim rear fuselage and the high-mounted horizontal stabilizer led to Northrop employees calling it the Scorpion—a name later formally adopted by the Air Force. Pending the availability of either turret under development, an interim six-gun fixed installation, with 200 rounds per gun, was designed for the underside of the nose. The thin wing had an
aspect ratio of 5.88, a
thickness-to-chord ratio of 9% and used a
NACA 0009-64 section, which was selected for its low drag at high speed and stability at low speeds. A further advantage of the straight wing was that it could accommodate heavy weights at the wingtips. The wing could not fit the circular-type (rotating)
spoilerons used in the P-61, so Northrop used the "
decelerons" designed for the unsuccessful
XP-79 prototype. These were clamshell-style split
ailerons, which could be used as conventional ailerons, as
dive brakes, or function as
flaps as needed. All flying surfaces, the flaps, and the landing gear were hydraulically powered. The thin wing dictated tall, thin, high-pressure () mainwheel tires, while the low height of the fuselage required the use of dual wheels for the nose gear. Several months earlier, the Air Force conducted a competitive evaluation of the three existing all-weather interceptor prototypes, the
XF-87, the XF-89, and the
US Navy's XF3D. although it was in last place in cockpit arrangement and ease of maintenance. During May 1949, the Air Force issued a cost-plus-fixed-fee contract, valued at roughly $48 million, which covered the modifications to the second prototype as well as the supply of the first 48 production standard aircraft, spare parts, tooling, ground-handling equipment, and a single static test frame. Two months later, the Air Force officially accepted the first prototype, roughly one year behind schedule. After repairs from a
crash landing on 27 June 1949, the XF-89 was flown to
March AFB to participate in the
RKO movie
Jet Pilot in February 1950. Shortly afterward, the aircraft crashed on 22 February, killing the observer, when
flutter developed in the
elevator, and the subsequent vibrations caused the entire tail to break off. and the addition of exhaust deflectors to the fuselage to reduce the turbulence and the consequent flutter. These modifications were initially applied to the second prototype to validate their effectiveness. ==Operational history==