The XF8U-3 "Crusader III" was a Chance Vought Mach 2-capable prototype series intended to fulfill the supersonic, all-weather, fleet defense interceptor role eventually undertaken by a design that would become the famous McDonnell Douglas F-4 "Phantom II". The Crusader III attempted to continue the line born with the original F-8 Crusader of 1957 which was eventually adopted by both the United States Navy and Marine Corps. The Crusader marked the end of the "gunfighter" jet - the period of American all-cannon fighters - as the Navy looked to missile-minded aircraft for future defense. The Crusader III retained some of the recognizable lines found in the original F-8 design but certainly became its own aircraft with an all-new profile and few commonality of parts between the expected successor and the original offering. However, the United States Navy elected to go a different route than the Crusader III presented and only five prototypes were eventually completed.
The original F-8 Crusader was later known as "Crusader I" with the arrival of F8U-2 (F-8C). F8U-2 then became "Crusader II" with its 2 x cannon armament, J57-P-16 engine of 16,900lb thrust, and lengthened fuselage. The II-model made up 187 of the total 1,219 Crusaders produced. With the arrival of the XF8U-3, the line added the "Crusader III" name - first unofficially, then officially.
Early work on the Crusader III yielded a design that offered improvements over the original Vought product. The resulting design was known by company engineers as model "V-401". The basic Crusader design form was held in check while a pointed nose cone assembly was added. The wings could pivot at different approach angles - known as a variable-incidence wing - as they could on the F-8. Large ventral strakes (fins) were added under the tail and the under-nose intake featured a sharp forward lower lip. The ventral strakes were too lengthy for the aircraft to land or take-off with so they were engineered to fold flat for the necessary clearance when on the ground. The engine of choice became a single Pratt & Whitney J75-P-5A turbojet offering 16,500lbs of thrust on dry and up to 29,500lbs of thrust with afterburner engaged. As with the original F-8, the Crusader III - prototype series name of XF8U-3 - was a conventional inline fighter design with a forward-set, single-seat cockpit and single-engine installation. The tail was capped by a single vertical tail fin and the mainplanes were high-mounted along the fuselage spine.
The XF8U-3 incorporated several technological features to help it fulfill its intended role. This included a radar system and fire control computer assist. The radar allowed for tracking of multiple targets and engagement of at least two of them. Proposed armament was to keep the 4 x 20mm cannon arrangement of the original but introduce broader support for the AIM-7 Sparrow medium-range radar-guided missile as well as the AIM-9 Sidewinder short-range missile. The aircraft would carry these across its seven hardpoints. There was also proposed thrust assistance envisioned through the installation of a Rocketdyne rocket motor to provide an additional 8,000lbs of thrust for a short burst of extreme speed - useful in reaching a target area in short order - though this was an optimistic measure considering the XF8U-3's conventional airframe design and construction.
With that, the XF8U-3 prototype went airborne for the first time on June 2nd, 1958. In a U.S. Navy competition against the McDonnell F-4 prototype, the Crusader showed some key qualities including speed and control. At least three of the completed five XF8U-3 prototypes were flown during testing. The aircraft that would eventually become the Phantom II was selected ahead of the Vought submission as it provided a much larger payload-carrying capability and included a second crewman to handle the radar system. As such, work on the XF8U-3 concluded with the five airframes completed. These were then passed to NASA for high-altitude testing until the airframes were dismantled and destroyed.
During its test phase, the XF8U-3 exhibited a maximum demonstrated speed of Mach 2.39 flying at altitudes of 50,000 feet. Its listed service ceiling was 65,000 feet but testing showed one greater at 76,000 feet - hence NASA's interest in the aircraft. Cruising speeds ranged around 575 miles per hour with a rate-of-climb nearing 32,500 feet per minute. Ferry ranges reached 2,045 miles with a combat range expected around 645 miles. Dimensions included a length of 58 feet, 8 inches, a height of 16 feet, 4 inches, and a wingspan of 40 feet.
In comparison, the F-4E Phantom II model showcased a maximum speed of Mach 2.23 with cruise speeds in the 585mph range. Ferry range became 1,615 miles with a combat radius of 420 miles. Rate-of-climb was 41,300 feet per minute. The E-model also could carry a combat load of 18,650lbs to include air-to-air missiles as well as air-to-surface missiles and conventional drop ordnance. The cannon-and-missile-minded XF8U-3 was only ever truly designed as a fleet defense fighter to which its ordnance-carrying capabilities would have proven extremely limited.
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