Between 1955 and 1958, the Air Force tested two X-13 vertical takeoff fighter demonstrators in response to an emerging demand for aircraft that could operate independently of nuclear-vulnerable airfield infrastructure. The lightweight designs took off and landed in a vertical orientation from a mobile launch and recovery platform, eliminating the excess weight of landing gear. This approach proved to be impractical from the pilot's perspective and the aircraft lacked the performance to be an adequate interceptor. While the Vertijet was an improvement over the other so-called "tailsitters" like the Convair POGO, it's operational profile has since been regarded as a technological dead-end for vertical takeoff and landing aircraft.
Long Description:
Ryan X-13 Vertijet
Throughout the 1950s, most major aircraft manufacturers in the United States were anticipating the application of Vertical Takeoff and Landing (VTOL) technology to many types of military aircraft. The armed forces expended considerable sums to develop VTOL aircraft that could remain safely dispersed at small operating sites without the need for cumbersome and vulnerable airbases or aircraft carriers in an age of intercontinental ballistic missiles and thermonuclear weapons. An aircraft with a thrust-to-weight ratio greater than one could launch vertically, and once airborne, transition to horizontal flight for completion of its mission, and return for a vertical landing without expensive, easily targeted runways. The Ryan Aircraft Corporation attempted to convert this idea into a practical fighter for the Air Force with its X-13 Vertijet. However, like most other VTOL aircraft, the performance compromises made for their unique capabilities did not warrant its introduction in place of more capable conventional aircraft.
The idea for the Vertijet originated just after World War Two when engineers for Ryan casually debated whether or not their FR-1 Fireball, which had a thrust-to-weight ratio of one at low fuel quantities, could take off vertically. The vertical take-off idea soon advanced beyond the discussion stage. In 1947, the Navy's Bureau of Aeronautics awarded Ryan a contract to investigate the technical challenges involved in the development of a vertically-launched jet fighter as part of a program to evaluate the feasibility of submarine-based aircraft. The Navy also funded a series of "tail-sitter" aircraft that centered on conventionally-configured airplanes, equipped with large counter-rotating propellers that would rest vertically on a strengthened tail section. These designs, which included the Convair XFY-1 (see NASM collection) and Lockheed XFV-1, were to use their high-thrust propellers to rise vertically from a broad range of naval vessels, thus allowing defensive air cover without aircraft carriers.
Ryan's engineering studies revealed that a similar jet-powered design was feasible with a reaction control system that diverted exhaust gasses in the appropriate direction to allow control during hovering and low-speed flight. A subsequent Navy contract funded construction of an unmanned flying demonstrator, which first flew on October 20, 1950. This ungainly contraption, powered by an Allison J33 turbine, and known affectionately as the "beast in the back yard," used a ball mounted nozzle to provide reaction control while hovering. Ryan engineers converted a B-47 fuel tank into a cockpit to allow test pilot Peter Girard to evaluate the test-bed's suitability as a manned research aircraft, which sat on its tail to take off vertically. On November 24, 1953, Girard made the first manned hovering flight in a jet aircraft with this unusual machine.
After Navy funding ran out, the Air Force became interested in Ryan's experiments and in July 1954 issued a contract to the company to construct two VTOL tail-sitter demonstrators, designated as the X-13 Vertijet. This project, based on the earlier Navy design proposal, was to demonstrate the suitability of easily dispersible VTOL fighters. The X-13, designed and built under the direction of Chief Engineer Curtiss Bates, emerged as a compact, single-engined delta-wing fighter. The only unusual feature visible to the casual observer was a set of winglets and the fixed landing gear. The Ryan Technical Section, led by Robert Fuhrman, designed the aircraft to travel on a special trailer, which would tilt vertically for the launch and recovery of the X-13 during vertical takeoffs and landings.
By late 1955, Ryan completed the first Vertijet (s/n 54-1619), and on December 10, Girard took off for its maiden flight. For its initial testing, the X-13 sported a fixed tricycle landing gear and flew as a conventional airplane. Fuhrman and his team did not want to risk vertical flight-testing until they had thoroughly explored the conventional handling characteristics of the X-13. After the installation of dampers had solved oscillation problems revealed during this phase of the testing, engineers added a temporary steel-tube truss with castering wheels to the rear of the X-13. This allowed the aircraft to sit on its tail during the vertical flight-testing phase without the need for the complex launch and recovery procedures inherent to the launch trailer. Pete Girard made the first vertical takeoff and landing on May 28, 1956. On the same day, the second X-13 made its first flight. The aircraft was agile and responsive in conventional flight.
In conventional flight, elevons and a rudder controlled the X-13. As the aircraft transitioned to a nose-high attitude to "hover" on the thrust from its own engine, a vectorable exhaust nozzle linked to the controls provided a simple and effective means of control. Small bleed-air thrusters mounted on the wingtips allowed for the small adjustments to the pitch and yaw of the aircraft when required by the tricky landing process. A stability augmentation system integrated the conventional and VTOL control systems together without requiring any abrupt changes to pilot control inputs. The vertical takeoff procedure consisted of elevating the bed of the launch trailer vertically, which allowed the X-13 to hang from a cable suspended by two arms on the top of the trailer, with a partially retractable hook. For vertical operations, a flat bumper replaced each of the main wheels on the fixed landing gear, which kept the underside of the fuselage from damage if it swung into the bed of the trailer, which made transport easier. The pilot then simply increased throttle until the hook lifted off the launch cable, backed away from the trailer and then accelerated vertically, smoothly pitching over to conventional flight.
However, vertical landings were more difficult and were probably the most impractical part of the Vertijet concept. Its greatest flaw, as with the earlier tail-sitters, was the obscuration of the pilot's vision by the airframe, which made it extremely difficult to judge the distance to the ground adequately without outside assistance. Although the pilot's seat pivoted 45 degrees towards vertical during landing, the pilot still had to approach the recovery trailer blind with the underside of the fuselage facing the surface of the trailer. Constant radio communication with a ground observer was essential to talk the X-13 into position during the cumbersome process. A 6 meter (20 foot) long folding pole with marked gradations attached to the top of the recovery trailer gave the pilot a clear indication of the distance remaining before he contacted the trailer. Once in position, the pilot slowly retarded the throttle until the nose hook caught the recovery cable.
On May 28, 1956, the X-13 made its first vertical hovering flight, and during that summer, Girard and fellow test pilot, Lou Everrett, began practicing the techniques required to catch the cable on the launch-and-recovery trailer by hooking a one-inch thick rope strung between two towers. For these tests, the Ryan engineers fitted X-13 with a wooden nose that was easily replaceable if damage occurred while the aircraft docked with the trailer. On November 28, Girard made the first transition from horizontal to vertical flight and back again in the X-13. On April 11, 1957, he launched from the trailer, transitioned to conventional flight, and returned for a vertical landing, thus completing the X-13's mission profile. On July 30, 1957, to illustrate the dispersed operating site concept the second X-13 put on an impressive display at the Pentagon for over 3,000 military officers and journalists.
However, competing programs reduced the funds available to continue the project, and the X-13 took to the air for the last time on September 30, 1957. While later programs, such as the XV-6 Kestrel (see NASM collection) and AV-8 Harrier, experienced greater success by resulting in operational aircraft, the X-13 was an effective solution to the problems of creating a VTOL fighter, given the limitations of jet technology at the time of its construction. The Vertijet accomplished all tasks specified for it and undoubtedly succeeded as an experimental demonstration aircraft, in spite of the inherent impracticality of the operational concept. Ironically, by the end of the twentieth century, the thrust vectoring system, initially pioneered on the X-13, would become an essential component of advanced combat aircraft. In 1960, Ryan donated the first X-13, along with its launch trailer, to the Smithsonian Institution.
