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X-Planes Experimental Aircraft

The unknown is inherently unpredictable. Wind tunnels, simulators and computers can only model what is known. That became clear by the mid-1940s as engineers began to probe the technological challenges of piloted, supersonic flight. By early 1945 the world's first experimental airplanes were under development: the rocket-powered XS-1 (later designated X-1), built under Army sponsorship by Bell Aircraft, and the turbojet-powered D-558-1 constructed by Douglas Aircraft under Navy patronage. With the supersonic X-1, flight testing assumed a distinctive process using highly experimental research programs -- such as with the X-3, X-4, X-5, and through dozens of subsequent X-series aircraft.

Research aircraft are the tools for exploration and discovery. Each is instrumented to acquire data about the aircraft, its systems and even the surrounding environment during research flights. These are typically flown by the Air Force in conjunction with NASA and conducted in a very methodical fashion to answer largely theoretical questions concerning innovative aircraft design principles. The X-15 explored hypersonic and exoatmospheric flight. Indeed, within an eight-month span in 1961, it became the first aircraft to exceed Mach 4, -5, and -6, and it later went on to become the first--and, so far, only--airplane to fly in near space as it soared to a peak altitude of more than 67 miles (354,200 feet). Until the 1970s, experimental planes (designated "X"-planes for "experimental") were the chief research tools for flight regimes that wind tunnels, simulators and production aircraft could not approach.

The X-Plane Program has evolved from being the first rocket-powered airplane to break the sound barrier (the X-1 on 14 October 1947) and included over 30 different major research designs, although not all were developed into flying prototypes . As the program progressed, other non-rocket-powered experimental aircraft were built and tested. These aircraft included: a range of vertical takeoff and horizontal landing vehicles; smaller, propeller-driven reconnaissance vehicles; and a series of unmanned missile testbeds of both single and multistage designs. Although the program grew to include conventional propeller-driven aircraft, all designs had in common the aspect of being highly valuable research tools for advancement of aerodynamics and astronautics.

Accomplishments of the X-Plane family have been many. The program included: (1) the first aircraft to break the sound barrier; (2) the first aircraft to use a variable-sweep-wing in flight; (3) the first to fly at altitudes in excess of 30,000, 60,000, and 90,000 m (100,000, 200,000 and 300,000Êft); (4) the first to use exotic alloy metals for primary structure; (5) the first to test gimbaled jet and rocket engines; (6) the first to use jet-thrust for launch and landing; (7) the first to fly three, four, five, and six times the speed of sound; (8) the first to test boundary-layer-airflow control theories over an entire wing at transonic speeds; (9) the first to successfully complete a 180-degree turn using a post-stall maneuver; and (10) the first missile to reach an intercontinental flight range.

Another member of the X-Plane Program would be the X-33. As a reusable spaceplane, the X-33 continues the research line developed by various components of the X-Program, such as the X-10 which tested cruise missile components; the X-12, the Atlas B missile which tested one-and-one-half propulsion staging and obtained the first intercontinental flight distance for a US missile; the X-15 which explored the problems of space and reentry at high speeds (Mach 6) and altitudes; the X-17 which explored high Mach effects on reentry vehicles; and the X-23A which was the first maneuvering lifting reentry vehicle. The X-17 was a multistage rocket design which transported various reentry vehicle configurations to very high altitudes to examine their reentry characteristics. The X-23A was launched by a modified intercontinental ballistic missile and utilized a “lifting body” design to glide back to earth. Information acquired from the X-23A was instrumental in later development of the Space Shuttle.

The majority of testing for the X-Plane family has occurred at Edwards Air Force Base (formerly known as Muroc Army Airfield). Hosts within Edwards include the Air Force Flight Test Center and Dryden Flight Research Center. Other sites which have served as X-Plane testing sites include: Langley Research Center and Ames Research Center; various Government owned ships; White Sands Missile Range, New Mexico; Wright-Patterson Air Force Base, Ohio; Cape Canaveral Air Station, Florida; Pinecastle Air Force Base, Florida; Buffalo, New York; and the National Aviation Facilities Experimental Center in Atlantic City, New Jersey. Edwards has seen more X-Plane programs and test flights than any other similar facility in the US.

As with every research program testing prototype equipment, the X-Plane Program has not been without technical glitches and equipment failures. Since the beginning of the program’s manned flight operations in 1946, approximately 15 major accidents and 4 fatalities (pilots) have been associated with manned vehicle tests. Three of these fatalities were from the X-2 Program, flown between 1952 and 1956, and the remaining fatality happened in 1967 during an X-15 research flight. Stringent range safety controls have resulted in no civilian property damage losses or fatalities being reported as a result of any X-Plane Program accident. Given the overwhelming number of test flights, the small number of accidents which resulted in loss of aircraft or life can be considered a remarkable program achievement.

Summary of the X- Plane Program

Model Manufacturer No. of Vehicles Built Years of Operation No. of Flights Primary Testing Facility Research Goals Program Achievements
X- 1 Bell Aircraft 3 1946- 51 157 Edwards AFB Investigate flight characteristics at greater than sonic velocities. Structural, physiological phenomena within transonic speed envelope First Mach 1+ flight; Maximum altitude of 71,902 ft
X- 1A Bell Aircraft 1 1953- 55 25 Edwards AFB Continue X- 1 goals at higher speeds and altitudes Obtained speed of Mach 2.44; Maximum altitude of 90,440 ft
X- 1B Bell Aircraft 1 1954- 58 27 Edwards AFB Exploratory aerodynamic heating tests; experimental reaction control system First reaction controlled flight
X- 1D Bell Aircraft 1 1951 1 Edwards AFB Continue X- 1 goals at higher speeds and altitudes No major milestones, Explosion during captive flight; vehicle jettisoned
X- 1E Bell Aircraft, Stanley Aircraft (wings) 1 1955- 58 26 Edwards AFB High- speed wing performance Mach 2.24, altitude 73,458 ft; first flight with ventral fins
X- 2 Bell Aircraft 2 1952- 56 20 Edwards AFB Swept- wing performance; higher speeds and altitude than X- 1 New altitude record of 126,200 ft; new speed record of Mach 2.87
Gasket explosion destroyed first X- 2; second aircraft lost to inertial coupling
X- 3 Douglas Aircraft 1 1954- 56 20 Edwards AFB High speed aerodynamic phenomenon; titanium construction; take off, land under its own power Led to understanding of inertia coupling
X- 4 Northrop Aircraft 2 1950- 53 82 Edwards AFB Test tailless, semi- tailless configuration at transonic speeds Showed tailless craft not suited for transonic flight
X- 5 Bell Aircraft 2 1952- 55 133 Edwards AFB Investigate aerodynamics of variable- seep- wing design Successful sweep- wing operation
X- 6 Convair Division, General Dynamics 1 shield- test aircraft (modified B- 36H) 1955- 57 47 Convair Testing Facility Test feasibility of nuclear propulsion Program terminated before prototypes constructed
X- 7A,
X- 7A- 3, X- 7B,
X- Q5
(unmanned)
Lockheed Missiles 61 1951- 60 130 New Mexico Test viability of ramjet engines for anti- aircraft missiles; modified to testing of powerplants Obtained Mach 4.31, first air- breathing full- scale research aircraft designed as Mach 3 testbed
X- 8A,
X- 8B,
X- 8C,
X- 8D
Aerobees
(unmanned)
Aerojet Engineering 108 (X- 8 designation)
800+ Aerobees
1947- 56 Unknown White Sands, Holloman AFB Upper air research, parachute recovery system Peak altitude of 121 miles
X- 9 (unmanned) Bell Aircraft 31 1949- 53 28 Holloman AFB Test air- to- surface missiles; guidance systems, etc. First chemical warhead test vehicle to test supersonic clusterable dispersion
9 unsuccessful flights
X- 10 (unmanned) North American Aviation 13 1955- 59 15 Edwards AFB Testbed for cruise missile components Established technology base for remote control; first Mach 2- capable target drone
3 unsuccessful flights Communications disruption; miswiring; autopilot malfunction
X- 11 (unmanned) Convair Astronautics Division 8 1956- 58 8 Cape Canaveral Provide flight data for Atlas missile First ICBM prototypes
X- 12 (unmanned) Convair Astronautics Division 5 1958 5 Cape Canaveral Test propulsion- staging guidance system, nose reentry configuration First intercontinental range mission of 6,325 miles
X- 13 Ryan Aeronautical Company 2 1955- 57 Unknown Edwards AFB Test pure- jet vertical takeoff and landing First successful VTOL flight on jet thrust alone
X- 14,
X- 14A,
X- 14B
Bell Aircraft 1 1957- 81 Unknown Moffet Field Test VTOL technology First VTOL aircraft using jet thrust diverter system for vertical lift
X- 15,
X- 15A- 2
North American Aviation 3 1959- 68 199 X- 15 High Range (Wendover, UT, to Edwards AFB) Explore problems of space and atmospheric flight at very high speeds and altitudes First manned hyper- sonic flight vehicle; altitude of 354,200 ft obtained; Mach 6.7 reached 4 Mid- flight explosions (2); loss of control (1); collapsed landing gear (1)
X- 16 Bell Aircraft Canceled None None None None High- altitude, long- range reconnaissance aircraft
X- 17 (unmanned) Lockheed Missiles 26 1955- 57 26 Holloman AFB Explore reentry characteristics High Mach effects on reentry vehicles
X- 18 Hiller Aircraft 1 1959- 61 20 Edwards AFB Explore large VTOL vehicles First tilt- wing usage for VTOL
X- 19 Curtiss- Wright 2 1964- 65 50 Caldwell; NAFEC, NJ Test VTOL technology using radial lift Dual- tandem tilt propeller use One Equipment failure
X- 20 Boeing Canceled None None None Piloted orbital flight Provided heat materials testing
X- 21A Northrop Corporation 2 1963- 64 Unknown Edwards AFB Test full- scale boundary control on large aircraft Proved Laminar Flow Control viable
X- 22A Bell Aerospace 2 1966- 84 501 Bell, Calspan Test Facilities Research dual- tandem- ducted propeller configuration; research V/ STOL handling using variable stability system design Ducted fan viability, advancement of VTOL technology; One hydraulic system failure
X- 23A (unmanned) Martin Marietta 4 1966- 67 3 Vandenberg AFB/ Pacific Ocean Test configurations, control systems, and ablative materials for hypersonic reentry vehicles First maneuverable reentry vehicle
X- 24A,
X- 24B
Martin Marietta 1 1969- 75 64 Edwards AFB Research of aerodynamics, flight characteristics of manned vehicle with FDL- 7 configuration Verified theoretical advantages of lifting body configuration for hypersonic trans- atmospheric aircraft
X- 25,
X- 25A,
X- 25B
Bensen Aircraft 3 1968 None Raleigh, NC Test discretionary descent vehicle designs Insight on pilot training
X- 26A,
X- 26B
Schweizer Aircraft, Lockheed Missiles 6 1967- 88 Unknown Vietnam Develop ultra-quiet surveillance aircraft Use as training vehicle; contributions to stealth designs
X- 27 Lockheed- California Canceled None None None Advanced, lightweight fighter
X- 28A George Pereira, Osprey Aircraft 1 1971 Unknown Philadelphia Naval Base, PA Explore usefulness of small, single- place seaplane for civil police patrol in Southeast Asia Unique contribution as home- built aircraft in X- Plane program
X- 29A Grumman Aerospace 2 1984- 90 Unknown Edwards AFB Test forward- swept wing design, advanced composites, other aerodynamic advances First FSW aircraft to fly supersonically in level flight
X- 30 None selected None None None None Serve as testbed for sustained hypersonic speeds within atmosphere or as space vehicles for orbital payload delivery
X- 31A Rockwell International, Deutsche Aerospace 2 1990- 95 523 Edwards AFB Break "stall- barrier," examine angles of attack 180 degree turn post- stall maneuver Failure of the pitot – static system: erroneous total pressure data
X-32
X-33 Lockheed- Martin Skunk Works 1 1999-2000 15 Edwards AFB Develop reusable single- stage- to- orbit transportation vehicle
X-34
X-35
X-36
X-37
X-38
X-39
X-40
X-41
X-42
X-43
X-44
Key to Acronyms:
AFB = Air Force Base
FDL- 7 = Flight Dynamics Laboratory- 7 (a prototype test craft of the Air Force's Flight Dynamics Laboratory, a predecessor to the X- 24B).
FSW = forward swept wing
ICBM = intercontinental ballistic missile
V/ STOL = vertical/ short takeoff and landing
VTOL = vertical takeoff and landing

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Updated Sunday, March 14, 1999 5:09:16 AM