North American Aviation YF-86D Sabre 50-577. (U.S. Air Force)
22 December 1949: North American Aviation, Inc., test pilot George S. Welch made the first flight of the YF-86D Sabre, 50-577 (c/n 164-1, at Edwards Air Force Base, in the high desert of southern California.
Based on the F-86A day fighter, the F-86D (originally designated YF-95) was a radar-equipped, rocket-armed, all-weather interceptor. Its first flight took place only nine years after the first flight of North American’s prototype NA-73X, which would become the famous P-51 Mustang fighter of World War II. This was an amazing jump in technology in just a few years.
The interceptor was intended to be an improved variant of the F-86A Sabre day fighter. During development, though, so many changes became necessary that the F-86D shared only about 25% of its parts of the F-86A. Essentially an new airplane, the Air Force assigned it the designation YF-95. It would revert to the F-86D designation before it actually flew.
North American Aviation YF-86D Sabre 50-577, the first of two service test aircraft, at the North American Aviation flight line, Los Angeles International Airport. (North American Aviation, Inc.)
The first YF-86D (still identified as YF-95) was rolled out at North American’s Inglewood plant in September 1949. In late November it was partially disassembled to be transported by truck to Edwards Air Force Base, about 120 miles (193 kilometers) away. The airplane was then reassembled and ground tested to prepare it for flight.
North American Aviation, Inc., F-86D-1-NA Sabre 50-456, the second production aircraft. (Ray Wagner Collection, San Diego Air & Space Museum Archives)North American Aviation, Inc., F-86D-1-NA Sabre 50-456, s/n 165-2, the second production aircraft (Ray Wagner Collection, San Diego Air & Space Museum Archives)North American Aviation, Inc., F-86D-1-NA Sabre 50-458, s/n 165-4. (Ray Wagner Collection, San Diego Air & Space Museum Archives)
The first two test aircraft carried no armament or fire control/radar system and retained the sliding canopy of the F-86A. This would be replaced with a hinged “clamshell” canopy in production models. The airplane was 40 feet, 3.1 inches (12.271 meters) long with a wingspan of 37 feet, 1 inch (11.294 meters) and overall height of 15 feet, 0 inches (4.572 meters). Its empty weight was 12,470 pounds (5,656 kilograms) and maximum takeoff weight was 18,483 pounds (8,384 kilograms).
The service test aircraft and early production airplanes were powered by a General Electric J47-GE-17 single-shaft axial-flow turbojet engine, producing 5,425 pounds of thrust (24.132 kilonewtons) at 7,950 r.p.m., or 7,500 pounds (33.362 kilonewtons) with afterburner. This engine was equipped with an electronic fuel control system which substantially reduced the pilot’s workload. The engine had a 12-stage compressor, 8 combustion chambers, and single-stage turbine. It was 226.0 inches (5.740 meters) long, 39.75 inches (1.010 meters) in diameters, and weighed 3,000 pounds (1,361 kilograms).
The first production aircraft, F-86D-1-NA Sabre 50-455 (manufacturer’s serial number 165-1) had a maximum speed of 614 knots (707 miles per hour/1,137 kilometers per hour) at Sea Level, and 539 knots (620 miles per hour/998 kilometers per hour)at 40,000 feet (12,192 meters). From a standing start, the interceptor could climb to 40,000 feet in 5 minutes, 54 seconds with a full combat load. The service ceiling was 54,000 feet (16,460 meters).
North American Aviation, Inc., F-86D-15-NA Sabre 50-574 (c/n 165-120), firing 2.75-inch FFAR rockets, circa 1950. (NASM)A production North American Aviation F-86D-60-NA Sabre, 53-4061, firing a salvo of 2.75-inch FFAR rockets. (U.S. Air Force)
The F-86D Sabre carried no guns. Instead, its armament consisted of twenty-four 2.75-inch (70 millimeter) Mk 4 Folding Fin Aerial Rockets (FFAR) with explosive warheads, carried in a retractable tray in the airplane’s belly. A Hughes electronic fire control computer was used to calculate an interception path and determine the firing point for the unguided rockets.
The aircraft was so complex that the pilot training course was the longest of any aircraft in the U.S. Air Force inventory, including the Boeing B-47 Stratojet.
The single-seat F-86D Sabre was nearly 50 knots faster than the contemporary twin-engine Northrop F-89 Scorpion and Lockheed F-94 Starfire, both of which carried a two-man crew. North American Aviation built 2,504 F-86D Sabres, and these equipped nearly two-thirds of the Air Defense Command interceptor squadrons.
North American Aviation YF-86D Sabre 50-577, NACA 149, at the NACA Ames Research Center, Moffett Field, California. (NASA)
After the Air Force service test program was completed, 50-577 was transferred to the National Advisory Committee on Aeronautics (NACA) Ames Aeronautical Laboratory at Moffett Field, California, and designated NACA 149. It was used as a variable stability aircraft for flight testing various control configurations for feel, sensitivity and response.
NACA 149 remained at Ames from 26 June 1952 to 15 February 1960.
North American Aviation, Inc., X-15A-1, 56-6670, at Los Angeles Division, October 1958. (Air Force Flight Test Center History Office)
20 December 1968: After 199 flights, the National Aeronautics and Space Administration cancelled the X-15 Hypersonic Research Program. A 200th X-15 flight had been scheduled, but after several delays, the decision was made to end the program. (The last actual flight attempt was 12 December 1968, but snow at several of the dry lakes used as emergency landing areas resulted in the flight being cancelled.)
The X-15A rocketplane was designed and built for the U.S. Air Force and the National Advisory Committee for Aeronautics (NACA, the predecessor of NASA) by North American Aviation, Inc., to investigate the effects of hypersonic flight (Mach 5+). Design work started in 1955 and a mock-up had been completed after just 12 months. The three X-15s were built at North American’s Los Angeles Division, at the southeast corner of Los Angeles International Airport (LAX), on the shoreline of southern California.
The first flight took place 8 June 1959 with former NACA test pilot Albert Scott Crossfield in the cockpit of the Number 1 ship, 56-6670.
Scott Crossfield prepares for a flight in the North American Aviation X-15A.
While earlier rocketplanes, the Bell X-1 series, the the Douglas D-558-II, and the Bell X-2, were airplanes powered by rocket engines, the X-15 was a quantum leap in technology. It was a spacecraft.
Like the other rocketplanes, the X-15 was designed to be carried aloft by a “mothership,” rather than to takeoff and climb to the test altitude under its own power. The carrier aircraft was originally to be a Convair B-36 intercontinental bomber but this was soon changed to a Boeing B-52 Stratofortress. Two B-52s were modified to carry the X-15: NB-52A 52-003, The High and Mighty One, and NB-52B 52-008, Balls 8.
From 8 June 1959 to 24 October 1968, the three X-15s were flown by twelve test pilots, three of whom would qualify as astronauts in the X-15. Two would go on to the Apollo Program, and one, Neil Alden Armstrong, would be the first human to set foot on the surface of the Moon, 20 July 1969. Joe Engle would fly the space shuttle. Four of the test pilots, Petersen, White, Rushworth, and Knight, flew in combat during the Vietnam War, with Bob White being awarded the Air Force Cross. Petersen, Rushworth and White reached flag rank.
One pilot, John B. (“Jack”) McKay, was seriously injured during an emergency landing at Mud Lake, Nevada, 9 November 1962. Another, Michael James Adams, was killed when the Number 3 ship, 56-6672, went into a hypersonic spin and broke up on the program’s 191st flight, 15 November 1967.
North American Aviation, Inc., X-15A-1 56-6670 on Rogers Dry Lake, Edwards Air Force Base, California. (NASA Image E-5251)
Flown by a single pilot/astronaut, the X-15 is a mid-wing monoplane with dorsal and ventral fin/rudders and stabilators. The wing had no dihdral, while the stabilators had a pronounced -15° anhedral. The short wings have an area of 200 square feet (18.58 square meters) and a maximum thickness of just 5%. The leading edges are swept to 25.64°. There are two small flaps but no ailerons. The entire vertical fin/rudder pivots for yaw control.
Above 100,000 feet (30,840 meters) altitude, conventional aircraft flight control surfaces are ineffective. The X-15 is equipped with a system of reaction control jets for pitch, roll and yaw control. Hydrogen peroxide was passed through a catalyst to produce steam, which supplied the control thrusters.
The forward landing gear consists of a retractable oleo strut with steerable dual wheels and there are two strut/skids at the rear of the fuselage. The gear is retracted after the X-15 is mounted on the NB-52 and is extended for landing by its own weight.
North American Aviation, Inc., X-15A-3 56-6672 just before touch down on Rogers Dry Lake. (NASA Image E-7469)
The rocketplane’s cockpit featured both a conventional control stick as well as side-controllers. It was pressurized with nitrogen gas to prevent fires. The pilot wore an MC-2 full-pressure suit manufactured by the David Clark Company of Worcester, Massachusetts, with an MA-3 helmet. The suit was pressurized below the neck seal with nitrogen, while the helmet was supplied with 100% oxygen. This pressure suit was later changed to the Air Force-standardized A/P22S.
X-15 cockpit with original Lear Siegler instrument panel. (NASA image E63-9834)
The X-15 is 50.75 feet (15.469 meters) long with a wing span of 22.36 feet (6.815 meters). The height—the distance between the tips of the dorsal and ventral fins—is 13.5 feet (4.115 meters). The stabilator span is 18.08 feet (5.511 meters). The fuselage is 4.67 feet (1.423 meters) deep and has a maximum width of 7.33 feet (2.234 meters).
Since the X-15 was built of steel rather than light-weight aluminum, as are most aircraft, it is a heavy machine, weighing approximately 14,600 pounds (6,623 kilograms) empty and 34,000 pounds (15,422 kilograms) when loaded with a pilot and propellants. The X-15s carried as much as 1,300 pounds (590 kilograms) of research instrumentation, and the equipment varied from flight to flight. The minimum flight weight (for high-speed missions): 31,292 pounds (14,194 kilograms) The maximum weight was 52,117 pounds (23,640 kilograms) at drop (modified X-15A-2 with external propellant tanks).
Initial flights were flown with a 5 foot, 11 inch (1.803 meters)-long air data boom at the nose, but this would later be replaced by the “ball nose” air sensor system. The data boom contained a standard pitot-static system along with angle-of-attack and sideslip vanes. The boom and ball nose were interchangeable.
NASA Research Test Pilot Neil A. Armstrong with the first North American Aviation X-15A, 56-6670, on Rogers Dry Lake after a flight, 1960. His right hand is resting on the rocketplane’s ball nose sensor. (NASA Image E60-6286)
The X-15s were built primarily of a nickel/chromium/iron alloy named Inconel X, along with corrosion-resistant steel, titanium and aluminum. Inconel X is both very hard and also able to maintain its strength at the very high temperatures the X-15s were subjected to by aerodynamic heating. It was extremely difficult to machine and special fabrication techniques had to be developed.
Delays in the production of the planned Reaction Motors XLR99 rocket engine forced engineers to adapt two vertically-stacked Reaction Motors XLR11-RM-13 four-chamber rocket engines to the X-15 for early flights. This was a well-known engine which was used on the previous rocketplanes. The XLR11 burned a mixture of ethyl alcohol and water with liquid oxygen. Each of the engines’ chambers could be ignited individually. Each engine was rated at 11,800 pounds of thrust (58.49 kilonewtons) at Sea Level.
Two Reaction Motors Division XLR11-RM-13 four-chamber rocket engines installed on an X-15. The speed brakes of the ventral fin are shown in the open position. (NASA)
The Reaction Motors XLR99-RM-1 rocket engine was throttleable by the pilot from 28,500 to 60,000 pounds of thrust (126.77–266.89 kilonewtons). The engine was rated at 50,000 pounds of thrust (222.41 kilonewtons) at Sea Level; 57,000 pounds (253.55 kilonewtons) at 45,000 feet (13,716 meters), the typical drop altitude; and 57,850 pounds (257.33 kilonewtons) of thrust at 100,000 feet (30,480 meters). Individual engines varied slightly. A few produced as much as 61,000 pounds of thrust (271.34 kilonewtons).
The XLR99 burned anhydrous ammonia and liquid oxygen. The flame temperature was approximately 5,000 °F. (2,760 °C.) The engine was cooled with circulating liquid oxygen. To protect the exhaust nozzle, it was flame-sprayed with ceramic coating of zirconium dioxide. The engine is 6 feet, 10 inches (2.083 meters) long and 3 feet, 3.3 inches (0.998 meters) in diameter. It weighs 910 pounds (413 kilograms). The Time Between Overhauls (TBO) is 1 hour of operation, or 100 starts.
The XLR99 proved to be very reliable. 169 X-15 flights were made using the XLR99. 165 of these had successful engine operation. It started on the first attempt 159 times.
The highest speed achieved during the program was with the modified number two ship, X-15A-2 56-6671, flown by Pete Knight to Mach 6.70 (6,620 feet per second/4,520 miles per hour/7,264 kilometers per hour) at 102,700 feet (31,303 meters). On this flight, the rocketplane exceeded its maximum design speed of 6,600 feet per second (2,012 meters per second).
The maximum altitude was reached by Joe Walker, 22 August 1963, when he flew 56-6672 to 354,200 feet (107,960 meters).
The longest flight was flown by Neil Armstrong, 20 April 1962, with a duration of 12 minutes, 28.7 seconds.
North American Aviation X-15A-1 56-6670 is on display at the Smithsonian Institution National Air and Space Museum. X-15A-2 56-6671 is at the National Museum of the United States Air Force.
A North American Aviation F-100 Super Sabre chase plane follows NB-52A 52-003 prior to launch of an X-15. (NASA)
Recommended reading:
Always Another Dawn: The Story of a Rocket Test Pilot, by A. Scott Crossfield and Clay Blair, Jr., The World Publishing Company, Cleveland and New York, 1960
At The Edge Of Space, by Milton O. Thompson, Smithsonian Institution Press, 1992
X-15 Diary: The Story of America’s First Spaceship, by Richard Tregaskis, E.F. Dutton & Company, New York, 1961; University of Nebraska Press, 2004
X-15: Exploring the Frontiers of Flight, by David R. Jenkins, National Aeronautics and Space Administration http://www.nasa.gov/pdf/470842main_X_15_Frontier_of_Flight.pdf
The X-15 Rocket Plane: Flying the First Wings into Space, by Michelle Evans, University of Nebraska Press, Lincoln and London, 2013
North American Aviation, Inc., X-15A-2 56-6671 accelerates after igniting its Reaction Motors XLR99-RM-1 rocket engine (NASA)
Apollo 17 (AS-512) on the pad at Launch Complex 39A, 21 November 1972. (NASA)
7 December 1972: At 05:33:00.63 UTC (12:33 a.m., Eastern Standard Time), Apollo 17, the last manned mission to The Moon in the 20th century, lifted off from Launch Complex 39A at the Kennedy Space Center, Cape Canaveral, Florida. The destination was the Taurus-Littrow Valley.
The Mission Commander, on his third space flight, was Eugene A. Cernan. The Command Module Pilot was Ronald A. Evans, on his first space flight, and the Lunar Module Pilot was Harrison H. Schmitt, also on his first space flight.
Gene Cernan, seated, with Harrison Schmitt and Ronald Evans. (NASA)
Schmitt was placed in the crew because he was a professional geologist. He replaced Joe Engle, an experienced test pilot who had made sixteen flights in the X-15 hypersonic research rocketplane. Three of those flights were higher than the 50-mile altitude, qualifying Engle for U.S. Air Force astronaut wings.
The launch of Apollo 17 was delayed for 2 hours, 40 minutes, due to a minor mechanical malfunction. When it did liftoff, the launch was witnessed by more than 500,000 people.
Apollo 17/Saturn V (AS-512) at Pad 39A during countdown. (NASA 72C-5901)
The Saturn V rocket was a three-stage, liquid-fueled heavy launch vehicle. Fully assembled with the Apollo Command and Service Module, it stood 363 feet (110.642 meters) tall. The first and second stages were 33 feet (10.058 meters) in diameter. Fully loaded and fueled the rocket weighed 6,200,000 pounds (2,948,350 kilograms). It could lift a payload of 260,000 pounds (117,934 kilograms) to Low Earth Orbit.
The first stage was designated S-IC. It was designed to lift the entire rocket to an altitude of 220,000 feet (67,056 meters) and accelerate to a speed of more than 5,100 miles per hour (8,280 kilometers per hour). The S-IC stage was built by Boeing at the Michoud Assembly Facility, New Orleans, Louisiana. It was 138 feet (42.062 meters) tall and had an empty weight of 290,000 pounds (131,542 kilograms). Fully fueled with 203,400 gallons (770,000 liters) of RP-1 and 318,065 gallons (1,204,000 liters) of liquid oxygen, the stage weighed 5,100,000 pounds (2,131,322 kilograms). It was propelled by five Rocketdyne F-1 engines, producing 1,522,000 pounds of thrust (6770.19 kilonewtons), each, for a total of 7,610,000 pounds of thrust at Sea Level (33,850.97 kilonewtons). These engines were ignited seven seconds prior to lift off and the outer four burned for 168 seconds. The center engine was shut down after 142 seconds to reduce the rate of acceleration. The F-1 engines were built by the Rocketdyne Division of North American Aviation at Canoga Park, California.
The S-II second stage was built by North American Aviation at Seal Beach, California. It was 81 feet, 7 inches (24.87 meters) tall and had the same diameter as the first stage. The second stage weighed 80,000 pounds (36,000 kilograms) empty and 1,060,000 pounds loaded. The propellant for the S-II was liquid hydrogen and liquid oxygen. The stage was powered by five Rocketdyne J-2 engines, also built at Canoga Park. Each engine produced 232,250 pounds of thrust (1,022.01 kilonewtons), and combined, 1,161,250 pounds of thrust (717.28 kilonewtons).
The Saturn V third stage was designated S-IVB. It was built by Douglas Aircraft Company at Huntington Beach, California. The S-IVB was 58 feet, 7 inches (17.86 meters) tall with a diameter of 21 feet, 8 inches (6.604 meters). It had a dry weight of 23,000 pounds (10,000 kilograms) and fully fueled weighed 262,000 pounds. The third stage had one J-2 engine and also used liquid hydrogen and liquid oxygen for propellant. The S-IVB would place the Command and Service Module into Low Earth Orbit, then, when all was ready, the J-2 would be restarted for the Trans Lunar Injection.
Eighteen Saturn V rockets were built. They were the most powerful machines ever built by man.
Apollo 17 launched 3 years, 4 months, 20 days, 16 hours, 1 minute, 0 seconds after Apollo 11, the first manned flight to The Moon.
Apollo 17 (AS-512) lifts off from Launch Complex 39A at 05:33:00 UTC, 7 December 1972. (NASA)Apollo 17 (NASA S72-55070)
5 December 1963: On Flight 97 of the X-15 Program, Major Robert A. Rushworth flew the number one aircraft, Air Force serial number 56-6670, to an altitude of 101,000 feet 30,785 meters) and reached Mach 6.06 (4,018 miles per hour/6,466 kilometers per hour).
The rocketplane was dropped from the Boeing NB-52B Stratofortress “mother ship” 52-008, Balls 8, flying at 450 knots (833.4 kilometers per hour) at 45,000 feet (13,716 meters) over Delamar Dry Lake, Nevada. Rushworth ignited the Reaction Motors XLR-99-RM-1 rocket engine, which burned for 81.2 seconds before shutting down.
The flight plan had called for an altitude of 104,000 feet (31,699 meters), a 78 second burn and a maximum speed of Mach 5.70. With the difficulties of flying such a powerful rocketplane, Rushworth’s flight was actually fairly close to plan. During the flight the right inner windshield cracked.
Bob Rushworth landed the X-15 on Rogers Dry Lake at Edwards Air Force Base, California, after a flight of 9 minutes, 34.0 seconds.
Mach 6.06 was the highest Mach number reached for an unmodified X-15.
56-6670 flew 81 of the 199 flights of the X-15 Program. It is in the collection of the Smithsonian Institution National Air and Space Museum.
From 1960 to 1966, Bob Rushworth made 34 flights in the three X-15s, more than any other pilot.
North American Aviation Inc./U.S. Air Force/NASA X-15A 56-6670 hypersonic research rocketplane on display at the National Air and Space Museum. (Photo by Eric Long, National Air and Space Museum, Smithsonian Institution)
North American Aviation’s NA-73X fighter prototype, engine idling, with Vance Breese in the cockpit at Mines Field, Los Angeles, 26 October 1940. (North American Aviation Inc.)
20 November 1940: North American Aviation’s Chief Test Pilot, Paul Baird Balfour, made his first flight in the NA-73X, NX19998, prototype for a Royal Air Force fighter, the Mustang Mk.I.
Vance Breese was the free-lance test pilot who made the first seven flights in the new airplane. Breese claimed to have made a bet with North American executives that Balfour would crash the prototype on his first flight.
Paul B. Balfour (1908–1951). This is Balfour’s NAA employee file card. (North American Aviation Inc.)
This flight was scheduled to be a high speed test. Edgar Schmued, the designer, offered to show Balfour around the airplane. “Before this flight, I asked Balfour to get into the airplane and go through the routine of a takeoff and flight. He responded that one airplane is like another and he would not need the routine checkout.”
The ground crew started the NA-73X’s 1,150 horsepower Allison V-1710-39 liquid-cooled V-12 engine at 5:40 a.m. and let it warm up to normal operating temperature. When it was restarted just prior to Paul Balfour’s flight, “it was a little hard to start,” according to Olaf Anderson, the airplane’s mechanic.
The prototype Mustang, NA-73X, lies upside down in a plowed field, 20 November 1940. (North American Aviation Inc.)
Balfour took off from Mines Field at about 7:10 a.m. After about twelve minutes of flight, the Allison stopped running. Balfour was too far from Mines Field to make it back to the runway. He landed in a plowed field west of Lincoln Boulevard. When the tires hit the soft surface, the prototype flipped over. Balfour was not hurt and was able to crawl out of the upside-down wreck.
The Civil Aeronautics Board report described the damage as “engine housing broken, both wingtips damaged, tail surfaces damaged, top of fuselage damaged, and other miscellaneous damage.” The NA-73X had accumulated just 3 hours, 20 minutes of flight.
Vance Breese won his bet.
Paul Balfour was not injured in the crash landing, but the NA-73X prototype was significantly damaged. (North American Aviation Inc.)
According to the C.A.B. investigation, the engine had stopped due to fuel starvation when Balfour neglected to select another tank.
The prototype was taken back to the factory and rebuilt. It would become the famous Mustang, one of the most significant aircraft of World War II.
Damage to the wingtips, tail surfaces, fuselage. (North American Aviation Inc.)
Robert C. Chilton was hired as the new Chief Test Pilot. He would continue testing the Mustang developments throughout the war. Chilton made his first flight in NA-73X on 3 April 1941.
The Mustang prototype was hoisted out of the plowed field and taken back to the factory where it was rebuilt. (North American Aviation Inc.)
Paul Balfour continued to work for North American Aviation, testing the NA-40 and NA-40B prototypes and the B-25 Mitchell medium bomber. He later served in the United States Air Force.
Paul Baird Balfour was born 5 July 1908 in Washington State. He was the son of Fred Patrick Balfour and Edna May Baird Balfour. Balfour attended two years of college.
Paul Balfour entered the U.S. Army Air Corps (prior to 1930). He was stationed at Rockwell Field, San Diego, California.
Balfour married Martha Lillette Cushman of Coronado, California, at Yuma, Arizona, 6 June 1930.
Balfour began working as a test pilot for North American Aviation, Inc., 1 March 1936.
On 2 July 1938, he married Lois Tresa Watchman at Kingman, Arizona. They would have two children.
Paul B. Balfour, center, with a North American Aviation B-25 Mitchell medium bomber. (Photograph courtesy of Neil Corbett, Test and Research Pilots, Flight Test Engineers)
On 9 November 1951, Major Paul B. Balfour, U.S. Air Force, attached to the 1002nd Inspector General Group at Norton Air Force Base, California, was flying a North American VB-25J, 44-30955, a transport conversion of a B-25J-30-NC Mitchell medium bomber.
Shortly after takeoff, at about 10:00 a.m., the airplane developed engine trouble. Unable to return to Norton, Balfour attempted a belly landing at a small private airfield. Witness saw that the airplane’s left engine was idling, and its propeller was feathered. As he approached, the airplane was blocked by a windbreak of eucalyptus trees bordering U.S. Route 66. Balfour banked away from the trees but the B-25 crashed in an orange grove along Bloomington Avenue in Rialto, approximately 7 miles (11 kilometers) west of Norton.
Balfour, still buckled in his seat, was thrown clear of the burning wreck and landed in the street. One man on board was killed and two others seriously injured. Balfour died in a hospital three hours later. He was 41 years old. Major Balfour was buried at the Inglewood Park Cemetery, Inglewood, California.
Burning wing of North American VB-25J 44-30955, near Rialto, California, 9 November 1951.
