The X-15 in flight. (Courtesy photo)

The rocket-powered X-15 was part of a fleet of X-plane experimental aircraft operated jointly by NASA and the U.S. Air Force. In the early 1960s, the X-15 set a number of speed and altitude records, reaching the edge of space (an altitude of more than 62 miles or 100 kilometers) on two separate occasions in 1963.

Currently, the X-15 still holds the official world record for the fastest speed ever reached by a manned aircraft: Mach 6.72, which is 6.72 times the speed of sound, or 4,520 mph (7,274 km/h).

The X-15 was retired in 1970, but the program featured many notable NASA and Air Force test pilots, including Neil Armstrong, the man who would go on to become the first person to step foot on the moon.

Interestingly, during the illustrious X-15 program, 13 flights by eight different pilots exceeded an altitude of 50 miles (80 km), meeting the U.S. Air Force’s standard for spaceflight. As a result, the Air Force test pilots were awarded Air Force astronaut wings, and the civilian pilots were granted NASA astronaut wings

SR-71 Blackbird

The SR-71 Blackbird was an advanced Cold War-era reconnaissance aircraft developed by Lockheed in the 1960s. The program was known as a “black project,” which meant it was highly classified. The twin-engine, two-seater aircraft was capable of outracing potential threats during reconnaissance missions, including being able to accelerate and out-fly surface-to-air missiles if it was detected.

SR-71 "Blackbird" testing
The SR-71 from Lockheed underwent test and development at Edwards Air Force Base, Calif., in the mid-1960s, and went on to surpass YF-12 records. For more than 25 years, the aircraft provided the nation with demonstrated strategic reconnaissance capability. (U.S. Air Force photo)

The SR-71 Blackbird could accelerate to Mach 3.3 (more than 2,200 mph, or 3,540 km/h) at an altitude of 80,000 feet (24,400 m).

The SR-71 made its first flight in December 1964, and was flown by the U.S. Air Force from 1964 to 1998. The Blackbird’s performance and achievements cemented the plane as one of the greatest triumphs in aviation technology during the Cold War.

Lockheed YF-12

The Lockheed YF-12 was a prototype aircraft developed by the Lockheed Corporation in the late 1950s and early 1960s. The massive two-person plane was built to intercept enemy aircraft at Mach 3 speeds.

YF-12 flies test mission
The Lockheed YF-12 flies test mission near Edwards Air Force Base, Calif. (U.S. Air Force photo

Testing of the YF-12 was completed over Area 51, the U.S. Air Force’s top-secret test and training range, located in a remote part of southern Nevada. Many of the YF-12 flights were used to hide the identity of the Lockheed A-12 reconnaissance aircraft, which was being tested at the same time for the CIA.

The YF-12 made its first flight in 1963, and had a reported top speed of Mach 3.2 (2,070 mph, or 3,330 km/h) at an altitude of 80,000 feet (24,400 m). The U.S. Air Force eventually cancelled the program, but the YF-12 made a number of research flights for the Air Force and NASA until 1978.

MiG-25 Foxbat

The Mikoyan-Gurevich MiG-25 Foxbat was designed to intercept enemy aircraft at supersonic speeds and to collect reconnaissance data. The plane is one of the fastest military aircraft to have entered operational service. The MiG-25 made its first flight in 1964, and was first used by the Soviet Air Defense Forces in 1970.

An air-to-air right underside rear view of a Soviet MiG-25 Foxbat aircraft carrying four AA-6 Acrid missiles.

The plane has an incredible top speed of Mach 3.2 (2,190 mph, or 3,524 km/h). The MiG-25 Foxbat is still in limited service in the Russian Air Force, but is also used by several other nations, including the Algerian Air Force and Syrian Air Force.

Bell X-2 “Starbuster”

The Bell X-2 was a rocket-powered research plane jointly developed by Bell Aircraft Corporation, the U.S. Air Force and the National Advisory Committee for Aeronautics (the precursor to NASA) in 1945. The aircraft was built to investigate aerodynamic issues with supersonic flight within the Mach 2 to Mach 3 range.

The X-2, nicknamed “Starbuster,” completed its first powered flight in November 1955. The following year, in September 1956, Captain Milburn Apt was at the controls when the X-2 reached Mach 3.2 (2,094 mph, or 3,370 km/h), at an altitude of 65,000 feet (19,800 m).

Shortly after attaining this top speed, however, Apt tried to turn the aircraft while it was still above Mach 3. The plane tumbled out of control, and Apt’s attempts to recover from the spin failed. This tragic accident ended the X-2 program, after a total of 20 test flights.

X-2 in the mid-1950s
An X-2 rocket plane in flight during the mid-1950s. (Courtesy NASA photo)

XB-70 Valkyrie

The mammoth six-engine XB-70 Valkyrie was designed by North American Aviation in the late 1950s. The aircraft was built as a prototype for a proposed nuclear-armed strategic bomber. The XB-70 Valkyrie achieved its design speed on Oct. 14, 1965, when it accelerated to Mach 3.02 (2,000 mph, or 3,219 km/h), at an altitude of 70,000 feet (21,300 m) over Edwards Air Force Base in California.

XB-70 Valkyrie accelerates to Mach 3.02
The mammoth XB-70 Valkyrie achieved its design speed Oct. 14, 1965 when it accelerated to Mach 3.02, or 2,000 mph at 70,000 feet altitude over Edwards Air Force Base, Calif. The aircraft weighed 500,000 pounds. (Courtesy photo)

Two XB-70s were built and used in supersonic test flights from 1964 to 1969. Whereas one of the prototypes was lost in 1966 after a midair collision, the other XB-70 is on display for the public to view at the National Museum of the United States Air Force in Dayton, Ohio.