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Why the Piloted Flight Speed Record Hasn’t Been Broken in 50 Years

It all goes back to the history of the X-15 A-2, an unmistakable rocket-powered product of its moment.
The X-15 in flight. Image: USAF/Wikimedia Commons.

This week marked the 50th anniversary of the fastest ever piloted aircraft flight, when Major William J. "Pete" Knight brought the rocket-powered X-15 A-2 to a speed of 4,520 miles per hour—over six times the speed of sound—in October 1967.

This is distinct from the air speed record, because this record necessitates a pilot. The X-43A jet has since reached speeds of 7,310 miles per hour, setting the air speed record in 2005, but the aircraft is unmanned. The famed SR-71 represents the fastest piloted jet flight since Knight's famous hypersonic flight half a century ago, though it never even reached 3,000 miles per hour.


We live in a time of militarized drone fleets, precision landings of orbital rockets, and soon, gigantic Mars-bound rockets (maybe). How has the flight speed record for a piloted aircraft held up for half a century?

The answer to this question is buried in the history of the X-15, an unmistakable product of its moment.

In 1967, the US military was still financing the military-industrial complex that emerged during World War II. While military spending in countries like the UK, France, and Germany plummeted to about one third of their peak war spending after 1946, the US maintained almost half of its military spending. In the 25 years after WWII, America funnelled about $328 billion (adjusted for inflation in the year 2000) annually to its military.

For agencies like NASA and the US Air Force, this spending was used to fund experimental vehicles and technologies.

The X-15 "research airplane" was one of over 30 "X-vehicles" produced by US military agencies like the Air Force, the Navy, and NASA between 1946 and 1971. These X-vehicles, mainly airplanes and missiles, were experimental technologies designed in hopes of achieving record altitudes and hypersonic speeds.

These extreme vehicles were tested in the remoteness of the Nevadan desert, with the aim of producing data that could be used to design spacecrafts—specifically, ones that could carry humans. In fact, Neil Armstrong trained for his Apollo 11 mission by flying the X-15.


In order to reach hypersonic speeds, the X-15 was designed with a rocket engine fueled by ammonia and liquid oxygen. This plane wasn't exactly fuel-efficient: In just 80 seconds, its engine could guzzle down 15,000 pounds of fuel.

Read more: Beware Indestructible 'Hypersonic Missiles'

After dozens of tests at increasingly ambitious speeds, the Air Force wanted to test the X-15's absolute speed limits. Out of the twelve people who flew the X-15, Major Pete Knight was a safe pick for the job. He already had three years of experience with the X-15 after being personally recruited from a training program at the Aerospace Research Pilot School.

(If Knight's name sounds familiar, it's probably because he's famously homophobic. He introduced the bill that made same-sex marriage illegal in California back in 2000 when he was a state senator.)

But despite Knight's experience as a pilot, the risks of the flight were clear. In June of 1967, Knight experienced engine failure while piloting the X-15 107,000 feet above Earth. He had to make an emergency landing in the middle of Nevada's Mud Lake.

Knight wasn't the only pilot who had to make emergency landings. In fact, they had been happening for years, and injuries were commonplace.

One of the most severe X-15 emergency landings was on November 9, 1962. Major Jack McKay experienced engine failure and was forced made a high-speed landing in Mud Lake. McKay jumped from the X-15 before its violent landing, but not without smashing his back and head into the lakebed. McKay eventually recovered and would fly the X-15 22 more times.


The damaged X-15 in Mud Lake, Nevada, on the day of Major McKay's crash in November 1962. Image: NASA

But when Knight set the piloted speed record, the flight went off without a hitch. On the October 3 flight, Knight accelerated to 4,520 miles per hour and was able to keep control of the aircraft.

Although the X-15 was a record-breaking aircraft, it wasn't safe to fly. Just weeks after Knight's flight, Major Michael Adams died during an experimental X-15 flight.

Adams was sent to test the X-15's altitude limits, and was able to climb 50 miles above Earth, into the Thermosphere. That's high enough for a satellite to enter orbit around the Earth, but the Air Force explicitly incentivized pilots to reach these dangerous heights. In fact, the Air Force awarded astronaut wings to pilots who reached fifty mile-altitudes.

But as Adams tried to descend, electrical disturbances damaged the X-15's guidance and reaction controls. Adams lost control of the plane, which spun at hypersonic speeds as it plunged to the ground below. Suffering from vertigo, Adams was unable to exit the plane and died when the X-15 crashed into the Mojave Desert. He was 37.

Schematic of the X-15 A-2 from a 1968 NASA memorandum about the plane's heat capacity.

Despite this tragedy, the X-15 was flown nine more times in the following year. Major Knight was scheduled to take it on its 200th flight in November 1968, but after a series of technical problems the flight was cancelled and the X-15 was officially retired. And while X-vehicle experimentation ran until 1971, no test was designed to break the speed record for piloted aircrafts set by the X-15.


So why haven't we broken the speed record for piloted aircrafts in 50 years?

It's possible that with the success of the Apollo program and the development of powerful nuclear weapons, the US simply lost interest in testing the boundaries of piloted flight speeds.

But after the loss of Adams's life and close calls with pilots such as McKay, NASA also started to more carefully evaluate when pilots are necessary.

This caution is evident in NASA's current approach to research aircrafts. A NASA fact sheet regarding its Prototype-Technology Evaluation and Research Aircraft (PTERA) explains that all testing is conducted in a laboratory:

"The ability to alter PTERA's configuration allows cost-effective testing of unconventional designs that might otherwise be too dangerous or expensive to test with a full-scale, crewed aircraft."

Pilots may be safe from dangerous hypersonic flight tests, but this risk may have shifted toward people on the ground. American, Chinese, and Russian air forces currently are all conducting hypersonic drone tests. In fact, these countries have already developed hypersonic missiles that can travel more than 3,000 miles per hour.

These weapons could become commonplace in future warfare, which is an unsettling prospect. Imagine cheaply made, unmanned aerial vehicles that bomb areas and leave, all before people on the ground even hear the detonation.

X-15 testing undoubtedly raised important ethical concerns about the relationship between humans and their hypersonic creations. Although its piloted flight speed record has stood for fifty years, these ethical concerns are far from obsolete.

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