Technically Speaking

The X-15: The Baddest Hot Rod in the Sky


On a hot day in October of 1967, a test pilot climbed out of a white NASA step van and walked toward the silver B-52 parked B-52 NASA hangarin front of the hangar at the NASA Flight Research Center at Edwards Air Force Base in California. He had made this walk many times before, but today was going to be different. The X-15 hanging under the right wing of the B-52 was waiting, surrounded by busy crew members doing their final pre-launch checks. Just a few hours later, they would all be tipping back beers at Club Muroc in celebration of a milestone achievement. Although this event occurred over 50 years ago, the record that was set that day still stands. October 3rd, 1967 was the day that NASA decided to pull out all of the stops and see how fast their X-15 could actually go.


THE X-15

The North American Aviation X-15 was a hypersonic rocket-powered aircraft operated by the U.S. Air Force and NASA. It was designed in the mid-1950s and flew it’s first flight in June of 1959. x-15 hypersonic jet diagramIt was designed to be carried aloft and drop launched from under the wing of the NASA B-52 mother ship. It’s shape resembled a rocket with stubby wings, thick dorsal and ventral wedge-fin stabilizers. A sizable portion of the fuselage’s outer skin was made from a heat-resistant nickel chromium alloy called Inconel X.

The landing gear was retractable, with a nose landing gear and two rear skids. The X-15’s single XLR99 engine was propelled by a combination of liquid oxygen and anhydrous (water-free) ammonia, and weighed a mere 910 lbs. It could burn 7 tons of fuel in just over a minute, generated half a million horsepower, and produced nearly 60,000 pounds of thrust. By comparison, a modern day fighter plane like the F-16 generates about 30,000 pounds of thrust in full afterburner.

The X-15 was arguably the most ambitious and successful flight test program in aviation history. So much rapid progress was made in the face of incredible risk with such undeveloped technology that no other program, with the exception of the Apollo missions, has even come close. It flew so high that it’s pilots earned astronaut wings: 280,500 feet or 53.1 miles above the earth. It flew nearly twice as fast as a bullet fired from a rifle. At the time, the X-15 was the most radical manned aircraft in history. Some might even say that to this day, it’s never been matched.


x-15 mockupThe X-15 with the tail number of 56-6671 is different than the other two X-15 aircraft at Edwards. It has no exterior markings.

The cockpit windscreens are covered, with small oval viewing windows for the pilot, making the plane look a bit like Snoopy from the Peanuts comic strip. It is covered with a milky white Martin MA-25S ablator to protect it from the higher aerodynamic heating loads.  An ablator is a coating that provides thermal protection, while being slowly consumed during the heat dissipation process. The original ablator applied to this X-15 was pink in color and gave the plane a rather odd appearance.

Fortunately, an application of a white sealer coating over the pink gave the aircraft a more proper look.  It also carried a pair of giant anhydrous ammonia tanks under its fuselage. These huge droppable propellant tanks allowed the  XLR-99 rocket engine to operate 60 seconds beyond the stock X-15’s 80-second burn time.  Among other modifications, the aircraft also carried a pylon-mounted dummy ramjet in the ventral region under the aft fuselage.

x-15 complete



Air Force test pilot William “Pete” Knight gets strapped into the X-15’s cramped cockpit. The plane is mounted on a pylon between the number 5 engine and william knight strapped into the X-15the fuselage under the right wing of “Balls ,” the massive NASA B-52 mothership. Today’s mission profile has only one objective. It is an attempt to set a maximum manned-flight speed record. The X-15 will be a piloted projectile blasting through a violent acceleration profile from 500 MPH to nearly 5,000 MPH in a mere 75 seconds.

After the fuel burn is complete, the X-15 will decelerate so violently that a rearward-facing crash pad is installed in the canopy, in front of the pilot.

Two sets of chase aircraft are needed, one pair at the start of the flight and one at the end. Nothing in the air can keep pace with the X-15 through it’s flight, not even a missile. The vast test range where the mission is being flown covers three states, Utah, Nevada and California. Knight will hurtle across these states in a blur only seconds long covering nearly a mile per second.



The primary risk of the X-15 concept, was that during the mission, it crossed from atmospheric flight to non-atmospheric near-space flight. Those two regimes are vastly different. One has air, the other doesn’t. In the atmosphere, aircraft steer themselves by moving air over actuated control surfaces. The rules change entirely in near-space. There is essentially no atmosphere or air moving over flight control surfaces. In that environment, the aircraft uses miniature rockets mounted in the nose and tail to control its yaw, pitch and roll. But in 1967, this was an imprecise science.

In the atmosphere, the X-15’s stubby wings and small control surfaces had very little effect. In near-space they had none. If there was an emergency, Knight would need to get the X-15 below Mach 4 and 120,000 feet to use his rudimentary ejection seat to escape. His chances of surviving an ejection in that environment were slim. Outside that environment, where the X-15 will set the speed record, the chances of survival are zero.


The launch of the B-52 goes perfectly. Knight stays busy by running through his checklist and verifying system status. Climbing to launch altitude of 45,000 feet takes a fair amount of time for the lumbering B-52.

B-52 turned loose
The X-15 continues to vent expanding gases from it’s rear vents during the climb, leaving a light contrail behind the aircraft.

x-15 venting

Once at the drop altitude, the final checklists and systems checks are completed, and the countdown starts. Knight lights the engine.

x-15 engine start

x-15 release
The X-15 was released by the B-52 at Mach 0.82.  It drops at a slight angle from under the wing of the B-52, and quickly rolls to level the wings. Once it gets stable, Knight takes the throttles to max power. There’s no turning back now.x-15 contrail
A bright white contrail pierces the sky in front of B-52. The supersonic chase planes struggle to try to keep pace with the X-15, but are quickly left behind. The white contrail curves even farther upward, getting smaller and smaller in the western sky. A concussive sonic boom occurs as the X-15 breaks the sound barrier.

x-15 brwaks sound barrier

At Mach 2 and 70,000 feet, the external tanks run dry and are jettisoned. There is no secondary sonic boom as Mach speed accumulates. The third stage stage of the rocket lights off and the aircraft continues to accelerate.


It quickly reaches Mach 5. Over five tons of anhydrous ammonia and liquid oxygen have already combusted in a controlled explosion just mere feet behind Knight’s cramped cockpit. Two tons of fuel still remain in the tank, but it’s draining fast. At Mach 5.5, the speed creates so much surface friction that the leading edges of the wings are glowing at over 1000 degrees. The aircraft is melting, and large chunks of Knight’s X-15 begin to burn and fall off. The aircraft is experiencing what is known as “Shock-Shock”, a condition where two shock waves intersect each other. When this occurs, it creates a a super heated jet of gas, essentially creating a blow torch.

Shock waves burn through the leading edge of the lower ventral fin, penetrating through the aircraft skin, igniting small fires in the engine housing. These fires are dangerously close to the explosive nitrogen tanks.

The X-15 reaches Mach 6, and Knight still has the throttle pushed all the way against the forward stop. At Mach 6.6, a large part of the aircraft’s ventral fin ignites and burns completely through. It flies off the aircraft, tracing a bright, burning arc to the desert floor. After what seems like an eternity, the aircraft tops out at Mach 6.7. He pulls the throttle back and the aircraft decelerates violently, but Knight retains control.


Powered flight is now terminated, and ballistic flight has been initiated. Knight is now at the controls of the world’s fastest glider. As the aircraft decelerates, Knight passes through Mach 5.5, and he sees an indication in the cockpit that a high temperature condition is occurring in the aircraft’s engine bay. He attempts to jettison the aircraft’s remaining fuel, but to no avail. The jettison tubes have become welded shut by the fire that is still burning in the engine bay. The extra fuel he has left on board means that he will have to land heavier and faster than usual.

The X-15 arcs over the Nevada-California border, traveling at nearly a mile a second, with leading edges of the wings and tail still glowing from the heat. Accumulated heat on the lower ventral inadvertently detonates the separation charges on the dummy scramjet engine that was being carried for test purposes. It explodes away from the X-15 as Knight decelerates through Mach 1 and 32,000 feet, sending more charred aircraft parts tumbling to earth. Knight continues to descend, with burning fragments continuing to drop off of the aircraft.


Over Barstow, California, the second set of recovery chase aircraft find Knight as he descends and decelerates to line up for landing. The X-15 has sustained serious damage. There are visible holes burned through the ventral tail. The chase pilots reporting the damage are concerned that due to the intense heat, the landing gear may not function and the nose tire may be melted.

x-15 landing skids

Knight extends his nose wheel, landing skids, speed brakes and sets flaps for landing. The chase pilot confirms that the landing gear appears to be intact. He skillfully touches down and a billowing plume of dust erupts behind his two rear skids as the X-15 slides to a stop on the 7.5-mile long lakebed. The flight lasts a total of 8 minutes and 16 seconds and covers over 213 miles. Knight’s rocket engine only burned for a fraction more than 2 minutes and 20 seconds of the flight. The fire crew on the ground race over to the aircraft to extinguish the flames.

Everyone on the ground is shocked at the sight of the missing parts and the holes burned in the fuselage. Leading edges of the wings and tail are charred. Every panel gap showed evidence of heat damage. The X-15 was literally minutes away from a catastrophic structure failure during the final stages of it’s flight.

x-15 crew inspection
knight x-15 on ground


A little over a month later, on November 15, 1967, Knight’s friend and colleague, U.S. Air Force test pilot Major Michael J. Adams died when his X-15 entered a violent spin at Mach 5 and disintegrated under crushing G-loads. At peak altitude, Adam’s X-15 began a yaw to the right, and it re-entered the atmosphere yawed crosswise to the flight path. It went into a spin and eventually broke up at 65,000 feet. The wreckage was strewn over 60 miles and two states. He is remembered as the only fatality in the nine years of the X-15 flight test program, which was terminated in October of 1968.
The X-15 program, with it’s speed and altitude records, would pave the way to the space shuttle program. It’s technology dividend is truly immeasurable, touching everything from the GPS constellation to cell phone communications and even the idea of civilian space travel.
William J. “Pete” Knight’s speed record remained unbroken by any winged craft until the space shuttle Columbia’s reentry from space on April 14, 1981. He had become the fastest pilot to fly inside the atmosphere in a powered, controlled flight, a record that officially remains today.

AND NOW FOR MY NEXT ACT…                                                                                                                   

 Pete Knight was awarded the Harmon International Aviator’s Trophy in 1969 for his record setting flight by then-President Lyndon Johnson. Following his test flight program completion in 1968, Knight transferred to an active combat unit and flew 253 combat missions over Vietnam in the supersonic F-100 Super Sabre. For Knight, flying the Super Sabre after the X-15 must have felt like going from a Top Fuel dragster to a Prius.

Pete Knight left the Air Force in 1982 and began a political career in 1984. He eventually became the first elected Mayor of Palmdale, California. Under Knight’s term, Palmdale was the fastest growing city in the United States. He was later elected in 1996 as a Republican Senator for the 17th District of California.



In 2003, I was part of the small Northrop Grumman team that built the first Production RQ-4A Global Hawk aircraft for the U.S. Air Force. Our roll-out event was a huge spectacle, signalling the beginning of what has now become a very successful Production program. The affair was attended by many of our state and local politicians and Air Force dignitaries. There was a long receiving line, where we shook hands with all of the VIPs. I had just shook the hand of our 21st District Senator, the incredible Sharon Runner (RIP), when I saw an extended hand come from my left side.

Instinctively, I extended my hand and turned to make eye contact only to see that I was now face to face with Pete Knight, my hero. He asked how I was doing, and I said, “Sir, it is an honor to finally meet you.” He gave me a very humble smile and thanked me. I wanted to get an opportunity to talk some more, but the line was moving quickly, and in a flash, he was gone.

I had just shaken the hand of the fastest man alive, and had no idea that when I met Pete Knight, he was dying of leukemia. Within a year, he was gone. Air Force Test Pilot, speed record holder and decorated combat fighter pilot William J. Knight died on May 7th, 2004 at the age of 74.



The actual X-15 flown by Pete Knight was restored after the record-setting flight. While restored back to it’s original raw black Inconel finish, it never flew again. Today, it is an artifact in the Air Force Museum at Wright-Patterson AFB in Dayton, Ohio. I visited the Museum about 10 years ago and made certain that I saw ‘ol girl. She’s enjoying the good life, nestled under the left wing of the last remaining XB-70 Valkyrie bomber and surrounded by scores of other fellow X-planes. And yes, I ran my hands all over it and imagined what happened on that historic day in October of 1967.

That is a memory that will stay with me forever.

X-15 museum
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Alex Welsh

Alex Welsh


  1. February 15, 2018 at 5:59 pm — Reply

    Absolutely incredible how far some of theses guys pushed the envelop……brave and skilled are not enough to decribe them

  2. February 16, 2018 at 5:53 pm — Reply

    Amazing story and a great read. This was one of my favorite aviation stories but I never knew as much detail as you so eloquently elaborated on. Well done!

    Chris Bennett

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