Sweeping majestically across a desert sky, a B-52 deploys its small slick black cargo. Within seconds this black projectile was gone, leaving just a long streak of white smoke behind. This was not a standard drop, but a specially built rocket plane. A research craft built to propel humanity forward into space.
Kissing the edge of space, it carried pilots to speeds unheard of before, or since. Not even the mighty Blackbird SR-71 could keep up. This was more like a rocket powered dart than an aircraft. Test pilot Joe Engle said, ‘It was the ultimate flying machine, no airplane can live up to what the X-15 did.’
This is the story of the X-15, the rocket plane.
Development of the X-15 began in 1954, on a joint research program sponsored by The National Adversary Committee for Aeronautics, the NACA that later became NASA, and the U.S Air Force.
The X-15 project ran concurrently with NASA’s Mercury program. The U.S was rapidly gearing up to launch a human into space but lacked critical data. North American Aviation and Reaction Motors were the two winning companies for the contract to build the new space plane; building the airframe and engines respectively.
The X-15 program was called upon to obtain all the missing pieces of research regarding aerodynamic heating, re-entry conditions, acceleration and deceleration forces, and the reactions of a human to weightlessness.
The only way of finding out what space had in store for their future astronauts was a manned vehicle capable of passing the boundary of space, or the Kármán line, 100 Kilometres or 62 miles above the surface of the Earth.
All of the unknowns of space travel needed to be investigated and it was down to the X-15 to find the solutions to problems that pilots would face in spaceflight.
From Nose to Tail
Now, to go over every detail of this plane we have to start with the basics.
At a length of 50 feet or 15.24 metres, a wing span of 22 feet, 6.7 metres and a height of 13.8 feet or 4.2 metres. This was a tiny aerial vehicle, certainly, compared to the monsterous B-52 Strata-fortress carrying it under its wing.
The X-15 would have to be made out of something that had never been tried before. The basic internal structure of the plane was formed from titanium. For the exterior, a special alloy comprised of nickel, chromium, iron and niobium, named Inconel X was used to fabricate the fuselage. Paired with the blunt shape of the plane, it was capable of withstanding temperatures of 1,200 degrees Fahrenheit or 649 Celsius.
Starting from the nose, debates raged between groups of the engineers on the design of the X-15. At the time, the excepted design of a hypersonic aircraft was to have a pointed nose to limit drag.
However, the director of NASA’s Ames Research Centre and Aeronautical Engineer, Harry Allen, argued for a blunt nose and body design. This would allow for excess heat to be directed away from the aircraft.
This design worked so well that it was used on all re-entry spacecraft that came after it.
Another early realisation in the design phase was that the X-15 would need more than traditional aircraft control surfaces. Due to the high speeds and altitudes it was expected to attain.
To solve this the reaction control system or RCS was installed in the nose. These small thrusters were fitted to the tip of the X-15 as well as under the wings. Once the X-15 had left the atmosphere, small bursts from these thrusters would be used to steer the aircraft.
This was an innovation that was applied to all spacecraft to follow and is still used today.
The cockpit of the X-15 was, as you might have envisioned, cramped and confusing. There were two flight configurations for the plane, one with a single joystick and another which had three. As well as many buttons, dials and switches the engineers could cram in there.
A quick side note: the seat for the first X-15s came from an unlikely source. The first test pilot, Scott Crossfield, had concerns over the seats . Knowing the speeds that this aircraft was expected to do, a normal jet fighter seat wouldn’t be able to give the support required.
After some thinking, Crossfield approached the International Harvester Company, the IHC, manufacturers of tractors and farm equipment. They had
conducted extensive testing and investigation on the response of the human body on a vibrating platform, and found the natural frequency of a human spine, the rate at which an object vibrates when it is disturbed.
The outcome of this research, was a seat that a person could sit in and withstand intense vibrations without compromising their spine.
As a result, the first X-15 to fly had a tractor seat!
The pilot seat was fitted with an ejection system but this could not be used when the plane was making hypersonic runs. If the pilot ran into difficulty, they would have to drop down to a more manageable speed before they could bail out.
With just two small windows, the visibility wasn’t the best but it was manageable. However, on 9th November 1961, during the X-15s first Mach 6, flight visibility for pilot, Robert White, went from being limited to dangerous.
During the high-speed run, everything was going smoothly. However, when White was throttling down the right window cracked and shattered.
This was due to thermal stresses being higher than engineers had expected. Thankfully, it was only the outer pane that shattered and White was uninjured and able to bring the plane down in one piece.
Safety concerns were raised about future hypersonic flights and solutions were compiled in the eventuality of a similar situation occurring. The simple solution the engineers settled on was replacing the window frame with a different metal to hold the window more securely.
This was not the only window pain encountered during the flights. Another issue came when testing for the top speeds of the X-15, with ablative materials added to the hull for additional thermal protection. A wise precaution, but had the side effect of burning off and attaching to the windows.
The engineers once again looked at solutions. One of which was exploding the outer pane of the glass of the affected window. With a series of small explosive charges attached to each window, if either were damaged then the pilot could simple blow the outer pane away, restoring visibility.
This idea was deemed too dangerous and a shutter for the left window was designed to remain covered during the high-speed runs. So at least the pilot had one window to see out of.
The main body of the X-15 was taken up by the fuel tank. Inside there were two tanks, one containing anhydrous ammonia, the other Liquid Oxygen.
These fuels were required to be kept at such cold temperatures that footage from the underside of a B52 carrying an X-15, showed that frost had formed on underbelly of the plane.
But, this cooling effect served a purpose. To stop the engines from overheating a system of regenerative cooling was deployed. This is where super cooled fuel is circulated around the exhaust to stop the engine from melting.
The two fuel tanks held a combined weight 8,165 kilograms, of fuel and Oxidiser which it burned through in 85 seconds, that’s 5,897 kilograms per minute! With additional external fuel tanks were required during the hypersonic runs because of this planes thirst.
Due to this high rate of fuel consumption, lead to the X-15 being airlifted by a B-52. It just would not have been able to get to the desired heights or speeds without being carried, despite its powerful engine.
One of the biggest challenges of the X-15 was the engine. A jet engine wouldn’t do because of the environment the plane would be entering. An air breathing engine simply wouldn’t work and using a traditional rocket engine would limit the planes capabilities.
The engineers calculated that the engine would require 240 Kilonewtons of thrust at sea level to be able to propel the aircraft to hypersonic speeds. Not only was that a ridiculous amount of power, but it also had to compact enough to fit in the narrow body of the plane.
To add to the various headaches of the engineers, the engine would also be required to have various thrust output, which would allow for different tests at different speeds and allow more control for the pilots.
A Turbo Pump powered by hydrogen peroxide was found to be the answer.
The Turbo Pump could vary it’s the speed at which oxidiser and fuel pump from their storage tanks into the combustion chamber.
As well as powering the Auxiliary Power Unit, the hydrogen peroxide was used for the thrusters on the wing tips and nose.
The way the Turbo Pump functioned is still used in the rockets to this day. Notably in SpaceX’s Merlin engine.
The development process of the engine was always throwing new obstacles in the way of progress. From having fuel mixtures that would corrode the gages in the cockpit to trying to keep the hull from melting, its safe to say the X-15 went over budget. It wouldn’t be a MegaProject without that.
The wedge tail design of the X-15 is one of the most distinctive elements of the aircraft. With its duel tail fin design. This was not just for symmetrical aesthetics but also served a purpose.
With the upper tail providing a control surface for atmospheric flight. Once the plane had gone hypersonic and left the atmosphere, this control was transferred to the reaction control thrusters located under the wing tips and nose.
During re-entry to Earth’s atmosphere, the angle at which the X-15 approached was so extreme that the upper tail became completely useless. This is where the lower tail came into effect and could control the X-15. However, when it came to landing the tail got in the way.
It needed to be ejected for the landing skids to deploy. The pilot would do this shortly before landing. Once ejected it would hopefully soft land with the aid of a parachute and be undamaged.
Each landing of the X-15 was unpowered. Having spent all its fuel, the plane would come in a low-lift glide. This technique of landing was later adopted and developed by space shuttle pilots.
The skill and precision needed to keep these planes stable in flight was unmatched. Only the best pilots got the job of flying hypersonic.
When the first test flights were conducted the final engine design wasn’t ready. So, the designers had to use two XLR-II engines that powered the Bell X-1 – the plane Chuck Yeager used to break the sound barrier.
On June 8, 1959, a sleek black aircraft was released from a B-52 flying at 37,550 feet/11,445 meters, with test pilot and engineer Scott Crossfield in the cockpit.
These first flights pushed the X-15 to Mach 3.3 and allowed some testing of the planes systems but it was way off Hypersonic flight.
That would not be achieved until March 7th, 1961 when pilot, Robert White took the X-15 to Mach 4. It was now a hypersonic plane. After this achievement, the X-15 appeared to be unstoppable. With fearless pilots jumping into the cockpit to smash more speed records and push the boundaries of what was possible in flight.
One of those pilots you may have heard of, Neil Armstrong. With his background in engineering, he was an ideal candidate for piloting the X-15.
On his flight on 20th April 1962, he took his X-15 up to 207,000 feet, 63km. Once there he held the aircrafts nose up for too long during the descent. This resulted in the plane ascending once again and a loss of control. Thankfully he regained control but missed his landing zone.
Zooming past it at Mach 3, he managed to turn the plane around and make a landing, narrowly avoiding some Joshua trees in the process.
The second plane, X-15-2, was rebuilt after a landing accident on 9th November 1962 which damaged the craft and injured its pilot, John McKay. Although McKay did take to pilot X-15 missions after this but his injuries contributed to his early death in 1975.
Following the crash, this X-15 was repaired and lengthened by 2.4 feet/ 73 cm, had a pair of auxiliary fuel tanks attached beneath its fuselage and wings, and a ablative coating was added. The plane was renamed the X-15A-2, and took flight for the first time on 25th June 1964.
It was in this X-15 on October 3rd, 1967, where William Knight reached Mach 6.72 which is 4519 Mph/ 7273 Kph, the fasted speed the X-15 reached. In fact, that speed has yet to be matched for a fixed wing aircraft to this day.
Joseph Walker made history, when he became the first X-15 pilot to pass the boundary of space twice, first on 19th July 1963, at a height of 105.9 Km/ 347,000 feet. He smashed this record a month later on 22nd August 1963, where he climbed to a height of 107.8km/ 353,000 feet.
Piloting of this plane was not risk free and a skilled pilot was not a guarantee of a successful flight.
Michael Adams was piloting his X-15 on November 15th, 1967. All appeared normal and the aircraft reached its targeted altitude of 266,000 feet/ 81km. During the decent problems began to arise. With the heading of the pane off by 15 degrees, it entered a spin at Mach 5. As it spun back towards the ground, 15g of force was put onto the hull. This was too much for X-15 to handle and it broke up 10 minutes after the launch, killing Adams. He was posthumously awarded astronaut wings for his final flight.
The 200th flight over Nevada was first scheduled for 21st November 1968, to be flown by William “Pete” Knight. Numerous technical problems and outbreaks of bad weather delayed the flight six times, and it was permanently cancelled on 20th December 1968. The X-15 was detached from the B-52 and then put into indefinite storage. The aircraft was later donated to the Smithsonian Air & Space Museum for display. The other surviving X-15 was donated to the National Museum of the United States Air Force.
To the future?
It cannot be understated how important this plane was to the space program. With the research flights it conducted, it laid the ground work for the Mercury, Gemini and Apollo programs and also the Space Shuttle and the SR-71 Blackbird.
By the time the nearly 10-year program ended with 199 flights, an X-15 had been flown to a blistering speed of Mach 6.7 that’s 4520 mph, a record that still stands for fixed winged aircraft. An X-15 also topped its altitude goal of 250,000 feet by nearly 19 miles. That record of 354,200 feet for a winged aircraft also remains intact to this day.
It wasn’t just breaking speed records that this plane was built for. With its pioneering work NASA learned, how to survive re-entry, stability and control in hypersonic flight, control systems for the vacuum of space and the first fully pressurised space suit. All down to the X-15.
Looking further ahead, the legacy of the X-15 lives on. With Virgin Galactic’s Spaceship 1 and 2 owning their design the to the X-15. Also being looked into is the deployment of microsatellites from the underside of a plane; just like this little Rocket plane did.
Sometimes, new ideas come from taking a look into the past.