X-15: The Other Spaceplane

0 Posted by - 9th September 2016 - Technology

When it comes to winged spaceships, the list of successful vehicles is rather small. The space shuttle is the most obvious example, with the Soviet Buran also making the cut. Let’s not forget the more recent examples of SpaceShipOne and SpaceShipTwo. But there is another, often overlooked spaceplane that began to take shape even before NASA existed. This unsung trailblazer, the North American Aviation X-15, carried eight humans into space (more than Project Mercury). Between 1959 and 1968 the X-15 completed 199 flights and achieved results that far exceeded the project’s original charter. The successes and failures of the X-15 program provided critical lessons that shaped the US spacecraft (and numerous airplanes) that followed.

The X-15 was a rocket-powered airplane that explored hypersonic flight and carried eight test pilots into space. (NASA photo)

Genesis of the X-15

The mid-20th century was an era of vigorous research and rapid discoveries in aeronautics. A series of American rocket-powered "X-planes" successively pushed top speeds and maximum altitudes ever higher. In 1952, even before Mach 2 had been exceeded, the National Advisory Committee for Aeronautics (NACA – the aeronautical research arm of the federal government and precursor to NASA) initiated a project to explore hypersonic flight (faster than Mach 5). This effort would ultimately result in the creation of the X-15.

While defining the requirements of the project, NACA planted a controversial seed that would later bear a bounty of fruit. In addition to sorting out the unknowns of flying at hypersonic speeds, the agency also dictated that the X-15 must also explore the mysteries of manned spaceflight. At a time when American families were still agape over the futuristic charms of color television, NACA envisioned a vehicle that could not only reach the unfathomable speed of Mach 6, but also fly higher than 250,000 feet.

In these pre-Sputnik days, many capable scientists believed that any form of spaceflight was still only a distant possibility. Indeed, few people foresaw the hectic space race that was looming just over the horizon. A handful of dissenters within the pool of X-15 stakeholders did not want to compromise their hypersonic ambitions by burdening the project with fanciful Zero-G distractions. NACA’s space faring requirement for the X-15 remained in spite of the objections – a sage decision.

Another important requirement of the X-15 project was that its objectives would be focused purely on the quest to advance the American scientific knowledge base. While the air force and the navy would pay much of the bill while also providing significant material resources and personnel, there would be no attempt to adapt the X-15 for military purposes. The project’s objectives were daunting enough without such diversions.

A Whole New Kind of Airplane

Even before engineers began putting pen to paper, they realized that the X-15 would have to be an airplane of many firsts. The demands of flying at hypersonic speeds and upper atmospheric altitudes were clearly beyond the capabilities of contemporary materials and designs. One of the primary problems they faced was how to handle the intense heat that would be generated as the X-15 blazed through the air at warp speed. Theorists pegged the Mach 6 temperature profile of the X-15 at around 2000 degrees Fahrenheit – or roughly the same temperature of lava spewing from a volcano.

The white color of this X-15 is attributed to an experimental heat-protective coating. Also note the large external fuel tanks that have been added. (NASA photo)

There were two schools of thought for building an airframe to sustain such high temperatures. One concept was to build the airframe out of carefully chosen alloys that could withstand the expected baking. The opposing idea was to coat a conventional aluminum and steel airframe with an ablative, heat-resistant coating. The sacrificial outer coat would burn away during high temperature operation and be reapplied for each flight. The X-15 design team chose the former "hot structure" approach.

The tremendous temperature extremes endured by the X-15 forced engineers to utilize materials and structures that were previously untried in aircraft. (NASA photo)

In the mid-1950s, only one material had sufficient heat tolerance to be considered for the X-15, a nickel alloy called Inconel X. This magical metal was applied in various thicknesses as the outer skin of the X-15. Inconel X was also used for some internal structures, along with titanium.

Although Inconel X existed prior to the advent of the X-15, adequate techniques for shaping, machining, welding, and riveting it did not. Engineers had to clear these endless hurdles to make the dream-like X-15 a tangible thing. Since different parts of the airframe would see different heat loads, the design team also had to come up with flexible joints that would allow differential expansion of various panels. These junctions also had to omit any gaps through which super-hot gasses could threaten the less-tolerant internal structure. The solutions implemented on the X-15 would set an important precedent for other spacecraft.

While the "hot structure" approach presented many challenges, it was ultimately proven to be successful. Interestingly, the X-15 would eventually be used as a test bed for ablative coatings as well. It was found to be extremely difficult and time-consuming to apply the coating in the precise thicknesses required. The coating also obscured access to the internal viscera of the airplane, making maintenance much more difficult. Residue from the burning of the material in flight blanketed the pilot’s windscreen and impaired visibility. The disappointing results seen with ablative coating on the X-15 were a prime factor in choosing ceramic tiles for outer surface of the space shuttle.

An interesting design element of the X-15 is its pair of distinctive vertical stabilizers. There is one on top of the fuselage and another on the bottom that pilots would jettison just before landing. While the rest of the airplane is sufficiently needle-like, these fat, wedge-shaped stabilizers look like a mistake.

The thick-wedge-shaped profile of the X-15’s vertical stabilizers was a key element that addressed stability concerns at very high speeds. (National Museum of the US Air Force photo)

The triangular cross section of the stabilizers was implemented to address stability issues that had plagued other X-planes even at speeds well below the hypersonic realm. In fact, some engineers believed that there was "stability barrier" that would stymie any attempts to go much beyond Mach 3. The stability barrier proved to be a non-existent boogeyman, just like the sound barrier that preceded it. The wedge shaped stabilizers on the X-15 were key to this breakthrough. Elements of the X-15 stabilizer design were incorporated into the space shuttle as well.

Although it was originally intended to be dropped from a B-36 bomber, the all-jet B-52 allowed the X-15 to be launched higher and faster. (NASA photo)

Original plans had the X-15 being dropped from the bomb bay of a modified B-36 bomber to begin each flight. This would have allowed the X-15 pilot to board his rocket just before starting the flight. As things turned out, the B-36 was being retired and the new B-52 bomber was available by the time the X-15 was ready to fly. The all-jet B-52 could carry the X-15 up to 45,000 feet and a speed of .8 Mach for the launch…a considerable boost over the B-36. One tradeoff was that the X-15 had to be attached to a pylon on the B-52’s starboard wing. So the X-15 pilot had to endure the long schlep to launch altitude while trapped in the cramped cockpit of his hitchhiking bird.

One of the greatest contributions of the X-15 is that it provided a vast catalog of real-world test data to compare against theoretical predictions. Prior to the X-15, engineers were forced to rely on wind tunnel tests of small-scale models and extrapolations of existing data to predict how things might behave at the edge of space. These processes often demanded broad assumptions and perhaps a fair amount of guesswork. Some figures, such as airframe surface temperatures proved to be significantly inaccurate (actual maximum temperatures were closer 1,400 degrees rather than 2,000 degrees), while other predictions were substantiated. The X-15 provided a yardstick for these estimation techniques and helped to improve the accuracy of future theoretical models.

Playing With Fire

Of all the unknowns that surrounded the birth of the X-15, the biggest gamble was that an adequate rocket motor could be produced. There was a variety of rocket motors already in production and various stages of development, but none satisfied the unique demands of the X-15. Most of the existing rockets had been created for use in ICBMs, where they only had to work once. NACA required a rocket that was robust enough for repeated use, could be throttled during operation (to include shut down and restart), and was safe enough to use with a human strapped to the same ship. It would prove to be a very tall order.

The XLR99 rocket engine that powered the X-15 could be throttled during flight. It required huge tanks of anhydrous ammonia and liquid oxygen. (NASA photo)

Multiple schedule slips and monumental budget overruns put the motor subcontractor, Reaction Motors, under the government’s microscope. It’s not that the company was incompetent or unscrupulous. It’s just that no one adequately predicted how challenging the development process would be.

The X-15 was ready to fly long before its XLR99 motor. In fact, the first 24 powered flights of the X-15 in 1959 and 1960 were made with a pair of XLR11 motors (the same motor that pushed Chuck Yeager and the X-1 past Mach 1 in 1947). The long-awaited XLR99 was finally completed in 1960 and installed in the three X-15 test articles.

The XLR99 proved to be worth the wait and expense. Its 57,000 pounds of thrust shoved the X-15 fleet to speeds up to Mach 6.7 (4,534 mph) and altitudes above 350,000 feet. The motor was fed by the X-15’s internal 1,400-gallon tank of anhydrous ammonia and a 1,000-gallon tank of liquid oxygen (both pressurized with helium at 3,600 psi). These tanks permitted engine runs lasting about 80 seconds. One X-15 was later supplemented with external drop tanks that boosted the run time to 150 seconds. Each motor was typically good for 20-40 flights before requiring an overhaul. The XLR99 was a revolutionary rocket motor that paved the way for the massive RS-25 engines used on the space shuttle and the Space Launch System still in development.

In addition to the XLR99, the X-15 was also equipped with an array of small hydrogen peroxide-fueled thrusters (aka – reaction control jets) in the nose and wingtips. These thrusters provided directional control in the uber-thin high-altitude air where conventional aerodynamic control surfaces were ineffective. The X-15 was the first vehicle to use reaction control jets as a matter of routine…but it certainly was not the last. Such controls became the norm for US spacecraft.

The Human Equation

While engineers pondered the technical enigmas of advancing aeronautics, medical experts tried to determine if it was all a moot point. There was some doubt whether humans would be able to perform adequately in the high and low gravity conditions they would endure while getting to and from space…especially while cocooned in the bulky pressure suits of the day. The X-15 served to answer many of those questions.

Pilot Jack McKay survived this emergency landing and flew the X-15 again. The program endured several "minor" crashes and one fatal incident. (NASA photo)

Studies had shown that test pilots had an average heart rate of about 75 beats per minute during flights in more-typical test aircraft. The pulse of those same icy-cool jet jockeys jumped to 145-185 bpm when they climbed into the X-15! Flights only lasted around 10 minutes, so the temporary cardio bump was not a health risk. Furthermore, experience with the X-15 indicated that the pilots’ peppy heart rate did not generally affect their performance either. This soon became comforting data to NASA flight surgeons as they monitored the mouse-like heart tempo of astronauts sitting on the launch pad awaiting liftoff.

There was significant uncertainty that a pilot would be able to maintain control of the X-15 during the hard accelerations they would experience. In addition to a conventional floor-mounted control stick, the X-15 also featured a small, secondary control stick near an arm rest on the right side of the cockpit. This secondary control allowed the pilot to fly the X-15 using only wrist movements while his arm was stabilized against G forces. Pilots generally preferred to use the wrist controller, although it would be some time before such controls were implemented in production aircraft. A similar control stick on the left side of the cockpit was used to operate the reaction control jets at high altitude.

The X-15 utilized a small wrist-actuated control stick that made it easier for pilots to fly during high G loads. (NASA photo)

Although the X-15 was not capable of orbiting the Earth, 13 flights exceeded the 50-mile (80 km) altitude that the air force considered the threshold for space. Two of those flights passed NACA’s more stringent 62.1-mile (100km) boundary of space. Air force pilots who flew the X-15 above the 50-mile border were summarily given astronaut wings. NASA relented some 40 years later and awarded astronaut wings in 2005 to the 3 NACA pilots (two awards were posthumous) who crossed the 50-mile line in the sky (but not 62.1 miles).

Flying the X-15 was a risky endeavor. The program endured numerous incidents that banged up the 3 test airplanes, and sometimes the pilots as well. Thanks to the program’s gradual, incremental approach to the unknown, new hazards were often detected and mitigated before they became overwhelming. On the 191st flight of the program, a string of equipment malfunctions, possibly compounded by pilot error, claimed the life of Test Pilot Mike Adams and destroyed one of the X-15s.

Before joining NASA as an astronaut in 1962, Neil Armstrong logged 7 flights in the X-15. (NASA photo)

Of the 12 pilots who flew the X-15, two went on to become astronauts in other vehicles. Joe Engle would later participate in glide tests of the space shuttle as well as two shuttle missions. Neil Armstrong’s exploits in the X-15 were largely eclipsed by that whole "first man on the moon" deal.

Scratching the Surface

Original cost estimates of the X-15 program were about $12 million. When all was said and done, more than $300 million had been spent. Yet, few can argue that the X-15 was one of the most successful and productive research programs that NACA/NASA ever managed. In terms of the data that subsequently shaped later projects, the X-15 was a bargain. More than 700 unique technical documents emerged from the X-15 effort, not to mention a pool of experienced test engineers that carried their skills to other programs.

It would take volumes to outline the achievements of the X-15 program and the contributions that this vehicle provided to the greater good of the aerospace community. Thankfully, the program has been well-documented by participants and historians. If you’d like to learn more about the X-15, I suggest starting with the following paper that was published by NASA in 2000. It provides an interesting cradle-to-grave summary of the program using layman’s terms:

Hypersonics Before the Shuttle – A Concise History of the X-15 Research Airplane – by Dennis R. Jenkins

NASA also has a great X-15 infographic that delivers an overview of the airplane and the program.

You can take a close-up look at the two remaining X-15 airframes. One hangs in the National Air and Space Museum in downtown Washington DC. The other resides at the National Museum of the United States Air Force in Dayton, Ohio.

Terry is a freelance writer living in Lubbock, Texas. Visit his website atTerryDunn.organd follow him onTwitterandFacebook. You can also hear Terry talk about RC hobbies as one of the hosts of theRC Roundtablepodcast.

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