Picture this: it’s 1955. The United States and Russia are beginning their forays into space travel. But how do you get about exploring a frontier no one has ever been to before? The U.S.’ answer: by getting as close as possible and collecting a bunch of data along the way.
But to get close to space, you have to have something really fast and powerful. Something which can exceed the speed of sound. For the NACA (National Advisory Committee for Aeronautics) – now NASA – that answer came in the form of the X-15 hypersonic rocket-powered aircraft.
Brian James McManus, creator of Real Engineering, took a deep dive into the design and creation of this powerhouse of a flying machine:
It wouldn’t be an exaggeration if you said the X-15 paved the way for space travel as we know it. Not only was this the United States’ first blueprint in terms of engineering for re-entry, atmospheric pressure, and the other unknown factors of space, but it is also a mission to gather data on its surroundings mid-flight which would later provide information on what to expect once the man left the confines of the planet.
The X-15 is interesting because each part of its design, from the engine to the frame, is connected and built for a specific purpose.
The Rocket Propulsion Engine
Take the engine, for instance.
There were a lot of unknown factors when making the X-15, but everyone knew the aircraft had to be fast. Think Mach 6.7 fast. This was a problem, because the X-2, the fastest plane at the time, could only reach speeds of Mach 2-3.
But the engine also had to be able to easily adjust its thrust output. To do this, engineers varied the speed of the turbo pump – which forces fuel and oxidizer from their respective containers into a combustion chamber – by using hydrogen peroxide as the pump’s dedicated fuel source.
Lastly, the engine also had to be slim enough to fit into the aircraft. This sounds easy to do on paper, but when you realize the engine has to burn 8,165 kg of fuel AND oxidizer in 85 seconds, putting that much power into a relatively small engine is a big task.
Fuel and Oxidizer
This brings us to what the engine ran on.
Finding a fuel source for a space aircraft was one of the biggest challenges for the NACA. Just like the engine, the fuel had to be powerful but also weight-efficient. There are many substances that can be mixed together to form a powerful fuel, but their mixing ratios and weight on a molecular level have to be taken into consideration so the X-15 won’t be held down by heavy fuel and oxidizer.
The NACA was looking for a substance with a high specific impulse (which is how efficient a fuel converts its mass into thrust) as well as one with regenerative cooling properties so the engine wouldn’t overheat and break down. Water, kerosene, and hydrazine were considered, before finally deciding on anhydrous ammonia as the fuel source for the X-15.
Combined with liquid oxygen as the oxidizer, the problem of finding propulsion substances seemed to be solved…
The Inconel X Frame
…But was it really?
Take note that ammonia is a toxic substance, which is one of many reasons why the metals used in the X-15’s creation had to be well-considered.
And then there was the problem of heat resistance. Not only did the frame of the X-15 have to be able to withstand the high temperatures of friction and re-entry, but it also needed to deal with the extreme cold coming from the internal cryogenic liquid oxygen tanks which house the oxidizer.
No natural metal can withstand both these high and low temperatures, so NACA employed the use of Inconel X – a mixture of nickel, chromium, iron, and niobium with extremely good heat resistance.
The metal was used on most of the aircraft, even on the joints. Using heat-sensitive paint, engineers were able to find temperature hotspots during testing and remedied them with Inconel X strips.
Not all of the frame was Inconel X, though. While the area surrounding the cockpit windows used to be made of the stuff, testing found that the metal would buckle due to thermal expansion. This would crack the window, leaving the pilot with very little visibility and could even suck them out of the aircraft if it was completely destroyed.
The metal around the windows was replaced with aluminum frames (which experience lower thermal expansion), while the rear framing was removed altogether.
Even with its good heat resistance, the Inconel X wouldn’t survive the Mach 6.7 flight. To help it, ablative plating was added on top of the Inconel X as a sacrificial layer that would absorb some of the heat.
There were still many problems that had to be addressed. From the ablative material falling from the aircraft, to the liquid oxygen mixing with the ablative, to the various issues with the plane’s geometry such as the tails and even the X-15’s nose shape.
Finally, after a decade of development, the X-15 was placed under the B-52 bomber, carried up to 45,000 feet, and carried out its 188th flight on October 3, 1967. Flown by William Knight, the X-15 broke into Mach 6 for 2.5 minutes before slowing down and coming back to Earth.
The aircraft would never fly again. The ablative coating didn’t fully protect the X-15 from the extreme heat, and some of the plane’s pieces had melted away. Two more X-15s flew for 11 more times before the project was ultimately shut down.
But it wasn’t all for naught. A lot of data was gathered both during the development of the aircraft and its many flights. With it, scientists and engineers were able to refine and develop better methods for space travel with their future endeavors.
The whole video is definitely worth a watch if you have the time. Brian McManus delves deep into the design of the X-15 and expertly connects each piece of the aircraft like a jigsaw puzzle. As the pieces fall into place, you get a better appreciation for the X-15 as a whole.