“It was the ultimate flying machine, No aeroplane can live up to what the X-15 did.” That’s what retired test pilot and astronaut Joe Engle said of the first real space plane and 50 years after its record-breaking flight of 3rd October 1967 when USAF test pilot William J. Knight achieved a top speed of MACH 6.72, 4519 mph or 7273 km/h, It’s still the fastest manned powered aircraft.
And if you thought the SR71 blackbird was the fastest jet, then you absolutely correct because the X-15 wasn’t a jet, it was rocket powered single seater aircraft which looked a bit like an oversized dart and had to launched from the underside of a modified B-52 at 45,000 feet, because the XLR-99 rocket engine would burn through all of its fuel in just 2 minutes.
Not only did the X-15 set speed records it also went past the point of where space officially starts at 100km, 62.1 miles on two occasions, both times piloted by Joseph A Walker at 105.9km, 347,000 feet and 107.8km 353,000 feet.
Although in the 1960’s the USAF considered space to start at 80km or 50 miles. Any crew that flew over the 50-mile limit where given Astronaut badges, 13 of the X-15 flights went higher than this and two of the pilots, Neil Armstrong and Joe Engle went on the become fully fledged Astronauts in the Apollo and Space Shuttle programs.
But apart from being a record-breaking aircraft, Research from the X-15 program lead to things like the first full pressure suit that would work in space, the first use of reaction controls, those are the little jets that position a spacecraft in space, the first use of superalloys in the structure of the plane that could withstand the heat of hypersonic re-entry and the Development of the first large restartable throttleable rocket engine, the XLR99.
These are a small selection of the developments and discoveries that would go on to contribute to later space programs including Mercury, Gemini, Apollo and the Space Shuttle.
In the early 1950’s, research which had started with the Bell X-1, the 1st supersonic plane, began looking into the problems that would be encountered by spaceflight.
At the time it was still unknown as to what would happen to the stability as well as other issues of the craft when travelling at hypersonic speeds, that’s between Mach 5 to Mach 10, or between about 3800 and 7,700 mph ( 6,200 to 12,400 km/h). This would be the type of speed that would be required to get to the edge of space and the re-entry.
In 1952 the National Advisory Committee for Aeronautics (N.A.C.A), NASA’s predecessor started looking into the problems and by 1954 they had contacted both the US Navy and Airforce to propose building a research aircraft which would become the X-15.
By 1956 the contract for the airframe had gone to North American Aviation and the rocket engine was to be built by Reaction Motors.
After the contract had been awarded to North American and before the launch of Sputnik in Oct 1957, North American had considered making an X-15B orbital space plane that could carry a crew of two by launching it into a low earth orbit on top of a pair of SM-64 Navaho missile boosters.
If this had been done, it could have predated the Space shuttle by over 20 years.
However, after Sputnik, the X-15B orbital space plane idea was shelved and revived several times until it was overtaken by the Mercury space program as the NACA became NASA in 1958.
In 1960 NASA considered using the B-52 and X-15 as a launch system for a Blue scout rocket to carry small satellites into orbit. The B-52 would be the first stage, with the X-15 being the second stage carrying the Blue scout, the third stage of 180,000 ft, before it was launched carrying the payload to orbit.
This is an idea using high-performance aircraft as launch platforms is now gaining interest once more as a method for launching the new generation of nanosatellites.
Three X-15’s were built and performed 199 missions over a 9 year period from 1959 to 1968. Due to delays in the engine development, the first 24 flights used two smaller XLR-11 engines but in 1960 the XLR-99 engines were fitted, over tripling the thrust to 57,000 lbs and this would be used throughout the remainder of the flights.
The X-15 generally performed two types of research flight paths, level high speed runs at around 100,000 feet and altitude runs where it would fly as high as possible.
Because of the speeds that the X-15 could reach, the temperature on exposed areas like the leading edges of the wings and the nose could reach 1200 degree Fahrenheit or 650 degree Celsius at Mach 6.
The fuselage was made from titanium covered in Inconel X, a nickel-chromium-based superalloy also used to make the thrust chamber for the Saturn F1 engines and could withstand the high temperatures without weakening.
Although Inconel was capable of withstanding the heat stress, the stresses that built up between the hot and cooler areas was causing concern as NASA was looking at testing a hypersonic RAMjet engine that could, in theory, push the X-15 to around MACH 8, around 6000 mph or 9900 km/h.
NASA also were looking for an ablative coating, that’s a layer of material that burns and turns to gas to protect the structure underneath and that could be easily applied to reusable spacecraft to cut refurbishment costs and turnaround times. The X-15 would be an ideal test bed for this type of heat shield.
After a minor crash in 1967, the second X-15 was rebuilt and renamed as the X-15A-2. It was extended by 28 inches (71 cm) for the extra Hydrogen tanks for the proposed RAMjet engine and fitted with detachable auxiliary fuel tanks that increased the flight time by 60 seconds. It was also coated with an experimental ablative coating.
It took 6 weeks to apply the spray on coating and when it was done the X-15A-2 was now white instead of black. It was also fitted with a dummy RAMjet to test the design. But during the record-breaking flight of Oct 3rd 1967, it also revealed majors issues.
Data from the flight showed that in places like the nose cone and the wing edges, whilst the coating had worked, it prevented the Inconel structure underneath to cool as it was designed and nearly brought about a structural failure of the X-15A-2 due to the uneven heat stresses that had built up.
Also when the craft reached Mach 6, the gases released from the ablator turned the cockpit glass opaque, so the pilot could no longer see out of it, luckily one of the two windows had been fitted with a metal eyelid which was raised before landing so he could use the other unaffected window.
Due to unexpected airflow problems, the temperature was so high the dummy RAMjet was seriously heat damaged and 3 of the 4 explosive bolts which held it to the mounting pylon ignited. It was then ripped from the aircraft as the final bolt failed, this put an end to the idea of a RAMjet powered X-15.
The X-15A-2 came back looking like a burnt out firework and whilst it was sent to be refurbished, the idea of the ablative coating was dropped due to the problems in getting adequate depth of the ablator over the structure.
Due to the research nature of the X-15, there were accidents and incidents with some of the test flights but there was only one fatal crash on 15th November 1967 when Air Force test pilot, Maj. Michael J. Adams lost control at 230,000 feet with the X-15 entering a Mach 5 spin.
There were no recommended techniques to recover from a supersonic spin as no one knew what the X-15 would do in such a situation, although Adams tried to recover it, at 65,000 feet whilst travelling at Mach 3.93 and tumbling through the air the X-15 broke up scattering the wreckage over a 50 square mile area.
As the 1960s drew to a close, and after the fatal crash, support in NASA for the X-15 program waned. Many of the major research goals had been completed in the years before and now spacecraft were the new priority. The last flight took place on 24th Oct 1968 by Bill Dana and within a year the remaining X-15s were retired and one of the most influential aircraft research programs was shut down for good.
The two remaining X-15’s are now on display at the National Air and Space Museum, Washington, D.C. and the National Museum of the United States Air Force at Wright-Patterson Air Force Base.