Modeller’s Guide

by Barry Davidoff
photos courtesy of NASA

September 1961. X-15 just landed after another test flight, with a B-52 mother ship flying over at low altitude.

World’s Fastest and Highest Airplane

An unofficial motto of flight research in the 1940s and 1950s was ”higher and faster.” By the late 1950s the last frontier of that progress was hypersonic flight, with speeds of Mach 4 and above, at altitudes nearing the edge of space. It required a huge leap in aeronautical technology, life support systems and flight planning. The North American X-15 rocket plane was built to meet that challenge – and it met and exceeded the expectations.

The X-15 become one of the world’s most important research airplanes. It was the first airplane to reach Mach 3, Mach 4, Mach 5 and Mach 6. On October 3, 1967 Pete Knight flew the X-15 and established the world absolute speed record for winged vehicles of Mach 6.70 or 4,520 miles per hour (7,232 km/h). This is twice as fast as a rifle bullet. More than 35 years later this speed record still stands for an airplane. Only the space shuttle has flown faster. The X-15 first flew in 1959 and made 199 flights during the next nine years. The X-15 was constructed to fly higher and faster than any other airplane. The X-15 exceeded 50 miles (80 kms.) on 12 occasions. On August 22, 1963 Joe Walker flew the X-15 to 354,200 feet (67 miles or 107 kms.) to set the world altitude record for winged vehicles. Research from the X-15 project was vital to the design and construction of the space shuttle.

During the 31st flight of the X-15 in 1962, it made an emergency landing while still carrying some of its fuel. The X-15 was severely damaged and rebuilt as the X-15A-2. The designation was derived from the X-15A since it was the first major modification of the airplane and -2 from the airframe (Serial Number 66671) being the second of the three X-15’s that was constructed. The modified X-15A-2 carried 70% more additional propellant in two external fuel tanks, which were jettisoned as the aircraft accelerated through Mach 2.5. The fuselage was extended by 29 inches and the nose landing gear lengthened. The X-15A-2 also was intended to carry a ramjet below its ventral stabilizer. Although research on a working ramjet was years away from fruition, the Air Force was interested in testing the aerodynamics of ramjets. On several flights the X-15A-2 carried a non-working ramjet.

The X-15A-2 with external fuel tanks in 1965.

MA-25S Ablative

The original X-15 outer fuselage was constructed of Iconel-X alloy which only could sustain aerodynamic heating below Mach 6.5. The heat load for the leading edge of the wing at Mach 6 is 9,500 Btu per square foot, but at Mach 8 it soars to 27,500 Btu per square foot. It was determined that in order to go faster that the X-15A-2 need to be covered with an ablative material to protect the fuselage. The ablative would erode slowly shedding the heat of aerodynamic friction in the process. After experimenting with many different types of ablative materials, NASA selected Martin Marietta’s MA-25S ablative, which has a distinctive pink color. The MA-25S is composed of phenolic resin, a catalyst and fine silicate glass beads. The ablative thermal protection paint created an extremely colorful and odd-looking aircraft.

The X-15A-2 was powered by the XLR-99 rocket engine which burned a mixture of liquid oxygen and liquid ammonia. The MA-25S ablative reacted when exposed to liquid oxygen. The pink ablator, therefore, was sealed under two layers of Dow Corning DC90-090 white paint. Since the ablator was covered by white paint before it flew, relatively few photos of the pink X-15 were taken.

X-15A-2 with full scale ablative coating on the ramp at Muroc dry lakebed, 21 June 1967.

The application of the MA-25S was a complex and detailed process. The X-15A-2 was originally flat black with extensive markings. After the entire airframe was washed thoroughly, the edges of removable access panels were masked with polyethylene tape. The airplane was then divided into 9 segments for painting with the pink ablative, which was applied with a spray gun. The edges of removable panels were then masked again with a black foam tape. The black foam masking tape created the sharp black lines that are seen in the photos and on the model. These panel lines are very noticeable on the two fuselage chines and the dorsal stabilizer. After the black foam masking tape was in place, the MA-25S was sprayed on the X-15. The thickness depended upon the heat loads at a particular point. Following spraying the ablator was sanded down to the exact thickness which varied from .015 inches to .265 inches at different locations. The nature and thickness of the ablator created several non-patterned shades of pink. When sanding was completed the X-15 was pink all over with black tape stripes over the panel locations.

X-15 under the application of the MA-25S ablator in the hanger

There have been stories that after painting the X-15 pink, that one of the X-15 pilots stated that he would not set the world speed record in a pink airplane. According to the story, the X-15 was then painted white. This story is an urban myth. The pink ablative needed to be covered with the white paint so that it would not react with any liquid oxygen that might leak or spill. Dow Corning DC90-090 white paint was used since it bonded well with the MA-25S. During painting the supply of DC90-090 paint was exhausted so another white paint, Dow Corning DC92-007 was used for part of the top coat. Limited stencils and markings mostly in red and orange were then applied on the white paint. Oddly, the plane that set the world speed record did not carry national markings or the NASA emblem. After the white paint was applied, both the black foam and polyethylene masking tapes were removed.

X-15A-2 following painting

The ablative paint also led to another distinctive feature of this version of the X-15A-2, which was the ”eyelid” over the left window of the canopy. The ablative paint on the nose during high speed flight would erode and splatter on the canopy windows. During a flight the eyelid would cover the left window and the pilot would use only the right window and instruments for the first part of the mission. Towards the end of the mission that right window would be so covered with ablator residue that visibility was severely limited. The pilot would then raise the eyelid and the left window would be clear for the final part of the mission and landing.

World Speed Record Flight

On October 3, 1967 the X-15A-2 piloted by Major Pete Knight of the Air Force was carried aloft by an NB-52B bomber. The X-15A-2 was released at 43,750 feet and accelerated rapidly when its XLR-99 rocket engine ignited. Pete Knight few a near perfect flight path. At over 100,000 feet the X-15A-2 established a new world speed record for fixed wing aircraft of Mach 6.70 or 4,520 miles per hour (7,232 km/h). The record still stands and since a new manned hypersonic aircraft is not even on the drawing boards, the record is likely to remain for at least another decade.

The X-15A-2 On October 3, 1967 as it is launched from the B-52 and begins its World Speed Record flight of Mach 6.70

Pete Knight flew 16 flights in the X-15 and went on to fly 253 combat missions in Vietnam. He accumulated over 6,000 hours in over 100 different aircraft. Following his retirement from the Air Force, his efforts turned towards politics. He was elected the Mayor of Palmdale, California, the home of the Dryden Flight Research Center, where he had been a test pilot. He went on to serve in the California State Assembly before moving up to the California State Senate.

Pete Knight poses before the X-15A-2 in 1965.

The X-15A-2 fared less well than its pilot. As a result of frictional heating during its speed record flight, the airframe suffered permanent damage. The X-15A-2 never flew again. Post flight analysis showed that the shock wave from the spike on the dummy ramjet caused unexpected heating on the leading edge of the ventral stabilizer. While Knight was setting a new speed record, the high heat burnt through the ventral stabilizer damaging the airframe and wiring. Approximately 25 seconds after the rocket engine shut down, damage to the ventral was so severe that the dummy ramjet was severed from the X-15. The ramjet was found later on the Edwards Air Force Base bombing range.

Damage to the ventral stabilizer The ramjet separated prematurely due to frictional heating

The ablator was significantly charred in many other places as well. Particles of ablator that were eroded on the forward sections of the fuselage impacted further downstream causing significant impact erosion. The dorsal stabilizer including the speed brakes and rudder had high degrees of charring. Various segments of the chines also were damaged due to high heat. Photographs of the X-15A-2 taken after the flight show the many areas of charring and damage.

The X-15A-2 following its World Speed Record flight. The ventral and dorsal stabilizers were charred heavily during the trial.

The X-15 program already was winding down at the time of the record speed flight, which was the 188th in the program. The X-15A-2 would require extensive refurbishment due to all the damage that had been caused by aerodynamic heating. It was therefore decided to retire the X-15A-2. All the white and underlying pink ablator were removed. The X-15A-2 was restored to its original flat black finish and today is displayed at the Air Force Museum at Wright Patterson Air Field in Dayton Ohio.

Sources and References

Literature:

• Hypersonic-The Story of the North American X-15 by Dennis Jenkis and Tony Landis, Specialty Press (North Branch, MN) 2003
• Aerofax Datagraph 2 North American X-15/X-15A-2 by Ben Guenther, Jay Miller and Terry Panopalis, Aerofax Inc. (Arlington, TX) 198
• The web site of the Dryden Flight Research Center at Edwards Air Force Base has many useful photos of the X-15

This article was originally published in IPMS Stockholm Magazine in March 2004.