By 1963, the design objectives of the X-15 had been met, and for the last six years of its life it served mainly as a testbed for other experiments. These experiments were mostly space-related and varied from a pod that was carried on one wingtip with which it was attempted to capture a micrometeorite to a top-looking camera for photographing stars. The next slide Slide 21is a photograph of the Colorado River Valley taken by a down-looking camera on an X-15 at 220,000 feet.
Looking down on earth. (Slide 21)
The most interesting X-15 follow-on experiment was the ramjet engine that was planned to be tested on the X-15A. Slide 23 This is a photograph of the X-15A, which was rebuilt from the X-15 that rolled on its back after the landing gear failure. The fuselage was lengthened 29 inches to make room for a hydrogen tank that was to provide fuel for an air breathing ramjet engine to be mounted on the ventral fin. The ramjet shown here is a boilerplate-dummy-engine used to obtain performance and stability data prior to installation of a working ramjet. Like the lower ventral fin, the ramjet had to be jettisoned prior to landing.
X-15 Ramjet. (Slide 23)
The ramjet rode well, but with the added structural weight and drag of the ramjet, the maximum speed of this configuration was just over Mach 5, and it was desired to test the ramjet at Mach numbers of 6 to 8. In order to accommodate this increased performance, two external fuel tanks were added (Slide 24)
X-15 Testing the Ramjet. (Slide 24)
These droppable tanks carried an additional 13,000 pounds of propellants. They were carried until empty at about 70,000 feet and Mach 2, at which time they were jettisoned, and the bare X-15 had full internal fuel with which to accelerate from that starting point.
One flight was launched in the configuration shown here, and flown to a Mach number of 6.33. The next flight was to exceed the original design speed of Mach 6.6, which meant that the structure had to be heat-protected. A silicone elastomeric was sprayed over the entire exterior of the airplane (Slide 25) This coating was designed to ablate during the high Mach portion of the flight and to carry some of the heat load with it.
X-15 New exterior coating. (Slide 25)
The white X-15 was flown one time to a Mach number of 4.94 to check the integrity of the thermal protection and then was flown, in the configuration shown here (Slide 26) to a Mach number of 6.70, the fasted speed achieved in the X-15 program.
X-15 Checking the integrity of the new exterior. (Slide 26)
Charring of the ablative coating (Slide 27) occurred in many areas of the X-15, but more serious damage occurred where the wake of the dummy ramjet impinged on the vertical fin.
X-15 Damage to the new exterior coating. (Slide 27)
Portions of the skin of the ventral were burned through and there were substantial damage to substructure and to subsystems enclosed in the ventral fin. The X-15A was returned to the North American factory at El Segundo for repair. Before the repair of the damaged structure was complete the X-15 program was canceled. The X-15A never flew again and the actual working ramjet was never flight-tested.
The flight resulting in all the heat damage occurred in October 1967. In November of that year X-15 No. 3 launched on what was planned to be a routine research flight to evaluate a boost guidance system and to conduct several other follow-on-experiments. During the boost, the airplane experienced an electrical problem that affected the flight control system and inertial displays. This distracted the pilot and may have caused him to misinterpret other displays. At peak altitude, the X-15 began a yaw to the right, and it re-entered the atmosphere yawed crosswise to the flight path. It went into a spin and eventually broke up at 65,000 feet, killing the pilot.
The tragic event, in retrospect, was the death knell for the entire project. Program management decided not to fly the X-15A again and to fly the X-15 No. 1 only for calendar year 1968.
On October 24, 1968, X-15 No. 1 flew a successful research flight to accomplish the 199th flight of the program. There still remained two months of the year in which to pass the 200-flight milestone. But a series of problems involving the experiment, the inertial reference system, and the weather combined to keep the X-15 on the ground, and time finally ran out on the most successful research airplane in history.
X-15 pilot Bill Dana
Special note: Four slides (Nos. 1, 20, 22, and 28) were deleted from this presentation because they simply showed a dark screen.
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