Components and Construction

The X-15 was designed with a hot-structure that could absorb the heat generated by its short-duration flight. Remember, the X-15 seldom flew for over ten minutes at a time, and a much shorter time was spent at the maximum speed or dynamic pressure. Development showed the validity of ground

damaging the landing gear, and in one case causing the gear to extend at Mach 4.2 (flight 2-33-56). Although the landing gear remained intact, the disintegration of the tires made the landing very rough. The need for very careful examination of all seals became apparent, and closer scrutiny of surface irreg­ularities, small cracks, and areas of flow interaction became routine. The lessons learned from this influenced the final detailed design of the Space Shuttle to ensure that gaps and panel lines were adequately protect­ed against inadvertent airflow entry.

Other problems from aerodynamic heating included windshield crazing, panel flutter, and skin buckling. Arguably, designers could have prevented these problems through more extensive ground testing and analysis, but a key purpose of flight research is to discover the unexpected. The truly significant lesson from these problems is that defect in subson­ic or supersonic aircraft that are compara­tively minor at slower speeds become much more critical at hypersonic speeds.37

One of the primary concerns during the X-15 development was panel flutter, evidenced by the closing paper presented at the 1956 industry conference. Panel flutter has proven difficult to predict at each speed increment throughout history, and the hypersonic regime was no different. Although the X-15 was conservatively designed, and incorporat­ed all the lessons from first generation super­sonic aircraft, the fuselage side tunnels and the vertical surfaces were prone to develop panel flutter during flight. This led to an industry-wide reevaluation of panel flutter design criteria in 1961-62. Stiffeners and reduced panel sizes alleviated the problems on the X-15, and similar techniques later found general application in the high speed aircraft of the 1960s.’■ The lessons learned at Mach 6 defined criteria later used in the development of the Space Shuttle.

The X-15 provided the first opportunity to study the effects of acoustical fatigue over a wide range of Mach numbers and dynamic pressures. In these first in-flight measure­ments, “boundary layer noise”-related stress­es were found to be a function of g-force, not Mach number. Such fatigue was determined to be no great problem for a structure stressed to normal in-flight loading. This knowledge has allowed for more optimum structural design of missiles and space cap­sules that experience high velocities.

On the X-15, the measurement of velocity was handled by early inertial systems. All three X-15s were initially equipped with analog-type systems which proved to be highly unreliable. Later, two aircraft, includ­ing the X-15-3 with the adaptive control sys­tem, were modified with digital systems. In the subsequent parallel evaluation of analog versus digital inertial systems, the latter was found to be far superior. It was far more flex­ible and could make direct inputs to the adaptive flight control system; it was also subject to less error. Thanks to advances in technology such as laser-ring gyros and dig­ital computers, inertial systems have become inexpensive, highly accurate, and very reli­able.3* In recent years they have been inte­grated with the Global Positioning System (GPS), providing three-dimensional attitude and position information.

During the early test flights, the X-15 relied on simple pilot-static pressure instruments mounted on a typical flight test nose boom. These were not capable of functioning as speeds and altitudes increased. To provide attitude information, the NACA developed the null-sensing “ball-nose” which could survive the thermal environment of the X-15. An extendable pitot tube was added when the velocity envelope was expanded beyond Mach 6. Thus far the ball-nose has not found subsequent application, and probably never will since inertial and GPS systems have evolved so quickly. Interestingly, the Space Shuttle still uses an extendable pitot probe during the landing phase.

The X-15 was the first vehicle to routinely use reaction controls. The HSFS had begun

research on reaction controls in the mid – 1950s using a fixed-base analog control stick with a pilot presentation to determine the effects of control inputs. This was followed by a mechanical simulator to enable the pilot to experience the motions created by reac­tion controls. This device emulated the iner­tial ratios of the X-1B, which incorporated a reaction control system using hydrogen-per­oxide as a monopropellant, decomposed by passing it through a silver-screen catalyst. Because of fatigue cracks later found in the fuel tank of the X-1B, it completed only three flights using the reaction control sys­tem before it was retired in 1958.44

As a result, a JF-104A with a somewhat more refined reaction control system was tested beginning in late 1959 and extending into 1961. The JF-104A flew a zoom-climb maneuver to achieve low dynamic pressures at about 80,000 feet that simulated those at higher altitudes. The techniques for using reaction controls on the X-15, and more importantly, for transferring from aerody­namic controls to reaction controls and back to aerodynamic controls provided a legacy to the space program.41

The X-15-3 was equipped with a Minneapolis Honeywell MH-96 self-adaptive control sys­tem designed for the cancelled Dyna-Soar. The other two X-15s had one controller on the right-hand side of the cockpit for aerodynamic controls and another on the left-hand side for the reaction controls. Thus, the pilot had to use both hands for control during the transition from flying in the atmosphere to flying outside the atmosphere and then back in the opposite direction. Since there was no static stability outside the atmosphere, the pilot had to count­er any induced aircraft motion manually using the reaction controls. The MH-96 had an atti­tude hold feature that maintained the desired attitude except during control inputs. The MH-96 also integrated the aerodynamic and reaction controls in a single controller, greatly improving handling qualities during the transi­tion from aerodynamic to space flight, as well as reducing pilot workload.42

But the basic feature of the MH-96 was auto­matic adjustment of gain (sensitivity) to maintain a desirable dynamic response of the airplane. The MH-96 compared the actual response of the airplane with a preconceived ideal response in terms of yaw, pitch, and roll rates. Initially, Milt Thompson stated that the system was “somewhat unnerving to the pilot” because he was not in “direct control of the aircraft” but was only “commanding a computer that then responded with its own idea of what is necessary in terms of a con­trol output.” However, pilots became “enthu­siastic in their acceptance of it” when they realized that the MH-96 resulted in “more precise command than was possible” with the reaction controls by themselves. Consequently, the X-15-3 with the MH-96 was used for all altitude flights planned above 270,000 feet.4’

There were some problems with the experi­mental system, including one that con­tributed to the death of Mike Adams in X-15- 3 on 15 November 1967. Nevertheless, the MH-96 constituted a significant advance in technology that helped pave the way toward fly-by-wire in the early 1970s. Today, most every aircraft, and several automobiles, fea­ture some variation of a fly-by-wire system with automatic rate-gain adjustment and sta­bility augmentation.44