MOCKUP INSPECTION

The previous year had resulted in some major configuration changes to the X-15. The wing size and shape were similar to those proposed by North American, but engineers increased the leading-edge radius (along with the radius on the empennage and nose) to satisfy aerodynamic heating concerns. The leading edge was also changed from replaceable fiberglass to a nearly solid piece of Inconel X. NASA had always harbored concerns about the use of ablative materials on the leading edge, but this change also eliminated the removable-leading-edge concept that was highly prized by Ames. The final configuration also increased the diameter of the fuselage by about 6% in order to increase the propellant capacity.-1541

A revised landing gear eliminated tail-strikes during landing and improved directional stability during slide-out. The side fairings, always a point of contention between North American and the NACA, were shortened ahead of the wing. The horizontal stabilizer was moved rearward 5.4 inches, the wing was moved forward 3.6 inches, and the center of gravity was brought forward 10 inches to improve longitudinal stability. However, perhaps the most visible change was that the area of the vertical stabilizers was increased from 50 square feet to 75 square feet. Full 10- degree wedge airfoils replaced the original double-wedge configuration for the vertical stabilizers. The area for the verticals was also redistributed (55% for the dorsal stabilizer and 45% for the ventral, instead of the original 73/27 configuration). In addition, both the dorsal and ventral stabilizers now had rudders that were nearly symmetrical and operated together at all times (except after the ventral had been jettisoned during landing). Originally, only the dorsal stabilizer had a rudder.-155

The development engineering inspection (DEI) took place in Inglewood facility on 12-13 December 1956. In the normal course of development, the Air Force inspected full-scale mockups to ensure the design features were satisfactory before construction of the first airplane began. Of the 49 people who took part in the inspection, 34 were from the Air Force, with the WADC contributing 22. The inspection committee consisted of Major E. C. Freeman from the ARDC, Mr.

F. Orazio of the WADC, and Lieutenant Colonel Keith G. Lindell from Air Force Headquarters. The NACA and the Navy each contributed a single voting member. Captain Chester E. McCollough, Jr., from the X-15 Project Office, Captain Iven C. Kincheloe, Jr. (already selected as the first Air Force X-15 pilot), and three NACA researchers served as technical advisors.-156

The inspection resulted in 84 requests for alterations, of which the board rejected 12 and deferred 22 others for further study. Surprisingly, the board rejected some of the more interesting of the proposed changes. These included suggestions that the aerodynamic center stick should be capable of controlling the ballistic controls at the press of a switch, the motions of the aerodynamic and ballistic side sticks should be similar, or a third controller that combined both functions should be installed on the right console. The committee rejected these suggestions since it seemed inappropriate to make decisions on worthwhile improvements or combinations before evaluating the controllers already selected under actual flight conditions. Given that two of the three controller suggestions came from future X-15 pilots (Iven Kincheloe and Joseph A. Walker), it appeared that improvements were necessary.-157

An even more surprising rejection occurred concerning changeable leading edges. North American had disclosed at the 1956 industry conference six weeks earlier that the leading edges were no longer removable, with little comment. Nevertheless, Harry Goett from Ames did not agree with the change. Goett wanted to widen the front spar lower flange and locate the ballistic roll thrusters at the back of the same spar. In addition, Goett argued that North American had initially proposed providing interchangeable wing leading edges. In spite of these logical arguments, the inspection committee decided the required changes would add 3 pounds to the design and rejected the request. At least one participant opined that deleting this feature would significantly decrease the value of the hypersonic research airplane.-158

The X-15 mockup as it was inspected in December 1956. At this point, the airplane looked substantially as it would in final form with short fuselage tunnels and shorter vertical surfaces. This inspection cleared the way for North American to produce the final manufacturing drawings and begin to cut metal. (U. S. Air Force)

Additional wind-tunnel testing resulted in modifications to the vertical stabilizer, but North American essentially built the configuration inspected in mockup form during December 1956. However, while the design and construction of the airframe progressed relatively smoothly, other systems were running into serious difficulties.