STRUCTURAL FABRICATION

The X-15 was breaking new ground when it came to structural materials, since it was obvious from the start that most of the wetted surface would be subjected to temperatures up to 1,200°F. Exotic materials made from the rare elements had not advanced sufficiently to permit quantity production of these expensive alloys, so the list of candidate materials was narrowed to corrosion resistant steels, titanium, and nickel-base alloys ("stainless steels"). The following table shows the strength properties of the candidate materials at room temperature; various aluminum alloys are included as a comparison. All properties are for bare sheet stock, except for the AM-355 bar stock. Materials marked with an asterisk were heat-treated.-1591

		Ultimate	Yield		Ultimate	Bearing Yield (ksi)	Modulus (x1000 psi)
	Material	Tensile Strength	Tensile Strength	Compressive Yield (ksi)	Shear Strength

		(ksi)	(ksi)		(ksi)
Nickel base	Inconel X *	155	100	105	018	186	31.0
Inconel	80	30	32	56	–	31.0
Corrosion- resistant steel	AM-350 CRES *	185	150	164	125	268	28.7
AM-355 CRES *	200	165	178	131	295	28.7
A-286 CRES *	150	95	99	91	136	29.0
4130(HT125- Mo)	125	103	113	82	180	29.0
Titanium	8-Mn	120	110	115	79	180	15.5
5A1-2.5Sn	115	110	110	72	175	15.5
6A1-4V*	160	145	145	99	230	16.3
Aluminum	2024-T4 *	62	40	40	37	63	10.5
7075-T6 *	78	69	70	47	110	10.3
6061-T6 *	42	36	35	27	58	10.2

Although 6A1-4V titanium and AM-350 CRES had good strength efficiencies over a wide temperature range, both of the alloys tended to fall off rapidly above 800°F. Inconel X, on the other hand, had only a gradual drop in strength up to 1,200°F. Because of this stability, North American chose Inconel X for the outer skin for the entire airplane. Regular Inconel (as opposed to Inconel X) was not heat-treatable, but it could be welded and was used in locations where high strength was not of paramount importance or where final closeout welds were necessary following heat treatment of the surrounding structures. To accomplish this, Inconel lands were incorporated into Inconel X structures prior to final heat treatment, and access-hole cover plates made from

Inconel were welded to these lands.-601

North American used high-strength aluminum (2024-T4) to form the inner pressure shell of the cockpit and part of the instrumentation bay. As a relief from high thermal stresses, the company used titanium for the structure of the fuselage and wings. Originally, the company used two titanium alloys: 8-Mn, which was the highest strength alloy then available but was not recommended for welding, and 5A1-2.5Sn, which had acceptable strength and was weldable. Later, North American began using a high-strength and weldable alloy, 6A1-4V, in some areas.

To combat the high concentrated loads from the engine, most of the aft fuselage structure used titanium framing. The majority of the structure used fusion welding, although the company also used a limited amount of resistance welding. North American radiographically inspected all critical welds to ensure quality.-611

The material that presented the most problems was probably the 5Al-2.5Sn titanium, which proved to have inconsistent tensile properties that made it difficult to work with. It also exhibited low ductility and notch sensitivity, and had a poor surface condition. These problems existed in both rolled and extruded forms of the metal. The surface condition was the most important factor governing the formability of titanium, so North American had to remove all oxygen contamination, inclusions, and grind marks by machining, polishing, or chemically milling the metal prior to the final finishing. As a result, North American procured titanium extrusions for the X-15 with sufficient extra material in all dimensions to allow technicians to machine all surfaces prior to use.-621

The limited amount of stretch and shrink that was possible with a titanium extrusion during stretch wrapping presented a different problem when North American went to form the side fairing frames. Each frame was composed of four titanium 5Al-2.5Sn extrusions. One of the problems was that the inside flanges were located in areas that had small bend radii, and it was necessary to prevent compression failure. The small bend radii were "relieved" (some material was removed prior to bending), and a gusset was later welded in to fill the relieved area. The alternative would have been to reduce compression by increasing the pull on the forming machine, thus shifting the bend axis closer to the inboard edge. This, however, would have resulted in a tension failure on the outboard flange.-631

North American found that one of the more interesting aspects of titanium was that a formed part was prone to crack until the residual stresses resulting from the forming had been removed. This delayed cracking could occur within a few minutes, or it might not become evident until weeks later. In response, North American initiated a process that provided stress relief for all parts except "slightly" formed parts, such as skin panels, since they exhibited few problems.-641

Forming the seven different pressure vessel configurations in the X-15 presented its own problems. When compatibility with the contained fluid permitted, titanium was the first choice of material. North American used a 26-inch Cincinnati Hydroform for the hemispherical ends of the 14-inch cylindrical nitrogen tanks with little difficulty. The company also attempted to form the 16-inch hemispheres for the helium tanks on this machine, but the optimum blank size was greater than the maximum machine capacity of 26 inches. Using a smaller-than-optimum blank required excessive hold-down pressure that resulted in small surface cracks. The alternative was to "spin form" the hemispherical ends. Engineers heated the blanks to approximately 1,600°F and used an internally heated spinning chuck to shape the disc. Unfortunately, this resulted in a surface with significant oxygen contamination, so North American used thicker parts and machined them to the correct thickness to eliminate the contamination. Machining was also required to match the hemispheres for each end of the tank prior to welding.-651

Finding the correct material for the main propellant tanks, especially the liquid-oxygen tank, took some investigation. Most steel and common heavy structural alloys gain strength but lose ductility when operated at low temperatures, although Inconel proved to be relatively insensitive to this. The martensitic alloys, such as heat-treated 4130 low-alloy steel and AM-350 CRES precipitation-hardened corrosion-resistant steel, followed predictable curves that showed severe ductility loss as the temperature decreased below -100°F. A titanium alloy containing 5% aluminum and 2.5% tin handled the low temperatures well, but did not have the requisite strength at 1,200°F. North American finally decided to manufacture the primary barrels of the tanks from Inconel X.1661

Initially, engineers used AM-350 CRES, formed on a 7,000-ton hydraulic press using a deep-draw process, for the 32-inch hemispheres of the main propellant tanks. Excessive thinning occurred until the optimum pressure on the press draw ring was determined. Even then, North American encountered some difficulty due to uneven forces from the pressure pins used to secure the blanks, resulting in non-uniformity around the periphery of the hemisphere. The engineers subsequently decided to discard the CRES hemispheres and to remanufacture them from Inconel X.1671

Inconel X proved to be remarkably easy to work with considering its hardness, although the engineers had to make severely formed parts in multiple stages, with annealing accomplished between each stage. Nevertheless, problems arose. One of the first concerned fabricating the large Inconel propellant tank hemispheres. The propellant tanks comprised a large portion of the fuselage and were composed of an outer cylindrical shell and an inner cylinder. Inconel X semi­torus hemispheres at each end of the tank joined these two parts. The hemispheres were formed in two segments, with the split located midway between the inner and outer cylinders. Technicians welded the inner torus segment to the inner cylinder, and the outer torus segments to the outer tank, before joining the two assemblies.1681

After initial attempts to spin the bulkheads from a single, heated Inconel X blank were unsuccessful, the technicians built up the cones by welding smaller pieces together, and performed a complete X-ray inspection of each weld. After the cones were formed to the approximate size, they went through several stages of spinning, with a full annealing process performed after each stage. The first spin blocks used for the hemispheres were made from hardwood, and cast iron was used for the final sizing. A problem developed when transverse cracks began to appear during the spinning of the hemispheres.1691

Both North American and the International Nickel Company investigated the cracks, but determined that the initial welds were nearly perfect and should not have contributed to the problem. Nevertheless, engineers tried different types of welding wire, and varied the speed, feed, and pressure of the spinning lathe, but the welds continued to crack. It was finally determined that the welds were—ironically—too good; they needed to be softer. North American developed a new process that resulted in slightly softer but still acceptable welds, and the cracking stopped.1701

Fabricating the X-15 gave North American engineers some of the first large-scale experience with the newest high-strength alloys of titanium and stainless steel. The main propellant tanks formed an integral part of the fuselage, and after a great deal of investigation, North American manufactured the barrels from Inconel X. The experience gained from building the X-15 provided lessons used during the construction of the Apollo capsules and space shuttle orbiters. (North American Aviation)

North American gained experience in manufacturing the propellant tanks and fuselage structure long before it manufactured the first flight airplane. The company constructed three partial fuselages as ground-test articles for the rocket engines. Reaction Motors at Lake Denmark received two of these, while the third went to the Rocket Engine Test Facility at Edwards. Although

not intended as "practice," they did allow the workers in Inglewood to gain a certain level of expertise on a less-critical assembly before building the real flight articles.-171!

Forming the ogive section of the forward fuselage also presented some problems for North American. The usual method to construct such a structure was to form four semicircular segments of skin and weld them together. However, due to the size of the structure and the need to maintain a precise outer mold line, the engineers decided that the most expedient production method was to make a cone and bulge-form it into the final shape in one operation. The initial cone was made from four pieces of Inconel X welded together and carefully inspected to ensure the quality of the welds. It was then placed in a bulge-form die and gas pressure was applied that forced the part to conform to the shape of the die. This process worked well, with one exception. For reasons that were never fully understood, one of the four pieces of Inconel X used for one cone had a tensile strength about 28,000 psi greater than the others. During formation this piece resisted stretching, causing the welds to distort and creating wrinkles. North American eventually discarded the piece and made another one using four different sheets on Inconel; that one worked fine.^

Both titanium and Inconel were hard metals, and the tools used to form and cut them tended to wear out faster than equivalent tools used in the production of steel or aluminum parts. In addition, it took considerably longer to cut or polish compared to other metals. For instance, it took approximately 15 times longer to machine Inconel X than aluminum. This did not lead to any particular problems during the manufacture of the X-15 (unlike some of the tool contamination issues faced by Lockheed on the Blackbird), but it did slow progress and force North American to rethink issues such as machining versus polishing.!73!

The windshield glass originally installed on the X-15 was soda-lime-tempered plate glass with a single outer pane and double inner panes. Engineers had based this choice on a predicted maximum temperature of 740°F. Data obtained on early flights indicated that the outer face would encounter temperatures near 1,000°F, with a differential temperature between panes of nearly 750°F. It was apparent that soda-lime glass would not withstand these temperatures. The engineers subsequently selected a newly developed alumino-silicate glass that had higher strength and better thermal properties as a replacement. The 0.375-inch-thick alumino-silicate outer pane withstood temperatures up to 1,500°F during one test. The next test subjected the glass to a surface temperature of 1,050°F with a temperature gradient from the outer to inner surface of 790°F without failure. In actuality, the thermal environment on the X-15 glass was more complicated, although slightly less severe. The outer surface could reach 800°F, while the inner surface could reach 550°F; however, the inner temperature lagged behind the outer temperature. During rapid heat build-up on high-speed missions, the maximum temperature differential reached 480°F at a time when the outer glass was only 570°F. At this point, both the outer and inner panes began to rise in temperature rapidly.774

Technicians at the Flight Research Center installed the alumino-silicate glass in the outer pane of all three X-15s, although they continued to use soda-lime plate glass for the inner panes until the end of the program. Corning Glass Company supplied all of the glass. The thermal qualification test was interesting. Corning heated an 8.4 by 28-inch panel of the glass to 550°F in a salt bath for 3 minutes, and then plunged it into room-temperature tap water. If it did not shatter, it passed the test.!75!

welding, and otherwise joining this material to make a practical machine." Storms described special techniques for contouring the skins that involved hot machining, cold machining, ovens, freezers, cutters, slicers, and rollers. For instance, one special tool fixture needed to control the contour during a heat-treating cycle of the wing skin weighed 4,300 pounds, while the skin it held weighed only 180 pounds. Despite the publicity normally associated with the use of Inconel X, Charlie Feltz remembered that titanium structures gave North American the most trouble. Fortunately, the use of titanium on the X-15 was relatively small, unlike what Lockheed was experiencing across town on the Blackbird.[76]