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Aviation History
1959
1959 - 1315.PDF
640 FLIGHT initially three slabs welded together edge-to-edge. Special techniques hadto be developed, including hot and cold machining, and ovens and freezers capable of taking major airframe sections. Owing to the necessity ofmaintaining very fine tolerances at all stages, "hard" jigging and tooling . has been employed throughout. As an example, one jig which determinesthe contour of the wing skins during their heat-treatment cycle weighs 4,300 lb, although the finished skins weigh but 180 lb. Most small partscan be heat-treated without retention in jigs, but during water-quenching a considerable amount of warpage was experienced in the early stages 'of development. Scale-inhibiting material is employed widely. In sharp contrast to normal practice, some 65 per cent of the airframejoints are welds. Here again very extensive research was necessary before the optimum fabrication procedures could be formulated. As might beexpected, the ductility of Inconel X is a function of the hardness, and the lattei must not be too high if the welded part is not to crack as a resultof weld shrinkage or stresses induced during the welding cycle. Certain sheet parts are defoimed to such an extent during their forming that theyhave to be annealed before being welded, and all bar stock is so treated to reduce its hardness. Since welding must be done before heat-treatment,the size of the heat-treat furnace imposes a limiting factor. For this reason ordinary Inconel (not Inconel X) is employed for the tank-endbulkheads. Inconel, unlike its more sophisticated partner, does not respond to heat-treatment and can be welded after it has gone throughthe Inconel X heat-treat cycle. A great deal of flame cutting and chemical milling is employed, sinceInconel X is not particularly easy to cut by conventional methods. Par- ticularly noteworthy members arc the leading edges of the wings andelevons, since during the re-entry these are expected to get hotter than any other part of the aircraft. Each is milled from a single slab ofInconel X; the final nose radius on the wing is O.375in and each leading edge is cut into 12in sections to minimize warping. A substantial proportion of the structure is stabilized by corrugations Aerodynamic and jet-reaction controls are disposed in an unusual manner (right). A, elerons; B, upper rudder; C, lower rudder (jettison- able); D, air brakes; E, landing flaps; F, roll jets; G, pitch jets; H, yaw jets; J, control-system H.T.P. tanks; K, pilot controller (left wrist) or by welded doubler sheets with parallel fluting. Most spar webs arecorrugations, and the tailplane spar is fabricated to form a single Inconel X unit, complete with the root rib and an inboard box section carryingthe tailplane pick-up hinges and actuator lugs. The complete aircraft is sprayed with a jet-black paint with a silicone-type base, which will withstand temperatures greater than 1,000 deg F for short periods. Although much thinner th&n the normal paint skin ona car, the black surface is expected to last for several nights. It has a high radiation rate, to disperse the heat generated during re-entry, and issprayed on over the oxide formed during the heat-treatment of the Inconel X skin. Propulsion. In its final form the X-15 will be powered by a singlerocket engine with a fixed chamber running on liquid oxygen and liquid anhydrous ammonia. Designated XLR-99-RM-1, this powerplant hasbeen developed for this application by the Reaction Motors Division of Thiokol Chemical Corporation. Propellants are stored in two large tanks within the fuselage.Unexpectedly these tanks are not integral but are retained within the outer skin by special pick-ups on the ring bulkheads and the main torusrings to which the wing A-frames are attached. Tank pressurization and purging is accomplished by a helium system fed by a 4,000 lb/sq in sphereat the rear of the aircraft. The propellants are fed to a turbopump driven by H.T.P., and at the maximum flow-rate of more than 10,000 lb/min therated sea-level thrust is "more than 50,000 lb" (implying a thrust well in excess of 60,000 lb in vacua). The LR-99 is designed to be fully con-trollable from 100 per cent thrust down to 50 per cent. X-15 . . .
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