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Aviation History
1959
1959 - 1314.PDF
FLIGHT, 8 May 1959 639 200 160 X-15 40 THE WORLD'S FASTEST AND HIGHEST-FLYING AEROPLANE WALKING around the X-15 one sees a rather missile-likeaircraft, with a long cylindrical body, small stubby wingsmounted well aft and a tail which would be fairly con- ventional were it not for the great size and extraordinary cross- section of the vertical surface, which terminates in a trailing edge almost 2ft thick. Altogether the aircraft is far from beautiful, and it gives little hint of its stupendous performance. In fact, it is designed to be more than twice as fast and fly several times as high as any other aeroplane yet built. Any vehicle is a compromise in design, and extreme performance can be obtained only at the expense of certain other factors. In the X-15 one limitation is that it has to be carried into the air by a parent aircraft, or by riding pick-a-back on a rocket package. Another is that the payload consists of one human pilot and a little over 1,300 lb of instrumentation. One might well ask why the X-15 has to be built at all, since most of the results which it will obtain could be either simulated in the laboratory or obtained by existing unmanned vehicles. The answer is simple; man has always wished to fly faster and higher, and the X-15 happens to be the vehicle chosen to obtain first-hand knowledge of what con- ditions are like beyond the atmosphere. It will carry equipment to measure the pilot's reaction on the controls and the effectiveness of these controls; it will measure structural and aerodynamic loads, and the kinetic heating and heat-transfer between components; and, being re-usable, it should provide in a few weeks more information on atmospheric exit and re-entry than could be obtained from a thousand pilotless and expendable missiles. The tables on pp. 642-3 outline the milestones in the develop- ment of this remarkable aeroplane and the monetary vote allocated to it. Contrary to what might be expected, the programme is a national effort, managed jointly by the United States National Aeronautics and Space Administration, Air Force and Navy. It was in the spring of 1952 that the National Advisory Committee for Aeronautics (as the N.A.S.A was then known) directed its laboratories "to study the problems likely to be encountered in flight beyond the atmosphere and recommend methods to explore these problems." After evaluating the suitability of simulated laboratory research, pilotless test vehicles and aeroplanes, the N.A.C.A. decided in favour of an aeroplane. Basic Design The diagram below indicates an approximate performance envelopefor the X-15, and emphasizes the dramatic increase in speed and altitude capability which this aircraft will achieve. It is therefore asine qua non that its design should pose entirely new problems, par- ticularly in the fields of aerodynamics and control, structures, accessorysystems and human factors. The configuration chosen is surprisingly conventional. The fuselageis a cylinder of oval cross-section. Along either side are disposed large fairings, which suit the aerodynamic profile and body/wing junction andprovide accommodation for the numerous fuel, control and instrumenta- tion lines. The stubby wings are sharply tapered and have a modifiedtrapezoidal section. The configuration of the empennage can be seen clearly in the illustrations. Details of the control system are related in alater section. Propulsion is provided by a single rocket engine, pump-fed with liquidpropellants from tankage occupying a major proportion of the fuselage volume. Immediately ahead of the tank bays is the main accessory andinstrumentation compartment, and forward of this is the cockpit contain- ing an upward-ejection seat. One of the most striking features of theaircraft is the thick wedge-section vertical tail. This presents reasonable drag at extreme Mach numbers and gready increases weathercock \ These curves give an approximate indication or the vast increase in speed I altitude capabi- lity of the X-15 over some of its experimental predecessors — whicn were themselves no mean performers stability, since at such speeds conventional sections have a low recoveryrate following yaw disturbance. At high angles of attack the upper sur- face is partially blanketed, and accordingly an under-surface is added, thelower rudder being jettisoned before landing. Landing is effected on rear skids and twin nosewheels. The latter are not fitted with either brakesor a steering system owing to the high-temperature environment; for the same reason main wheels could not be employed. Structure. In the design of the X-15 airframe the most severe problemwas posed by the fact that almost the entire aircraft might experience peak skin temperatures as high as 1,000 deg F. In fact, one of the mosttrying aspects of the environment was the impossibility of predicting the expected temperatures with any high degree of accuracy. A numberof types of structure were investigated, some in considerable detail, but in the event the choice fell upon an airframe fabricated from a mixtureof stainless steels and titanium beneath an outer skin of Inconel X. Aluminium alloys are restricted to internal applications where theambient temperatures and stresses permit their use. Inconel X, produced by the International Nickel Company, has beenwidely employed in gas turbines and in many other hot and corrosive environments, but its use in an airframe—and on such a scale—isunprecedented. According to the type of heat-treatment and fabrication cycle employed, the alloy can be made to show optimum mechanicalproperties above 1,100 deg F or below. In the X-15 the greatest stresses should be experienced within the atmosphere at relatively low tempera-tures, and for this reason the moderate-temperature form of the alloy has been adopted. Sheet and plate are received by North American after annealing at1,900/2,000 deg F for 15 to 30 min followed by quenching. International Nickel then suggest stress-equalizing at 1,625 deg F for 24 hr, ageing at1.300 deg F (N.A.A. retain this temperature for 3 hr) and further stress- relieving at 1,625 deg F for 4 hr. The latter heating has to be extremelyrapid to prevent the alloy from becoming age-hardened while in the low- ductility temperature range for age-hardened alloy (1,200 to 1,550 deg F),and this in turn places an upper limit on the material sections which can be employed. Inconel X is an extremely strong and tough material and it is appliedin relatively large sheets. Four are used to make each main tank section and the upper and lower surfaces of the right- and left-hand wings are -" X-15
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