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Aviation History
1968
1968 - 0226.PDF
220 BY THE MACH 6 ... fore, fitted to one windscreen transparency so that it could be closed at high speed and opened during approach and landing. A retractable pitot has been incorporated, also for approach and landing. During the climb to 40,000ft the internal fuel-jettison system is checked out, the X-15 lox tank then being topped up from a reservoir aboard the B-52. The X-15 is normally launched over Mud Lake, some 200 miles north of Edwards AFB. Maj Knight described various emergencies which could happen at separation as a result of engine or fuel-flow failure; for instance, if the fire went out, instant action with the tank jettison switches was necessary to save the aircraft. Assuming all went well, the X-15 was allowed to drop 2,500ft-3,500ft before pulling up into the climb. Although originally a 7.33g airframe, the 50 per cent increase in launch weight restricts this to 2.5g and, in practice, the pull-up is made at 2g and takes about 20sec. The engine (which has a design life of 30min, with a time between overhauls of lOmin) consumes fuel at the rate of 2191b/sec and the external tanks are jettisoned about 60sec after ignition. A speed restriction, applicable with externals, necessitates closing down the engine and abandoning the flight should the tanks fail to jettison. Maj Knight compared the X-15 with a three-stage rocket: the B-52 first stage, a booster, provided Mach 0.8 at 40,000ft; the external tanks were equiva- lent to a second stage giving Mach 2.2 at 70,000ft; and the clean aircraft was the third stage and instrumented capsule. Great care was necessary, said the lecturer, to adhere to the planned flight profile if the calculated speeds and heights were to be realised; there was no room for seat-of-pants flying. Inertial readings of speed, altitude and rate-of-climb were used during the high-speed stage of flight, with reversion to "suck- blow" instruments during the terminal stages. The aircraft was flown largely on angle of attack, measured (together with yaw) by a sensing ball mounted on the nose of the fuselage. During flight, maximum q (dynamic pressure) values of about 4001b/ sq in have been recorded and the ball glowed red hot, although it couldn't be seen from the cockpit, a fact for which the speaker was grateful. No autostability was fitted but pitch and roll damping was provided. An excellent adaptive flight control system com- manded a rotation rate proportional to stick displacement. In general, said the lecturer, the X-15 handled as well as any other aircraft he had ever flown; only at low angles of attack was lateral control rather sensitive. During the main part of the flight the pilot is unable to judge a pattern which will enable a safe landing to be made at any given location. This is due mainly to the very low values of L/D which are characteristic of hypersonic aircraft such as the X-15. Real-time energy-management techniques arc employed throughout the flight to decide which landing areas FLIGHT International. 15 February 194, are available and to compute the flight-path necessary j0 position the aircraft for landing. It was Maj Knight's opini0. that aerodynamic, orbital re-entry glide vehicles, with even worse glide ratios, would have to be fitted with auxiliary engines to correct errors in the final approach and ensure consistently safe landings. Disorientation could also be a problem, said the speaker i reply to a question. Until the push-over (at the top of the climb) was well advanced, there was almost no view of the ground. The loss of the No 3 aircraft may have been partly attributable to this cause, compounded by the characteristics of the control system used. Cockpit film revealed that a yaw angle of 90° had occurred, after which the aircraft entered a Mach 4.8 spin at about 230,000ft. Height was lost at the rate of about 30,000ft per turn, but the aircraft partially recovered at about 120,000ft. A divergent control characterise then caused break-up through overstressing. SCORCHING SHOCK-WAVE The X-15's total engine/burning time was about 150see, continued the lecturer. The final speed attained depended largely on the trajectory and it was planned to increase the former by Mach 0.5 per flight. Stagnation temperatures oi almost l,800°F were comfortably below the limiting tempera- ture—about 2,500 °F—of the protected airframe. Shock-waves impinging on the airframe, however, caused very much greater rises in temperature. During the record-breaking flight at the end of last year a shock-wave shed from the ramjet spike had intercepted the lower fin, causing a large area of structure to be melted away, thus indicating a temperature of about 3,000'F. The hydraulic circuits were damaged, so that the flaps could not be lowered for landing. The white finish clearly showed up the hotspots over the airframe. While the plastic skin underwent a number of distinct changes, dejpending on temperature, from crazing through charring to ablation, transition of the airflow from laminar to turbulent was clearly revealed by burn marks on the skin. A normal dead-stick pattern is flown at 350kt, reducing to 210kt over the boundary. Since the surrounding country is so featureless, estimating of altitude during finals is difficult; black lines have been painted on the lake bed to provide a reference. The glide characteristics of the X-l 5-A2 are not i good as those of the other aircraft, due partly to the increased airframe weight (the finish alone puts on 4251b) and p< to the poor flow over the erosion-pitted wing. Actually, said the lecturer, the worst part occurred just ate coming to rest at the end of the flight. A myriad vehicles, i seemingly at 100 m.p.h. and coming straight for the aircraft, was a horrifying sight. M.W. [The two remaining X-15s are to be retired at the end of this year,following cuts in the 1968-1969 American budget. See last week'i Flight, page 174.—Ed] X-15 No 2, with external M* and oval windscreen, seen befort application of the ablative ft® ^
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