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Aviation History
1968
1968 - 0225.PDF
filGHT International. IS February I96S 219 By the Mach 6 X-15 and "The High and the Mighty One" pulling away from Edwards AFB. The symbolism above the "USAF" insignia represents the number of successful missions (vertical lines) and the number of abortive flights (horizontal lines) THE LECTURE BY MAJ WILLIAM J. KNIGHT, USAF, tO the RoyalAeronautical Society on January 30 was attended by agoodly audience, as is usually the case when flying men gather to talk about their pet subject. Especially so as the audience happened to be the RAeS Test Pilots Group and the lecturer was the World's fastest non-spaceflight pilot. Maj Knight has been associated with the North American X-15 over a two-year period and, in particular, with the No 2 aircraft, the X-15-A2, in which he reached 4,534 m.pJi. (Mach 6.72) on October 3 last year. A fortnight later, flying another X-15, he obtained an astronaut rating by reaching 280,000ft, thus exceeding the minimum 50-mile requirement. Three X-15s were built by North American in the late 1950s. The first is flying in virtually the standard in which it was delivered. The third had an experimental flight-control systemI which was to be eventually coupled with on-board energy- management display. This vehicle was lost, together with its pilot, in an accident last November. The subject of the lecture was the second aeroplane, which had been extensively modified for flight at speeds up to Mach 8 in support of an experimental ramjet programme. While it was being rebuilt, following a crash in 1962, the opportunity was taken to add a 29in long section to the fuselage to provide Maj William J. Knight, USAF—in flight-configured, non-lecture clothing space for the ramjet's liquid-hydrogen tank. In order to boost the X-15 to Mach 8, provision was made for the carriage of two external drop tanks, each measuring some 25ft long and 40in in diameter. These are mounted one on each side of the fuselage and carry a total of 13,5001b of propellants. Much of the leading edge of the lower fin has been removed in order to accommodate the ramjet. These modifications, said Maj Knight, had adversely affected stability about all three axes. Although the external tanks were dimensionally similar, thek different weights when fuelled (one with liquid oxygen, the other with anhydrous ammonia) caused a lateral unbalance. Also, since there were no baffles in the tanks, the effect of forward acceleration and the climb angle caused the liquids to settle to the rear of the tanks, resulting in a rearward e.g. shift. Finally, the cutting away of a large section of under-fin resulted in a loss of directional stabiUty, particularly at high angles of attack, when the upper fin was partially blanketed. These changes have increased the gross weight from 35,0001b to 53,0001b; the original 200 sq ft wing has been retained. A new surface finish was developed by Martin to protect the structure from the effects of the greater kinetic heating— up to 2,500 °F—at the higher speeds, as the Iconel-X skin lost integrity after l,300°F. The entire airframe was chemically cleaned and sprayed with a silicon plastic coating, which was next trimmed and sanded to correct thickness (depending on temperature) and profile. The airframe was then sprayed with a white sealant which not only preserves the lox-sensitive plastic from contamination but also hides its garish pink colour. After painting, the aircraft undergoes a weight and balance check and an engine run to test the 57,OOOlb-thrust Thiokol YLR-99 powerplant and also ensure that the fuel transfers satisfactorily from the drop tanks. The X-15 is then mated to the B-52, after which the Marquardt ramjet engine, of 2,0001b thrust, is mounted on the former. The pilot enters the X-15 about an hour before take-off to go through the check list. Visibility from the X-15-A2 is con- siderably worse than with the other aircraft of the series because, as part of a thermal-stress relieving exercise, the wind- screen pillars were rounded off, necessitating smaller, oval transparencies. "The design team asked if I thought the original rectangular transparencies were adequate. I said yes, so they went away and cut the area down by half," commented the lecturer dryly. It was found that the plastic coat disintegrated to some extent at high speed, covering the windscreen with an opaque film; fragments also found their way down the pitot tube ("Both of which stimulated adrenalin flow but were other- wise depressing.") A mechanical eyelid-type shutter was, there-
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