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Aviation History
1961
1961 - 1867.PDF
Above, simulated kinetic heating test on a Bloodhound rear-fuselage. Right, a launcher with its power supplies undergoing a rainstorm test at the outdoor test-site rubber cords in the normal manner for structures of this size,and is then sweep-frequency excited with constant force by small electro-magnetic vibrators. Vibration parameters are measuredby piezo-electric accelerometers, in conjunction with manual or automatic resolving and plotting equipment. In missiles, vibration as an environmental condition is a reliabilityhazard comparable in severity to kinetic heating. The principal sources of this vibration are the periodic and random engineforces, plus radiated sonic energy and aerodynamic turbulence or boundary-layer "noise." Propulsion, guidance and control elements are tested in a simu-lated flight vibration environment in a sound-enclosing chamber which houses two large electro-magnetic vibrators and theirforced-air cooling systems. The larger vibrator, driven from two lOkVA audio amplifiers in series, develops a peak thrust of 10,0001bat frequencies up to 1 kc/s; the smaller, driven from a single 1 OkVA amplifier, develops a peak thrust of 3,0001b with a level responseto 5kc/s. Each vibrator is rated for a maximum peak acceleration of lOOg, vector. Both vibrators are mounted on a 40-ton concreteblock sunk to floor level and isolated from the building foundations by supporting rubber sections. Each component or assemblyis operated in a controlled vibration environment with frequency and amplitude characteristics representative of in-service conditions,often combined with low or high temperature conditions or with temperature cycling. The flight vibration environment is determined from magnetic-tape recordings or telemetered accelerometer signals, supplemented from instrumented ground-level engine runs and missile staticfirings. A number of recordings taken from several general areas within the missile body are analysed by playback through a seriesof narrow-band filters. An envelope containing the acceleration/ frequency relationship is derived, which, after the addition ofsafety factors, is applied as the acceptance test specification. Current official environmental specifications and normal design-proving checks are met by subjecting the specimen to a programme of sine-wave excitation, wherein the applied peak acceleration isvaried appropriately as theifrequency range is continuously swept. "'Equivalent sine-wave " testing is an expedient adopted to enableenvironmental vibration tests to be made with the basic minimum of equipment. It is sufficient to meet most current environmentalspecifications, but it is recognized that the vibration experienced by most missile components is random with time rather thanperiodic, with the characteristics of a continuous frequency spec- trum. Random motion excitation is therefore almost certain to gainacceptance in the near future as the preferred technique. Random motion is already applied to Bloodhound components inresearch and development testing as a complement to, and confirma- tion of, equivalent sine-wave tests. The noise signal is normally thatgenerated by an electronic (i.e., "'white noise") generator, rather than a playback of recorded in-flight noise. Kinetic heating, vibration, shock and sustained acceleration areall typical induced active-life environments. The active-life of a guided missile is, however, only a very small part of its totalin-service life. Indeed, the fatigue and wear design cases of a missile are provided by the ground drill and performance checkingneeded to maintain its state of readiness. The natural environments in which this state of readiness must be maintained become, there-fore, an important area of study. The climatic simulation section of Bristol Aircraft's laboratory investigates the effects of dry anddamp heat, sunshine, dust, driving rain, salt spray, low tempera- tures, snow, and deterioration due to storage and age, such ascorrosion and fungus growth. The problems may often be aggra- vated by storage under tarpaulins or by transportation in a ship'shold, and the performance and durability of all types of mechanical and electrical equipment, materials, sealants, finishes and packag- ing must be proved under many combinations of conditions.Climatic extremes at ground-level ambient pressure are repro- duced in one chamber, in which the air temperature can be variedunder fully automatic control between ±70°C. With all conditions, vigorous air circulation reduces the temperature gradients in theworking area to ±1°C. Relative humidity is controlled up to approximately 95 per cent RH within the dry-bulb temperaturerange 20 to 50°C, and is generated by water injected as a fine vapour into the airstream. Saturation is possible. To simulatedriving rainstorms, water can be discharged into the chamber at up to 25Ogal/min through 44 nozzles. A smaller chamber has a temperaure range of —75° to 100 C,and the same humidity range as the larger chamber, but can also be evacuated to simulate altitude conditions up to 120,000ft. Twin45 h.p. evacuation pumps can simulate rapid rates of climb, and automatic pressure control enables pre-selected levels to be main-tained. This chamber is used for sub-assembly and equipment testing, and for materials evaluation. Dust storms or the dust clouds caused by travelling over dirtroads can cause excessive wear or seizure of moving parts. These conditions are simulated in another chamber. Finely groundquartz dust is uniformally distributed by fans housed in the dust hoppers beneath the grating floor. During a test the internal airtemperature is raised (to a maximum of 40 C) and the dust con- centration, distribution and particle size are checked by sampling. In another chamber, the salt-laden atmosphere prevailing atmany coastal sites is reproduced. An aqueous solution of salts is sprayed into the chamber, and the missile remains for four weeksin temperature and humidity conditions conducive to corrosion. Smaller, specialized chambers provide for the study of suchproblems as resistance to mould growth and the effects of hydraulic oil contamination. Tests are also carried out on the effects of longexposure to tropical day and night temperatures and humidity variations, icing conditions, exposure to sunlight, and otherclimatic conditions. Weathering trials of a long-term nature are made at outside sites, where the launcher and its power suppliesare mounted on foundations representative of an actual launching site, with similar natural drainage, and checked daily throughouta month of operation. Finally, the laboratory is equipped with a wide range of apparatusdesigned specifically for the realistic development and acceptance of missile packaging. Shocks caused during rail shunting or by acrane swinging the package against the side of a ship's hold are simulated by placing the package on a low carriage which runs downa railway inclined at 10° and fetches up against a buffer. Measure- ments of the induced shock to the missile at impact are taken atseveral points on the airframe and in each plane. Rough handling shocks, such as might result from topplingfrom the tailboard of a lorry, are simulated in drop tests, and bump test machines subject specimens to repetitive decelerations ofabout 40g severity at a frequency of 2-3 bumps per second to simulate the bumping and bouncing of an unsecured lorry load.A mechanical low-frequency vibrator reproduces the considerable vibration to which rail and road packages are exposed. Compre-hensive testing on these machines is punctuated by periods of storage in dry heat and damp heat in the climatic chambers, and'ofexposure to the water spray in the rainstorm facility. The missile normally travels uncovered during local transportation and groundhandling. The trolleys and mechanical handling equipment are proved to establish their ability to withstand fully-laden towing atspeeds of up to 30 m.p.h. over third-class roads.
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