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Aviation History
1958
1958 - 0464.PDF
11 April 1958 479 HIGH-ALTITUDE FACILITY BRISTOL AERO ENGINES' NEW PLANT; 100.000FT AND MACH 3.5 CONDITIONS AS the aircraft designer's sights are raised from target speeds/\ of Mach 1, through Mach 2 to the verge of the so-called JL -^- "thermal thicket," so has the engine designer's interestin the ramjet increased proportionately. There are two reasons for this, one positive and the other negative: the ram compressiondiffusible from the high-velocity airflow reaches really worthwhile values, and at the same time the heat of compression becomesembarrassing to the turbine powerplant. Add to this the lightness and simplicity of the ramjet and it becomes an intrinsicallyattractive proposition for the flight regimes of the immediate future. Perhaps one may be permitted a brief recapitulation on theramjet (or athodyd), the rotationless virtues of which haye lured designers since Rene Lorin first propounded the propulsive ductalmost half a century ago. Rene Leduc succeeded in propelling an aeroplane by ramjet soon after World War II, and has sinceproduced a classic series of all-ramjet machines. The Arsenal de I'Aeronautique has done much work on subsonic ramjets, cul-minating in the supersonic dual-power Nord-Ayiation Griffon intercepter and the Nord ST450 supersonic ramjet test vehicle.German wartime propulsive-duct schemes, notably by Pabst, were ambitious, but aeroplane speeds did not then provide the ramcompression necessary for practical economy. This initial work was elaborated in the U.S.A. by Marquardt, McDonnell andWright in the supersonic field, whose ramjets are respectively fitted to the Boeing Bomarc, Bendix Talos and N.A.A. Navahomissiles. In Great Britain, Bristol Aero-Engines has been developing theramjet for the last decade, concurrently with the guided-weapon development of Bristol Aircraft, and the companies have achievedthe joint success of the Bloodhound contract for the R.A.F.—a significant foot in the door of Mr. Sandys' all-missile defence era.In parallel, here has also been the N.G.T.E./Napier basic ramjet research programme, which recently achieved the record altitudeof 114,000ft, with combustion data recorded at 70,000ft and Mach 2.3. This work has, however, been largely subsequentto the Bristol programme, in which, apart from some initial help from Boeing, the production design has been based squarely uponthe results of several hundred Bristol test-vehicle (Bobbin and other types) firings. Bench running has hitherto been limited tosynthesizing supersonic flight conditions up to 30,000ft because the jet outlets have had to exhaust into sea-level atmosphere. Two years ago it became obvious that the manned aircraft couldconceivably follow the winged missile into the M = 2.5 to M=4.0 zone in the upper atmosphere, which suggested a requirement forcontinuous running, long-life ramjets. While the firing of expend- able and recoverable test vehicles had made possible the develop-ment of operational engines with a life of a few minutes, something more was required for, say, a high supersonic dual-power fighter Steam from the silencing tower billows away towards the River Severn, carrying off waste heat from a Bristol supersonic ramjet under test. powerplant. Today one envisages an even longer endurance ram- jet application in the high supersonic transport. Of course, in terms of aero-engines as we think of them tdday, flight times will be brief—under two hours for the North Atlantic. It was to keep Britain in the forefront with such developmentsthat Bristol Aero-Engines designed and built the High-Altitude Test Plant, at Patchway near Filton. The problem with facilitiesof this nature is to apply what B.E.A.'s chief engineer calls "crystal- ballmanship" to the needs of the next generation of engines, andso strike such a balance that the heavy engineering of construction can be completed in time for the research to be usefully applied.The ingenious Bristol solution was to swap the doubtful advantages (in the ramjet context) of continuous operation for the certain onesof halved construction time and a comparatively modest capital investment barely reaching seven figures. The technical convolu-tions necessary to provide an extremely low-density atmosphere and, at the same time, a large airflow at high velocity requires acomplex circuit and considerable infrastructure. Add to this large-capacity heating and cooling requirements and even asupersonic wind tunnel begins to appear simple by comparison! In Bristol's case, a source of compressed air for feeding thetest engine was already available from a centralized supply in the air compressor house (No. 1 Air Station) which feeds several testfacilities.* The first problem, therefore, was to establish a means of evacuating the test cell, and this has been done by an ingeniousadaptation of the blow-down wind tunnel technique. Steam is ejected through six nozzles connected to the test cell so that theair therein is entrained and the required depression induced. Steam was chosen in preference to compressed air because it is cheaperto produce and is relatively easy to store. In the present installa- tion there are three oil-fired boilers, together capable of generatingsteam at the rate of 38,000 lb/hr; 98,000 lb of steam can be stored at 250 Ib/sq in, giving a seven-minute run at the maximum dis-charge rate. This system is well suited to extension in parallel or series—there is ample open space adjoining the test house—andMr. Greenhough, Bristol Aero-Engines' engineer-in-charge of ramjet test facilities, considers that the HATP is capable of expan-sion to the foreseeable limit of air-breathing engine test require- ments. Equally, should the volume of the work increase, thefacility could be extended by the addition of a second test cell. The basic portions of the circuit can simulate the supersonicairstream encountered by the intake up to a Mach number of 3.5, and the low-density air of the upper atmosphere up to 100,000ft.There remains the effect of aerodynamic kinetic heating. Acceleration of the hot, high-pressure air to a supersonic airflowby delivering it through a venturi nozzle also reduces its tempera- ture. It is therefore necessary to pre-heat the air, so that the engineintake will meet it in conditions representative of a ramjet travelling rapidly through thin air—not those of a stationary engine in ahigh-velocity airstream. It is one of those confusing seeming contradictions which one sometimes encounters in the AliceThrough the Looking Glass of wind-tunnel work. The total tem- perature includes the kinetic energy present in the flying engineplus the Shockwaves of its centrebody intake, whereas the test engine has to have the kinetic heat energy supplied artificially tosupplement that from its own conical and normal intake shock- waves. For example, to simulate a flight speed of M=3.0 in thestratosphere (—56.5 deg C ambient temperature) the blowing nozzle must deliver air at 330 deg C. Parallel installations, or by-passes, in the air-delivery systemare used to adjust the delivery temperature. On the one hand there are two rube-matrix water coolers (also in parallel) which canreduce the full mass flow of 150 lb/sec from 260 deg C to 95 deg C for an indefinite period. On the other hand, there is one largeelectric-matrix storage heater of 4.5m C.H.U. (8.1m B.Th.U.) capacity, which is charged in four hours by l,500kW elements.At maximum rate this unit will raise an 80 Ib/sec airflow from 260 to 450 deg C for a period of ten minutes. Reference to the perspective and schematic diagrams show thatthe foregoing statement of requirements and the way they have been met is an over-simplification. In practice, there is a great dealof interdependence—and, of course, numerous safety precautions. What is not apparent in either diagram—although the data andsome of the photographs help—is the truly massive scale of the "boilery." Air arrives from the compressor house, through a 36in steelpipe carried on reinforced concrete bipods, and enters the upper story of the main building. Here, in parallel, are the twin coolers,the storage heater and a direct-delivery pipe. The massive piping and ship's boiler-like drums of the coolers and heater are laggedto a thickness of several inches with asbestos compound. The room is largely glass walled, but at one end are batteries ofLockheed hydraulic accumulators for some of the valve-actuating *"Flight," May 9, 19S2, page SS9.
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