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Aviation History
1956
1956 - 1014.PDF
160 FLIGHT, 27 July 1956 ARMSTRONG SIDDELEY SCREAMER A Powerful Rocket Motor from Coventry : - IT is a curious fact that, outside the U.S.A., only one com-pany has any considerable background of experience withliquid-oxygen aircraft rocket motors. This company is Armstrong Siddeley Motors, Ltd., whose Rocket Division is based at Ansty, near Coventry. In the years following World War 2 their embryo rocket staff—under Mr. S. Allen, chief engineer—"got their feet wet" with a 2,000 lb-thrust unit named Snarler. This motor ran on liquid oxygen and methanol, the drive to the pumps being taken from an external source, such as the wheel-case of a turbojet. The Snarler was flight tested in 1950 and a full description appeared in our issue of August 6, 1954. No production order was placed for the Snarler but the Ministryof Supply asked Armstrong Siddeley to develop a larger unit capable of meeting the requirements of future designs of super-sonic fighter. The new project was given the name Screamer and development went ahead upon an engine incorporating its ownpump drive, so that, if necessary, it could be used as the sole means of propulsion for an aircraft. From the outset the normalrequirements for engines intended for piloted, aerobatic aircraft had to be met in full, and a further requirement was that thethrust of the Screamer should be variable over a wide range. At the start of design in 1950 the Screamer was intended to give4,000 lb thrust at sea level. The first work—which preceded the development contract—was concerned with an up-rated Snarler,using liquid oxygen and methanol fed by pumps driven from an external source of shaft power. The next step in developmentwas to incorporate a gas generator and turbine to fulfil the latter function, thus making the unit self-contained. Following thisstep it was decided to redesign the motor to enable it to run on liquid oxygen and kerosine or wide-cut petrel; although seem-ingly unimportant from the design point of view, diis actually required more development man-hours than did any other singledesign change, largely owing to the significantly increased flame- temperature. The first run with kerosine took place in 1951. Shortly afterwards an aircraft application became foreseen. Asis frequently the case when this occurs, the thrust requirements began to rise, first to 6,000 lb and then to 7,000 and 7,500 lb. Atone period in the development the Screamer design specified two combustion chambers, the larger chamber being required onlyfor periods of maximum thrust. By June 1952, however, the design was frozen with a single chamber which reduced theweight and base drag, more than counteracting the slight rise in cruising fuel consumption. The high design pressure in the com-bustion chamber is an aid to obtaining good specific fuel consumption at partial thrust and, at the high altitudes wheresuch low thrust would be most probably required, the low atmospheric pressure assists in keeping the expansion efficiencyat a high value. The Screamer chamber was designed with an expansion cone correct at full thrust at an ambient pressure of6.67 lb/sq in, corresponding to about 20,000ft altitude. At mini- mum thrust the pressure corresponds to 68,000ft. As the total technical and design staff at this time numberedbetween 20 and 25, the magnitude of the task of developing a motor of the calibre of the Screamer could be described as awe-some. In order to attempt to quicken development and get a prototype Screamer into the air, it was decided to employ wateras a third propellant. During the early years of Screamer develop- ment there was very little design knowledge of gas generators—for driving turbines—burning a mixture of liquid oxygen and hydrocarbon fuel, and die injection of a suitable amount of water(to form super-heated steam) resulted in a final flow of gas to drive the pump turbine more akin to that from high-test peroxide,albeit with some carbon dioxide present. Because water was being carried for the gas generator, it wasdecided to use its excellent cooling properties in the combustion chamber jacket, the water then being injected into the com-bustion chamber as a third propellant. The advantages obtained in cooling were considered to outweigh the loss in specific im-pulse due to lowering the temperature of the combustion gas. In any case, this loss was only slight because of the effect ofwater in reducing the mean molecular weight of the exhaust gas. It was envisaged that, shordy after the first flight, the watersystem would be deleted, and by the time the Screamer was cleared for flight at the end of last year, the gas generator hadbeen operated with an excess of oxygen and the combustion chamber had been cooled by hydrocarbon fuel. Conversely, ofcourse, the use of water increased the general circuit complexity by more than 50 per cent, and, since no restrictions were placedon the type of water employed, deep anodizing and chromate sealing was necessary to prevent corrosion of aluminium parts. Particularly in the development of the pumps, valves and seals,the previous experience of the Snarler programme proved invalu- able and, in fact, Snarler pumps were used as a starting point.All three pumps are designed to run at a maximum of about 20,000 r.p.m., at which speed the delivery pressure is 900 lb/sq infor the liquid oxygen and 850 lb/sq in for the water and hydro- carbon fuel. The liquid-oxygen pump has a five-vane impellerwith a screw booster, in S.I 10 steel (unlike the Snarler pump, the impeller is not shrouded), running in a cast, stainless casingincorporating a vapour bleed valve similar to that on the Snarler. Owing to the high rubbing speed, it was decided to employ aface seal of fluon-impregnated sintered bronze, 0.025in thick, on a steel backing, spring-loaded against a fixed Hecla washer groundoptically flat within one Newton ring. Whereas in the Snarler one of the bearings was unlubricated, all bearings in the Screamerliquid-oxygen pump are lubricated by E.E.L.3 synthetic oil circu- lated from a pump on the gearbox, with precautions to preventit from coming into contact with the liquid oxygen. Special Durestos sleeves and washers are employed to insulate the coldestportions of the pump from the remainder. The fuel and water pumps both employ similar open, five-vane impellers in alum-inium-alloy volute casings with a stationary carbon-faced seal and bellows. Pump development was largely done with electricallydriven rigs pumping water. Early development of the turbine was done at Ansty's steamplant used for gas-turbine research. Initially much effort was expended upon high-speed rotors resembling sophisticated Peltonwheels, but the final design is a more conventional axial-flow impulse turbine of the supersonic Laval type. Owing to thehigh blade loads the design was more arduous than for correspond- ing gas turbines, the gas velocity being about 4,000 ft/sec (at625 deg C) and the peripheral speed of the turbine rotor being 1,100 ft/sec. The final design employs a Rex 448 disc (of about13in diameter) broached to receive the fir-tree roots of 43 1.75in Nimonic 80 blades. At 20,000 r.p.m. this wheel delivers some350 s.h.p. It drives a gearbox with 25-deg pressure-angle 10 d.p. teeth loaded at 1,000 lb per inch of face-width. On the boxare mounted pumps for the three propellants, together with scavenge and pressure pumps for the E.E.L.3 oil, a tachometer/generator and, should the application require them, a pair of
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