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Aviation History
1956
1956 - 1015.PDF
FLIGHT, 27 July 1956 161 ARMSTRONG SIDDELEY SCREAMER. . . Dowty Vardel hydraulic pumps. The oilcircuit includes a heat exchanger cooled by the hydrocarbon fuel. Gas to drive the turbine is supplied bya gas generator upon which many develop- ment man-hours were expended, largely inconverting this unit from liquid-oxygen/ methanol to liquid-oxygen/hydrocarbonfuel. In its essentials it resembles a minia- ture combustion chamber. At its head isan igniter body, forming a throatless cham- ber of 0.375in diameter. On either side ofthe igniter is mounted a high-frequency plug, the electrode of each being hollowand terminating at a 0.02in nozzle through which fuel is sprayed to form an impinge-ment fan in the centre of the igniter. Liquid oxygen is sprayed from a 0.07in central hole in the head, so forming an oxygen-richmixture capable of being easily ignited by the high-frequency discharge from the plug-body to the electrode. The resultant jet of flame escapes through the water-cooledigniter body into a cup at the head of the gas generator proper. Here the flame meets liquid oxygen injected through two tan-gential swirl holes and fuel from three swirl injectors, forming a fairly oxygen-rich mixture which burns in the main body ofthe generator. An accompanying diagram shows the flow of water through the cooling scrolls in the chamber walls and alsoclarifies the method by which water is injected at the exit from the chamber. The resulting gas-flow has been found not toimpair turbine life and to eliminate flame or smoke from the exhaust; furthermore, in spite of the quenching with water, nocarbon is formed. Injection of water reduces the gas temperature to 625 deg C, the overall mass flow being about 2 lb/sec. Thegas duct feeds three nozzles spaced around 120 deg of the turbine casing. From the turbine the gas is exhausted through twinducts, via a snail-shell exhaust volute. It is in the combustion chamber that the Screamer showsparticularly marked departure from previous practice. Originally the work was centred upon orthodox throated chambers but itwas soon found that good results could be obtained from a throat- less chamber which, provided adequate performance can beobtained, is the optimum for low weight and good cooling. Theoretically the specific impulse is slightly reduced (comparedwith a throated chamber) owing to the fact that the heat is added to a rapidly moving gas stream. Various forms of chamber wererun on liquid-oxygen and kerosine in 1951 and 1952 and the throatless chamber was the pattern which posed the fewestproblems. The initial chambers were rated at 4,000 lb thrust and were fed from electrically driven pumps. Later, scaled-upchambers of 8,000 lb thrust were successfully tested, this thrust being first reached in September 1954. Many variations were studied, with a view to determining theoptimum method of injection of the water. One typical pattern of Screamer chamber, which is shown in the large drawing, acceptsthe water at the nozzle end; the water then passes round a cooling scroll in the divergent portion of chamber and back along theparallel section to a row of holes at the head of the chamber, through which it is injected to form a cooling film along the innerwall of the liner. In some designs of chamber axial coolant pas- sages were formed using soft iron wire stretched along the diver-gent portion of nozzle. Yet another type of chamber had five separate stages of waterinjection at various axial positions some 2-3in apart, fed with water from four external water pipes. Finally it was decided toinject all the water at the head of the chamber, with reverse flow through the double walls from an inlet at the nozzle end. Develop-ment was conducted with tubular slave chambers (without a divergent portion) machined from steel bar. To these were boltedflat back-plates drilled with radial or concentric rows of injection holes for the liquid-oxygen and hydrocarbon fuel. The finaldesign of Screamer injector has like-on-like impingement jets in the centre and parallel (non-impingement) shower-head injectorsaround the outer portions. At various stages in development the Screamer chamber wasmade in two portions bolted together at the junction of the parallel and divergent sections. In all designs the inner shell and outercase are made of S.21 mild steel, which is readily welded and has good thermal conductivity. The inner shell was hard-chrome-plated to resist erosion and other parts cadmium-plated to pre- vent rusting. Production chambers would have been tubularforgings but the development chambers were machined from solid. The back-plates in the Screamers so far built have alsobeen machined from solid, the material being S.110 steel. The starting and control system of the Screamer is relatively .:"•'.•••••; This diagram shows the type of combustion chamber :v•"-.- ;. used in late-model Screamers. It is throatless and has; . a double wall with scrolls to constrain the cooling water. complex. For this reason no attempt is made here to describethe complete system in detail but the following is a full outline of the manner in which the motor is started and sustained.Prior to firing, the three propellant feed valves are opened by energizing the suction-valve solenoid. This allows the propellantsto flow from the main airframe tanks through filling valves to the starting tanks, and also to fill the main engine lines up to thestop and by-pass valves. One starting tank is provided for each of the three propellants, their purpose being to run the gasturbine up to speed during the starting cycle. The liquid- oxygen starting tank is a tubular steel cylinder containing a free-floating steel piston fitted with two Duaflex rings. Like the other starting tanks it is pressurized to 450 lb/sq in from a 3,000 lb/sq innitrogen bottle, and it has an excess capacity in case the gas generator should be slow in starting. The tank automaticallyrefills after each start, since it is connected to the pump suction and the head of liquid oxygen from the airframe tank is sufficientto refill the starting tank by pushing back the floating piston to its starting position. The fuel and water starting tanks are likewisemachined from solid steel with an interior polished to a very high surface finish so that the synthetic rubber diaphragm withineach tank shall not stick to the inner wall. At each end is a perforated plate, shown clearly in the large drawing, to preventthe diaphragm from being blown out of the tank. Vapour formed by the liquid oxygen boiling in the warm pipesis released through priming valves on the starting tank and oxygen pump and also through by-passes from the valves associated withthe gas generator and combustion chamber. Air trapped in the water and fuel lines can be bled off at suitable points. TheScreamer starting-cycle is initiated by energizing the starting solenoid which allows nitrogen to open two paralleled valveswhich admit nitrogen pressure to the three starting tanks. At the same time nitrogen is used to purge various portions of the engine.From the starting tank the liquid oxygen is piped first to the stop and by-pass valve and igniter stop valve on the gas generator;fuel and water are each fed to the gas-generator and igniter valves and, via a pressure-balance valve, to the gas generator mixingsection. The pressure of the water injected into the gas generator opens an air valve which allows nitrogen to open the main oxygenand fuel igniter stop valves; the combustible mixture then flows into the gas-generator igniter where it is immediately lit by thehigh-frequency plugs. Should the igniter pressure not rise to its normal value withintwo seconds dangerous accumulations of propellants are prevented by a delay switch. Assuming normal ignition the rising igniterpressure opens a valve to admit the main gas-generator propellants, and also operates a switch which breaks the two-second delaycircuit and at the same time makes the high-frequency circuit for the main combustion-chamber spark. A nitrogen-operated switcheffects the change-over in ignition from the gas generator to the main combustion chamber. Gas from the gas generator accelerates the turbine so that thethree pumps feed propellants back to the main tanks via the by- pass lines from the three main stop and by-pass valves. In the sameway as for the gas generator, the main flows of fuel and water are fed to the main combustion chamber through a pressure-balancevalve (39-41 in the key). Water pressure immediately downstream of the pump energizes a switch and solenoid circuit, allowingnitrogen to admit igniter fuel and oxygen to the main chamber. Propellants fed to the main-chamber igniter are lit by sparks fromthe high frequency plugs and the resulting build-up of pressure opens another valve, allowing nitrogen to operate the main waterstop and by-pass valve and also a pressure switch which breaks the delay circuit and switches off the plugs—just as in the circuitsassociated with the gas generator. Water pressure downstream of the main valve operates the main liquid-oxygen valves and theresulting oxygen pressure then operates the fuel valve, the pro- pellants being fed in this sequence to the combustion chamber.
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