947,557. Fluid-pressure servomotor control systems. ROLLS-ROYCE Ltd. July 2, 1962, No. 25341/62. Heading G3P. [Also in Division F1] In a gas turbine jet propulsion engine having reheat combustion equipment mounted in the jet pipe, the reheat equipment comprising main and pilot burners, main and pilot fuel supply control units are provided so as to ensure that the reheat fuel/air ratio in the jet pipe does not exceed the stoichiometric ratio. Manually operable means are provided for selecting as desired the amount of fuel supplied to the main burners, the construction being such that throughout the operation of the reheat equipment the pilot burners are supplied with an adequate amount of fuel to keep them alight no matter what quantity of fuel is being supplied to the main reheat burners. The reheat combustion equipment is shown in Fig. 8 and comprises pilot burner manifolds 191 supplied with pilot fuel through line 189 the fuel being injected into combustion stabilizing troughs 194. Main reheat fuel is supplied through lines 242, 243, 244, 245 to manifolds 246, 247, 248, 249 which discharge fuel into the jet pipe 5 upstream of the pilot burner assembly. Fuel supply to the lines 242, 243, 244, 245 is controlled by a staging unit shown in Fig. 5. The jet pipe is provided with a variable area nozzle which is controlled by a series of rams such as 350, Fig. 8. The main reheat fuel supply control unit 73 and the pilot reheat fuel supply control unit 74 are shown in Fig. 4 in which is shown also the reheat fuel supply pump 11 which comprises a centrifugal pump 12 which is driven by the turbine 13, the turbine being supplied with air under pressure through the line 15. Fuel is supplied to the pump through inlet 10 and discharges therefrom through line 68 in which are disposed a non-return valve 69 and a heat exchanger 70. The fuel then passes into the interior of the control unit. The main reheat fuel control unit comprises a sleeve or throttle valve 88 which is formed with triangular shaped slots 89, the area of which increases axially to the right. The throttle valve is connected to a shaft 84 which is connected by epicyclic gearing to an engine-driven shaft 75, whereby the valve is rotated during operation. The centrifugal governor 85 is also driven by the epicyclic gearing and the governor acts to control the axial position of the throttle valve, the governor arms engaging a collar 87 on the shaft 84. A sleeve 95 is connected to the throttle valve by a shaft 93 which is splined to the sleeve so that the sleeve rotates together with the throttle valve but may move axially independently thereof. The sleeve 95 is formed with triangular-shaped slots 157 the area of which increases axially to the left, the slots co-operating with an annular slot 128 of axially variable width so as to constitute a metering orifice. The sleeve 95 is connected by a bearing arrangement 96 to a pair of capsules 97 which are disposed within a common chamber 98, the interiors of the capsules being connected by a line 103, chamber 104 and line 105 to air at pressure P 6 within the engine jet pipe 5. The chamber 98 is connected by line 118 chamber 113 and line 115 to air at pressure P 3 , the pressure at the outlet of the high pressure compressor of the engine. An orifice 114 controlled by a needle valve 119 is disposed between the chamber 113 and the line 115, and a fixed orifice 112 is provided between the chambers 104 and 113. The capsules act to position the sleeve 95 in dependence on the function (P 3 /P 2 Î P 3 ) - P 6 , P 2 being the pressure at the downstream end of the low pressure compressor. The annular slot 128 is provided by a pair of sleeves 126, 127 disposed around the sleeve 95, the sleeve 127 being constituted by a part of the control unit casing and the sleeve 126 being movable towards and away from the sleeve 127 so as to vary the width of the annular gap 128 by a bell-crank lever 137, 139. The bell-crank lever is controlled through linkages 50, 150 by the pilot's throttle lever 51, Fig. 3. The sleeve 126 is biased to the right by a spring 129. The movement of the arm 137 of the bell-crank lever is limited by stops 155, 156. Thus fuel supplied through line 68 into the casing 72 of the control unit passes through the metering orifice 128, 157 into the space within the sleeve 95 and throttle valve 88 and discharges through the orifice afforded by the triangular slots 89 and the cooperating annular recess into the main reheat fuel line 90, through the non-return valve 220 and line 221 to the burner staging unit 224, Fig. 5. The pilot reheat fuel supply control unit 74 is incorporated in the assembly shown in Fig. 4 and comprises a sleeve 166 formed with triangular-shaped slots 182, the sleeve 166 being connected to the sleeve 95 of the main reheat fuel control unit by a pull rod 168 whereby the sleeve 166 has the same rotational and axial movement as the sleeve 95. Fuel passes to the chamber 173 of the pilot control unit from the line 68 by way of a filter 172 and line 171, the fuel passing into the space within the sleeve 166 through a metering orifice afforded by the co-operation of the triangular slots 182 and the annular orifice 183 which is provided by the co-operating sleeves 176, 177 as in the case of the main control unit. An axially movable cup-shaped throttle valve 184 is disposed within the sleeve 166, the valve being in splined connection with a fixed rod 185 so as to prevent rotation but to permit axial movement thereof. The valve 184 defines a chamber 164 with the sleeve 166,the chamber 164 being in communication with the space within the sleeve 95 and throttle valve 88 by way of a duct 163. The pressure within the sleeve 166 at the left and right-hand sides of the piston 184 will be equal, axial movement of the piston ensuring this. Fuel which has passed through the metering orifice 182, 183 into the space within the sleeve 166 discharges therefrom through the triangular-shaped orifices 186 the open area of which is controlled by the throttle valve 184, the fuel discharging through the pilot fuel duct 187, valve device 188 and line 189 to the non-return valve 190, Fig. 5 and thence to the concentric pilot fuel manifolds 191, Fig. 8. The position of the throttle valve 88 of the main control unit is dependent also on the pressure drop across the metering orifice 128, 157. Main reheat fuel in the line 221 passes to the chamber 222 within the burner staging unit 224, Fig. 5. This unit comprises cylinders 225, 226, 227, 228, 229 within which are disposed piston valves 232, 233, 234, 235, 236, the lower sides of the pistons being acted on by fuel at high pressure supplied through line 253 from line 68, see Fig. 4. The piston valve 232 controls flow of fuel from the chamber 222 to the line 239 which leads through line 241 back to the suction side of the reheat fuel pump 12, see Fig. 4. The burner staging unit comprises also a control valve 270, the axial position of which is controlled through the linkage 50 by the pilot's control lever 51. In the position shown, high pressure fuel from the line 253 is applied to both sides of the piston 232 and the piston is therefore at its lowermost position closing off the outlet to the return line 239. High pressure fuel is also supplied through lines 264, 265 whereby the piston valves 235, 236 are in their lowermost positions closing off the outlets to the lines 244, 245 and so to the manifolds 248, 249. The lines 261, 262 however are connected to the low pressure fuel within the line 239, the piston valves 233, 234 are therefore in their uppermost positions and main reheat fuel can pass from the chamber 222 into the lines 242, 243 and so to the manifolds 246, 247. When in the reheat off position the valve 232 will be open and the remaining valves closed, fuel in the chamber 222 then flowing through line 239 back to the suction side of the pump. As the control valve 270 is moved'to the right, the valve 232 is closed and the valves 233, 234, 235 and 236 progressively opened. The reheat fuel system also comprises a hot streak type ignition unit which causes a "shot" of fuel to be injected into the main combustion equipment of the engine the fuel causing a streak of flame to pass through the turbine to the reheat combustion equipment in the jet pipe. The unit is shown in Fig. 5 and comprises a casing 291 within which are disposed a piston 304 having an orifice 323 which co-operates with a valve 324; a piston 330 having a bore 331 therethrough and carrying a valve 336; also a diaphragm 297 mounted on a rod 296 which carries a valve 295 which cooperates with a bore 294. The valve 336 cooperates with a duct 334 which leads to the hot streak fuel injector in the engine combustion chamber. The left-hand side of the diaphragm 297 is connected to the pressure P 6 in the engine jet pipe, and the right-hand side thereof is connected to the pilot fuel pressure. The unit is also connected through line 289 to the suction side of the reheat fuel pump 12, and through line 311 to a valve body 315, see Fig. 4. When the reheat system is about to be brought into operation the piston 304 will be disposed at the right-hand end of the chamber. At this time, the valve member 316, Fig. 4, will be open so that fuel at the pressure in the main fuel passage 90 will be supplied via the lines 314, 313, 311 so as to fill the hot streak ignition unit. The valve 295 is closed at this time since there is no pressure in the pipe 301. As soon as the reheat system is brought into operation however, fuel will flow through the pilot fuel passage 189 and will pass through the pipe 301 and will act on the diaphragm 297 to open the valve 295. The pressure in the space 306 to the left of the piston 304 will therefore fall since it will be connected to the line 241 which connects to the suction side of the pump 12. The piston 304 will therefore move towards the left to the position shown i