will also cause a change in pH number.) Normal values for the viscosity and pH number of the unused fluid are as follows: • Viscosity (fluid temp. 100ºF): 185 to 210 SSU • pH number: 8.8 to 9.8 NOTE Catapult hydraulic fluid, NAEC P/N 217768-3 is nontoxic and requires no special clothing or handling procedures. If splashed into eyes, flush with water. Accumulator and Air Flasks The accumulator receives the hydraulic fluid delivered to the system by the hydraulic pumps and delivers it to the retraction engine cylinder, bridle tensioner, and other components of the catapult where high-pressure fluid is required. The two 50-cubic-foot air flasks are installed in the base of the retraction engine. The air flasks contain no moving parts. Drain valves are provided for maintenance. Figure 4-47 illustrates the two types of accumulators used with linear retraction engines. Main Hydraulic Pumps The main pump sets (figs. 4-44 and 4-45) deliver hydraulic fluid to the retraction engine accumulator. The hydraulic pumps are connected in parallel. The intake line to each pump is provided with a strainer. Each pump discharge line is fitted with a delivery control unit, which has a built-in relief valve. When the hydraulic fluid leaves the pumps, the delivery control unit directs it either through the bypass line back to the gravity tank (pump offstroke), or through the pressure line to the main accumulator. This pressure line is equipped with one-way check valves to prevent the backing up of fluid from the accumulator when the pumps are offstroke. The gravity tank is designed to fit the individual installation. Baffles are installed to minimize entrapment of air in the hydraulic fluid. The fluid stored in the gravity tank enters the pumps through the pump intake screen, which filters out any large particles of foreign matter that may be in the fluid. The fluid goes into the main pumps, which are connected in parallel. The intake line to each pump is fitted with a shutoff valve and a strainer. The strainer assists in removing dirt from the hydraulic fluid. The fluid then passes through the pumps, which generate a pressure in the fluid. As it leaves the pumps, the fluid is fed through the delivery control unit, which incorporates adjustable relief valves. The fluid then passes through the pressure line, which carries it to the retraction engine accumulator. The retraction engine accumulator operates to maintain the pressure of the fluid within the required pressure range (2700 psi on C-7/C-11, 2500 psi on C-13). Hydraulic fluid under pressure from the pumps is forced into the main accumulator. This fluid pushes a piston against preset air pressure. The force of the air against the piston keeps a constant pressure in the hydraulic lines. Normally, the pumps operate automatically. For the C-7 and C-11 catapults, this is accomplished through the action of a tailrod switch. When the hydraulic fluid chamber is filled to maximum capacity, the tailrod is withdrawn into the accumulator, and the cam trips the lever on the tailrod switch, de-energizing the solenoids on the delivery control unit. This stops the flow of fluid to the accumulator and directs fluid from the pumps to the gravity tank. When a hydraulic component is operated, air pressure in the accumulator pushes the piston toward the hydraulic-end cover, forcing hydraulic fluid to the component. As the hydraulic fluid is used, the tailrod is extended from the accumulator until the cam on a second guide trips the switch lever, energizing the solenoids on the delivery control unit to direct fluid to the accumulator. Fluid is then pumped to the accumulator until the cam on the first guide trips the tailrod switch. NOTE The two cams on the tailrod guide are spaced far enough apart to prevent the pumps from going onstroke every time a small amount of hydraulic fluid is discharged from the main accumulator. In addition, a hydraulic volume normal limit switch is installed to maintain an adequate amount of fluid to complete a launch valve cycle. 4-36