PRINCIPLES OF NAVAL ENGINEERING 



at the moment of launch into the power cylin- 

 ders. Steam pressure acts directly on the 

 pistons and propels the piston- shuttle assembly 

 through the cylinders. A sealing strip closes the 

 slot in each cylinder as the pistons are driven 

 forward, thus preventing the escape of steam 

 from the cylinder slots through which the con- 

 nector moves. 



Prior to a launch, the engines of an airplane 

 must be operating at full power. A holdback 

 device is utilized to prevent the airplane from 

 being moved forward by the thrust of its own 

 engines, until the time of launch. The holdback 

 device hooks into a fitting in the flight deck. 



The piston-connector-shuttle assembly is 

 stopped at the end of its launching run by a water 

 brake. The brake consists of two cylinders of 

 water locatedco-axially with the power cylinders 

 at the forward end of the catapult. The spear tips 

 of the pistons ram into the water-filled cylinders. 

 As the spear tips penetrate the water, pressure 

 builds up and stops the assembly. (See fig. 21-14.) 



The principal unit in the shuttle retraction 

 and tensioning systems is the grab. This unit is 



essentially a spring-loaded latch mounted on a 

 wheeled frame just aft of the shuttle. The grab 

 is driven along the shuttle track through a system 

 of cables by hydraulic force. The hydraulic re- 

 traction engine consists of two cylinders. In one 

 cylinder, hydraulic pressure is converted into 

 the mechanical motion of a piston rod which is 

 installed in this cylinder. The other cylinder is 

 an accumulator in which hydraulic liquid is 

 stored under pressure. The motion of the piston 

 rod is transmitted to a device called a crosshead 

 to which the drive cables of the grab are at- 

 tached, (See fig. 21-15.) 



Prior to a launch, the grab is moved forward 

 by a hydraulic cylinder-piston assembly. This 

 assembly is located aft of the grab. (See fig. 

 21-16.) When liquid is introduced into the cyl- 

 inder, the piston pushes the grab forward. The 

 grab, in turn, exerts force on the shuttle so that 

 it moves forward enough to place tension on the 

 towing bridle which connects the shuttle to the 

 airplane. When the launch is made, the grab re- 

 leases the shuttle and it is driven through the 

 power cylinders of the catapult by steam pres- 

 sure. After the launch is made, the grab is 



147.132 



Figure 21-14.— Water brake of a steam catapult. 



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