MK 26 GMLS JETTISON
The Mk 26 GMLS has two jettison devices. They are deck-mounted at an angle to the launcher platform at the A and B dud-jettison positions. The two units are hydromechanically extended and retracted. They use an explosive gas generator to provide the ejecting force. Both the missile and an expendable piston assembly jettison over the side. Jettison operations start with a preparatory order from the ship's combat system (SCS). The main control console (MCC) operator in the integrated control station (ICS) controls all subsequent actions. The jettison devices of the various Mk 26 GMLS mods are identical.
Above the deck, the jettison mounting bracket supports, encloses, and protects the upper jettison components (fig. 8-2). Anti-icing fluid is circulated around the expendable piston cap to prevent ice buildup. A locking post and screw secure the gas generator cover to the bracket.
Below the deck, the 4-foot jettison housing cylinder contains the extender mechanism and the expendable piston assembly. The upper end of the cylinder is thicker than the lower end. It must be thicker to withstand the explosive force developed by the gas generator. A motor housing is bolted to the back of the cylinder. It contains a small bidirectional hydraulic motor that is controlled by a solenoid valve assembly.
The extender mechanism consists of an extender screw, extender nut, and extender sleeve. The extender screw is coupled to the output shaft of the hydraulic motor. The extender nut is threaded onto the screw and bolted to the extender sleeve. The extender screw is rotated counterclockwise (to extend) or clockwise (to retract) by the motor. The extender nut and sleeve (with the expendable piston assembly) travel out or in on the threads of the screw.
The extender sleeve serves as the barrel for the expendable piston assembly. A key and keyway prevent the sleeve from turning as the mechanism is extending or retracting. Gas ports in the aft section of the sleeve permit expanding gases from the fired gas generator to enter the sleeve. Gas pressure falls the sleeve up to the expendable piston. Only the piston assembly is propelled overboard.
The expendable piston assembly weighs about 75 pounds and fits inside the extender sleeve. A split lock ring is bolted to the piston cap. It attaches the cap to the piston sleeve and the piston assembly to the extender sleeve. A piston plug is inside the piston sleeve. It serves as a guide for the positioner rod of the safety mechanism. The piston plug is also a header for the expanding gas pressure.
The gas generator contains an electrically ignited explosive charge. When the generator is fired, expanding gases propel the piston assembly and a missile from the guide rail. The generator is a one-shot device that must be replaced after each firing.
Figure 8-2.-Mk 26 GMLS jettison device.
What if a gas generator accidentally fired while the jettison device was retracted? A pressure safety relief mechanism is built into the unit. Components of the mechanism will safely vent the expanding gases to the atmosphere. They also prevent the piston and other metal pieces from ejecting outward.
When the extender sleeve is retracted, a blow-in plug assembly in the wall of the sleeve aligns under the gas generator. The assembly is designed to collapse into the forward chamber of the expendable piston. Gas pressures act only on the front side of the piston plug. That keeps the piston assembly inside the extender sleeve.
Two headless straight pins are staked at right angles to the center of the positioner rod. They serve as a capture cage for pieces of the blow-in plug assembly. A rupture disc (or plug) is in the center of the piston cap. It breaks (from internal pressure) and allows the gases to escape. Normally intact, the ruptured disc keeps moisture out of the jettison device.
When ordered to prepare for jettison operations, the MCC operator presses the RSR/RAIL SELECTION-A or -B and POINTING SELECTION-JETTISON push buttons. Automatically, the launcher slews to the correct position and readies itself for jettisoning. For an antiair warfare (AAW) missile, the AAW arming device disarms. The two firing contractors and the AAW contactor retract. For an antisubmarine warfare (ASW) missile, the ASW contactor retracts. The retractable rail extends, and the ASW fire through latch unlocks.
EXTEND JETTISON DEVICE.- As the launcher synchronizes to the jettison position and the guide arm components are correctly positioned, jettison device operations begin. Jettison control circuits automatically energize control valve solenoids. The solenoids start the hydraulic-mechanical actions to extend the extender sleeve and piston. When the sleeve leaves its retracted position, the train and elevation power-drive brakes set. They hold the launcher aligned to the jettison position. The extender mechanism drives the expendable piston to within one-half inch of the tail cone of the missile and stops (fully extended).
JETTISON FIRING.- With all jettison circuit interlocks satisfied, a ready-to-jettison indication is given on the MCC. When the jettison order is received, the operator depresses the JETTISON FIRE push button. The firing circuits apply 20 VAC to the gas generator squibs. The generator ignites and jettisons the missile and expendable piston assembly.
RETRACT JETTISON DEVICE.- The MCC operator starts retract operations by depressing the JETTISON DEVICE-RETRACT push button. The extender mechanism returns the sleeve to its retracted position. When the sleeve is fully retracted, control system circuits automatically release the train and elevation power drive brakes. The ASW or AAW aft shoe latch retracts. The AAW rail segments also retract to clear the guide arm for the next missile. The MCC operator may return the system to normal operation.
Jettison operations are usually performed in the auto-load mode. For maintenance purposes, the step-load mode is used. All component operations must be initiated manually. The firing circuit to the gas generator may be checked but the unit is not ignited. The gas generator is classed as a high-explosive hazard. Handling and stowing procedures are conducted within strict adherence to applicable safety regulations.