Engineering Training

ASSIGNMENT SHEET

CATAPULTS

Assignment Sheet Number 3.8

INTRODUCTION

–The effectiveness of an aircraft carrier depends upon the speed of its airplane launching operations. Therefore, a compact and efficient device for getting all airplanes into the air within a short time is needed. This requirement is met by the modern carrier catapult.

LESSON TOPIC LEARNING OBJECTIVES

Terminal Objective:

–3.0 Supervise the operation and maintenance of shipboard auxiliary systems.

Enabling Objectives:

–3.52 Describe the function and operation of the catapult system.

–3.53 Explain the impact of catapult operations upon the ship's engineering plant.

–3.54 Explain safety precautions associated with the operation and maintenance of the catapult system.

STUDY ASSIGNMENT

1. Read Information Sheet 3.8.

2. Outline Information Sheet 3.8 using the enabling objectives for lesson 3.8 as a guide.

3. Answer study questions.

STUDY QUESTIONS

1. Describe the catapult steam system.

2. Describe the operation of the catapult system from readying of the aircraft to retraction of the shuttle.

INFORMATION SHEET

CATAPULTS

Information Sheet Number 3.8

INTRODUCTION

REFERENCES

•(a) Principles of Naval Engineering, NAVEDTRA 12960

• (b) Catapults, NAVAIR 51-15ABD-1

(c)USS Nimitz (CVN 68) Ship Information Book Vol 2 Pt 1 Bk 3 Piping Systems

–

INFORMATION

–General

– Catapults installed on aircraft carriers are steam-powered, direct-drive, flush-deck type catapults used to launch aircraft from the carrier deck. The catapult consists of two rows of slotted cylinders side-by-side in a trough under the flight deck. Pistons within these cylinders connect to a shuttle that tows the aircraft. The catapults are installed as electrically interlocked pairs and can be fired alternately or independently.

–Each catapult consists basically of eight major systems:

– Launching system - This system may be defined as those components to which access can be gained at the flight deck level.

– Steam system - There are two basic steam systems associated with steam catapults. They are the dry receiver system and the wet receiver (constant-pressure) system. The main difference between the two is that the constant-pressure system uses a capacity selector valve (CSV) to control the steam pressure to the catapults for launching while the dry receiver system must have the pressure selected for each launch.

– Retraction system - A rotary type retraction engine system provides the power to retract the shuttle and the launching engine pistons after the catapult has been fired. It is also used to advance and maneuver the grab forward and aft.

– Drive system - This system provides the means of transferring the motion of the retraction engine to the grab for advance and retraction of the shuttle and piston assemblies.

– Hydraulic system - This system supplies pressurized fluid to the hydraulic components of the catapult.

– Bridle tension system - This system exerts a force on the shuttle to tension the aircraft before launching.

– Lubrication system - This system provides lubricating oil for launching engine cylinder walls by injecting lubricating oil through the cylinder covers with a spray pattern that ensures even lubrication of the cylinder walls before passage of the launching engine pistons.

– Control system - The control system provides for the control of the catapult during all phases of operation. There are three modes of control for the catapults:

• Normal operation - The catapult is controlled from the integrated catapult control station located at the flight deck level.

• Emergency mode I - The catapult is controlled from the emergency deck edge control panel in conjunction with the central charging panel located below deck in the retraction engine compartment.

• Emergency mode II - The catapult is controlled from the central charging panel.

–Main components (Figure 3.8.1)

– The principle component of the steam catapult is a cylinder-piston assembly with two power cylinders and two pistons per catapult. The spear-shaped pistons, which in the launching operation are forced at high speed through the cylinders by steam pressure, are solidly interconnected by means of a connector shaped like an inverted T. The vertical leg of the inverted T extends upward through a slot in the flight deck, and serves as the hook to which the aircraft's towing bridle is connected. The piston connector is attached to the shuttle. The shuttle is a small roller-mounted car which moves (during the launch) on tracks installed just under the flight deck.

– Power to drive the shuttle and its aircraft load comes from expanding steam piped to the catapult from the main steam system (Figure 3.8.2). This steam is placed under pressure in large tanks (called accumulators or receivers) located under the launching engine. Steam is admitted to the receivers through the flow control valve (sometimes called steam charging valves). From the receivers, the steam is transferred at the moment of launch into the power cylinders. Steam 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 connector moves.

• The launching cylinders are preheated by using an internal and external preheating system which prevents thermal shock and minimizes possible damage to the launching engine when superheated steam is admitted through the launching valves into the launching cylinders. This process is called "soaking" and can take many hours to accomplish. This also calls for the cooperation between the ship's engineering department and air crews.

• The steam supply piping must also be warmed up before use. Before opening the catapult system valves, the 10-inch machinery space cut-out valves, the 2-inch bypass valves, and the catapult cross-connect valves must be checked to ensure they are closed. This is necessary to avoid imposing an unexpected large steam demand on the reactor plant.

• After system pressure is reached, and after sampling has been performed to determine purity, the catapult steam drains can be lined up to the high pressure drain system.

– Prior to a launch, the engines of the aircraft must be operating at full power. A holdback device is utilized to prevent the aircraft 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 (Figure 3.8.3). The brake consists of two cylinders of water located co-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.

– The principle unit in the shuttle retraction and tensioning system 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 provided by the retraction engine (Figure 3.8.4). The hydraulic retraction 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 fluid 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 attached.

–Catapult operation (Figure 3.8.5)

– The aircraft is prepared for launch. The aircraft to be launched is spotted just aft of the launching shuttle at the battery position. The aircraft is attached to the shuttle, and a holdback unit is installed to hold the aircraft in place during tensioning. The tensioner is then activated to apply pressure against the grab and move the shuttle forward to tension the aircraft.

– The catapult is fired. After tensioning is completed, the catapult is fired by opening the launching valve assembly and permitting steam to surge into the cylinders. The force of the steam pushes the pistons in the cylinders, breaking the holdback. The steam then forces the pistons forward, towing the shuttle and aircraft at ever- increasing speed until takeoff is accomplished.

– The pistons and shuttle are halted by the water brake. The forward motion of the shuttle is stopped when tapered spears attached to the pistons plunge into the water-filled cylinders of the water brake.

– The grab advances and latches to the shuttle. As the pistons approach the water brakes, a switch is actuated. The launching valve assembly closes and the exhaust valve opens to exhaust the spent steam. The retraction engine is set in rapid motion, causing the advance of the grab. At the forward end of the shuttle track, the grab latches to the shuttle.

– The grab retracts the shuttle to the battery position. The retraction engine is reversed and returns the grab, shuttle and pistons to the battery position. The catapult has completed a full cycle and is in position to launch another aircraft.

–Safety precautions

– During launching operations, sound-powered phones shall be used for essential communications, to ensure that signals between operators are not misunderstood.

– Blowers or ventilation ducts shall not be trained directly on the launching valve assembly or steam supply area.

– Pressures shall be maintained at predetermined settings.

– The catapult shall not be operated with any known broken safety wires, loose or cracked components, major hydraulic leakage, or defective communication or electrical control equipment.

– A hangfire (a condition where the catapult partially fires, leaving the aircraft midway on the catapult) can leave the launching valve control valve in the stroked or non-stroked position. If a hangfire occurs, no one shall pass forward of the aircraft until all danger of a delayed launching has passed.

– All pumps, limit switches and safety valves shall be inspected regularly to reduce the possibility of injury to personnel or damage to equipment.

– During launching, live steam escapes from the track and brake areas. Therefore, personnel shall avoid contact with the steam and avoid all exposed metallic areas that can get hot due to steam temperatures. Personnel shall also be equipped with required protective clothing.

– Ear protection shall be worn in all areas of high noise levels. This includes the flight deck as well as the catapult machinery and control spaces.