Fig. 14.21. Cartridge-pneumatic starter scheme

In connection with high energy of powder gases (p = 9 MPa) the powder turbine starter rotor can get impossibly quick turning, during which its destruction under affection of centrifugal forces is possible. Therefore in a powder turbine starter the limiter of maximum rotational speed is stipulated (relief valve 12, aerodynamic braking fan 14, switch 21 with flyweight 19 of maximum rotor rotational speed in Fig. 14.21).

High temperature of gases (T_g =1700…1900 C) and their corrosion activity do not allow to ensure for powder turbine starters continuous service life. In this connection starters of this type are applied for starting of gas turbine engines of one-time operation.

The important feature of a powder turbine starter is a capability of gas turbine engine starting within very small time – 2...3 seconds for smokeless powder pyrocartridges and 15…20 seconds for ammonium nitrate pyrocartridges.

The powder turbine starters are applied when it is necessary to create large power – from 200 up to 280 kW.

The principle of air-powder turbine starters (catridge-pneumatic starter) operation consists in the following. A charge is inserted in the breech and ignited electrically. The relatively slow-burning propellant produces gases at approximately 930 °C and 8 MPa.

The mix of powder gases with air moves to a starter turbine (Fig. 14.21) and turns the starter for about 15 s. It has lower temperature in comparison with powder gases. It allows to increase duration of starter service. Air in the mixing chamber of an air-powder turbine starter is supplied from an outside source of compressed air. Atmospheric air supply to the mixing chamber at the expense of powder gases flow ejector affection is probable.

In recent years the pneumatic-cartridge starter has achieved considerable use in the Air Force, primarily because of its inherent characteristics of a lightweight, self-contained system with the extremely high torque value of over 800 Nm plus the option of quick engine starts and simulta­neous multi-engine (gang) starts from the high-pressure, high-temperature cartridge gases, or from low pressure supplied from a running engine, conventional ground support equip­ment, or airborne starting units.

For a cartridge start, a standard cartridge is first placed in the breech cap 2. Next, the breech cap is closed down on the breech chamber by means of the breech handle 3 and rotated a part-turn to engage the lugs between the two breech sec­tions. This rotation allows the lower section of the breech handle to drop into a socket and completes the cartridge ignition circuit (up to this point, it would have been impos­sible to fire the cartridge). As shown in Fig. 14.21 the car­tridge is then ignited by applying voltage to the connector 4 at the base of the handle, thus energizing the insulated ignition contact 5 at the top of the breech cap, which touches a point on the cartridge itself. The circuit is com­pleted to ground by the ground clip 6 (a part of the car­tridge), which contacts the inner wall of the breech cap. Upon ignition, the cartridge begins to generate gas. The gas is forced out of the breech to the hot gas nozzles 7, which are directed toward the buckets on the turbine rotor 8, and rotation is produced. Gas emerging from the opposite side of the wheel enters an exhaust ring 9 in the exhaust duct, is collected, and passes out of the starter via the overboard exhaust connector 10. However, before it reaches the noz­zle, the hot gas passes an outlet leading to the relief valve 12. This valve ports hot gas directly to the turbine, bypass­ing the hot gas nozzle, as the pressure rises above the preset maximum, Therefore, the pressure of the gas within the hot gas circuit is maintained at the optimum level.

For a pneumatic start, compressed air from a ground cart is led by dueling on the aircraft to the compressed air inlet 13. It passes into the compressed air nozzle ring and is directed against the buckets of the turbine rotor by vanes placed around the ring. Rotation is thus produced in essen­tially the same manner as in the cartridge start. Compressed air leaving the turbine rotor collects in the same exhaust ring and is ported overboard via the overboard exhaust connector.

Whether starting is accomplished by cartridge or com­pressed air, some opposing force is required to keep turbine speed within safe limits. This opposing force is provided by the aerodynamic braking fan 14. The fan is connected directly to the turbine shaft. It is supplied with air from the aircraft nacelle, and its output is carried off by an exhaust ring 16 concentric with, and located within, the turbine exhaust ring. Hot gas (or compressed air) exhaust and aero­dynamic braking fan output are kept separate up to the over­board exhaust connector. At this point, they merge, the cool air from the fan cooling the hot exhaust gas.

The gearshaft 17 is part of a two-stage reduction that reduces the maximum turbine speed of 67500 r/min to an output of approximately 4000 r/min. The large gear of the final output turns the output spline shaft 24 through an overrunning clutch 18.

The clutch is situated in the output area between the gear shaft, on which the final drive gear is located, and the output spline shaft. It is a pawl- or sprag-type, one-way overrunning clutch, and its purpose is to prevent the engine, once operat­ing under its own power, from driving the starter, thereby possibly driving the turbine rotor at a speed above its safe limit. The nature of a pawl or sprag clutch is such that it can transmit torque in only one direction. That is, the driving member will operate through the clutch, delivering full torque to the driven member. But the driven member cannot become the driver – even though revolving in the same direction – and transmit torque back into the original driver. Any tendency for it to do so would disengage the clutch. When the engine has been started and the starter has finished its cycle and stopped, only the output spline shaft and the outer (driven) part of the clutch will be revolving. The balance of the starter will be at rest.

The starter is equipped with an output spline shaft having a shear section that permits the shaft to shear if torque to the engine during the starting cycle is excessive. When the shaft shears, torque to the engine is stopped, thus preventing dam­age to the aircraft engine gearbox. The output spline shaft will also shear during the overrunning cycle (engine started and operating) if the starter malfunctions in such a manner as to develop a frictional resistance to torque from the air­craft engine gearbox.

In the event the clutch and spline shaft fail to operate and the turbine is driven beyond burst rotational speed by the aircraft engine, the containment clamp provides addi­tional strength to the starter turbine area, preventing damage to the aircraft.

A vent 23 through the clutch and output shaft eliminates internal pressure buildup. Centrifugal force caused by out­put rotation prevents oil leakage through the vent.

The starter is lubricated by a splash system. Oil slingers 25 attached to the clutch output race pick up oil from the sump 26 and distribute it throughout the interior of the starter as the output spline revolves. A catching cup con­struction in the housing carries the oil into the overrunning clutch and other difficult-to-reach areas. Since the part to which the slingers are attached is constantly spinning, even after the starter has completed its cycle, starter lubrication continues as long as the aircraft engine is operating. The oil sump contains a magnetic plug 27 to collect contaminants.

In liquid monopropellant starter a charge of liquid monopropellant (a mono-propellant fuel is one that requires no separate air supply to sustain combustion) is decomposed to produce the high-energy gas needed for turbine operation. Monopropellants that can be used include highly concentrated hydrogen per­oxide, isopropyl nitrate, and hydrazine. All are difficult mate­rials to handle, and principally because of this problem there has been little operational installation of such equipment.