Re: How is a jet engine started?

Greetings:

This information is edited from portions of text at the referenced URL.You 
can find detailed drawings of these various starting systems and their gear 
assemblies at the following URL:  

http://www.atsc-army.org/cgi-bin/atdl.dll/fm/1-506/ch8.htm#fig8_1


"Basic types of starters developed for gas turbine engines are air turbine 
(pneumatic), electric motor, hydraulic, combustion, and cartridge 
pnuematic. Air turbine starters are the most commonly used. Smaller engines 
generally use electric starters. Hydraulic starters are frequently found in 
helicopters and some marine gas turbine installations. Combustion and 
cartridge-pneumatic starters are generally special application devices that 
may require a self-contained starting system. An impingement starting 
system is sometimes used An impingement starter consists of simple jets of 
compressed air piped to the inside of the compressor or turbine case. The 
jet air blast is directed onto the compressor or turbine
rotor blades and causes them to rotate. 

Air-turbine starters (which are also called pneumatic starters) are used 
more than any other for starting jet aircraft engines, particularly larger 
engines. A small geared air turbine is attached to the engine starter pad 
located at the accessory drive gearbox. Air-turbine starters must receive 
compressed air from an external power source. A compressor mounted on a 
ground unit or onboard the aircraft is one such source. A small turbine 
engine usually drives these units. On multiengine aircraft, air is often 
bled from the first engine started and used to operate the starters for the 
remaining engines. With an air-turbine starter, the air supply must be of 
sufficient volume and pressure to meet starter requirements. Otherwise, the 
starter torque may not produce consistently successful starts within an 
acceptable time limit. When bled air from another operating engine is used, 
the engine being used for a compressed air supply must be turning over fast 
enough to ensure adequate air pressure to the starter of the engine being 
started ."


Best regards, your Mad Scientist
Adrian Popa