These are mounted to the front of jet engine
s. They may be driven by means of a high-pressure fluid (compressed air or other gas, or oil), or electricity. A jet engine requires a considerable amount of moving air in order to start and maintain combustion
of the jet fuel
- whatever air may be sitting around in the combustion chamber
is not enough. Therefore, it is necessary to spool up
the engine's intake turbine
somehow. This is where the starter bullet comes in.
Usually an elongated cylinder with a dome tip (for improved airflow), the starter bullet, when activated, engages the intake turbine's drive shaft and starts spinning it faster and faster. When the engine reaches a certain RPM range and/or internal pressure, the engine control unit starts spraying jet fuel into the combustion chamber through one or more nozzles and firing one or more spark plugs. At this point, the starter bullet disengages from the intake turbine's drive shaft in order to avoid getting reamed out when the combustion starts - because when it starts, the intake turbine, linked mechanically with the outlet turbine, will increase rotational speed much faster than the starter bullet can cope with. This is because the outlet turbine is being spun up very quickly by exhaust gas from the combustion chamber.
If you want to know what a starter bullet sounds like, go to a small airport (or a web site) and see if you can listen in while they start up a jet engine. The whine you hear before the engine suddenly picks up rotation and starts to roar is the starter bullet.