An explanation of the JFS

eleazar

之前CN101讨论过，这个实在BMS论坛看到的，需要的存起来了慢慢看

    The F-16 was designed first and foremost to be an ultra-maneuverable WVR (Within Visual Range) aircraft. The American Military in the 1960s relied very heavily on technology, and suffered for it. In the early years of Viet Nam it was thought that technology would make dogfighting a thing of the past. But the failure rate of air-to-air missiles was very very high. Many aerial combat battles came down to a guns fight. So, a group unofficially called the "Fighter Mafia" lobbied Congress and the Pentagon for a maneuverable guns platform aircraft. The F-16 was born.

    Now for an airplane to be maneuverable, the less it weighs the better. And an electric starter motor powerful enough to turn the great big heavy Pratt & Whitney or GE engine (depending on which block we're talking about) is hundreds of pounds, not to mention the battery and cabling required to drive it. And it's bulky as well. So the engineers who designed the F-16 came up with a cool idea: The Jet Fuel Starter.

    The Jet Fuel Starter or JFS is a small jet engine, mounted in the fuselage, kind of under the left wing. It runs on jet fuel (DUH) and it produces enough power to turn the big heavy main turbine engine on the F-16. But what makes the JFS run?

    Stored hydraulic pressure. That's what. Now the main engine on the F-16 is mechanically connected to two (2) hydraulic pumps, pump A and pump B. So anytime the main engine is running these pumps are pumping, creating hydraulic pressure. Some of this pressure is stored in what are called "hydraulic accumulators". A hydraulic accumulator is a way to store hydraulic pressure. Picture a coffee can, sealed on one end, with a piston in the middle. Between the piston and the sealed end is a lot of nitrogen. Now pump hydraulic fluid into the open end. The pressure of the hydraulic fluid pushes the piston back against the nitrogen. Shut off the valve, and you now have stored hydraulic pressure.

    When you select JFS Start 1 or Start 2, it opens the valve on the hydraulic accumulator (or both of them) and allows the hydraulic fluid to flow to the JFS. The hydraulic pressure turns a fuel pump which pressurizes jet fuel to the very high (maybe 600 PSI) pressures needed to run a jet engine. At the same time, the same fuel pump mechanically spins the small turbine spool in the JFS (It's really a small jet engine!) fast enough to get air moving through at rates required for combustion. Add the fuel, and a spark, and you have a small jet engine running.

    Now the JFS main spool is mechanically linked to what is called the accessory gearbox on the main engine in the F-16. This mechanical linkage actually turns the N1 spool inside the main engine, spinning it fast enough to create air flow and fuel pressure (remember the low- and high-pressure fuel pumps on the F-16 are attached to the accessory gearbox, so when the engine is turning so are the fuel pumps) to sustain ignition. Once the N1 spool reaches an RPM of roughly 21%, the pilot moves the throttle forward into the idle detent, fuel flows into the burner can, and Presto! The main engine is running!

    Now if for some reason the engine doesn't start properly, there is no good way to get the pressure back into the hydraulic accumulators. The Crew Chief has to come out with a great big wrench and crank on a bolt for about 20 to 30 minutes to get the pressure back in the accumulators. There is no GPU connector, no motive air flow connector, no jumper cables. And then the Crew Chief is owed a nice bottle of aged distillate of malted barley, preferably from the land of the Scots.