Victor wrote:I'm finding it difficult to understand what you wrote so bear with me.
The start-up/shut-down sequence as I understand it is:
Main engine engage
Main engine spool-up/JFS shut off
Main engine ignition & light off
Main engine shut off
Main engine spool-down
Main engine turbine stop
Is this sequence completed in 70-75 seconds before the next JFS start or is next JFS start initiated before the sequence is complete? If so, at which point in the sequence do we fire up JFS again?
As I said, -15ºC (+5ºF) is not particularly cold for western countries where all current fighters are made. The bigger problem would be the rarefied air at altitude and maybe that was the problem the teens faced in Leh. Otherwise, these jets operate fine in US, Canada and Sweden where it gets much colder.
The sequence is not completely correct. The JFS doesn't power the engine all the way till it spools up to idle rpm. Let me explain once again. The JFS only turns the High pressure shaft. The high pressure compressors attached to this shaft start turning as well starting the airflow in the engine. When the RPM reaches the 'start rpm' (typically about 20%), the flow fuel is started into the combustion chamber and an ignition is provided. If the combustion starts, then the exhaust gases power the HP turbine (which are connected to the HP compressors through the HP shaft) and also the LP turbine (which is connected to the LP compressor). At this point the combustion spins the turbines and the turbines spin the compressors, and the JFS can disconnect. The engine continues to spool up, i.e. the rpm of both the LP and HP stages continue to rise till they reach 'idle' rpm.
Now, starting up the engine can fail at various stages.
1. The fuel flow is started before the start RPM is reached (called a hot start), ignition will fail (not enough air).
2. Even after the flow of the fuel is started and the ignition provided, combustion may not start (due to various environmental conditions, temperature of fuel).
In either case, you have to stop the process and go back to the point where you can restart (does not always need the engine RPM be 0). This generally puts a minimum time required, typically about 20 secs. I do not know what makes ADA put an upper limit of 75 seconds on this. But if they can keep this interval down to 75 seconds, they can make three relight attempts.
Victor wrote:On starting the JFS, it seems a more judicious use of energy to use the battery to fire a compressed air cylinder than to actually turn the JFS turbine directly, although that can certainly be done. Start 1 & Start 2 refer to the number of Compressed air bottles used but these can be recharged manually if the engine doesn't start on the first try. How do we recharge a dead battery? This assumes no external help is available. Do we know why we chose this method instead of compressed air?
Of course not. If you change from electrical energy to potential energy, then potential energy to kinetic energy you will always have more loses. No engineer will do this! The air cylinders in the teens are not filled up using batteries either. The are are filled up using hydraulic pumps powered by the engine, when it is running. Both the battery method and the compressed air cylinders can fail to start the JFS. For the teens, you have at most 2 chances. ADA designers have ensured that LCA has at least 3 chances. If the battery is dead, you can use an external battery in parallel, pretty much like jump starting your car. In the case of teens, if compressed air in both the cylinders are used up, one has to power the hydraulic pumps using an external source to build up pressure in the cylinders again.
If batteries are required to start JFS then how are they kept in optimal condition? Don't batteries loose charge in cold, specially if complete unit in soaked overnight in -15 or below?
Maybe thats the reason others use explosive cartriage/ compfessed air to negate any enviromental issues or maintenance issues?
You are right the batteries under-perform when it is really cold. And that is what they were checking. Do the batteries have enough juice to start the JFS. The answer seems to be yes even after soaking the plane overnight at -15 C. That should be good.
Victor wrote:Regarding the intake at the base of the vertical stabilizer, maybe it is a ram air inlet for an EPU (the hump) that powers the ECS behind the cockpit. The dorsal spine has an "engine bleed air duct to environmental control system" per the cutaway posted earlier.
EPU and ECS have no connection whatsoever.
1. EPU stands for Emergency Power Unit, which is used only when the engine flames out in mid-flight. Since the engine drives the hydraulic pumps, the hydraulic pressure starts to fall. If it falls below a limit, the EPU kicks in, and turns the turbine which is connected to the hydraulic pump and the hydraulic pressure is maintained so that the flight control surfaces remain operable till engine relight occurs.
2. ECS is Environment Control System which is working all the time. It takes care of the cockpit environment, i.e. pressure, temperature, humidity etc. and also temperature of the environment around various avionics-bays.
The ECS gets its air from the engine bleed air, which is why the duct is called "engine bleed air duct". The duct carries the bleed air from the 6th stage of the engine to the ECS (housed behind the cockpit). There it goes through a series of heat exchangers and cooling unit to bring the air to the required temperature and pressure. I think I am pointing this out for the third time, once with pictures with the duct outlined. Here it is again!