delta2014 wrote:F119 Doctor,

Thank you very much for explaining this to me.

Here are some questions:

(1) If the J79-19 was tested on top of a 40,000 foot mountain (just pretending), would it still be able to run at zero airspeed at 40,000 feet?

(2) If the J79-19 is able to run at zero airspeed at 40,000 feet, would thrust be reduced down about 3,798 lbs? That would be 32% of its 11,870 thrust at sea level. Does 3,798 sound about right?

(3) Now, in flight, flying at sea level, at about what airspeed would the J79-19 be able to produce its maximum MIL power thrust? It will definitely be less than 11,780, because you only get 11,780 when testing on the ground at sea level with zero airspeed. So would you think in flight the engine might be able to produce, maybe, 10,000 lbs of thrust? If 10,000 was the maximum amount of thrust the engine was capable of producing in flight at sea level, at what airspeed would the F-104 need to be flying at to get the J79-19 to produced its maximum MIL thrust?

Thanks,

Delta2014

1) I don't have any personal experience with the J79 engine, but it should be able to run at Mil power at 40K, zero airspeed, at least with a test cell bell mouth on your hypothetical 40K mountain. With it installed, the altitude independent horsepower extraction will push the engine operating line closer to the compressor stall line, and the aircraft inlet will restrict the flow, also pushing it toward stall. Any angle of attack will further distort the inlet flow, resulting in compressor stall and flameout.

2) According to my handy P&W Vest pocket handbook, inlet pressure at 40K, 0 Mn would be 2.72 psia. Ratio the sea level thrust of 11,780 lbf by that pressure over standard sea level pressure of 14.7 psia would give you a thrust of approximately 2180 lbf. This assumes that the J79 doesn't increase its airflow significantly below standard inlet temperature. Since the inlet temperature has dropped from 59F to -70F at that altitude, the thrust would be a little less since the EGT would also be lower, and exhaust velocity drops with the square root of the temperature ratio (in absolute degrees). I would give a guess that Mil thrust would be somewhere between 2000-2200 lbs under these conditions.

3) If you are flying at sea level at Mn of 1.0, you have an inlet pressure of 27.82 psia. If there were no engine limits, 27.82 / 14.7 x 11780 = 22293 lbf thrust. But your inlet temperature has gone up to 163F, more than 100F above standard day conditions. Engine limits on rotor speed, turbine temperature, and main burner pressure all will reduce the "corrected" rotor speed and airflow through the engine. Assuming the engine maintains 100% rpm on the gauge, the corrected rotor speed would only be 91%. I would provide a wild guess that the engine would be putting out something in the order of 16K-18K thrust under these conditions, with fuel flow around 15,000 lbs / hour (reflecting the 0.85 Mil SFC). Of course, airframe drag is also huge under these conditions, so you might never get to 1.0 Mn at sea level with Mil power. The other effect that occurs with increasing speed is the decreasing delta between exhaust velocity and airspeed - not a huge effect subsonic for fighter engines, but significant in transonic and more so supersonic

As I noted in the beginning, I don't have any direct J79 experience, so this is sort of a generic answer for jet engines in general, using the J79 thrust numbers as a basis.