"Joe Delphi" wrote in message link.net...

Page 1-58:
"The airplane is powered by two General Electric J79 engines. Aircraft
153071z thru 153087aa have J79-GE-8 engines installed, with a thrust
rating of 10,900 pounds each. Afterburner operation increases the
maximum thrust to 17,000 pounds. Aircraft 153088aa and up have
J79-GE-10 engines installed, with a thrust rating of 11,870 pounds
each. Afterburner operation increases the maximum thrust to 17,900
pounds each."

Seems strange that the -10 has only 2,000 more pounds thrust w/o burner and
only 900 pounds more with burner. I know that each aircraft has two engines
so that comes out to an additional 4,000 pounds thrust w/o burner.

Does that sound right?

Well, 11,870 - 10900 = 970, so _that_ part's not too close.

But it's really not too odd, one you look at the fundamentals.
I'm going to be simplifying a bit, for those who don't like
Thermodynamics.

Basically, a jet engine develops thrust by squirting hot air
out the back. The hotter the air, the more thrust for a given
amount of airflow. So far, that's pretty straightforward - you
pull in a bunch of air, squish it so that you can burn more fuel
in it, burn the fuel to heat it up. aand squirt it out the back.
However, making it work is a little more complicated. It takes
a lot of power to compress the air. The best way to get that
power is to stick a turbine in the hot gas comin out of the
burners, and use that to drive the compressor. So far, so good,
but the turbine blades can only get so hot before they deform and
fail. So, you can only heat the air up a certain amount.
(Using the turbine to extract energy from the hot gas also cools it
down quite a bit, too.) This maximum Turbine Entry Temperature is
basically what drives the amount of unaugmented Or, as its sometimes
called, Dry) thrust that a jet engine can produce.
One solution to get more thrust is to heat the air up after it has
flowed through the turbine. (Afterburning) The amount of heat that
can be added is much greater, being limited by either the tail pipe's
materiels, or by how much fuel you can pump in. As you can guess,
though, you end up burning an awful lot of fuel.

For an F-4J, sitting on the runway, is burning about 10,000#/hour/engine
at Military (Max. unaugmented) thrust, and pretty close to
36,000#/hour/engine with the Afterburners operating.

If you like, you can think of an afterburning turbojet as two engines:
The turbojet itself, and a ramjet downstream. The amount of thrust
produced by one is only indirectly related to the amount of thrust
produced by the other.

--
Pete Stickney