? for the F-4B pilots

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?

Brett- 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, BRBR

There ya go, thanks..answers all my questions.


P. C. Chisholm
CDR, USN(ret.)
Old Phart Phormer Phantom, Turkey, Viper, Scooter and Combat Buckeye Phlyer

"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

"Peter Stickney" wrote in message
om...
"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

I know you are trying, but don't give up your day job. :-) There are some
problems and misconceptions with your simple explanation on how and why it
works.

Red Rider

"J" wrote in message . rr.com...
"Peter Stickney" wrote in message
I'm going to be simplifying a bit, for those who don't like
Thermodynamics.
I know you are trying, but don't give up your day job. :-) There are some
problems and misconceptions with your simple explanation on how and why it
works.

One of the dangers that comes from playing to the audience, RR.
I'd rather run the risk of over-simplicating for those who aren't
technically inclined, vs. drowning them with a firehose of Tech
Stuff. (Think of it as bait - we'll suck 'em in, get 'em hooked,
and then gaff 'em with the numbers.)

Why turn 'em off with a lot of True Stuff about Turbine Stresses,
Mass Flow, Pressure Ratios, Fuel/Air ratios, Compressor & Turbine
Efficiencies & suchlike, if it only makes their eyes glaze over?

(Oh, and as for an afterburning turbojet being considered 2 separate
engines, there have been engine installations that did just that.
The powerplant for the Republic XF-103 Mach 4 interceptor. (Cancelled
in the late '50s, but they'd cut metal for it, and the powerplant
had been tested at the N.A.C.A. tunnels and the tunnels at (I think)
Tullahoma. Basically, it was a Wright J67 (Bristol Olympus "fixed" by
Curtiss-Wright, just like they did to the Sapphire to get the J65,
with a big afterburner spaced way back in the tail. At low speeds,
it was pretty much a normal gas generator/AB combination, but as
speed picked up, and the gas generator output started decreasing,
somewhere around Mach 2, they'd divert the inlet flow around the
turbojet, shut the turbojet down, and keep going on just the AB,
using it as a ramjet.) It's just crazy enough to work.

--
Pete Stickney

"Peter Stickney" wrote in message
om...
"J" wrote in message
. rr.com...
"Peter Stickney" wrote in message
I'm going to be simplifying a bit, for those who don't like
Thermodynamics.
I know you are trying, but don't give up your day job. :-) There are
some
problems and misconceptions with your simple explanation on how and why
it
works.

One of the dangers that comes from playing to the audience, RR.
I'd rather run the risk of over-simplicating for those who aren't
technically inclined, vs. drowning them with a firehose of Tech
Stuff. (Think of it as bait - we'll suck 'em in, get 'em hooked,
and then gaff 'em with the numbers.)

Why turn 'em off with a lot of True Stuff about Turbine Stresses,
Mass Flow, Pressure Ratios, Fuel/Air ratios, Compressor & Turbine
Efficiencies & suchlike, if it only makes their eyes glaze over?

(Oh, and as for an afterburning turbojet being considered 2 separate
engines, there have been engine installations that did just that.
The powerplant for the Republic XF-103 Mach 4 interceptor. (Cancelled
in the late '50s, but they'd cut metal for it, and the powerplant
had been tested at the N.A.C.A. tunnels and the tunnels at (I think)
Tullahoma. Basically, it was a Wright J67 (Bristol Olympus "fixed" by
Curtiss-Wright, just like they did to the Sapphire to get the J65,
with a big afterburner spaced way back in the tail. At low speeds,
it was pretty much a normal gas generator/AB combination, but as
speed picked up, and the gas generator output started decreasing,
somewhere around Mach 2, they'd divert the inlet flow around the
turbojet, shut the turbojet down, and keep going on just the AB,
using it as a ramjet.) It's just crazy enough to work.

--
Pete Stickney

Whoooooo! Don't include me in your post. I don't agree with you at all. You
are throwing a bunch of big words in there like you know what you are
talking about, which I don't think you do at all. Your statement of

"If you like, you can think of an afterburning turbojet as two engines: The
turbojet itself, and a ramjet downstream."

Is a crock. And the J-67 was a different concept. Enough said!

"PLONK"

Red Rider

J wrote to Pete:
Whoooooo! Don't include me in your post. I don't agree with you at all.
You are throwing a bunch of big words in there like you know what you are
talking about, which I don't think you do at all. Your statement of

"If you like, you can think of an afterburning turbojet as two engines:
The turbojet itself, and a ramjet downstream."
Is a crock.

Actually, Pete's statement is absolutely true. The reason apparently
escaped you, though.

If you like, you can think of the previous sentence as absolutely false.
:-)
--
John Miller
My email address: domain, n4vu.com; username, jsm

You can write a small letter to Grandma in the filename.
-Forbes Burkowski, CS, University of Washington

"J" wrote in
news

"Peter Stickney" wrote in message
om...
"J" wrote in message
. rr.com...
"Peter Stickney" wrote in message
I'm going to be simplifying a bit, for those who don't like
Thermodynamics.
I know you are trying, but don't give up your day job. :-) There
are
some
problems and misconceptions with your simple explanation on how and
why
it
works.

One of the dangers that comes from playing to the audience, RR.
I'd rather run the risk of over-simplicating for those who aren't
technically inclined, vs. drowning them with a firehose of Tech
Stuff. (Think of it as bait - we'll suck 'em in, get 'em hooked,
and then gaff 'em with the numbers.)

Why turn 'em off with a lot of True Stuff about Turbine Stresses,
Mass Flow, Pressure Ratios, Fuel/Air ratios, Compressor & Turbine
Efficiencies & suchlike, if it only makes their eyes glaze over?

(Oh, and as for an afterburning turbojet being considered 2 separate
engines, there have been engine installations that did just that.
The powerplant for the Republic XF-103 Mach 4 interceptor. (Cancelled
in the late '50s, but they'd cut metal for it, and the powerplant
had been tested at the N.A.C.A. tunnels and the tunnels at (I think)
Tullahoma. Basically, it was a Wright J67 (Bristol Olympus "fixed"
by Curtiss-Wright, just like they did to the Sapphire to get the J65,
with a big afterburner spaced way back in the tail. At low speeds,
it was pretty much a normal gas generator/AB combination, but as
speed picked up, and the gas generator output started decreasing,
somewhere around Mach 2, they'd divert the inlet flow around the
turbojet, shut the turbojet down, and keep going on just the AB,
using it as a ramjet.) It's just crazy enough to work.

--
Pete Stickney

Whoooooo! Don't include me in your post. I don't agree with you at
all. You are throwing a bunch of big words in there like you know what
you are talking about, which I don't think you do at all. Your
statement of

"If you like, you can think of an afterburning turbojet as two
engines: The turbojet itself, and a ramjet downstream."

Is a crock. And the J-67 was a different concept. Enough said!

"PLONK"

Red Rider

Well, Red, he got it pretty close to right on the XF-103.

From http://home.att.net/~jbaugher1/f103.html
*****
Specifications of the XF-103:
Engines: One Wright XJ67-W-3 turbojet, rated at 15,000 lb.st. dry and
22,000 lb.s.t. with afterburning, plus one XRJ55-W-1 ramjet rated at
18,800 pounds of thrust. and

This phenomenal performance was to be achieved by the adoption of a dual-
cycle propulsion system. Takeoff and normal cruise were to be powered by
a Wright XJ67-W-3 turbojet, rated at 15,000 lb.s.t. dry and 22,000
lb.s.t. with afterburning. The XJ67 was a license-built version of the
Bristol Olympus. At high speed, the thrust of the turbojet was to be
augmented by a XRJ55-W-1 ramjet, capable of delivering 18,800 pounds of
thrust. With both powerplants operating, a total thrust of 37,000 pounds
could be achieved at altitude.
*****

From http://www.fas.org/nuke/guide/usa/airdef/f-103.htm
*****
The Republic XF-103 Model AP-57 had a Delta wing swept-back 55 degrees at
the leading edge and incorporates a variable incidence feature. The
horizontal tail is of the Delta configuration with a sweep-back angle of
60 degrees at the leading edge. The alighting gear was of the tricycle
type and retracts into the fuselage. The turbo-jet was a Wright
Aeronautical Corporation XJ-67-W-1 power plant with afterburner. A Ferri
type two-dimensional engine air inlet is used. The thrust of the turbo-
jet engine was limited at high Mach numbers by the allowable turbine
inlet temperature. The air bypassed the engine compressor and turbine,
and using the afterburner as a ram-jet combustion chamber, the available
thrust is greatly increased above a Mach number of 2.0
*****

As for the concept "you can think of an afterburning turbojet as two
engines: The turbojet itself, and a ramjet downstream.", if you think
about it, that's exactly how it works - feed the exhaust of the jet with
more fuel, light it and get thrust, no different than a ramjet. The force
of the incoming air keeps the flame front from blowing forward.

Regarding the remainder of his explanation, where are all the big words?
Seems like a pretty reasonable explanation of the basic operating
principles. All engines are basically air pumps; more air throughput,
more power.

Dave in San Diego

--
-
"For once you have tasted flight, you will walk the earth with your eyes
turned skyward;
For there you have been, and there you long to return."
Leonardo da Vinci