Forum: Technology

I had gotten to wondering about something when it comes to military thrust versus afterburning thrust. Suppose you have what we will call Engine 1. It's military thrust is 18,000 pounds uninstalled at sea level, and its afterburning thrust is 25,000 pounds. Over time, technology gets better and it is substantially upgraded until it becomes Engine 2. Its military thrust has been increased to 25,000 pounds and there is a corresponding increase in afterburning thrust. The dimensions of the engine have not been changed from Engine 1 to Engine 2.

Now, the question is: Can an aircraft fitted with Engine 2 go just as fast as the same aircraft fitted with Engine 1 while using military thrust only, since its military thrust now equals the afterburner thrust of its more primitive cousin? I suppose the closest to real life I could compare this is that of the F-15C which has a top speed of Mach 2.5 with 25,000 lb. afterburning engines, but what if an improvement on the F100 allowed it to make 25,000 lb. thrust with military power (which might be possible, since TEG once spoke of F100s that could reach 37,000 in afterburner)? Could it still reach Mach 2.5 without 'burners?

It seems like I must be missing something...since the F100s have been uprated since then and I have yet to hear about Eagles supercruising (well, maybe marginally in a clean configuration). Would turbine inlet temperature become too large? Is this actually decreased with afterburner use because the heating can be done after the turbine section? Thrust lapse shouldn't be the issue, since thrust is a product of exhaust velocity and mass flow. Given that both engines have the same size and area, exhaust velocity must be the same to produce the same thrust.

Help me out here in understanding this?

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Kryptid wrote:

Now, the question is: Can an aircraft fitted with Engine 2 go just as fast as the same aircraft fitted with Engine 1 while using military thrust only, since its military thrust now equals the afterburner thrust of its more primitive cousin?

(Simply stated)
It depends on the speeds we are talking and the exhaust velocity of engine. When using afterburner, the exhaust temperature and velocity increases dramatically. This gives a high specific excess thrust necessary to accelerate the aircraft to the desired speed. As an aircraft seeds up, the resulting acceleration of the engine's exhaust will drop. (Thrust Lapse) The engine's thrust drops accordingly.

I'll cite a non-afterburning example; the KC-135Q versus the KC-135R. While the later has the F108 Turbofan (CFM-56) with more thrust, the J57 turbojet powered KC-135Q was faster. The Qs were not reengined until after the retirement of the SR-71. The Qs had a higher top end that made the Blackbird's refueling easier. So while the Rs would take off with more fuel, accelerate and take-off on a shorter field, and be more efficient for long duration flights; the Qs would fly closer to MACH 1 after achieving their top speed for the KC-135's given amount of drag at altitude.

So too, a higher MIL trust will not yield the same specific thrust, as an engine in AB, thrust being equal. With the MIL engine, you're not getting near the acceleration on the air, where in the AB that same volume of air is greatly accelerated.

Kryptid wrote:

Could it still reach Mach 2.5 without 'burners?

No. While the the engine will accelerate quicker without the AB, and may be able to hold MACH 1.1 or 1.2 in MIL, MACH 2.5 generates a tremendous amount of drag that an engine without the extra velocity (from the afterburner) will not likely be able to overcome.

Kryptid wrote:

Would turbine inlet temperature become too large?

To get an F100 sized engine to produce enough excess thrust to drive the F-15 to MACH 2.5 without afterburner, yes

Kryptid wrote:

Is this actually decreased with afterburner use because the heating can be done after the turbine section?

Typically AB use had little to no effect on the engine's operation above MIL. The TIT may increase or decrease a little, but this is small compared to the operating temperatures involved. So if your TIT is 1000* at MIL, and you engage AB, the TIT will still be near 1000* but the exhaust may jump from 650* at the exhaust nozzle to 3000*+ and gain the resulting velocity from the added heat/expansion.

Kryptid wrote:

Thrust lapse shouldn't be the issue, since thrust is a product of exhaust velocity and mass flow. Given that both engines have the same size and area, exhaust velocity must be the same to produce the same thrust.

It wouldn't be an issued based on inlet drag, but if you use a larger bypass ratio to achieve higher thrust for the same inlet diameter, the result would be a lower net exhaust velocity at MIL when combining the core and fan stream flows. The exhaust may have higher flow, but be exiting the engine at a lower overall velocity. Better during takeoff and acceleration, but worse at high speed.

Read through these from NASA, they will explain in more detail.

Specific Thrust: http://www.grc.nasa.gov/WWW/K-12/airplane/specth.html
Turbofan Trust: http://www.grc.nasa.gov/WWW/K-12/airplane/turbfan.html
Afterburner Thrust: http://www.grc.nasa.gov/WWW/K-12/airplane/turbab.html

Play with this jet engine sim from NASA, see what you can get?
http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html
It even has an F100 based model that the AB can be switched on/off.

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TEG

Thanks for the input, TEG!

So how are engines usually improved in thrust over their lifetime? By increasing their ability to withstand higher operating temperatures? Is that how they've improve the thrust output of the F100 series over time?

Kryptid wrote:

So how are engines usually improved in thrust over their lifetime? By increasing their ability to withstand higher operating temperatures?

Depends; some get larger fans, or more efficient fans, some get increased airflow, some get higher TIT, some get higher operating pressure ratios, some get new FADEC software for more efficient control, etc. The design of a specific engine will lend it to be modified/improved in some details but not others. (IE you can't make the F110 or F100 fan diameters larger)

Kryptid wrote:

Is that how they've improve the thrust output of the F100 series over time?

The F100-PW-229 has a smaller BPR than the PW-100/200/220, but a much higher OPR, It also has increased airflow due to a more efficient fan, and higher RPMs on N1 and N2 spools. The FTIT (Fan Turbine Inlet Temp) was increased slightly, while combustion temps are much higher. The higher CC temp is controlled by better cooling in the CC. The augmentor was modified for more efficiency and smoother transition between 'segments'. The engine's DEEC (FADEC) also allows the engine to spool up from IDLE to MAX faster than any other engine of it's time. (Not counting the F119 or F135)

So a little here and a little there can add up to roughly a 20% improved thrust at MIL and MAX.

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TEG

TEG,

Thanks for your never ending educational posts. Always well worth the read, much appreciated..

And I have to ask: has there been any renewed developments as of recent, that you might be aware of, with regards to PW-232 program? It just seems like such a no-brainer, as long as air forces X,Y,Z around the world, including USAF, are apparently geared up to be operating F-16 and F-15 for the next 20 years +/-, in a 'soon-to-be' globally engine upgraded (T/W upgraded) environment. Shouldn't one plan to be competitive then and stay relevant? Thanks...

geogen wrote:

has there been any renewed developments as of recent, that you might be aware of, with regards to PW-232 program? It just seems like such a no-brainer, as long as air forces X,Y,Z around the world, including USAF, are apparently geared up to be operating F-16 and F-15 for the next 20 years +/-, in a 'soon-to-be' globally engine upgraded (T/W upgraded) environment. Shouldn't one plan to be competitive then and stay relevant? Thanks...

IMO the air forces of the world think 29K is enough to hurtle the Viper through it's flight envelope just fine. The ONLY 32K sales to date (Viper or Eagle) have been the GE-132s for UAE's aircraft. Most sales of late have been the PW-229EEP (giving more TBO than thrust to keep life-cycle costs lower)

One big thing to remember with the 32K thrust engines is more stress on aircraft. If the Eagle could execute maximum performance with the PW-100, why wouldn't the PW-229 suffice? (without putting undue stress on the airframe) Likewise the PW-200 of the original Block 5 Viper made for eye-watering performance. The PW-229 or GE-129 can both do the same with the now heavier Block 50+/52+ or later. Why put the undue stress cycles on your aircraft when there isn't a need? Lately program/life-cycle costs have been more important to world air forces than T/W ratio. The PW's EEP has been outselling the GE over 2/1 in open competition on the fighter market.

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TEG

Thanks for reply, TEG. But (like there isn't always a 'but' after a: thanks for...) doesn't the PW-232 have the capability to be modulated by the user between either TBO settings and full blown performance settings, depending on the requirement of the day?

geogen wrote:

...doesn't the PW-232 have the capability to be modulated by the user between either TBO settings and full blown performance settings, depending on the requirement of the day?

It would be the requirement of the air force in question.

To my knowledge the 'trust level' would have been set in the software of the DEEC. Something that couldn't be changed on the fly, but maybe after the DEEC was changed, or new software uploaded into it while 'on wing' (installed).

Look at this from PW (or any other air forces' perspective) What is cheaper?

Producing/purchasing additional PW-229s from an existing production line, with existing technical data, logistics, parts suppliers, Engines that are almost common to the USAF and numerous other operators worldwide. Infusing just enough technology from the F119/F135 to make the engines even MORE reliable and durable extending their TBOs by 50% all for about the same cost? (and speaking of 'upgrades'. will product 29K trust in any large or small mouth Viper or Eagle)

...or...

Starting PW-232 production, paying for flight testing, developing technical data, retooling the production line, establishing new contracts with parts suppliers, crafting new logistics chains, providing new tooling for maintenance, etc..... ALL without the support (technical or financial) of the US DoD or USAF; to gain a little less than 10% trust at MIL/MAX? All for a production run of less than 100 engines? (that will only produce up to 32K trust in a large mouth Viper or Eagle, so only applicable to current GE Viper upgrades)

Ask GE how that worked out for them and the limited run of the GE-132; I haven't noticed them flying off the shelves.

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TEG

Well as usual, that's about as good as a concise answer as one could get, thanks.. Smile

But if I could shoot one more from the hip out to you then... in your educated guesstimation, about how big a market - i.e. many actual PW-232 orders (either domestic and/or FMS) on the table - could justify PW in plopping down the $ and interest w/ a restarted -232 Program? A market for 250 engines? 350? 500??

I doubt anything less than 250 in firm (money being exchanged) orders, with some follow on options.

Since the FMS countries didn't beat down the doors for PW-229s to install in the MLUs, nor did the USAF for PW-220 powered Vipers/Eagles (besides the ANG Block 42s) I figure PW doesn't anticipate great interest in 're-engining' the Vipers/Eagles of the world with PW-232s either.

That's just my Two Cents

TEG

Kryptid wrote:

Now, the question is: Can an aircraft fitted with Engine 2 go just as fast as the same aircraft fitted with Engine 1 while using military thrust only, since its military thrust now equals the afterburner thrust of its more primitive cousin? I suppose the closest to real life I could compare this is that of the F-15C which has a top speed of Mach 2.5 with 25,000 lb. afterburning engines, but what if an improvement on the F100 allowed it to make 25,000 lb. thrust with military power (which might be possible, since TEG once spoke of F100s that could reach 37,000 in afterburner)? Could it still reach Mach 2.5 without 'burners?

In this hypothetical engine, yes. The F-15 certainly can go to that speed without afterburning. The basic idea of afterburning is jet engine combustion chamber is not 100% efficient. After the first burn burn, there is still a lot of oxygen left. So if one add fuel once more it will burn again, adding more kick. The root of this unefficient burn is material science limitation. If one can create a turbine fan inside a 100% efficient combustion chamber, it will have to withstand temperature around 2500 degree. Best current composite only work up to around 1600 degree. I would imagine if in the future there is a 100% burn engine, afterburner won't exist anymore. Adding fuel will simply spew out black smoke.

however, normal jet engine can't go faster than mach 2.2 or so. At this speed, air starts behaving strangely, those fan blades stop being a compressor and everything turns into gigantic turbulent everywhere. Good combustion stops and the engine sputters. Hence the need for different type of engine for supersonic speed.

@bigbird2

Did you even bother to read this thread :>
TEG explained it very well. Even if you upgrade mil power to AB level it still wouldnt go fast as plane with older engine on AB. He gave very good example of same tanker with different engines.

Here is excellent story about Tumansky R-15 engine. It explain lot of things.

http://translate.google.com/translate?js=n&prev=_t&hl=en&ie=UTF-8&layout=2&eotf=1&sl=cs&tl=en&u=http%3A%2F%2Fwww.mzak.cz%2Fmotory%2Fr-15%2F

Some in Bigburd's defense - he did say "IF" a whole lot. IF we ever get advanced materials that can withstand being spun at 15K+ RPM @ 3000* temperatures it COULD be possible.

We won't see it in our lifetimes though.

The big advancements in aviation jet propulsion IMO are; Geared-Fan Technology, Hydrocarbon fueled SCRAMJET engines, and Pulse Detonation Engines. All three are revolutionary, but if they get PDEs to work for military applications it will be like the leap of technology between piston an turbine engines.

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TEG

Here's for hoping PW gets that first operational PDE, eh... and not, God forbid, 'that other manufacturer'?? Doh

From what I've seen both companies are going separate ways on PDEs. Unc|@$$ sources indicate PW is going for a pure PDE engine that uses the power of the detonation to provide thrust. GE on the other hand is using the PDE as a combustion chamber of sorts to provide 'power' through a conventional turbine.

Time will tell... Shrug

But the evolution (if it works) has been compared to the birth of the gas-turbine aviation engine.

TEG

Forum: Technology

Jet Engine Question