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LightningZ71
Posts: 540
Joined: Sat Aug 27, 2016 10:59 pm

Re: USAF PCA and other 6th gen fighters

Mon Jun 29, 2020 3:09 pm

Adding to Ozair's statement, with regard to the B-52, the only reason that an engine swap is even being discussed there is that supporting the TF-33 is getting more and more difficult as the platform ages. Going to a modern engine will certainly bring greater efficiency with respect to fuel and emissions, but the B-52 is not effectively limited in its operational capability due to a lack of range from a closest friendly refueling point point of view. For the change to be deemed worthy, it needs to provide enough savings for the lifetime of the engines on the wing from reduced maintenance costs and fuel consumption savings (more efficient engines use less fuel, requiring less fuel for equivalent distances flown) requiring lower take off weights (which reduce wear and tear on the frame, admittedly a very minor factor) and fewer expensive aerial refuelings, against the cost of purchasing all new engines, spares, support equipment unique to those engines, training existing maintainers on the new engines, overhauling the cockpit, adding digital engine management systems, retraining the air crews, going through the design work, purchasing the engine control systems, running the new wiring, testing all of the individual components, and requalifying the whole airframe with the new engines. None of that is free, and much of it costs lots and lots of man hours, which also isn't free. After all of that, what have you gained for the B-52 operationally? It can fly a bit farther on a full load of fuel, but the extra range wasn't really a need before. IT won't be able to carry MORE bombs as it was already at structural capacity as it stands. It won't be able to go appreciably faster or higher, as that's more of an airframe thing and not an engine power thing. It can already loiter longer than you really want a crew to be loitering. All in all, its not an appreciably better bomber from the standpoint of being a bomber.

The F-35 is a different story. There is a noted need for more electrical power. It stands to gain substantially from greater engine efficiency as it allows it to fly farther without requiring external tanks, and is especially valuable for the C given naval ranges, and the B, given its limitations due to the extra mass of the SVTOL equipment. It also stands to gain in maneuverability, as it will have more engine thrust to help overcome the loss of energy it experiences in high G turns (more valuable in the A as it is rated for higher G turns).

The F-18 is kind of in the middle. Greater engine thrust can make take offs safer as it will have more thrust available in a single engine failure on take off, and will have more thrust available in the case of an unexpectedly low power cat launch. The greater thrust will allow it to carry more fuel aloft to allow for greater range with the same bomb load, though, you do run into other limitations there. It won't have a much higher top speed. IT will be able to cruise slightly faster for the same current fuel burn, or cruise at the current speed for longer. Coupled with CFTs, though, it can make a wider variety of missions available to it without requiring drop tanks. It's not transformative, just slightly additive. And, that then calls for the next question, is it worth it? Or, do you spend that money elsewhere, like on autonomous tanker drones that can provide for aerial refueling for a handful of fighters to give them essentially the same as above capabilities, while also being able to refuel F-35s, making them more capable as well?

Engine upgrades when you were moving from old, low pressure low bypass turbofans and turbojets, to modern high bypass turbofans, like on the KC-135, or the B-52 moving to the TF-33s in production at the end of the line, make a lot of sense. The improvements are vast, and the capabilities of the craft greatly enhanced. Incremental upgrades, in the order of a few tens of %, take a lot more convincing and have much, much longer pay-off times.
 
RJMAZ
Posts: 1961
Joined: Sat Jul 09, 2016 2:54 am

Re: USAF PCA and other 6th gen fighters

Tue Jun 30, 2020 8:44 am

744SPX wrote:
I would like to believe the F-35 will be getting the AETP engine, but based on recent AF and Navy acquisition history I don't see it happening.
Since the installation of upgraded engines on the F-14, F-15E, F-16 and Legacy Hornets in the late 80's/early 90's, both the Navy and AF have refused to purchase improved engines for their aircraft despite their availability from GE and Pratt and this in the face constant weight gain and higher drag on all fighter aircraft..

Actually this is completely opposite from the truth. Every single fighter you mentioned got newer engines fitted.

The F-14's went from Pratt TF30's to GE F110.
The Hornet went from the F404-GE-400 to the 402 which was a huge upgrade in thrust and durability.
The newest F-16's in the USAF have way more thrust than the early builds and they even got an option for a second GE engine after 10 years of service. New international F-16 customers even have a third engine to chose from.

I would go as far as saying every US fighter in history that was in production for more than 20 years got a new engine.

I never said that the USAF would re-engine existing F-35's. This would be silly considering the engine would still have thousands of flight hours left. In 10 years time it is a near certainty that the F-35 rolling off the production line will have a different engine to the one fitted today. It will have improved thrust and fuel burn. The F-35 will be in production for 40+ years so it will probably even get a third engine just like the F-16.
 
744SPX
Posts: 198
Joined: Mon Jan 27, 2020 6:20 pm

Re: USAF PCA and other 6th gen fighters

Tue Jun 30, 2020 9:21 pm

Yes, they all did get newer engines fitted, but my point was it was 30 years ago and all these aircraft have continued to gain weight while thrust upgrades have been available for a very long time. The US F-15E and F-16 fleet could have been fitted with 32-34k versions of the F100/F110 20 years ago. The Super Hornet has received no thrust upgrades despite the fact that it needs it more than the C/D ever did. The F-15C never got a thrust upgrade.
 
Ozair
Topic Author
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Joined: Mon Jan 31, 2005 8:38 am

Re: USAF PCA and other 6th gen fighters

Tue Jun 30, 2020 10:43 pm

744SPX wrote:
Yes, they all did get newer engines fitted, but my point was it was 30 years ago and all these aircraft have continued to gain weight while thrust upgrades have been available for a very long time. The US F-15E and F-16 fleet could have been fitted with 32-34k versions of the F100/F110 20 years ago. The Super Hornet has received no thrust upgrades despite the fact that it needs it more than the C/D ever did. The F-15C never got a thrust upgrade.

What is missing is your justification that the extra thrust is needed. You have claimed the F-15 and 16 fleet need the extra thrust but the evidence isn’t present to support that. Sure the aircraft have increased marginally in weight but additional top end thrust likely doesn’t provide the performance gains you think it does.

Again if you consider the time the aircraft will spend in that maximum thrust zone, likely just tens of seconds out of a total flight of one to two hours it just isn’t worth the upgrade cost. GE doesn’t list a dry thrust for the F110-GE-132 but the F100-PW-229A has the same dry thrust as the standard 229 with only the wet thrust increased to 32.5k. (specs for both engines here, http://www.jet-engine.net/miltfspec.html)

Same for the SH, to claim it needs more thrust you need to provide a statement from the USN asking for it. With the Classic Hornet the Kuwaitis paid for the thrust improvements to the F404 that the USN subsequently benefitted from, the USN didn’t pay for that work themselves. Additionally Kuwait have ordered the SH but haven’t asked or are paying GE for an engine upgrade for the aircraft.
 
744SPX
Posts: 198
Joined: Mon Jan 27, 2020 6:20 pm

Re: USAF PCA and other 6th gen fighters

Wed Jul 01, 2020 2:33 pm

I get what you are saying about cost benefit, I'm just always of the mind that you can't have too much thrust. Also, considering how much DOD money is uselessly thrown away, I think thrust upgrades are more than justifiable.

Also, I'm pretty sure both the F100 and F110 32k versions have proportionately increased dry thrust, between 19,000 and 20,000 lbs I believe. Both engines have higher airflow fans so its not just about afterburner thrust. Also, the F100-232/229-EPE use F-119 tech not just for more power but also more fuel efficiency.
 
Ozair
Topic Author
Posts: 5100
Joined: Mon Jan 31, 2005 8:38 am

Re: USAF PCA and other 6th gen fighters

Wed Jul 01, 2020 10:27 pm

744SPX wrote:
I get what you are saying about cost benefit, I'm just always of the mind that you can't have too much thrust. Also, considering how much DOD money is uselessly thrown away, I think thrust upgrades are more than justifiable.

Again, the time spent in that thrust region is so small the improvement is minuscule and aerodynamics plays a far more significant role than the additional thrust. For example if you take the F-16C powered by the F100-PW-129 the difference from MIL to Max AB on turning agility is negligible. A Max AB jet clean at 22,000lbs has only a .5 degree better turn rate than the same jet at MIL. So despite the addition of over 60% more thrust the pilot gets only a 2.5% better turn rate. That turn rate advantage from using Max AB also reduces further with weight gain of the aircraft. At a representative operational configuration weight of 26,000lbs the turn rate with Max AB is only 0.3 degrees better. That turn rate advantage is now within the margin of pilot skill and not airframe performance.

Above numbers are taken from this source doc https://info.publicintelligence.net/HAF ... lement.pdf using 10,000ft MIL and Max AB charts. Start at page A8-33.

Where the higher thrust versions improve performance is on take-off, acceleration and energy retention but I expect, given the minimal agility differences, that a lower thrust F110/F100 at Max AB is going to also provide essentially the same advantages except for take-off where the thrust will really matter. For the UAE, the only user of the F110-GE-132, who have only three AAR tankers and yet nearly 150 fighter aircraft being able to take off in hot and dry conditions with greater weights is worth the additional sustainment cost and likely fuel use of the 32k engine. The USAF doesn’t have that issue.

744SPX wrote:
Also, I'm pretty sure both the F100 and F110 32k versions have proportionately increased dry thrust, between 19,000 and 20,000 lbs I believe. Both engines have higher airflow fans so its not just about afterburner thrust. Also, the F100-232/229-EPE use F-119 tech not just for more power but also more fuel efficiency.

Did you actually read the source link I posted above on engine specs? There is no dry thrust or SFC value listed for the F110-GE-132 but the F100-PW-229A lists the same dry thrust as the 229. No SFC for either of the 32k versions, 132 and 229A, but for the F100 the SFC is listed across a number of versions, all within a small variance of values (0.720 to 0.735). I highly doubt the 32k versions of both engines have a significant enough difference in SFC to their lesser powered counterparts.

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