How have your opinions of the JSF changed over the years?

Discuss the F-35 Lightning II
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caux

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Unread post16 Aug 2011, 20:50

wrightwing wrote:Which means that they achieved that milestone, over a year ago, in other words. As for the engine weights, the discrepancy may be that one weight is the dry weight vs. the weight with all of the oil/lubricants.


680Kg of oil, lubrificants???!! No, I don’t think so…
I think that probably F-135 is heavier because of single engine installation requirements (it is more reliable!) and probably the actual trust/weight ratio is more or less 8.5.
In that situation the engine respect trust requirements and also has a huge growing potential that could be used in the future...and P&W is working for proposing it. That is in according with both articles.
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sewerrat

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Unread post17 Aug 2011, 02:39

Unfortunately, (as I've thought many times) the Marines STOVL requirements screwed the CTOL variants. STOVL and affordability necessitated the requirement for one very big powerplant in place of 2 smaller engines along with the built in safety of 2 turbofans.

The JSF couldn't have been designed with 2 engines because of the lift fan required. Actually... it could have been designed with 2 F414 sized powerplants, but then there would have been 2 unique powerplants: one for each side, which would have added a lot of money to ER&D, and unit fly-away price.

And unfortunately that one large powerplant spools slower than would 2 F414 sized engines. You've got all that damned rotational mass, plus the larger diameter, and hence its not going to spool as fast.

Can't quantify the "spool up", obviously, other than to say physics says that 2 smaller engines would spool faster.
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SpudmanWP

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Unread post17 Aug 2011, 03:25

The USAF only wanted ONE engine too (ala F-16).
"The early bird gets the worm but the second mouse gets the cheese."
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battleshipagincourt

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Unread post17 Aug 2011, 04:00

Whether the USAF wanted a single engine or not doesn't change the fact the VSTOL feature limited the range of options available for the power plant of either service. If the ideal power plant was a low-bypass turbofan like the F119, that wasn't going to happen. Single engine restricted the navy's desires as well.

While probably cheaper in comparison to two engines, the F135 has been a disaster. They took an F119 engine and mounted an external fan for the X-35. How the **** does this 'derivative' of the same engine cost so many billions MORE to develop?!
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SpudmanWP

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Unread post17 Aug 2011, 04:45

There are several ways that two engines would have worked for STOVL. They could have mounted a transmission (ala V-22) that allowed both engines to feed the lift fan. If they had gone with a ducted-air lift device (forgot who had that, MD maybe), two engines would have worked also.

The fact is that the USAF (the largest customer by a large margin, IIRC 8x the orders of the USN) was only going to be part of the program on the condition that it only had a single engine.
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caux

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Unread post17 Aug 2011, 11:47

STOVL requirements heavly influence an aircraft.
A jet configuration does non have the excess power tipical of a 2 engine helicopter (or "similar") and so it is not possible to meet STOVL requirements with an engine out of order (a two engine aircraft not able to land without an engine is a nonsense from reliability point of view).
That is true for all STOVL configuration proposed for JSF program.
MDD proposal was this:

http://www.aerospaceweb.org/aircraft/fi ... mdd_01.jpg

So, a single relatively high BPR engine is the best choice for a “simple” strike fighter both for USAF and Marines.
USN has obviously a different point of view, but we must say that in the past also US Navy used a lot of single engine aircraft and today engine reliability is higher than in the past.
The US Navy preference for 2 engine aircraft was a strong selling point for F-18 concept against a naval F-16, but we but say that US Navy in that period was having a bad experience with F-14/TF-30.

Now the situation is different and probably the F-135 weight is the best confirm that the engine is designed to be reliable.

PS. Sorry for my English: as you can understand that is not my language… ;-)
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mave

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Unread post17 Aug 2011, 16:28

Surely the F135 has a worse power to weight ratio than the F119 because fundamentally it needs to carry around a higher pressure ratio turbine, and higher torque shaft line to drive the lift fan?
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sewerrat

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Unread post17 Aug 2011, 18:08

battleshipagincourt wrote:Whether the USAF wanted a single engine or not doesn't change the fact the VSTOL feature limited the range of options available for the power plant of either service. If the ideal power plant was a low-bypass turbofan like the F119, that wasn't going to happen. Single engine restricted the navy's desires as well.

While probably cheaper in comparison to two engines, the F135 has been a disaster. They took an F119 engine and mounted an external fan for the X-35. How the **** does this 'derivative' of the same engine cost so many billions MORE to develop?!


Not only did STOVL limit powerplant options, but it also greatly limited the planform confirguration of the JSF into having "only" 2 large weapons bays on the sides of the fusealage, and NOT having a central bay with 2 smaller side bays. That caused the somewhat portly dimensions of the JSF because without STOVL there could have been a central weapons bay holding 3-4 amraam, while the 2 side bays could have been reduced in volume to hold a single 2000lb "bomb" and either another amraam or a sidewinder. Those 2 large bays are rather bulbous as they are now.

Again, STOVL really yanked in the options on the JSF's configuration/dimensions/weights. Still the F-35 is a good airplane I'll admit, but its not what could have been if were not for the damned STOVL requirements calling for a large lift fan in the fusealage. At least the Maries are getting into the 5th gen fighter business.
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river_otter

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Unread post18 Aug 2011, 11:27

battleshipagincourt wrote:While probably cheaper in comparison to two engines, the F135 has been a disaster. They took an F119 engine and mounted an external fan for the X-35. How the **** does this 'derivative' of the same engine cost so many billions MORE to develop?!


Although as I understand it the core including the burners is essentially identical (saving considerable cost over new development), the rest of the turbines and the afterburner are different diameters and had to be designed new. And the afterburner is stealthy, which definitely wasn't cheap to develop new. It is a new engine, derived from but not the same as the F119. Plus it should cost more to develop, because it's a significant improvement over the F119 in many ways:

It's a more powerful engine. The larger diameter and higher bypass ratio gives substantially higher thrust until the larger diameter compressor tips begins to stall from Mach effects at somewhat lower speeds than the F119. It's not designed as a supercruise engine, but below that regime, where nearly all flying activity occurs (unless you're a Concorde, F-22, or SR-71), it's a better engine. Sea level thrust of 43,000 lbs. vs. 35,000 lbs. And that from (remember, identical core...) probably very comparable fuel consumption per hour at most efficient cruise to a single F119. And, although the F119 almost certainly exceeds its rated thrust (at least some quotes put it as high as 39,000 lbs.), so does the F135, and by a much wider margin. The F135 has been tested to 50,000 lbs. while still meeting durability requirements. (Though the F-35 was designed around a 43,000 lbs. thrust F135 and the aircraft can't exceed that without some modification there's no will to fund the development for right now. The lift fan and swivel duct of the B model can't take it at all, and the A model's exhaust duct would need to be redesigned to handle it. Maybe a future air superiority D model based on the A? But I digress...) Nevertheless, that means that at 43,000 lbs., the F135 should greatly exceed durability requirements, and has a future growth potential the F119 does not have. http://www.aviationweek.com/aw/generic/story_generic.jsp?channel=defense&id=news/asd/2010/08/27/01.xml&headline=Pratt And although I can't find specific figures, it's nearly a given the larger-diameter F135 has an even greater margin of higher dry thrust compared to the F119.

Power take-off (35,000 shp/20,000 lbs. lift thrust) for the lift fan, ducting for the roll posts (3,900 lbs. thrust), and different cycles to allow it to generate more aft thrust dry for STOVL lift. The F119 can't do that, period. Basic economy of scale, the F-35A and B use the exact same engine, so the engine has to be able to do that. The B's engine just has a lift fan drive bolted onto the front, and air bleed open for the roll posts, but is the same engine otherwise. You don't like the STOVL, but still it was a requirement that the engine could do it, and that's not free to develop. It's not a disaster, it's still far cheaper than developing two entirely new engines. And remember, the F119 has less thrust, and worse low-speed performance, so it was not suitable for the F-35A either. It would be two new different engines, or one new engine that could do both.

Navalized version. The F-35C uses an entirely different engine from the A and B models. It's essentially identical to the others in an aerodynamic sense, saving a fortune in designing the basic shape, but it's made of different materials with more resistance to corrosion in a sea environment. (The C's engine actually can't much exceed the rated 43,000 lbs. thrust due to the reduced thermal resistance of its different alloys.) There is no navalized F119. Those changes took metallurgical, engineering, and testing work in addition to the work on the A/B engine, and that's not free. The F119's development cost would've been considerably higher if they also had to develop a navalized F119.

Variable afterburner. Although little discussed (and I don't know why; it's a major development), Jeff Knowles has said this is the first engine with a progressively variable afterburner rather than strict wet/dry thrust transition, or a tiny handful of discrete stages lighting off. So maybe the F-35 can't supercruise (I personally think it can based on extrapolation from other performance measures released to the public, but certainly not in the Mach 1.5+ range of the F-22), but even if it can't, it may be able to cruise supersonic quite efficiently in a minimum afterburner setting that no other engine including the F119 has available to it. It also would improve reliability/safety if the afterburner can better adapt to differences in the dry engine state with altitude, speed, etc. vs. an on/off afterburner that's more dependent on having a specific set of start conditions. And think about what that would mean against other fighters that have to keep going to full afterburner to keep up with an F-35 burning just a few percent extra fuel. And what it means in terms of infrared signature management to go to partial (and variable) afterburner. I'm sure that brand new technology, including new hardware and new control software, was definitely not cheap to develop from scratch.
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caux

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Unread post18 Aug 2011, 14:11

mave wrote:Surely the F135 has a worse power to weight ratio than the F119 because fundamentally it needs to carry around a higher pressure ratio turbine, and higher torque shaft line to drive the lift fan?

Yes, and not only that, I suppose.
It has also a bigger fan and consequently a bigger casing.
So, F-135 volume is more or less 35% bigger than F-119 one and that could justify the increase of weight.
Yes, not all the 680kg because F-135 is not simply bigger but it has similar core and BPR bigger and the conventional axisymmetric nozzle probably is a lighter solution than a 2D vectored nozzle used by F-22, but I think that the rest could be explained by a request of more reliability and big growing potential that are vital, especially for STOVL variant that must increase trust every time weight increases (Harrier docet).
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Unread post18 Aug 2011, 14:37

'river_otter' thanks for your explanations about F-135 engine differences. One of the stories about the JBD testing of the F-35C talk about the inbuilt variable afterburner on the catapult. Due to experience with the Super Hornet family (which uses a different 'after market' solution apparently) through that experience with heat effects on the JBD - or whatever - the F-35C engine (and I guess all engines) were developed with this function from the start. I'll look for reference again. To me that backs up the vairable afterburner of the F-135 function you mention. Thanks again for bringing that to our attention.
_______________

F-35C Lands at Lakehurst For Testing

From this thread: http://www.f-16.net/f-16_forum_viewtopic-t-15767.html

F-35C completes first jet blast deflector testing

http://www.navair.navy.mil/index.cfm?fu ... ry&id=4691

"...Each Nimitz-class aircraft carrier has a JBD for each of its four catapults. The size, cooling configuration and angle to the catapult vary slightly between the four, so the test team had to repeat various tests – military and limited afterburner power takeoffs – for the various JBD configurations...."
____________________

Amy Butler had a story with more detail also on same thread page (scroll down).

JBD Testing A Key Step For Joint Strike Fighter Aviation Week & Space Technology Jul 18, 2011 p. 84 by Amy Butler | Joint Base McGuire-Dix-Lakehurst, N.J.

http://www.navair.navy.mil/lakehurst/nl ... esting.pdf (125Kb)

"...Lockheed Martin test pilot Dan Canin commanded different levels of engine power for various intervals. One example of a cycle is 10 sec. of standard military power, 30 sec. of limited afterburner and another 60 sec. at idle...."
&
"...Even without the more extensive data provided by today’s sensor array, Super Hornet engineers gained valuable experience during JBD trials that led to a change in how the aircraft is launched. During testing, hot air was inadvertently recirculated into the air intake of the Super Hornet, prompting a “pop stall,” or hiccup in the airflow for the propulsion system. The result was a dangerous fireball coughing from the back of the Super Hornet, says Briggs.

The design fix was the creation of a limited afterburner setting for launch. Engineers crafted software such that the engine is at 122% of military power when a pilot sets it to afterburner. By the time the jet reaches the edge of the deck, the system automatically opens the throttle to full afterburner at 150% of power without intervention by the pilot, says Briggs.

Having completed the first phase of JBD trials with a single F-35C, engineers are eager to test a more realistic scenario with one aircraft in front of the deflector and one behind.

Because of this lesson, the limited afterburner setting was designed into the F-35 in its infancy...."
RAN FAA A4G Skyhawk 1970s: https://www.faaaa.asn.au/spazsinbad-a4g/ AND https://www.youtube.com/channel/UCwqC_s6gcCVvG7NOge3qfAQ/
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southernphantom

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Unread post18 Aug 2011, 19:40

First impression: awesome stealthy SHornet. Second impression: expensive, semi-LO F-16 with tiny payload before becoming a more-expensive Desert Falcon.
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wrightwing

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Unread post18 Aug 2011, 20:09

southernphantom wrote:First impression: awesome stealthy SHornet. Second impression: expensive, semi-LO F-16 with tiny payload before becoming a more-expensive Desert Falcon.


Care to elaborate?
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shep1978

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Unread post18 Aug 2011, 20:16

southernphantom wrote: Second impression: expensive, semi-LO F-16 with tiny payload before becoming a more-expensive Desert Falcon.


Semi-LO? How can you claim that when it has 'all aspect' stealth?

Tiny payload? I take it you're not aware that it can carry munitions externally as well as internally and is actually capable of carrying more than an F-16 of any block.
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Unread post19 Aug 2011, 17:59

If an F-16 wants to carry the same payload as an F-35 does internally and keep the same fuel fraction it has to carry two 370 gal bags on the wings and a jammer on centerline increasing its drag by over 50%.

in short, the F-16 will have less range, less SA, and a worse RCS on the way in. And assuming it punches tanks after dropping bombs it still has to drag that ECM pod everywhere which still increases drag by 10%

Then of course paperwork must be done on why the pilot felt the need to ditch a few thousand dollars of equipment.
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