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

Discuss the F-35 Lightning II
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andreas77

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Unread post14 Aug 2011, 18:14

battleshipagincourt wrote:...but having all the mission equipment within a clean airframe is better for performance and range than mounting it externally.


Well thats not really true either, an airframe with internal stores and sensors gets larger and adds extra weight (doors, actuators, ejectors etc.) and volume (which tends to be much bigger than the actual volume of the bombs/missiles that you put in there), there are no free lunches.

I mean, just look at the F-35, with the size constraints that was given there could not have been a surprise for anyone that the internal weapons bay and sensors would result in a chubby airframe and relatively small wings, thats not really what you want when you are designing a fighter.

And if internal weapons bay and sensors improves on range, why hasnt there been any non-stealth fighter built this way? The range of the F-35 has more to do with its fuel fraction than the fact that it has internal weapons bays. And if the F-35 is so aerodynamic, why is so heavily punished by external stores (8% extra range from 30% extra fuel)?
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SpudmanWP

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Unread post14 Aug 2011, 18:30

The small amount of extra range derived from external tanks has to do with the fact that those external tanks contribute a smaller percentage of the overall fuel carried (thanks to the F-35's MASSIVE internal fuel load) when compared to other 4th gen fighters.

There is very little information about the differences in internal vs external range when it comes to the F-35. Based on your 8% number, I can assume you are referring to the Norway briefing (673 internal vs 728 external). Assuming your numbers are accurate, the one thing that it does prove is that there is a TREMENDOUS drag & range penalty for carrying external loads.
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shingen

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Unread post14 Aug 2011, 19:52

I read somewhere that about half of the fuel in external tanks extends range and the other half pays for the drag of the tank and the weight of the tank and fuel. If a plane has a small fuel fraction ET's will extend the range more than if it has a large FF. Also think about what happens to a large FF aircraft if they don't drop the tank. They add a few % of fuel, then carry the weight and drag of the tank long after the fuel is gone.
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andreas77

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Unread post14 Aug 2011, 20:02

Yes, those are my sources. If they are correct or not, I dont know.

I do think that there is only so much an aircraft of a given size can carry. Since the F-35 has such a huge internal fuel capacity and internal sensors, quite a bit of that capacity is already consumed. The high wing loading also points in this direction.

Anyhow, I think it is possible to build someting like the F-35A with better performance, but the A seems to suffer from some design constraints given by the B-model.
Two GE414 and more freedom in the design (no lift-fan space, the freedom to chose canards etc.) would result in a much better aircraft IMO.
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SpudmanWP

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Unread post14 Aug 2011, 21:12

How would it have been "much better"?

It would have been more expensive (two engines as opposed to one) and wider. Having two engines also means having to feed them with a higher flow of air, which means larger tunnels and less internal fuel (ie lower range).

They dropped canards early for complexity & stealth reasons, not due to -B version.
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battleshipagincourt

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Unread post14 Aug 2011, 22:45

andreas77 wrote:Well thats not really true either, an airframe with internal stores and sensors gets larger and adds extra weight (doors, actuators, ejectors etc.) and volume (which tends to be much bigger than the actual volume of the bombs/missiles that you put in there), there are no free lunches.


You do have a point, although you're missing some critical details. While I agree that mounting everything internally adds weight and drag that you wouldn't otherwise have with external weapons and pods, you have to take into consideration what mission you're using the aircraft for.

The F-16 can theoretically carry a ten ton weapon load, but its small fuel fraction means having to load external tanks(reducing the number of hardpoints as well). Mounting weapon pods for precision-targeting adds significant drag compared to a more 'flush' design. The EOTS system of the F-35 does add more drag than designers originally expected, but certainly only a small fraction of what a weapons pod would cause. With a high fuel fraction the F-35 isn't as sleek, but that allows missions to be done without any external fuel.

Seriously compare this to a fully-loaded F-16 with three fuel tanks, two weapons pods, two 2,000 Ib JDAM's, and four AIM-120's. This would be about what you'd get from a clean F-35, and its performance doesn't diminish much unless you start mounting external stores. Unfortunately I'm unsure how well an F-35 does when you start doing this, as I've heard that doubling the weapon load and adding two external tanks only increases range by about 8%. I don't even think they can jettison the pylons after the weapons are spent, making this even less attractive.

andreas77 wrote:I mean, just look at the F-35, with the size constraints that was given there could not have been a surprise for anyone that the internal weapons bay and sensors would result in a chubby airframe and relatively small wings, thats not really what you want when you are designing a fighter.


Supposedly it can match an F-16's agility, but I still doubt it. Regardless, the aircraft is well designed for its mission. It's very hard to pick up on radar or IR, has very impressive sensors and weaponry, and carries all the mission systems internally. While it certainly doesn't have the fighter-like design of the F-16, it certainly has the stealth and strike capabilities all packed into a single airframe.

andreas77 wrote:And if internal weapons bay and sensors improves on range, why hasnt there been any non-stealth fighter built this way? The range of the F-35 has more to do with its fuel fraction than the fact that it has internal weapons bays. And if the F-35 is so aerodynamic, why is so heavily punished by external stores (8% extra range from 30% extra fuel)?


???

Don't external stores punish all fighters in this way? Add just 2 x 2,000 Ib JDAM's to the F-16 and see how well it does compared to a clean configuration. The F-35 flourishes most from its internal weaponry... those two AMRAAM's and JDAM's come with no drag penalty at all. It might have benefitted more to have semi-recessed weapons as well, but bombs and tanks hanging off wings are just terrible where drag is concerned.

At least with internal weapons, the added drag they incur are significantly less than external racks. And conformal fuel tanks are vastly better than external tanks because the penalty throughout the entire mission is minimal. The Eurofighter I believe has the ability to supercruise (mach 1.5) with a weapons load of four semi-recessed AAM's, but not with much else hanging off the wings.
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andreas77

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Unread post14 Aug 2011, 23:10

SpudmanWP wrote:How would it have been "much better"?

It would have been more expensive (two engines as opposed to one) and wider. Having two engines also means having to feed them with a higher flow of air, which means larger tunnels and less internal fuel (ie lower range).

They dropped canards early for complexity & stealth reasons, not due to -B version.


The 414 offers equal or more (EPE) thrust with lower weight. Regarding cost I am not so sure one is cheaper than two. What are the manufacturing costs of the two engines respectively? One F-135 costs more than one GE414, thats for sure. And the F-135 development program is now at $8.5 billion (according to flightglobal).

Its true that the fuselage would be wider, but it would also be lower. That could open for dedicated AA-stations under the engines with narrower bomb-bays on the sides compared with the current layout so that might even out?

How can canards add complexity to the design if it has nothing to do with the B-model, just asking?
Last edited by andreas77 on 14 Aug 2011, 23:11, edited 1 time in total.
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grinner68

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Unread post14 Aug 2011, 23:10

At some point the cost of the aircraft will exceed it's capabilities.
The F-35 may have hit that mile post before it's even entered service.
Last edited by grinner68 on 14 Aug 2011, 23:31, edited 1 time in total.
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andreas77

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Unread post14 Aug 2011, 23:19

battleshipagincourt wrote:...

andreas77 wrote:And if internal weapons bay and sensors improves on range, why hasnt there been any non-stealth fighter built this way? The range of the F-35 has more to do with its fuel fraction than the fact that it has internal weapons bays. And if the F-35 is so aerodynamic, why is so heavily punished by external stores (8% extra range from 30% extra fuel)?


???

Don't external stores punish all fighters in this way?

...


Absolutely not! Remember that 30% extra is quite little, the Gripen NG for example can take almost 100% extra fuel externally and most other 4th gen fighters flies with similar loads regulary (and they would not do that if they did not benefit from it).

Also remember that you gain more from the first extra fuel tank than from the last so if the gain is 8% with 30% extra fuel for the F-35, how much is the gain with 60% extra fuel (4 tanks i guess?)? Its not 16%, thats for sure!
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Unread post14 Aug 2011, 23:37

grinner68 wrote:At some point the cost of the aircraft will exceed it's capabilities.
The F-35 may have hit that mile post before it's even entered service.


At FRP the F-35A will cost about 15% more than current 4th gen US jets (F/A-18E). If you think that 6xF-15As can do more and survive longer than 7xF/A-18Es, then it is worth it.
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Unread post14 Aug 2011, 23:40

What's the NG's range clean vs combat loaded (with and without external tankage)?
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battleshipagincourt

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Unread post15 Aug 2011, 01:03

andreas77 wrote:Absolutely not! Remember that 30% extra is quite little, the Gripen NG for example can take almost 100% extra fuel externally and most other 4th gen fighters flies with similar loads regulary (and they would not do that if they did not benefit from it).


Having a high internal fuel fraction is a positive attribute for any fighter. The fact that the F-35 can only take 30% external fuel because it carries so much within the airframe itself. Definitely it incurs a fixed drag penalty (wider aircraft and thicker wings) but that penalty is vastly less than carrying external weapons/fuel.

Take the F-15E, which introduced the idea of conformal tanks... the tank's drag/fuel penalty was something like 1/10 compared to an external tank's 1/2. That turned out to be so successful that the F-16, F-18E, and even the Eurofighter are all either using CFT's or under consideration for them.

Ever since the F-15 Eagle, fighters were built under the concept that they maximize performance by only equipping them with ample fuel for 20 minutes of combat, favoring external tanks because they can be jettisoned. Experience however showed that tanks take up valuable pylons and incurred significant penalties from the added drag. So when the F-35 came into consideration, they decided that it made a lot more sense to just design it around its primary mission... which meant 2 JDAM's, 2 AIM-120's, internal sensors, and fuel for a 590-mile combat radius.



The F-22 and the F-35 benefit greatly by having internal weaponry, as opposed to carrying external stores. Compare a battle-ready F-35 to a similarly-equipped F-16 and ask which one will perform better. As stated before, external tanks constitute only about half their value in fuel, because they also add drag and weight.
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Unread post15 Aug 2011, 04:18

andreas77 wrote:The 414 offers equal or more (EPE) thrust with lower weight. Regarding cost I am not so sure one is cheaper than two.


I thought the f135 had a MUCH better thrust to weight ratio than two f414's. 3800lbs vs 4900lbs for very similar thrust produced.
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Unread post15 Aug 2011, 04:46

1. Ten years ago (under the assumption that the F-15 would soon be replaced by the F-22), I considered this program to be the next logical step after the Raptor, continuing the proven high/low concept of fighter procurement. I was also excited at the prospect of finally having the front-line STOVL fighter that all the cold-war era books said we should have.

2. Two years ago, I was angry that the F-22 was being canceled in favor of the F-35, which I considered to be a sad substitute (inferior and more vulnerable).

3. Recently, I have quit fretting over APA's panic mongering about the F-35's performance (though I'm still upset over the F-22). I'm no longer worried about F-35s being shot down in their droves by S-300/400s, PAK-FAs, or J-20s. What concerns me today are the costs and LM's continued failures to execute on time. I now fear that the numbers ordered will be cut far short of what was originally planned, leaving our own forces dependent on ageing legacy platforms and hurting our reputation with invested allies to whom we made promises regarding affordability.
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Unread post15 Aug 2011, 04:59

^^

My feelings exactly! Everything you've written is exactly how I felt about the Raptor being taken down by the technically inferior and more expensive Lightning II program. Now I don't doubt its capabilities, but fear it will be too expensive by the time it's ready. Quite upset they cancelled the Raptor over something unproven and as yet a money pit.
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