F-35 internal fuel, range

Discuss the F-35 Lightning II
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mixelflick

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Unread post12 Oct 2017, 16:02

As I've become more familiar with it, the F-35 is starting to look like a real winner. Beyond the stealth, sensor fusion and maneuverability though, I'm fascinated with the gas.

So I understand it's 18,000lbs, which is 11,000lbs more than an F-16 and still quite a bit more than the Eagle (13,850), and she has two engines. SU-35 has almost 21,000 but again 2 engines..

So my question is: How they heck did LM get 18,000lbs of gas into such a small airframe? Is every nook and crannie not taken up by sensors packed w/ fuel?
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playloud

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Unread post12 Oct 2017, 16:19

That's why the F-35 is a little pudgy.
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steve2267

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Unread post12 Oct 2017, 16:21

playloud wrote:That's why the F-35 is a little pudgy.


That and internal weapons bays for 2K bombs...
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steve2267

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Unread post12 Oct 2017, 16:23

mixelflick wrote:So my question is: How they heck did LM get 18,000lbs of gas into such a small airframe? Is every nook and crannie not taken up by sensors packed w/ fuel?


More or less...

Joint strike fighter fuel tank layout BAE.gif
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sprstdlyscottsmn

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Unread post12 Oct 2017, 17:13

mixelflick wrote:So I understand it's 18,000lbs, which is 11,000lbs more than an F-16 and still quite a bit more than the Eagle (13,850), and she has two engines. SU-35 has almost 21,000 but again 2 engines.


can we please drop the "one engine vs two" nonsense?

Fuel flow is TSFC multiplied by Thrust. When in a cruise condition thrust is equal to drag. The factors that will impact range the most are L/D in cruise, fuel fraction, and TSCF.

What makes the F-16 so much more fuel efficient (nm per lb of fuel) than the F-15 is not having one engine, it's a smaller and lower drag airframe. That smaller airframe is due to having one engine, as the smallest plane you could wrap around two of those engines would be an F-15.

Su-27 is a big plane so it has two big engines.

The F-22 is a big plane so it has two big engines.

The F-35 is sized such that two F414s would equal the thrust, but would change the outer mold to be wider at the tail and narrower (top to bottom). A single engine was required by the Marines IIRC (or was it the AF, I can't recall at the moment).

The F-35 will never have the nm/lb that the F-16 has because it is BIGGER! Even with a similar L/D it needs so much more L that it gets that much more D as well. That much more D means more fuel flow. Now, it has so much fuel that it can double the fuel flow of an F-16 and still fly 32% further, and a similar L/D means the F-16 is carrying next to nothing while the F-35 can be carrying full fuel and 5k of weapons without changing L/Dmax (really takes configuration changes to adjust that). In fact, an F-35A with full fuel and 5k internal weapons weighs in at roughly 52,500lb. An F-16 with full fuel and nothing else is roughly 27,000lb. Meaning if they have the SAME L/D (and TSFC) when clean the F-16 will have fuel flows 51% that of the F-35, but it only carries 38% of the fuel, so it only gets 74% of the flight time (range depends on speed at which the L/D being discussed is hit).

The F-35 may have some performance issues that are less than stellar when compared to clean aircraft, but Range isn't one of them.
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mixelflick

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Unread post12 Oct 2017, 17:32

OK well, I didn't know that. Thought 2 engines means it'd burn more fuel..

In any case, it really looks like the F-35 has some great legs. You always hear about how much gas Flankers carry/great range but you never hear about how much drag there is from lugging a meaningful weapons load around. Their AAM's look to be real draggy, hung off of a draggy airframe.

The F-35 sure is impressive...
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zhangmdev

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Unread post12 Oct 2017, 17:48

I think that has something to do with F-35 uses fuel as hydraulic fluid and coolant for environment control. A first in fighter jets?
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steve2267

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Unread post12 Oct 2017, 17:52

zhangmdev wrote:I think that has something to do with F-35 uses fuel as hydraulic fluid and coolant for environment control. A first in fighter jets?


Not sure about hydraulic fluid. All the flight controls use self-contained electrohydraulic actuators -- that is, they are sealed, and have their own hydraulic fluid / own pump etc. It was considered a technical risk at the beginning of the program, but appears to have worked very well. I am unsure if there is even a central hydraulic system for anything, and if so, if it uses fuel for the working fluid. I kind of doubt it... if you burned ALL your fuel, you would no longer have hydraulics. But I may be wrong.

I do believe fuel is used for cooling, though.
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neptune

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Unread post12 Oct 2017, 18:17

steve2267 wrote:... I kind of doubt it... if you burned ALL your fuel, you would no longer have hydraulics. But I may be wrong.


....if you burn all the fuel, you don't need hydraulics.......OTOH you will arrive at the crash site, first!

....maybe you will alarm when the fuel quantity depletes to the minimum fueldraulic limit???
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vanshilar

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Unread post12 Oct 2017, 18:35

mixelflick wrote:OK well, I didn't know that. Thought 2 engines means it'd burn more fuel..


Well, it sort of does, but not directly because of the number of engines. Two engines tend to mean a bigger aircraft, which means more weight to move around and more drag, thus more fuel consumed. Or more accurately, a bigger aircraft tends to mean two engines, and a bigger aircraft tends to men more fuel consumed.

The SR-71 also used its fuel as a heat sink, which helped keep the skin relatively cool at Mach 3. "Relatively cool" meaning relative to the amount of heat at Mach 3 speeds.
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sprstdlyscottsmn

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Unread post12 Oct 2017, 18:39

mixelflick wrote:OK well, I didn't know that. Thought 2 engines means it'd burn more fuel..


I apologize if the post sounded angry. I hate the loss of nonverbal communication in text. I only wanted to pass along the knowledge.
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zhangmdev

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Unread post12 Oct 2017, 18:40

I searched fueldraulics. Seems it is used only on STOVL version, swiveling the three-bearing nozzle. So it has nothing to do the internal fuel/range. EHA does save a lot of piping.
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steve2267

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Unread post12 Oct 2017, 18:47

zhangmdev wrote:I searched fueldraulics. Seems it is used only on STOVL version, swiveling the three-bearing nozzle.

Makes sense to use fuel as the working fluid for the swiveling nozzle.

zhangmdev wrote:So it has nothing to do the internal fuel/range. EHA does save a lot of piping.


In addition to saving weight (e.g. the piping you note), it should increase damage tolerance -- if a hit takes out an EHA survo, it is not going to rupture a hydraulic line resulting in loss of an entire hydraulic system. Also, less risk of fire as there is not an entire hydraulic system at risk of burning.
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Unread post12 Oct 2017, 21:11

Thanks 'zhangmdev' and also the fuel storage diagram is no longer correct; 'quicksilver' says fuel is not stored in the tails.
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F-35amendedFuelTankLayoutBAE.jpg
RAN FAA A4G Skyhawk 1970s: https://www.faaaa.asn.au/spazsinbad-a4g/ AND https://www.youtube.com/channel/UCwqC_s6gcCVvG7NOge3qfAQ/
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Unread post12 Oct 2017, 23:35

Salute!

Not so sure about your "analysis", Spurts.

1) I cannot find a F-16 GTOW you mention that only has internal fuel.

2) The two engine discussion is familiar to this old fart. It turns out that you can only pull the throttles back so far and you still have "x" pounds per hour, total. So in our over-powered A-37 we would have about 1500 - 1600 PPH total to maintain a good loiter AoA/speed. If we shut one down and pushed up the single engine we saw about 1300 PPH or so. The motor was running at a better point on the "curve" at a higher rpm.

The Hornet guys I first met said their fuel flow was about like an Eagle!! I discount the higher drag from more skin friction, but you could convince me.

3) I have a problem with asserting F-35 weight requiring lots more lift and associated induced drag. I'll bet that the sucker will come close to the 7 pounds per mile I saw in the Sluf and Viper, and that was total gas from climb to cruise and descent at home. e.g. RTB from 300 miles away using 2100 pounds plus reserve once there. worked every time.

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