From the Dod Buzz version of the story provocatively titled "The F-35’s legs might not be long enough", and based upon the deceptively-named Federation of American Scientists' release of the DoD's F-35 Selected Acquisition Report for 2010, we discover that the F-35A model is apparently estimated to be 6 nautical miles short of its Key Performance Parameter (KPP) Combat Radius (584 instead of the KPP's 590 nautical miles). In engaging the hand-wringers I purposefully did not make a point (but I dropped lots of hints and typed 'estimate' as often as I dared). The weekend has come and gone and no one I saw picked up on what the story was really about. Which is amusing, because the DoD Buzz story practically spelled it out at the end:
But programme officials are also debating whether to change how the range of the F-35A is calculated, the source said. The equation does not include a buffer margin of 5% of fuel capacity, which is intended to be preserved through the end of the flight test period in 2016. Eliminating the buffer margin adds another 72.4km to the aircraft’s combat radius, the source said.This paragraph could have been written in Linear A as far as the Anti-JSF crowd was concerned. Let's take a moment to decompose what the paragraph actually says and implies.
RE: But programme officials are also debating whether to change how the range of the F-35A is calculated, the source said. The equation does not include a buffer margin of 5% of fuel capacity, which is intended to be preserved through the end of the flight test period in 2016So evidently:
1) The program had a conservative methodology in place to help ensure the KPP was achieved.
2) Part of that methodology was installing a 5% margin above and beyond that needed to achieve the KPP.
3) The Program planned to use the buffer until 2016.
4) It seems that the purpose of the extra 5% margin was established by the program to act as a tripwire for taking action.
RE: Eliminating the buffer margin adds another 72.4km to the aircraft’s combat radius, the source said.Now we see:
1) In reality, even the 'estimated' combat radius really doesn't break the KPP metric based upon expected aircraft performance, but only breaks a program-instituted fudge factor.
2) This fudge factor when added to the KPP threshold means the REAL number 'not being met' via actual performance-based factors in the estimate is ~629nm and not the 590nm KPP.
So the JSF-Haters spent an ENTIRE weekend venting over a 'scare piece' claiming a KPP wasn't being met as it is currently measured, when in reality a fudge-factor based tripwire instituted by the program was barely breached and is still well above the KPP. Instead of observing and noting the wisdom of the program's approach, the Anti-JSF crowd beats them up over a faux "issue" (vs. a risk being managed). I'll be interested in knowing what the program comes up with as a solution. I would think the fuel-level sensor adjustement (software or hardware) will be the most attractive. I can't help but think a realtively easy answer could be found in tweaking the FADEC at the margins, but the division of labor between Airframe and Powerplant contractors could make it impractical. In any case the program should continue to work to the current methodology and use the tripwire for the original purpose: as a reason to take action as the prudent thing to do.
Be sure and visit the threads, they're a riot -- Including one little (OK, a 'complete') troll I 'Pwned' and his associated meltdown. He was last seen begging for my attention and futilely downrating my comments. If you run into him, and just can't or don't want to ignore him, call him 'Sweetheart'. He likes that.
What do you think about the conjectural prospect of the Air Force adapting a land version of the C, instead of the A? It trades thrust-to-weight and an internal gun for range and lower wing loading, but I think it may be a worthwhile choice, given the fewer airbases that the USAF has to work with.
ReplyDeleteI think the AF would be crazy to adopt the C, as the Navy essentially had to have the C configuration not for range, but for the low-slow controllability on carrier approaches/landings. The thrust edge is more useful on a clean outer mold line (OML) found on LO aircraft than the edge you get with slightly lower wing-loading . We don’t even really know the wing loading of the F-35 (any model) because modern fighters also generate lift via their fuselage. Recall that the Israelis discovered the f-15 would fly without most of one wing. Even the early F-15s and F-16s were close in dogfighting capability, I worked with and for Test Pilots and TPS grad flight engineers who regaled me with tales of the light F-16s being able to out turn an F-15 below 20K ft but the situation was reversed once you went higher, so there’s no one best configuration. I saw a question over at SNAFU (a couple of weeks ago?) where Solomon had a post quoting the usual suspects claiming the F-35 would turn like a F-105. Hilarious. The problem with the C model IMHO is that it is the most off-optimal model. The STOVL concept was conceived first, and the A and B were optimized in the original design to have very similar weight and CGs at their mid-mission weights. The B model’s fan installation weighs just about exactly the same as a half-full fuel tank it replaces. As the C has the same engine as the A but weighs more, it probably has a slightly higher instantaneous turn rate-- but not by much (not as important in the era of low observable aircraft ability to get first ‘eyes-on’ any less-stealthy hostiles, and availability of off-boresight targeting) due to the slightly lower wing loading, but gives up thrust-weight ratio an gains drag during maneuvering (wing size penalty) which hurts in the sustained turn. The A (and B) should be able to power around a turn faster than the C. Add to that the A’s will be much cheaper, we can buy more – and all is right in the world
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