Wednesday, March 25, 2009

This JSF Post Is Hot

This JSF Post Is Hot: "

This item caught my eye from Dave Fulghum's post, referring to a certain'all-singing, all-dancing omnipotent fighter aircraft:

‘Thermal problems were first identified in hot-weather ground operations,’ he says. ‘The problem was that the fuel-air heat exchanger was not providing enough cooling. In the short-term, at least, they’ll just limit hot weather activity.’

It's just as well that US forces won't have to operate in hot climates any time soon. According to highly placed sources, the Obama administration expects that the Schleswig-Holstein question, and the assurance of a Protestant successor to the Elector of Hanover, will be the big conflict flashpoints of the next two decades.

Speaking of hotspots, take a look at these details of'F-35 AA-1 on its ferry flight to Edwards last fall:

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JSF program office pic - full image here.

That is the'exhaust from the JSF's integrated auxiliary and emergency power and thermal management unit, located next to the left vertical tail to conceal its radar and infra-red signature - but apparently hot enough to discolor paint.

On BF-1, the first production-type JSF, the exhaust has been relocated to the underside of the aircraft.

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JSF program office pic - full image here.

Time for the JSF enthusiasts hereabouts to show us an example of a similar, apparently round-edged and hot nozzle on the visible lower surface of any stealth aircraft.

In any event, the JSF has a thermal management problem, and one design change - moving the exhaust - hasn't fixed it, since for the time being it appears that hot-weather operations are being restricted.

The issue is a basic one for stealth aircraft:' lots of vents, grills and inlets can't be tolerated because they create radar and infra-red hotspots, so instead the heat generated by electronics and other systems (such as the F-35's electrically powered actuators) is dumped into the fuel by heat exchangers and lost from the aircraft as the fuel is burned.

The toughest conditions for this process are ground idle (where the fuel is not being used fast) and end-of-mission, where there is not much fuel to absorb the heat and the aircraft is at low altitudes where temperatures are higher.

Most problems of this kind can be fixed, perhaps with larger and more efficient heat exchangers and other changes - a new fuel pump is in the works for JSF.

The question is how much it will cost (in terms of money and weight), how long it will take - that is, which production lot will be the first to get the modifications - and how easy or otherwise it is to retrofit. The fuel pump itself will be available on LRIP 3, according to the Government Accountability Office, which says that 'Thermal management challenges hamper the ability to conduct missions in hot and cold environments.' (Emphasis added.)

Show-stopper? Probably not. A delay factor? Quite possibly, and one to keep an eye on.

Updated:' Dave's comment below about the actuators is valid. Hydraulic actuators are inherently cooled by circulation of fluid, but the electro-hydrostatic actuators on the F-35 are not on a fluid circuit. Here's a flaperon actuator on the Moog booth at Farnborough 2008:

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AvWeek/Bill Sweetman

There's nothing in the picture to give scale, but think of a lawnmower engine. There are four of these actuators on JSF, plus many smaller actuators.'

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The primary actuators have'to be electro-hydraulic, not electric, because a pure electric actuator can seize. The actuator itself is in the middle, flanked by two electric pumps. The actuator has to be on two independent cooling loops - it's possible that the two black-capped connectors on the nearer end of each pump assembly are cooling lines. There is also a possibility that the unit would have to be heated in high-altitude cruising flight.


(Via Ares.)


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