Weapons bay thermal environment

[steve2267]

Have specifications and/or requirements for the thermal environment of the F-35 weapons bays been published? If so, can someone point me to them, or answer the following questions? (My search on +thermal +environment came up short.)

What is the max-sustained temperature of the weapons bays? One part of my brain is whispering that I read once-upon-a-time and somewhere that it is on the order of 4-500°F. Another part of my brain says that's way too high -- that's as hot as a kitchen stove cranked to max!

F-35 Bay Presents Challenges To Weapons
Amy Butler | Aviation Week & Space Technology | Sep 17, 2012

... the F-35 is expected to carry far more weapon types in its bay, which has a challenging thermal and acoustic environment. Although the bay has not presented developers with conditions beyond the specifications of weapons slated for use in the F-35, engineers acknowledge there is little margin. “We are within about 10-12 degrees in most cases. But it is close” to the design specifications of some weapons, says Charlie Wagner, weapons integrated project team lead for the F-35 Joint Program Office. “It is not that simple, though. Maybe I can get [a weapon] that hot. But can I get it hot for an hour? Or can it be that hot for two days?”

Source: F-35 Bay Presents Challenges To Weapons

aviationweek.com-F-35 Bay Presents Challenges To Weapons.pdf

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The following details discoveries in F-35A flight sciences testing:
- Testing to characterize the thermal environment of the weapons bays demonstrated that temperatures become excessive during ground operations in high ambient temperature conditions and in-flight under conditions of high speed and at altitudes below 25,000 feet. As a result, during ground operations, fleet pilots are restricted from keeping the weapons bay doors closed for more than 10 cumulative minutes prior to take-off when internal stores are loaded and the outside air temperature is above 90 degrees Fahrenheit. In flight, the 10-minute restriction also applies when flying at airspeeds equal to or greater than 500 knots at altitudes below 5,000 feet; 550 knots at altitudes between 5,000 and 15,000 feet; and 600 knots at altitudes between 15,000 and 25,000 feet. Above 25,000 feet, there are no restrictions associated with the weapons bay doors being closed, regardless of temperature. The time limits can be reset by flying 10 minutes outside of the restricted conditions (i.e., slower or at higher altitudes). This will require pilots to develop tactics to work around the restricted envelope; however, threat and/or weather conditions may make completing the mission difficult or impossible using the work around.

Source: DOT&E 2015 F-35 Annual Report.pdf (page 10)

DOT&E 2015 F-35 Annual Report.pdf

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I haven't heard Gilmore jumping up and down over these temperatures... so I am guessing they are a non-issue?

Also, if a weapons bay can become too hot sitting on the deck in the hot sun (e.g. > 90°F), I am guessing the temp limit is somewhere between 1-200°F?

Regarding the temperature limits exceeded during flight at less than 25,000ft altitude, I was reminded of a passage from Ben Rich's book, Skunk Works:

Kelly thought it was more practical to try special iron ferrite paints that would absorb a radar ping rather than bounce it back to the sender: The paints were moderately effective but inhibited heat dissipation through the airframe's outer skin and we experienced overheating engine problems. But the paint lowered the radar cross section by one order of magnitude, so we decided to give it a try. ...

Source: Rich, Ben R., Janos, Leo, Skunk Works, Little, Brown and Company, New York, 1994, p. 153

So I wonder if the higher bay temps are due to a combination of being insulated on one side by RAM coating and proximity to a large heast source -- the F135 -- on the other?

[optimist]

AFAIK the f-35 gets rid of heat by sinking it into the fuel, and in the bypass air

[blindpilot]

Other operational aircraft have done well with similar types of limits.

First warning: this is an old man`s 45 years ago memory - third hand (my roommate`s dad`s) report.
That noted-supposedly according to those reports, the B-58 was technically capable of speeds never explored, because it was expected to melt and fall apart somewhere in the Mach 3 plus area, or be ready to fall apart for the next flight if flown very long at speeds in the Mach 2+ ranges. As such, I was told it had a temp timer to avoid bad things resulting from pressing beyond the design limits.

We had limits we worked around with water injection, and some planes have had artificial "stick shaker" systems installed to keep away from sneaky quiet departure limits. etc. etc.

My question is that with FBW, flight control laws et al, I wonder if these types of issues are not just controlled by the flight computers these days. For example full throttle could still autopull you back to 500-550-600 knots if the deviation times out. With today`s smart planes, pilots shouldn`t have to worry/think about it other than being aware of the constraints imposed by the control laws. I would think this is no more than limits like an AoA limit. You might not be able to chase a Thud on the deck, but why would you need to? - and how much longer than ten minutes would a mission be capable of anyway? Ten minutes dumps a lot AB fuel in the tail pipe? Somewhere around the 11th minute you might run out of fuel. Go vertical instead, lose airspeed quickly or punch all the way up to 25,000 ft.

Even if this persists as a limit, I don`t see much problem. Most low level work is usually in the 450-500 knot range and 10 minutes seems like a long time for a specific mission manuever...
but the fighter guys would need to weigh in.

BP

[Dragon029]

Just a reminder, but the thermal limits pertaining to >X knots at <Y altitude for 10 minutes were caused by a piece of electronics being moved to the weapon bays for ease of maintenance. Those electronics weren't initially rated for the temperatures experienced in the bays, so they had to impose the restriction while those electronics were tested / requalified to the necessary thermal standard. There's been no word on whether those tests have taken place yet and if so, whether they were successful or not, but throwing some extra thermal insulation over a box or swapping some standard sub-system electronic components for higher temperature versions shouldn't be an issue.

[steve2267]

Just a reminder, but the thermal limits pertaining to >X knots at <Y altitude for 10 minutes were caused by a piece of electronics being moved to the weapon bays for ease of maintenance. Those electronics weren't initially rated for the temperatures experienced in the bays, so they had to impose the restriction while those electronics were tested / requalified to the necessary thermal standard.

Thanks for posting; I was not aware of that. Makes sense.

... but throwing some extra thermal insulation over a box or swapping some standard sub-system electronic components for higher temperature versions shouldn't be an issue.

My response is: it depends. Without numbers, requirements, and other context, we are just blowing smoke. Unless the insulation is perfect, and without active cooling, any item you wrap in some thermal insulation will eventually reach the same temperature as the surrounding environment.

Chicken Little Gilmore is still not screaming about the thermal sky falling, so I take that to mean it is a non-issue, or it has been resolved.

My original reason for starting this thread was motivated by engineering curiousity: just how hot does it get in the bays? Are those temps primarily caused by air flow (skin friction heating), the F135 (thermal conduction through internal structure), or both? Also, has the F-35 program revisited a thermal issue Kelly & Co discovered 50+ years ago with the first ferrite paints applied to the U-2?

[Dragon029]

I saw an interview with a guy who was an ex-A-10 pilot who had also been involved with the F-35 program in some secondary or tertiary manner; he had said the weapon bays get above 80C (176F), IIRC at cruise airspeeds & altitudes. Down low and at higher speeds like in the DOT&E report, they'd naturally get hotter still. 85C / 185F is one industrial thermal standard; I'd bet that's what they were rated for.

[popcorn]

Related to the topic but no specifics.

http://aviationweek.com/awin/f-35-weapo ... cus-amraam

Although with the bay doors open, the F-35 generates vibration levels exceeding those against which the Amraam has been tested, “right now I don't have any indication this is an aircraft problem,” Wagner says. “Because the bay doors are open for only a very limited amount of time, . . . that short duration is significant, but it is not something that I am too worried about.”...

Engineers are continuing to refine data on potential thermal issues in the weapons bay as it carries ordnance. “The data that we have gotten so far show that the bay can get hot,” Wagner says. “But it is only in specific parts of the flight envelope on specific days where we start seeing temperatures that are high enough that cause us concern.”

He notes that the team is incorporating lessons from the storage and use of weapons in Afghanistan and Iraq, both extreme heat environments, including how hot weapons can be before they are loaded into the bay and called upon during a mission.

Wagner says the thermal and vibration analysis should be complete in the next 15 months, in line with wrapping up testing for the USMC IOC.

[count_to_10]

Given the proximity of the weapon bays to the engine, I'm left wondering a) why they didn't foresee this problem, and b) why they didn't have some vent coming off the intakes or the fan duct to circulate cool air into the weapon bays.

[eloise]

Given the proximity of the weapon bays to the engine, I'm left wondering a) why they didn't foresee this problem, and b) why they didn't have some vent coming off the intakes or the fan duct to circulate cool air into the weapon bays.

They did have some kind of cooling as far as i can remember

[juretrn]

Has there been any development regarding this issue?
Has it just been quietly resolved? Or is the soultion still ongoing?

[neptune]

[quote="eloise"]....They did have some kind of cooling as far as i can remember...[quote]

...I've noticed the cooling scoops before but have yet to discern the vents/ exits for the air flow. Any ideas???

"There, near the engine nozzle"

Thks,

[eloise]

...I've noticed the cooling scoops before but have yet to discern the vents/ exits for the air flow. Any ideas???

There, near the engine nozzle

[steve2267]

If they are venting the cooling air in a concentric [i]sheath[i] around the engine nozzle, that could potentially greatly reduce the IR signature of the engine.

[Dragon029]

Has there been any development regarding this issue?
Has it just been quietly resolved? Or is the soultion still ongoing?

Considering that the 2016 DOT&E report doesn't mention those speed / altitude limits, I think it's safe to assume that additional testing showed the electronics (that weren't officially rated for the bay's temperatures) are safe to operate in those conditions. Or perhaps it was actually deficient and they made the necessary modifications. The electronics had been moved to the bays for greater accessibility, so it wouldn't have taken long to retrofit any deficient components.

[juretrn]

Considering that the 2016 DOT&E report doesn't mention those speed / altitude limits, I think it's safe to assume that additional testing showed the electronics (that weren't officially rated for the bay's temperatures) are safe to operate in those conditions. Or perhaps it was actually deficient and they made the necessary modifications. The electronics had been moved to the bays for greater accessibility, so it wouldn't have taken long to retrofit any deficient components.

Then the whole issue must have fizzled out, doesn't stop desperate trolls from trying to claim F-35 "can't fly with bomb bay closed"