APG-81 power output wattage

Cockpit, radar, helmet-mounted display, and other avionics
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stealthflanker

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Unread post20 Apr 2019, 06:09

SpudmanWP wrote:One thing you are missing is the Heat-Exchanger capacity for fuel-based coolers, yet another unknown component in the equation.


You have suggestion for the possible value ? Or should i add 2.3 KW to include the air cooling ?


Eloise wrote:Cooling of Mig-35 is only 1/2 of F-18E?


Well that's what suggested. Early Zhuk AE program faced difficulties as the MiG appears to not able to provide required cooling, with the plan to accommodate 1100 modules.

If you browse brochures about Zhuk-AE you may notice that there were 2 versions. One being 5 KW version while the others 3 KW. as far as i remember. The 3 KW version appears to be the one flying in the MiG-35 demonstrator. The Russians were generous enough to provide their module PAE (25%) All that needed is then to guesstimate the operating frequency and duty cycle. Thus where i arrived at 7.9 KW/sqm value for MiG-35 radar cooling.

Regarding the ambient air. Do you have suggestion on possible value that can be added ? Like what i should add.
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eloise

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Unread post20 Apr 2019, 15:38

stealthflanker wrote:
Well that's what suggested. Early Zhuk AE program faced difficulties as the MiG appears to not able to provide required cooling, with the plan to accommodate 1100 modules.

If you browse brochures about Zhuk-AE you may notice that there were 2 versions. One being 5 KW version while the others 3 KW. as far as i remember. The 3 KW version appears to be the one flying in the MiG-35 demonstrator. The Russians were generous enough to provide their module PAE (25%) All that needed is then to guesstimate the operating frequency and duty cycle. Thus where i arrived at 7.9 KW/sqm value for MiG-35 radar cooling.

Regarding the ambient air. Do you have suggestion on possible value that can be added ? Like what i should add.

9F9CD13B-2297-416E-BCFA-A30FE6C67FBE.png

According to your source, the cooling capacity is 2kW/m3, is this 2kW per 1 cubic meter of ambient air passing through the heat exchanger per second? If that is the case, i think the engine fan heat exchanger can provide better cooling capacity.
Secondly, this value is based on sea level, is sea level ambient air better at provide cooling or is high altitude air better at provide cooling? Sea level is very dense, so that could transfer heat quicker, on the otherhand, high altitude is extremely cold, which seem that is beneficial for cooling?
Finally, does it make sense to use the full fuel load as coolant ? If that how they do it then does that means fighter radar will get weaker the further they fly?
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eloise

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Unread post20 Apr 2019, 15:55

stealthflanker wrote:Rather crude estimate could be made.

Based on approach here :
viewtopic.php?f=38&t=53650

Unfortunately there are no information released on cooling type or cooling capacity of the F-35, the one that can be spared for radar. One assumption that could be made is to use fuel as the heatsink. Thus roughly more fuel means more cooling capacity.
As posted in the thread that one example is AN/APG-79 which requires about 15 KW of cooling. Another one i tried to estimate is the improved MiG-35 which have about half (7.9 KW) of cooling capacity. Putting them on linear with their respective fuel capacity allows regression equation to be developed. The equation is kinda crude Y=0.003X-6.7112 Where X is the amount of fuel and Y is the cooling capacity. Only based on 2 data unfortunately. At the moment i'm still searching for more possible data or maybe a method for better accuracy.

The aircraft chosen would be F-35B as it appears to carry the least amount of fuel (5996 Kg) compared to other F-35 variants (about 8000 Kg). Thus the cooling requirement for the APG-81 would likely be based on it. Plugging the variable in one may got around 11.3 KW of cooling capacity.

Then one could attempt to estimate the limit of average power output of the radar. We may assume similar parameters in the thread that i posted. Except that the number of modules is 1676 for APG-81 and cooling capacity is 11.3 KW.

The calculation yield the limit of average power output of 4416 Watt. If we assume, 25% duty cycle of pulse doppler operation. The peak power of the radar is about 17.6 KW.

I want to point out something:
F-35B internal fuel capacity is 6125 kg
9C8188EA-1684-4605-874C-F9DEFDB38276.jpeg

F-18E internal fuel capacity is 6780 kg, F-18F internal fuel capacity is 6354 kg, both use APG-79
9D54E449-A087-4D98-9C9A-13997B50D1D0.jpeg


APG-79 cooling requirement as you mentioned is 15.6 kW
2062E00C-E117-4FC2-8F24-5776AE0468D9.png


F-35B only carry 3.7% less fuel than F-18F but F-35 has both ambient air heat exchanger and engine fan heat exchanger, i don't think it is reasonable to assume F-35 cooling capacity is worse than F-18 super hornet by over 38%
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stealthflanker

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Unread post20 Apr 2019, 17:23

eloise wrote:According to your source, the cooling capacity is 2kW/m3, is this 2kW per 1 cubic meter of ambient air passing through the heat exchanger per second? If that is the case, i think the engine fan heat exchanger can provide better cooling capacity.
Secondly, this value is based on sea level, is sea level ambient air better at provide cooling or is high altitude air better at provide cooling? Sea level is very dense, so that could transfer heat quicker, on the otherhand, high altitude is extremely cold, which seem that is beneficial for cooling?
Finally, does it make sense to use the full fuel load as coolant ? If that how they do it then does that means fighter radar will get weaker the further they fly?


F-35B only carry 3.7% less fuel than F-18F but F-35 has both ambient air heat exchanger and engine fan heat exchanger, i don't think it is reasonable to assume F-35 cooling capacity is worse than F-18 super hornet by over 38%


Um it's square meters of antenna aperture not cubic meter of volume.

The thing is regarding high altitude however is that there is less air, and as the airplane move there would be increase in the cooling air intake temperature. Plus i assume full fuel load because there are no real "percentage" on how much is actually need to "buffer" the heat load without "boiling the fuel". I would say tho it might be like 10-20% but i have no real means to prove that number.

In my view it will constrain the range by some amount. So the flight plan will include "allowance margin" for cooling.

and thanks for the data. I might have used the old version of the chart.

The serious problem in my regression equation is obvious. That there are only 2 sets of datapoints. As you see it's basically extrapolating, which may result in error underestimates or greatly overestimates. This is the point where i would like to have additional data set or maybe some good insight on how to actually predict the cooling capacity.
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eloise

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Unread post21 Apr 2019, 17:22

stealthflanker wrote:Um it's square meters of antenna aperture not cubic meter of volume

In short, what their value only represent bigger antenna can cool down easier? Without taking into account the speed involved? Logically shouldn't moving faster (but not too fast) help dissipate the heat better? Same effect as having a cooling fan.
I think the estimate missing too many variable to be accurate
Isn't air cooling adequate for ALQ-99 and NGJ ?

stealthflanker wrote:The thing is regarding high altitude however is that there is less air, and as the airplane move there would be increase in the cooling air intake temperature.

The air temperature will increase as plane go faster, but since ambient air is extremely cool at high altitude, they will be cooler than sea level ambient air still
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marauder2048

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Unread post21 Apr 2019, 23:22

eloise wrote:Isn't air cooling adequate for ALQ-99 and NGJ ?


No. NGJ is ethylene-glycol and water.
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SpudmanWP

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Unread post21 Apr 2019, 23:58

NGJ is liquid cooled, but everything is contained in the pod.
"The early bird gets the worm but the second mouse gets the cheese."
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eloise

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Unread post22 Apr 2019, 10:12

After some digging around:
This may interest you @stealthflanker
engine heat exchsnger.PNG


In ground mantainance operation test, a modified F119 engine with fan duct heat exchanger system can provide 27 kW of electrical power and 12 kW cooling. That is from the fan duct exchanger alone as it seem. F-135 with higher bypass ratio should be better i think
2.PNG

cooling.PNG
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