The F-35 Heat Sink "TMS" probably can't work. If so, then why do they lie about how it works?

Over the last two decades, research in (hybrid-)electric powertrains as an alternative to gas turbines has significantly increased. One of the key challenges for both realizing a theoretical benefit on aircraft level and successfully implementing first demonstrations is the thermal management of up to multi-megawatt electric powertrains [6,7]. Besides the high efficiency of electric components compared to gas turbines, they have no natural large heat rejection system such as the engine exhaust, so only small amounts of heat can be dissipated naturally via conduction through the structure. Therefore, the TMS has to manage their entire heat load. Additionally, electric components typically have low operating temperatures compared to combustion engines, which result in only small available temperature differences to ambient conditions for the TMS.

www.mdpi.com...

The above excerpt is so important because the F-35 claims to use an air-duct heat exchanger but the bypass ratio of the turbo jet is far too small to allow this to be an effective method of cooling. Somehow all the heat energy in the compressor stage has to be removed or the airplane itself will overheat.

aircraft-database.com...

The F-14's engine for comparison had a bypass ratio of .86:1 and couldn't get an air duct heat exchanger to work. A civilian aircraft for fuel efficiency might have bypass ratios of 5:1 or 8:1, as an example.

The bypass ratio is how much fan-driven air goes around the outside of the jet engine to cool it and because it's accelerated by the fan (powered by the turbine) it also acts as additional thrust hence better fuel consumption.

Fighters want power so don't want much bypass at all.

Any ideas why the F-35 claims it can do something it probably can't?

a reply to: DarthTrader

I had come across this on reddit some time ago. The post talks about heat management on the F-35. Interesting read, using the fuel as a heat sink. Apparently while flying overheating is a non issue. But when fuel is low and they fly closer to the ground. Heat can become an issue.

np.reddit.com...

The F-35 uses fuel as a heatsink. The PTMS (power/thermal management system) system is fiendishly complicated and robust so I wish I had a block diagram to be a little more accurate here, but all you really need to know is this: under typical circumstances, avionics are cooled with PAO (polyalphaolefin, liquid coolant) which is pumped through heat exchangers to keep it below a certain temperature. The radar system and DAS especially are two of the hottest-running systems in the jet. Heat is dumped into fuel to keep the PAO cool. Fuel is subsequently cooled via the Fuel-Air Heat Exchanger (FAHX), which is fed with ram air. This heat exchanger and ram air duct are located above the right intake, opposite the gun.

The problem occurs when your fuel reaches thermal saturation. Now the F-35 carries tons of #ing fuel. The thing is a giant flying fuel tank. It carries more fuel than the F-22, and has half as many engines. It carries an F-15's plus an F-16's worth of internal fuel. If you asked me to describe the F-35 in one word it would be 'fuel'. So under typical flight (it's cold as # in the air and the fuel-air heat exchanger works great) this isn't an issue.

Where problems arise is when the aircraft land after flying a sortie. Because they land with very little fuel, it means that the fuel has diminished ability to sink the PTMS heat. Less fuel = heats up faster. Furthermore, without the ram air and with the substantially hotter atmospheric temperatures at ground-level, the FAHX has to rely on the substantially less powerful (compared to ram air) suction of the IPP (integrated power package, basically the beating heart of the aircraft) to move air across the FAHX. (Sidenote: remember that I said the PTMS system is fiendishly complicated? There's something like eight different modes of operation for the system all for various states of thermal saturation or failure. In the event that the IPP can't draw air across the FAHX, there's a little fan in there that will kick on in a last-ditch effort to help keep the fuel cool. It looks like a huge heavy-duty version of the heatsinks you put on a PC CPU). When the fuel gets too hot the jet enters Thermal Management mode and shuts down the hotter avionics to avoid damage. Like I said, not an issue when flying, but an issue when they're at 1,000 pounds of gas or less and landing after a day of flying. This means that the post-flight Vehicle Systems test (VSBIT) is going to fail, which is a code-3 grounding situation. But it's a recognized problem so not much thought is put into it.

The problems are exacerbated when you fuel them up from a fuel truck, however, and they already have fuel in them. Air Force fuel trucks are a dark evergreen color which absorbs a lot of thermal radiation from the sun, which makes the fuel in the truck heat up. You pump all this hot fuel into a jet that already has lots of hot fuel in it, and simply put, you aren't going to be able to run the jet until that fuel radiatively cools down enough to adequately sink avionics heat. Until that fuel is cool enough (remember that the larger the difference in temperatures, the faster and more efficiently heat can be swapped), you can't get the jet off the ground because it won't pass its preflight VSBIT.

This is where the story about the Air Force painting the fuel trucks white came from, in order to keep the fuel in the trucks as cool as possible to help the jet cool down and be able to quick-turn it if need be. It's a problem in-work, and it's simply because the F-35 avionics are so advanced and powerful that they generate a lot of heat. This actually is a rather interesting problem and I would therefore hazard a guess and say that if other fifth-gen-plus aircraft that are still in development (like the T-50) aren't having thermal management issues, it reflects that the avionics are substantially less powerful than what the F-35 has at its disposal.

Even the F-22 had thermal management issues and the military-hating media was all over that too, so this isn't a new problem. All that said, personally I haven't seen a single jet miss its second sortie of the day because it was still too hot from its first flight.

So basically we're dealing with Mechwarrior-style heat issues. Bottom line: thermodynamics ain't noone's fool.

a reply to: grey580

That's one reason that you see them taxi with the weapons bay doors open. They have problems with thermal management on the ground, and opening the doors helps to dissipate heat somewhat. Once they're in the air, where the temperature drops significantly, the heat exchanger starts to get more efficient, and the fuel cools quite a bit. When I used to help recover KC-135s, they'd land with ice still on the wings where the fuel was. The empty tanks didn't have any ice on them, but the ones that were full had a little rime ice on the skin. You can see it in some U-2 pictures of them landing as well. The F-35 probably won't see ice like that, because the fuel acting as a heat sink, but being that cold outside helps a lot.

a reply to: DarthTrader
Because they're not lying


Just because you haven't figured it out doesn't mean they havent

In 2014, BF-05 was hot soaked and solar load tested. The aircraft underwent a baseline engine run at 59 degrees. Then it was hot soaked to 103 degrees for 24 hours before a second engine run was performed. Finally it was pushed up to 120 degrees before an engine run was performed.

After the hot soaking was performed, a solar array was placed over the jet to simulate a full diurnal cycle, including heat reflected from the ramp around the aircraft. The test simulated a full day of the aircraft not flying, and sitting parked on the ramp. The test began with the aircraft hot soaked to 80 degrees, before being raised to 95, with the aircraft fluids playing catch up to the temperature. At each temperature, the aircraft performed a normal start up, VSBIT, simulated short take off, normal climb out with afterburner, simulated mission, and simulated vertical landing. At 120 degrees, the fuel was 25 degrees from its fuel-hot caution limit of 145 degrees, and the aircraft handled the temperatures.

Imagine if they added water injection.

originally posted by: ManBehindTheMask
a reply to: DarthTrader
Because they're not lying


Just because you haven't figured it out doesn't mean they havent

What a brilliant discussion. You should provide more like this. It's an amazing reply: 5 stars.

originally posted by: grey580
a reply to: DarthTrader

I had come across this on reddit some time ago. The post talks about heat management on the F-35. Interesting read, using the fuel as a heat sink. Apparently while flying overheating is a non issue. But when fuel is low and they fly closer to the ground. Heat can become an issue.

np.reddit.com...

The F-35 uses fuel as a heatsink. The PTMS (power/thermal management system) system is fiendishly complicated and robust so I wish I had a block diagram to be a little more accurate here, but all you really need to know is this: under typical circumstances, avionics are cooled with PAO (polyalphaolefin, liquid coolant) which is pumped through heat exchangers to keep it below a certain temperature. The radar system and DAS especially are two of the hottest-running systems in the jet. Heat is dumped into fuel to keep the PAO cool. Fuel is subsequently cooled via the Fuel-Air Heat Exchanger (FAHX), which is fed with ram air. This heat exchanger and ram air duct are located above the right intake, opposite the gun.

The problem occurs when your fuel reaches thermal saturation. Now the F-35 carries tons of #ing fuel. The thing is a giant flying fuel tank. It carries more fuel than the F-22, and has half as many engines. It carries an F-15's plus an F-16's worth of internal fuel. If you asked me to describe the F-35 in one word it would be 'fuel'. So under typical flight (it's cold as # in the air and the fuel-air heat exchanger works great) this isn't an issue.

Where problems arise is when the aircraft land after flying a sortie. Because they land with very little fuel, it means that the fuel has diminished ability to sink the PTMS heat. Less fuel = heats up faster. Furthermore, without the ram air and with the substantially hotter atmospheric temperatures at ground-level, the FAHX has to rely on the substantially less powerful (compared to ram air) suction of the IPP (integrated power package, basically the beating heart of the aircraft) to move air across the FAHX. (Sidenote: remember that I said the PTMS system is fiendishly complicated? There's something like eight different modes of operation for the system all for various states of thermal saturation or failure. In the event that the IPP can't draw air across the FAHX, there's a little fan in there that will kick on in a last-ditch effort to help keep the fuel cool. It looks like a huge heavy-duty version of the heatsinks you put on a PC CPU). When the fuel gets too hot the jet enters Thermal Management mode and shuts down the hotter avionics to avoid damage. Like I said, not an issue when flying, but an issue when they're at 1,000 pounds of gas or less and landing after a day of flying. This means that the post-flight Vehicle Systems test (VSBIT) is going to fail, which is a code-3 grounding situation. But it's a recognized problem so not much thought is put into it.

The problems are exacerbated when you fuel them up from a fuel truck, however, and they already have fuel in them. Air Force fuel trucks are a dark evergreen color which absorbs a lot of thermal radiation from the sun, which makes the fuel in the truck heat up. You pump all this hot fuel into a jet that already has lots of hot fuel in it, and simply put, you aren't going to be able to run the jet until that fuel radiatively cools down enough to adequately sink avionics heat. Until that fuel is cool enough (remember that the larger the difference in temperatures, the faster and more efficiently heat can be swapped), you can't get the jet off the ground because it won't pass its preflight VSBIT.

This is where the story about the Air Force painting the fuel trucks white came from, in order to keep the fuel in the trucks as cool as possible to help the jet cool down and be able to quick-turn it if need be. It's a problem in-work, and it's simply because the F-35 avionics are so advanced and powerful that they generate a lot of heat. This actually is a rather interesting problem and I would therefore hazard a guess and say that if other fifth-gen-plus aircraft that are still in development (like the T-50) aren't having thermal management issues, it reflects that the avionics are substantially less powerful than what the F-35 has at its disposal.

Even the F-22 had thermal management issues and the military-hating media was all over that too, so this isn't a new problem. All that said, personally I haven't seen a single jet miss its second sortie of the day because it was still too hot from its first flight.

So basically we're dealing with Mechwarrior-style heat issues. Bottom line: thermodynamics ain't noone's fool.

This mostly describes a heat storage system, but not the actual heat rejection system. That's more specifically what I'm calling bull***t.

Transferring heat into the fuel is a bit misleading, although the guy says they do - he doesn't point to the system that does. But knowing that the Avionics are liquid cooled, what most likely is happening is it's a heatpipe system that transfers heat to a storage reservoir for dumping.

Normally the heat rejector is a RAM Thermal Management System. The F-35 specifically claims to not use this, but to instead use a heat exchanger in the duct. This means they would be dumping heat through the bypass air going around the jet engine.

Remember the jet engine needs to dump heat as well. For instance let's say the combustion chamber reaches 3500-F. That will melt the engine. The engine may run fuel rich and the excess fuel becomes a heat sink of its own and is rejected out the exhaust.

This is crucial for a turbojet with low bypass ratios such as the F-35 which has half the bypass ratio of the F-14s engine (see citations).

That brings the engine temp down to say ~2200-F which can protect the engine components. But that heat doesn't 100% go out the exhaust. It also is moved into the RAM Thermal Management System, where the avionics also dumps into, and that is distributed across the airframe where turbines force air cooling inside the jet to reject heat.

Understanding the process in general now?

I challenge that the F-35 cannot do this with a heat exchanger in the bypass airflow, because there's not enough bypass air to reject enough heat from a ~2000 degree engine amidship.

All that heat is being dumped into the airplane.

Now there's some claim to some thermal management by the VTOL system but that's not on every aircraft so I don't buy that either.

originally posted by: ManBehindTheMask
a reply to: DarthTrader
Because they're not lying


Just because you haven't figured it out doesn't mean they havent

See how worthless your response is? Get out of here.

Oh - what I propose instead is that the F-35 skin is a conductor and that the F-35 dumps heat from the skin of the aircraft by electrical resistance in the conductive skin. This makes sense with the idea of stealth tech via radar conductivity. The F-35 is an antenna that can be electrically tuned to the attacker's frequency. Being an antenna (conductor) the skin can adjust its range to absorb whatever radar is hitting it.

This adds heat to the skin, but the heat rejection from the plane's skin far exceeds it, enough to allow an electrical heat exchanger to dump heat through the conductive surface from all the plane's systems.

It's possible the phenomenon of leaving weapons bay doors open and fuel temps being insufficient reservoirs comes from the fact that like any air cooled engine, without airflow there's not a lot of cooling. And the skin of the aircraft doesn't have a fan to force air flow over it. It has to happen by motion.

It was my understanding that the majority of the cooling issues with the F-35 come from the avionics, not the engine. Unless they are trying to do something about the aircraft's IR signature, engine heat shouldn't be much of an issue while in flight.

a reply to: DarthTrader

The F135 uses fan duct heat exchangers and bleed manifolds for engine heat management. As part of the J/IST program, they developed the Thermal/Energy Management Module, which was integrated with the engine. T/EMM was made more efficient and evolved into the PTMS system that is currently flying. As a result of J/IST, the PTMS developed an integrated compact heat exchanger that uses multiple cores in a single assembly, to make it more efficient. They also removed several portions of the original system. The PTMS dumps waste heat in flight through engine mounted heat exchangers in the engine fan air duct. The coolant system for the avionics runs lines through the fuel tanks, and dumps its heat into the fuel to assist with cooling.

a reply to: JIMC5499

The engine is used as a heat sink for some of the avionics cooling, in addition to the fuel being used as a heat sink.

If they "lie" about it then its probably to keep China and Russia in the dark about the actual technology behind it..

originally posted by: Zaphod58
a reply to: JIMC5499

The engine is used as a heat sink for some of the avionics cooling, in addition to the fuel being used as a heat sink.

Uh no, read and respond to my response about this false concept. First of all the engine cannot be used for cooling as the engine itself is +2,000 deg F.

Secondly, the fuel isn't cooling anything because of reasons I mentioned above as well. It's acting as a heat reservoir, which is different. There is a kind of "average temperature load" that can be used with Phase changing materials or other reservoirs but in this case the F-35 isn't using those as far as can be told, and I doubt it is sufficient anyway because it doesn't do anything about the excess engine heat.

originally posted by: Zaphod58
a reply to: DarthTrader

The F135 uses fan duct heat exchangers and bleed manifolds for engine heat management. As part of the J/IST program, they developed the Thermal/Energy Management Module, which was integrated with the engine. T/EMM was made more efficient and evolved into the PTMS system that is currently flying. As a result of J/IST, the PTMS developed an integrated compact heat exchanger that uses multiple cores in a single assembly, to make it more efficient. They also removed several portions of the original system. The PTMS dumps waste heat in flight through engine mounted heat exchangers in the engine fan air duct. The coolant system for the avionics runs lines through the fuel tanks, and dumps its heat into the fuel to assist with cooling.

I know it uses fan duct heat exchangers (the claim). What puzzles me is that you just take this at face value without even considering the design implications.

Did you just copy paste this from some conventional website or LMT or NOC's websites?

There isn't enough bypass air to do what they claim. It is a design limitation of aircraft with even greater bypass ratios. I've stated this several times. Think outside the box.

originally posted by: JIMC5499
It was my understanding that the majority of the cooling issues with the F-35 come from the avionics, not the engine. Unless they are trying to do something about the aircraft's IR signature, engine heat shouldn't be much of an issue while in flight.

Engine heat is always a huge issue for any jet engine that doesn't have a high enough bypass ratio. Otherwise you're just taking air into the engine, super heating it, then before it is ejected some of that heat transfers to the engine without ANY airflow to take the heat away.

a reply to: DarthTrader

I actually talk to people that work on the F-35, and have discussions about them, instead of deciding that everything published about them is a lie, and that I'm smarter than the people that work on them. Oh wait, that's just anecdotal, so it's irrelevant, as is everything published about them. So sorry to bother your "discussion" about them.

Many ways to cool down things in planes...Supermarine piped coolant through the wing skins and fuselage sides on its seaplane racers.
Higher you get the air temp drops so heating efficiency goes up.Its all a balancing act.