Chengdu next gen combat aircraft (?J-36) thread

BoraTas

BoraTas

Major
Registered Member
jvodan said:
As I understand the nose angle is one of the limiting factors on speed in that you dont want the shock wave crossing the skin of your nose cone,
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The geometry of the plane suggests a sub 2 mach airframe speed.
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The nose angle is only relevant for efficient cruise and even then there are exceptions to the rule. This aircraft below (F-104) was some of the most efficient aircraft in Mach 2.
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tankphobia

tankphobia

Senior Member
Registered Member
sunnymaxi said:
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With the factory yellow paint it really does look like a Dorito chip.
 
A

AndrewS

Brigadier
Registered Member
superdog said:
Still much easier than managing heat in space lol. It's when you go supercruise (especially > Ma 1.5) that heat dissipation through atmosperic cooling (i.e. ram air, engine bypass, surface heat exchanger) becomes difficult and using fuel as heat sink becomes critical. All higher Ma supercruise aircraft has faced this challenge from the Concorde to the SR71.

You're right that having 1MW power generation ability (and we don't know if it does) doesn't mean it can and would sustain 1MW output, I think it certainly would not, just like that 1kw power supply in my PC may never reach its rated power, but it still gives the headroom to run more powerful hardware running at a few hundred watts, and can cover the occasional peak when needed.

Even half of 1MW still mean a gigantic leap from 5th gen fighters. To take that kind of heat away from your electronic components is not difficult with the appropriate space for cooling component (essentially more fancy car radiators), but to get rid of that heat from your plane entirely could be tricky if 1) you need to do this for a prolonged period of time, and 2) you need to do this while supercruising at Ma 2. In that case you better have very large fuel capacity, not because of the energy consumption (still tiny compared to thrust), but because you'll need the cooling capacity offered by tons of fuel. Good thing jet fuel can take a lot of heat with the right tank design, they can run at above 100C.

Some additional thermal management tricks are also available, for example you can trade speed with more atmospheric cooling, you can "supercharge" your cooling ability with expandable heat sinks (like a tank of LNG), and you can "precharge" your cooling capacity by refrigerating the fuel during "less hot" stages of the flight, or just getting precooled fuel from a tanker.

Here is a fun and in-depth read on this exact topic from Beihang Univeristy:
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View attachment 142415

Also from Beihang, a patented precharge cooling system designed for China's "new generation high performance supersonic aircraft":
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(in Chinese, use your preferred translator if needed)
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The focus on electronics and cooling makes a lot more sense now.

At Mach 0.9
- fuel accounts for about 27% of the overall cooling capacity.

But at Mach 1.8 and during manoeuvres
- cooling capacity drops 4x
- and all cooling has to be with fuel as a heat sink
- but presumably this is when electricity demand is at its peak

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But at Mach 1.8, what happens when the fuel heats up too much? The outside air is too hot, so there's nowhere for heat to go except the engine.

---

I see there is an assumption in the paper that there is a single fuel tank as a heat sink with a maximum fuel temperature of 36C. But the engine does benefit from incoming fuel being as hot as possible as this will contribute to increased thrust.

So you mention that Jet Fuel can be stored at 100C+ , which is above the flashpoint but below the autoignition temperature of 210C. How high could Jet fuel safely be heated prior to the engine?

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So ideally, you want to have 2 separate fuel flows:

1. A fuel flow fed directly to the engine which is as close to 210C as safely and practical as possible. That would maximum the energy and thrust contained in the fuel. At the same time, it would maximise the amount of heat removed per unit of fuel burned, and maximise radar output at Mach 1.8

2. A larger fuel tank whose temperature stays as low as possible to efficiently cool the electronics. The paper has a fuel tank maximum temperature of 36C (below the flashpoint). Commercial electronics runs at 70C, but milgrade up to 125C?

So the elegant answer to both these requirements is a heat pump to [chill the fuel tank] and [heat the engine fuel line]

@tphuang

What's your guess as to the weight and efficiency of such a heat pump?
 
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FairAndUnbiased

FairAndUnbiased

Brigadier
Registered Member
AndrewS said:
The focus on electronics and cooling makes a lot more sense now.

At Mach 0.9
- fuel accounts for about 27% of the overall cooling capacity.

But at Mach 1.8 and during manoeuvres
- cooling capacity drops 4x
- and all cooling has to be with fuel as a heat sink
- but presumably this is when electricity demand is at its peak

---

But at Mach 1.8, what happens when the fuel heats up too much? The outside air is too hot, so there's nowhere for heat to go except the engine.

---

I see there is an assumption in the paper that there is a single fuel tank as a heat sink with a maximum fuel temperature of 36C. But the engine does benefit from incoming fuel being as hot as possible as this will contribute to increased thrust.

So you mention that Jet Fuel can be stored at 100C+ , which is above the flashpoint but below the autoignition temperature of 210C. How high could Jet fuel safely be heated prior to the engine?

---

So ideally, you want to have 2 separate fuel flows:

1. A fuel flow fed directly to the engine which is as close to 210C as safely and practical as possible. That would maximum the energy and thrust contained in the fuel. At the same time, it would maximise the amount of heat removed per unit of fuel burned, and maximise radar output at Mach 1.8

2. A larger fuel tank whose temperature stays as low as possible to efficiently cool the electronics. The paper has a fuel tank maximum temperature of 36C (below the flashpoint). Commercial electronics runs at 70C, but milgrade up to 125C?

So the elegant answer to both these requirements is a heat pump to [chill the fuel tank] and [heat the engine fuel line]

@tphuang

What's your guess as to the weight and efficiency of such a heat pump?
Click to expand...
can you explain why you think that is the case? I am having trouble understanding why this would be the case and not the opposite.

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N = 1 - T1/T2 where T1 is intake temperature and T2 is exhaust temperature.

If heated fuel is injected prior to the combustion chamber at higher temperature than usual, wouldn't the effective intake temperature T1 be higher than usual, so efficiency is lower than usual?
 
M

Moonscape

Junior Member
Registered Member
FairAndUnbiased said:
can you explain why you think that is the case? I am having trouble understanding why this would be the case and not the opposite.

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N = 1 - T1/T2 where T1 is intake temperature and T2 is exhaust temperature.

If heated fuel is injected prior to the combustion chamber at higher temperature than usual, wouldn't the effective intake temperature T1 be higher than usual, so efficiency is lower than usual?
Click to expand...
the PV diagram refers to the working fluid, which is air. the inlet air temperature stays the same, but the outlet air temperature will be higher if the fuel is preheated since more of the thermal energy from combustion will go towards heating the air vs warming up the fuel to combustion temperature
 
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