J-20 5th Gen Fighter Thread IV (Closed to posting)

Status
Not open for further replies.
thunderchief

thunderchief

Senior Member
delft said:
I was thinking about the claim mentioned recently in this thread that F-22 is able to achieve 6 g at a formidable altitude. I think J-20 might be able to do the same if the thrust of the engines can be increased. Two ways to do that were used in piston engines during WWII: injecting a nitrogen oxide or a mixture of water and methanol to improve take off performance and, for fighters, increase power during combat at high altitude. The second was often used in early turbojet and turboprop engine to improve take off performance (
Please, Log in or Register to view URLs content!
). J-20 might have adapted engines and carry enough water methanol mixture to achieve the higher thrust during a few minutes. Compare that with using TVC with its added weight and reduced thrust which for a US engine is less of a problem than for a Chine engine at this point in the development.
Click to expand...

I'm certainly no expert on aircraft engines , but nowadays main problem is not how to temporarily increase thrust (various stages of afterburner could do that) . Instead , main problem is how to make durable enough engine with special alloys to sustain high temperatures developed during the work cycle . That is way there is so much effort in metallurgy and materials science ( single crystal blades for turbines etc ... )

p.s. I forgot to mention , unlike piston engines in ww2 , power(thrust) of jet engines depends on temperature . Ideal jet engine would have maximum temperature in combustion chamber and absolute zero at exhaust .
 
Last edited:
thunderchief

thunderchief

Senior Member
vesicles said:
"absolute zero"? I hope you don't mean 0 Kelvin...
Click to expand...
Yes , exactly 0 Kelvin . Bear in mind , this is ideal or better said "ideal" engine with maximum efficiency , not a practical solution ! Main point is , you need as large as possible difference in temperature between combustion chamber and exhaust.
 
Quickie

Quickie

Colonel
My take is the temperature would only go down to the ambient air temperature, but, by the time, the exhaust would be fully expanded at some distance from the nozzle. But I can understand the idea thunderchief is trying to convey, which is basically a correct one.
 
L

latenlazy

Brigadier
What thunderchief said isn't that absurd. IF (BIG IF) if you could get maximum temperature in combustion chamber and zero kelvin at exhaust, assuming that you get zero kelvin without taking energy away from the exhaust (cooling), that means that your exhaust is able to maximize conversion of energy into motion and not heat. Of course, reality doesn't work that way, and specific conditions need to be met (that zero kelvin exhaust cannot come from taking energy away from the closed system), and in some ways perfect absolute zero is a bit of an impossibility, since that exhaust is still moving.
 
Last edited:
thunderchief

thunderchief

Senior Member
Quickie said:
My take is the temperature would only go down to the ambient air temperature, but, by the time, the exhaust would be fully expanded at some distance from the nozzle. But I can understand the idea thunderchief is trying to convey, which is basically a correct one.
Click to expand...

latenlazy said:
What thunderchief said isn't that absurd. IF (BIG IF) if you could get maximum temperature in combustion chamber and zero kelvin at exhaust, assuming that you get zero kelvin without taking energy away from the exhaust (cooling), that means that your exhaust is able to maximize conversion of energy into motion and not heat. Of course, reality doesn't work that way, and specific conditions need to be met (that zero kelvin exhaust cannot come from taking energy away from the closed system), and in some ways perfect absolute zero is a bit of an impossibility, since that exhaust is still moving.
Click to expand...

As I said this engine is just a thought experiment ;) , not a practical solution , especially in atmosphere of a planet . On the other hand , similar principle could be used for a rocket engine in outer space with ambient temperature close to 0 K.

To clarify what I meant , let us take ideal gas law P*V=n*R*T . Right side of the equation could be interpreted as internal energy of gas . If we could temperature close to 0K , internal energy would go close to zero . And where would rest of the energy go ? It will become kinetic energy pushing our aircraft (space ship , whatever ;) )

Anyway , that is way you need large temperatures in jet aircraft engines . You store the energy in the gas (air in this case) and then you release it .
 
plawolf

plawolf

Lieutenant General
blacklist said:
exaust at 0 kelvin would make ir-missile obsolete... awesome stealh engine it is... ;)
Click to expand...

Not really. If there was such an engine, people will just design and program missiles to homing in on the highly focused extreme cold signature instead and plane would be dropping ice cubes as decoys. ;)
 
Status
Not open for further replies.
Top