Afterburners: Why the Nozzle Opens Wider with Afterburner On

[@achalorocks]

So basically, you have the gasses from the engine normally, which has a smaller exit area to stay choked and get the most velocity, and then you add the extra gasses from the afterburner, which means you have to open the nozzle to account for the extra gasses trying to exit the engine and prevent back pressure while gaining thrust?

[@JoshTheEngineer]

I would say that's a pretty good summation. Another way to think about it is to consider the engine without the afterburner on and the nozzle in the optimum choked position. What happens when you close the nozzle even further? You'll still have choked flow, but your mass flow rate goes down and so does your thrust.

[@solarsynapse]

@@JoshTheEngineer How about an engine that does not have an afterburner which has a wider nozzle for takeoff woosh (I know, technical term.) and then chokes down for speed?

[@joshnc101]

As someone who’s a high school dropout and can’t wrap my head around this sort of math, it was a fascinating explanation.

[@JoshTheEngineer]

Thanks for watching! I hope it wasn't too crazy. I have a lot of background derivation videos too in case you're wondering where some of the equations are coming from.

[@commonsense4993]

In other words: 1) Without afterburner, a smaller opening gets more speed (thrust) from the the exhaust 2) With afterburner, the opening has to be larger, but just enough based on the temperature to keep the back pressure the same. Thereby keeping the intake flow the same during the transition to AB while the added AB heat expands the exhaust increasing the output flow (thrust). This keeps the engine from stalling. Is that correct?

[@michaeltalbot8242]

Just as an FYI don't start at this end of the engine at art at the front end and work backwards only getting into reheat if you engine needs it

[@demikaung6609]

Sir, this is a very good explanation with thermodynamic system.

[@Azzer1989]

this massively helped with my assignment and it all just clicked after so thank you very much!

[@JoshTheEngineer]

You're welcome, glad I could help!

[@theeddytor3490]

damnn this was something i wasn't looking for but i glad i did. i mean that was a lot more in depth about how variable nozzles work and how it effects the thrust but this was completely a different ball game. got to learn from math perspective. really helpful.

[@DaylightDigital]

Nice video, this is how I like to reason it out: For the (roughly) fixed mass flow assumption, the product of the sonic area and the stagnation pressure divided by the square root of the stagnation temperature must be constant, whether the A/B is lit or not. From Rayleigh Flow analysis we know that the stagnation pressure will fall and the stagnation temperature will rise (i.e. subsonic heat addition). Both of these effects will decrease the magnitude of the term multiplied by A-star, so A star must increases proportionately to keep the combination constant. We probably don't need any calculations at all to reason out that the sonic area must increase. It's a very similar analysis that leads to the constant stagnation pressure sonic area product when analyzing shock waves, except now we allow the stagnation temperature to vary as well.

[@KikoGuimaraes75]

Very well explained procedure and problem. Thanks for this!

[@JoshTheEngineer]

You're welcome!

[@PhillMagGamerDad]

How is this calculated in real life though, considering the fact that the Inlet Pressure is lower if the AB is switched on from stationary start on the runway, compared to doing Mach 2 at 40000ft altitude? Does this mean the Nozzle will be opened more when travelling at speed compared to ground level? I'd have to assume there are sensors in the tailpipe to constantly measure Mass Air Flow for every potential scenario of the flight envelope?

[@dray22222]

Apparently its temperature...

[@suserman7775]

Compared to the non afteburning state, the afterburning state has about 57 percent of the density and 133 percent of air velocity. So 1/(.57x1.33) is about 1.3 and that represents the ratio of Area with afterburner compared to non afterburning. Or put another way, even though AB velocity is higher and that would act to REDUCE the needed area to maintain the flow rate as is, that is MORE than countered but a huge increase in volume (same as reducing density).

[@gazlink1]

Yup.. that explains it in a nutshell

[@aidannzchch476]

What I'm trying to understand is where the afterburner mechanically applies the thrust force to the engine. I know with a non afterburning turbofan, the forward force is applied at the intake fan, compressor, diffuser and combustion chamber. In other words a forward pressure vector on the internal surfaces of the engine. But how does the afterburner add to this? Where is the force applied? Not the flameholder or jet pipe. If it doesn't increase mass flow rate through the rest of the engine then there is no extra force applied on the components mentioned earlier. What am i missing?

[@frederf3227]

Momentum. 1kg/s mass flow is twice as much force at double the delta v. Afterburning is still internal to the engine just post turbine. Pressure surface integral still equals the higher force.

[@FlyNAA]

Yes momentum 100% accounts for the thrust, in a mathematical sense. But ultimately, 100% of the thrust must also make it into force on the pylon, and prior to that, on a certain distribution on the engine (including inlet, basic engine, and nozzle) skin. So the question is, what is this distribution exactly, and where does afterburner add to it?

[@clementfortin4315]

is the converging nozzle exclusively for after burner use or it also varies depending on speed in flight. My theory is that when the jet is flying slow the nozzle would stay wider for lower engine load and fast response and as the jet goes faster, the nozzle gets smaller to produce faster thrust. A bit like a gearbox, where open wide is first gear and choked is 6th gear. Or would it be better to have a fixed nozzle size if not for afterburner? Thanks

[@maxwelmax9609]

Please do one for thrust vectoring.

[@m.a.4794]

Can you explain to me what happens to the mass flow rate if the nozzle throat stays fixed even when the AB is on? will the flow be choked at the throat in that case?

[@JoshTheEngineer]

It's a little hard to analyze exactly what happens in real life, but if we make a simplifying assumption, I can answer your question. Let's assume that for case 1, the afterburner is on and has the P0 of 192 kPa. As I go through in the video, the nozzle opens up wider. For case 2, let's assume that we didn't open up the nozzle wider, but also that the stagnation pressure in the afterburner is still P0 = 192 kPa. If this is the case, then by keeping the nozzle closed at its lower area position, you would decrease the mass flow rate through the engine. This would obviously cause problems upstream in the engine, but that's why we made the assumption that the stagnation pressure was the same in both cases.

[@IllilliiililiIIliliI]

5:35 "corrected" mass flow dont increase after choked point,

[@Anurag_077]

Love you Josh. You are a good teacher.

[@JoshTheEngineer]

Thanks, I appreciate it!

[@johannwegmann4365]

Thankyou Friend, I didnot know, a nightmare for construction indeed.May I use an afterburner convergent nozzle as a booster to start a Ramjet?We need to reach a critical inlet speed air, fuel injection,and once combustion chamber ignites then self sustaining and put AB off? If you please ,I am thinking in a 500 Newtons thrust small ramjet for sailplanes.Bye and many thanks.You are great, but go slower, I am FEA Professor for long,think this is a class or lecture.Next try to lecture Thrust comparison AB/ versus off AB.Bye.

[@asdjefndk1]

Great Explanation. Thanks a alot.

[@FlyNAA]

Wouldn’t the stagnation pressure be higher in the AB case, due to the higher speed and dynamic pressure? Or did you make them the same (AB is adding temperature but not velocity) as a simplification that still demonstrates the point?

[@FlyNAA]

Or, is it that in adding temperature the density is decreased by the exact amount that offsets the increase in speed, such that dynamic pressure stays the same?

[@santhoshinib8684]

I want to know why we have to keep pressure drop as small as possible? .... So that we need to increase nozzle area

[@faisalshaikh6450]

What’s up steam and downstream in nozzle plz?

[@Raptorman0909]

Josh, what additional work is needed to calculate the true values given the fact that the mass flow IS higher with afterburner -- with AB on a much larger amount of fuel is added to the flow even though the air flow is the same.

[@JoshTheEngineer]

You're correct that with an afterburner, it's harder to justify neglecting the added mass flow rate of the fuel in the analysis. Just as a quick aside, the main increase in thrust comes from the increase in temperature from the burning of the added fuel. While the thrust also does increase due to the added mass from the fuel itself, it's pretty small compared to the temperature increase. That being said, it would be more accurate to include the fuel mass flow rate as well. The easiest way to include the fuel mass flow rate in this analysis is to specify the flow rate (in kg/s), and then add it to the flow rate without the AB on. Then you would use this new mass flow rate instead of the old one and continue the analysis as is shown in the video.

[@faisalshaikh6450]

Brother we are working on a project on jet engine nozzle for 3D modelling we require the real dimensions of jet engine nozzle but we aren’t getting it on Internet or anywhere. Do you have the real dimensions? Or you know where we can get it?

[@BillyReed68]

I just watched this once but Did I see that you were increasing the flow rate in m/s by almost a factor of 3 with AB? 237m/s to 732 m/s?

[@nischaysingh8132]

Thanks this is very informational

[@JoshTheEngineer]

You're welcome!

[@ronaldpiper4812]

Way intense

[@garygrose9188]

There haven't been afterburners since the 70's. Afterburner is the practice of reburning he remaining exhaust from the engine. The engine in your illustration uses fresh air from the N1 or fan, this is called augmentation. The air used in augmentation is bypass air that flows past the core engine and acts as more of a ram jet than afterburning core exhaust.

[@solarsynapse]

Really? F-35 Afterburners inject extra fuel after the fan. en.wikipedia.org/wiki/Afterburner

[@garygrose9188]

@@solarsynapse Hay Zac, no kidding?

[@89apaul]

Fantastic video

[@JoshTheEngineer]

Thank you!

[@mykiscool]

Thanks for sharing. I don't understand this very well, but one thing I do understand is that this is why my rocket engine exploded!

[@JoshTheEngineer]

My pleasure. Gotta be careful with those afterburners.

[@mecarloschavez6281]

Hey -basically afterburners increase temperature and velocity , but keeps mass flow rate to avoid back pressure on the turbine wheel. ---I'm actually designing a small variable nozzle for a jet engine - I'll try to make the math for my self - :) need to view the video several times to get note of the ecuaciones

[@joefarah06]

Awesome stuff man. Thanks 👍

[@JoshTheEngineer]

No problem, thanks for watching!

[@shubhamchauhan3773]

Great work Sir...

[@JoshTheEngineer]

Thanks!

[@groomlake51]

🤯so good it made my brain hurt🤓

[@summerraiin90]

great explanation!

[@JoshTheEngineer]

Thanks!

[@waitemc]

Who is Kelvin?

[@VictorChristopherOng]

You are so amazing!

[@JoshTheEngineer]

Thanks!

[@stevelewis7263]

No afterburner, nozzle constricts to increase flow of pressure, with after burner there is ample thrust and nozzle doesn't need to constrict, if you constrict nozzle with afterburner too much pressure is produced and you blow the end off your engine...seemples

[@TenzinDorjee]

Subscribed! You're way better than my professors. Lmao

[@JoshTheEngineer]

Ha, thanks a lot!

[@96Kaafarani]

Thank you Josh

[@JoshTheEngineer]

No problem!

[@paulmoore5401]

Ummm ….yeah ….I was hoping for the “pilots” explanation of why the nozzles get bigger in AB.

[@powertool4uau]

Yes but this still doesn't explain why my toast always lands buttered side down when l drop it.

[@Bleideris0]

He explained on heis equasions, you just didnt understood.

[@JoshTheEngineer]

You forgot to strap a cat to your toast.

[@kenjohnson6603]

The nozzle must vary to control engine temperature. The J-78-5B operates at 596 (+/_) 11 C

[@Dunes4321]

Wow, just wow..........

[@joebob4579]

why so serious baby?

[@michaeltalbot8242]

Good stuff Josh but you need to slow your ninformation flow as it is Avery technical subject and you will lose your audience other wise

[@JoshTheEngineer]

Thanks for the suggestion Michael. I've always spoken pretty fast, so it's hard to change that now.

[@davidlegg9690]

Not you again

[@JoshTheEngineer]

This has always confused me; no one is forcing you to watch any video on KZbin. You can go watch any other afterburner video.

[@jonasreuterbring5193]

Maby less caffeine before a video!!! Montonic voice = boring

[@justanotherguy469]

You are too cool for us. The excitement is not in his voice, the excitement is in the concept that his monotonic voice is explaining.

[@DanSlotea]

Set your playback speed to 0.75x, it's way better. I don't understand the rush, just like on Engineering Explained