# F-35 engine exhaust

1. Jan 18, 2013

### RandomGuy88

When the F-35 is taking off or landing vertically, are any of the jet exhausts supersonic? I have seen the amount of thrust they produce but nothing about the Mach number. I am not looking for anything specific just whether or not the exhausts are supersonic.

2. Jan 19, 2013

### etudiant

The vertical lift fan that sustains the F35B is not supersonic afaik. I do not know about the jet engine exhaust component.

3. Jan 19, 2013

If you know the thrust (roughly) already, see if you can find the engine mass flow listed anywhere. Sometimes you can find that stuff.

4. Jan 19, 2013

### AlephZero

If you take a simple view of this, the flow through a jet engine combustor is certainly not supersonic (it is limited by the flame speed of the fuel) and the turbine is a diverging duct, so there is nothing to make the engine exhaust supersonic - except maybe the afterburners, but afterburners are not useful for VTOL.

As for the lift fan, for efficiency in converting energy into thrust (= change of momentum) you want to move a large mass of air slowly, not a small mass of air fast, so there would be no reason to design it for supersonic flow.

5. Jan 19, 2013

That's not a valid line of reasoning here. The conclusion may be correct, but the reasoning isn't. Just because the combustor is subsonic doesn't mean the exhaust is, especially if you are modeling the turbine as a diverging duct. At that point it all comes down to the combustor pressure and whether or not it is high enough to choke the combustor outlet.

Really though, it is all about the pressure at the turbine exit. The nozzle on many fighter engines is (or has the ability to be) convergent-divergent, so as long as the turbine exit pressure is higher than that required to choke the throat of the nozzle, the exhaust will be supersonic at least somewhere in the nozzle. Generally it needs to be slightly higher than that so that there isn't a shock in the nozzle, which is extraordinarily inefficient. Many fighters operate this way, as evident by the shock diamonds you can see in the exhaust.

6. Jan 19, 2013

### RandomGuy88

Boneh3ad is correct. Whether or not the exhaust is supersonic depends on the nozzle geometry and the pressure ratio. Afterburners are not required for supersonic exhaust. In fact I believe it is pretty typical for military aircraft operating at subsonic speeds and low altitude to have supersonic exhaust.

I did manage to find a reference that indicates the exhaust velocity for the F-35 while taking off or landing vertically is Mach 1.

Vertically oriented high speed exhaust impinging on the ground is a serious problem for vehicles like the f-35 because the high speed and high temp exhaust can damage runways and even cause the flight decks of aircraft carriers to buckle. The V-22 Osprey has similar problems.

7. Jan 19, 2013

### AlephZero

But choking the combustor outlet doesn't seem like a good idea in practice, unless you can keep it choked for all engine operating conditions, and run the whole turbine supersonic. If you don't like shocks in the nozzle, you like shocks in the turbine stages even less. That seems like doing it the hard way to me.

I take the point about con-di nozzles though. I mostly work on big quiet engines not small noisy ones, so that dropped off my radar.

8. Jan 19, 2013

### RandomGuy88

The combustor outlet is not choked. If there is choked flow anywhere it occurs at the minimum area in the nozzle downstream of the turbine.

Fighter jets are typically capable of adjusting the nozzle outlet area in order operate efficiently at various operating conditions.

9. Jan 20, 2013

My point about choking the combustor only holds if you are modeling the entire turbine/nozzle assembly as a simple diverging duct since that would presumably effectively be the throat. I agree that doing this in practice sure seems like a silly idea. In reality, all jet engines save scramjets have subsonic combustors. The real question then is whether or not the turbine outlet pressure is high enough to choke the throat of the propelling nozzle.

Actually, the more I think about it, the more I feel like the jet being supersonic might actually be advantageous for the case of the STOVL systems on the F-35. I am certainly not an expert on STOVL applications, but heat on the landing surface can definitely be an issue and the LiftFan doesn't have to worry about it because it is essentially just a ducted, two-stage, contra-rotating propeller system. No combustion takes place. The main exhaust, though, seems like it could benefit from expanding to supersonic speeds since the flow will cool off quite a bit as it expands and even at the stagnation point will not fully recover the total temperature. Then again, I have no idea how a supersonic exit would affect the stability of the aircraft while in that mode.

10. Jan 20, 2013

### RandomGuy88

Interesting point about a possible benefit of expanding to supersonic speeds. Although from what I have seen the recovery factor of a supersonic impinging jet is typically around 1, but that is small scale lab experiments.

Here are two articles discussing the impact of these jets on the surface. One of them mentions the exhaust of the f-35 is roughy 1700 F

http://theaviationist.com/2010/11/24/the-f-35b-heating-problems/#.UPt2cGt5mSO

http://www.aviationweek.com/Blogs.aspx?plckBlogId=Blog:27ec4a53-dcc8-42d0-bd3a-01329aef79a7&plckController=Blog&plckScript=blogscript&plckElementId=blogDest&plckBlogPage=BlogViewPost&plckPostId=Blog%253A27ec4a53-dcc8-42d0-bd3a-01329aef79a7Post%253Afd6afd6b-9176-4003-8ea6-08049291946b [Broken]

I do know that the supersonic jet is a significant noise source. There are several mechanisms in the supersonic impinging jet that generate noise at very high amplitude and over a wide range of frequencies.

The jets themselves are influenced by various instabilities but I am not sure how that affects the stability of the aircraft. That would be interesting to know though. The f-35 has two small jets located outboard on the wings to aid in roll stability while moving vertically.

I know that when the jet is impinging on the ground there is a significant amount of entrainment of the ambient air. This can actually reduce the lift of the jet because the air under the aircraft begins moving due to entrainment which lowers the pressure and essentially sucks the aircraft towards the ground.

Also the recirculation that is set up by the jet impinging on the ground can result in high temperature air being ingested by the engine which can hurt performance.

Last edited by a moderator: May 6, 2017
11. Jan 21, 2013

### HowlerMonkey

How will a non-supersonic jet exhaust propel a plane to supersonic speeds?

12. Jan 21, 2013

### RandomGuy88

It's all about the momentum flux. So speed of the exhaust is not the only parameter.

Assuming the nozzle is perfectly expanded:

Thrust = (mass flow) * (change of velocity of air through engine)

So by giving a large mass of air a relatively smaller delta V it is possible to produce a significant amount of thrust. This is the concept behind modern high bypass turbofans. So the exhaust velocity does not necessarily have to be supersonic.

13. Jan 21, 2013

### qumf

“supersonic speeds” ia relative to local condition. mainly to local temperature for air/gas.
supersonic speed for a plane is subject to outside atmosphere. the ourside temperature is low, so the sonic speed should be low.

"non-supersonic jet exhaust" is subject to inside gas after burning. the gas' temperature is rather high. the gas' sonic speed is also very high. even though the speed is under supersonic. the absolute speed of exhaust gas is much higher than the speed of plane.

14. Jan 22, 2013

### HowlerMonkey

So you can power a plane beyond mach 1 with a subsonic jet exhaust?

I guess I'll pose that at kennedy space center for a laugh next time I'm there testing.

15. Jan 22, 2013

### qumf

The thing happen is relative to many factors. such as the mass of the plane.
it is possible, It is not definitely

16. Jan 22, 2013

Yes you can propel a vehicle to supersonic speeds with subsonic exhaust.

Think about it in terms of force, not exhaust velocity. If you want to accelerate something, you need a force, not a velocity. The force generated by a jet engine, or thrust, is given by $F = \dot{m}v$. In othe words, if you have a high enough mass flow rate out of the engine, you can have very high thrust out of low velocities, as alluded to earlier by RandomGuy88.

17. Jan 22, 2013

### HowlerMonkey

18. Jan 22, 2013

### RandomGuy88

Would you care to offer an explanation as to how we are wrong? I am no propulsion expert but I see no reason why we would be wrong.

19. Jan 22, 2013