F-35 Jet Exhaust: Supersonic Takeoff/Landing?

[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.

 

[etudiant]

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

 

[boneh3ad]

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

 

[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.

 

[boneh3ad]

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.

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.

 

[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.

 

[AlephZero]

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.

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.

 

[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.

http://m.youtube.com/watch?v=35-8B7yGGnY

 

[boneh3ad]

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.

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.

 

[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

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.

 

[HowlerMonkey]

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

 

[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.

 

[qumf]

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

“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.

 

[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.

 

[qumf]

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.

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

 

[boneh3ad]

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.

 

[HowlerMonkey]

http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html

 

[RandomGuy88]

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.

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.

 

[boneh3ad]

So... what does this engine applet have to do with anything?

 

[HowlerMonkey]

Still waiting for data on a plane that can be powered over supersonic speeds in Earth's atmosphere at standard pressure and density (sea level) with a subsonic exhaust velocity.

 

[RandomGuy88]

Are you expecting us to provide this data?

Nobody ever said anything about there being a specific plane that can fly at supersonic speeds at seal level with subsonic exhaust velocity. All we said is that flying at supersonic speeds does not require supersonic exhaust velocity.

The thrust produced by an engine depends on the momentum fllux. Which means you can produce large thrust with low exhaust velocities if the mass flux is high. Its really that simple.

For an aircraft to fly at supersonic speeds you only need enough thrust to overcome the drag at those speeds. It does not matter where the thrust comes from. In fact you don't even need thrust, as evidenced by Felix Baumgartner's recent stunt during which he reached M = 1.25 in free fall.

So the force to propel something to supersonic speeds can be produced by any means. So why then would a propulsion system capable of producing enough force with subsonic exhaust not be able to propel an aircraft to supersonic speeds?

Keep in mind I am not saying that this is the best or even a good way to fly at supersonic speeds, or that there is even an available engine capable of doing this. I am not a propulsion person so I have no idea.

We are just saying there is no reason it can't be done.

 

[HowlerMonkey]

I was hoping for at least a single real world reference of subsonic exhaust velocity propelling a jet aircraft to supersonic speeds at what is considered normal operational altitudes.

 

[RandomGuy88]

Again, nobody said anything about their actually being an aircraft that does this. You made the claim that this is not possible and we said you are wrong.

 

[HowlerMonkey]

Prove me wrong.

 

[berkeman]

Thread closed temporarily for Moderation...

Thread re-opened. I think it would be good if we could find out if supersonic aircraft typically use supersonic exhaust velocities. Even though it should be theoretically possible to achieve Mach 1 with subsonic exhaust velocities, it may not be a practical possibility. Let's see if we can find that info...

 

[RandomGuy88]

I wonder if the confusion lies in the thrust equation for an air breathing engine.

For a perfectly expanded air breathing engine such as a turbofan or ram jet the thrust is given by

T = mdot_e*Ve - mdot_0*V0

Where:
mdot = mass flow
V = velocity
e = exit of engine
0 = inlet of engine

The difference between mdot_e and mdot_0 is due to the fuel that it injected in the combustion chamber. Typically this is considered negligible and instead thrust is approximated by

T = mdot*(Ve - V0)

This equation says nothing about whether or not you can fly at supersonic speeds with subsonic exhaust. It simply states that the exhaust of the engine must be greater than the speed of the aircraft. Since the temperature of the exhaust gas is very high, the local speed of sound within the gas will also be very high. So even though the exhaust velocity must be greater than the velocity of the aircraft, I do not see anything saying that the Mach number of the exhaust must be greater than the Mach number of the aircraft.

Now, as I have said before I am no propulsion expert. Perhaps there is a way to prove that an aircraft cannot fly at supersonic speeds with subsonic exhaust but I have not seen it and I do not see why that must be the case. I also have no idea if any aircraft fly like this or whether or not it would even be efficient.

Prove me wrong.

While my previous post may have been a little rude, I find this post to be very immature. A question was asked "How will a non-supersonic jet exhaust propel a plane to supersonic speeds?" Several users offered reasonable explanations. You then chose to mock us with your Kennedy space center comment and then post "Prove me wrong" all while never offering any explanation of your own or attempting to refute our claims. If my explanation above and my understanding is wrong I would certainly love to know, but you have not even made an attempt at offering an explanation.

 

[AlephZero]

Assuming still air and measuring velocities relative to the ground, the inlet air velocity is 0, therefore to generatre thrust (= change in momentum) the exit velocity must be opposite to the aircraft's direction of flightl.

So for supersonic flight, the exhaust velocity must be supersonic relative to the aircraft.

This is fundamentally different from a non-air-breathing rocket engine, where the initial propellant velocitiy is 0 relative to the rocket not relative to the ground. A rocket engine can work perfectly well if the exhaust gas is traveling in the same direction as the rocket (but slower), relative to the ground, and the nozzle velocity does not have to be supersonic relative to the rocket.

 

[boneh3ad]

Assuming still air and measuring velocities relative to the ground, the inlet air velocity is 0, therefore to generatre thrust (= change in momentum) the exit velocity must be opposite to the aircraft's direction of flightl.

So for supersonic flight, the exhaust velocity must be supersonic relative to the aircraft.

I do not agree with this line of reasoning. It works in the sense that it requires that the exhaust velocity must be greater than the plane's velocity (in the frame of reference of the plane), a fact reflected in the thrust equation as well. It says nothing about Mach number, which has that temperature dependence. As RandomGuy88 pointed out, the exhaust is so much hotter than the speed of sound in the exhaust is going to be substantially higher than that at the inlet. With that in mind, the exit velocity could certainly be faster than the plane velocity, even if the inlet flow is supersonic and the outlet flow is subsonic, meaning you could generate thrust at supersonic speeds with a subsonic exhaust. That fits neatly within your discussion of momentum conservation.

All that said, I am not so sure any plane would every actually be operated that way, but it is theoretically possible.

 

[HowlerMonkey]

The F16 plane pictured generating "mach diamonds" is limited to mach 1.2 at sea level.

It certainly won't do mach 1.2 at sea level with a subsonic exhaust velocity considering it requires supersonic exhaust velocity to fly significantly slower.

I said I would pose the question to the guys next time I am at NASA and I will...but I can't be held responsible for their response.