the power of jet engines...
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THRUST
I think I have an answer to your query.Yes,it is v.interesting.It is a concept/fact we should be aware of.It really suprised me.
The RB-211 is rated 58,000lb. SL.By-pass ratio 4.3
35,000ft. 0.85 M max cruise thrust 12,708lb.
Official Rolls Royce figs.
I think I have an answer to your query.Yes,it is v.interesting.It is a concept/fact we should be aware of.It really suprised me.
The RB-211 is rated 58,000lb. SL.By-pass ratio 4.3
35,000ft. 0.85 M max cruise thrust 12,708lb.
Official Rolls Royce figs.
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jtr,
we also had some discussions about power/thrust conversion. The interesting and somehow irritating fact is that a jet engine produces maximum thrust ( full rated T.O.) while at zero speed, thus has NO power in means of physics; while it accelerates the thrust is decreasing, but the power equivalent is increasing, and there are formulas to calculate the power needed to sustain a certain speed.
With all the factors supplied for a certain engine type You can figure out a quite accurate conversion rule, e.g. for the CFM 56 engine it is something like fuel flow (kgs/h) x 4,3 = Horsepower equivalent; but as said, this number only applies to this engine. Older engines with a much worse fuel efficiency need a smaller number to convert.
Hope it helps
we also had some discussions about power/thrust conversion. The interesting and somehow irritating fact is that a jet engine produces maximum thrust ( full rated T.O.) while at zero speed, thus has NO power in means of physics; while it accelerates the thrust is decreasing, but the power equivalent is increasing, and there are formulas to calculate the power needed to sustain a certain speed.
With all the factors supplied for a certain engine type You can figure out a quite accurate conversion rule, e.g. for the CFM 56 engine it is something like fuel flow (kgs/h) x 4,3 = Horsepower equivalent; but as said, this number only applies to this engine. Older engines with a much worse fuel efficiency need a smaller number to convert.
Hope it helps
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The easiest way to demonstrate thrust is to blow up a balloon and let it go. It will fly all over the place. The most common thought as to why the balloon is propelled forward is that the air is flowing very rapidly from the the narrow neck of the balloon. Some people will extrapolate that the ballon example can be applied to a jet engine. If they did, they would be wrong. An example of this error would be to have them explain how a rocket engine can work in outer space where there is no field of air to react against. The real thrust comes from the pressure exerted on the forward wall and side walls of the combuster. Each can or on an annular can represents a rocket engine. The exhaust from the combuster flows through a turbine which drives the rest of the engine (Fan and Compressor). There is a pressure differential between the forward wall of the can and the rear end of the can which resultst in a pressure differential. The pressure on the forward wall of the can is much greater than the pressure at the rear of the can. Since the combuster can(s) are attached to the engine and the engine is attached to the wing this pressure differential forces the airplane forward. The fan acts as a propeller and it too is attached to the engine and it provides an added force moving the aircraft forward just like a propeller on a conventional aircraft. Use the formulae to calculate the thrust assuming you have all the figures, but if you want to explain a jet or, a rocket engine to your friend use this.
Here is a little gem. An Atlas rocket engine that generates 150,000 pounds of thrust only has an internal pressure of about 600 PSIA. This pressure working against the surface area of the forward or, upper wall of the engine and the wall of the exit nozzle is what creats the force. Do the math. The forward or upper wall has a surface area of 250 square inches or there abouts. Also the higher the rocket goes until they get up to the top of the atmosphere the faster the gas flows out of the nozzle creating an even higher differential increasing the thrust over the level generated at sea level which is somewhat lower than the rated 150,000 pounds.
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The Cat
[This message has been edited by Lu Zuckerman (edited 23 October 2000).]
[This message has been edited by Lu Zuckerman (edited 23 October 2000).]
Here is a little gem. An Atlas rocket engine that generates 150,000 pounds of thrust only has an internal pressure of about 600 PSIA. This pressure working against the surface area of the forward or, upper wall of the engine and the wall of the exit nozzle is what creats the force. Do the math. The forward or upper wall has a surface area of 250 square inches or there abouts. Also the higher the rocket goes until they get up to the top of the atmosphere the faster the gas flows out of the nozzle creating an even higher differential increasing the thrust over the level generated at sea level which is somewhat lower than the rated 150,000 pounds.
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The Cat
[This message has been edited by Lu Zuckerman (edited 23 October 2000).]
[This message has been edited by Lu Zuckerman (edited 23 October 2000).]
Nice thought Lu, but a balloon would work even better in space than in your average room.
The jet of air from the neck of a balloon is the same as a jet of air from a jet engine (that is why it is called a jet engine!)
The jet of air from the neck of a balloon is the same as a jet of air from a jet engine (that is why it is called a jet engine!)
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To: Checkerboard
Dear Check,
The air rushing out of the neck of the balloon is not reacting against the surrounding air. It is the pressure differential between the front of the balloon and the rear of the balloon. What you said about the balloon in outer space is true. The pressure differential would be greater due to a lack of surrounding atmosphere and the absence of any drag.
When fluid is contained under pressure the internal pressure acts at right angles to the containing wall. That means that when the container is sealed the pressure on the containing walls is equal. When you open up one end of the container the fluid under pressure flows out of the opening and the differeential pressure between the front and rear causes the container to move in the direction of the highest pressure.
Whether you are talking about a balloon, a rocket engine or a jet engine the principle is the same.
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The Cat
Dear Check,
The air rushing out of the neck of the balloon is not reacting against the surrounding air. It is the pressure differential between the front of the balloon and the rear of the balloon. What you said about the balloon in outer space is true. The pressure differential would be greater due to a lack of surrounding atmosphere and the absence of any drag.
When fluid is contained under pressure the internal pressure acts at right angles to the containing wall. That means that when the container is sealed the pressure on the containing walls is equal. When you open up one end of the container the fluid under pressure flows out of the opening and the differeential pressure between the front and rear causes the container to move in the direction of the highest pressure.
Whether you are talking about a balloon, a rocket engine or a jet engine the principle is the same.
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The Cat
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Lu,
You're right, the jet is not reacting against the air outside and rockets, balloons and turbine engines do operate off the same principle, but you're wrong about the pressure differential providing the thrust.
The engines and balloons that you talk about derive their thrust by the momentum change, or accelaration, of a mass of working fluid. At the end of the day, you cannot go faster forwards tham you are propelling gas backwards. Previous threads have discussed how thrust decreases with increasing forward speed. This is precisely because the momentum change imparted is decreasing, and the exhaust gas is leaving backwards at speed cancelled out by the forward speed
Supersonic jets have to propel gas backwards fast (e.g. Concorde, military jets)because their ultimate speeds are higher (that's also why they are so noisy when they pass you on final approach - the high jet speed creates lots of turbulence which you hear as rumbling noise), but Commercial airliners impart lower accelerations to larger amounts of gas - higher bypass ratios gas turbines and propellers.
The engine is not "sucked forward" as other threads have alluded to - they are propelled forward by the acceleration of gas through some kind of nozzle (momentum change).
This is also demonstrated by the reaction on a constricted hosepipe. Where is the pressure on the "forward wall" of a hose? At the pumping station. Where is the acceleration of the water? In the nozzle. So where is the force developed? In the nozzle, which is why the nozzle flicks around, not the rest of the hose.
You're right, the jet is not reacting against the air outside and rockets, balloons and turbine engines do operate off the same principle, but you're wrong about the pressure differential providing the thrust.
The engines and balloons that you talk about derive their thrust by the momentum change, or accelaration, of a mass of working fluid. At the end of the day, you cannot go faster forwards tham you are propelling gas backwards. Previous threads have discussed how thrust decreases with increasing forward speed. This is precisely because the momentum change imparted is decreasing, and the exhaust gas is leaving backwards at speed cancelled out by the forward speed
Supersonic jets have to propel gas backwards fast (e.g. Concorde, military jets)because their ultimate speeds are higher (that's also why they are so noisy when they pass you on final approach - the high jet speed creates lots of turbulence which you hear as rumbling noise), but Commercial airliners impart lower accelerations to larger amounts of gas - higher bypass ratios gas turbines and propellers.
The engine is not "sucked forward" as other threads have alluded to - they are propelled forward by the acceleration of gas through some kind of nozzle (momentum change).
This is also demonstrated by the reaction on a constricted hosepipe. Where is the pressure on the "forward wall" of a hose? At the pumping station. Where is the acceleration of the water? In the nozzle. So where is the force developed? In the nozzle, which is why the nozzle flicks around, not the rest of the hose.
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I think there is a tad more to it than thrust always drops off from the static case as aircraft speed builds up.
The intake has a part to play in obtaining the eventual compressed air that the engine uses to keep itself pumped up (using the everlasting balloon analogy) While the increase in efficiency of overall compression due to the intake as speed initially builds may not be huge, the compressor can still do better when it does not have to suck all the air in. A bit of ram is helpful.
Once the speed builds the increasing drag effects will quite soon overwhelm the benefits of feeding the compressor better. Once you are supersonic though its all change and the intake provides perhaps 40% of the necessary compression – check with WOK if you doubt that.
The intake has a part to play in obtaining the eventual compressed air that the engine uses to keep itself pumped up (using the everlasting balloon analogy) While the increase in efficiency of overall compression due to the intake as speed initially builds may not be huge, the compressor can still do better when it does not have to suck all the air in. A bit of ram is helpful.
Once the speed builds the increasing drag effects will quite soon overwhelm the benefits of feeding the compressor better. Once you are supersonic though its all change and the intake provides perhaps 40% of the necessary compression – check with WOK if you doubt that.
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If what you say is true then you are stating that the jet exhaust propels you forward and not the pressure differential between the forward wall of the combustor can and the lower pressure due to mass acceleration. If you had a means of generating gas pressure separate from the nozzle and introduced that pressure the gas would be expelled from the nozzle and the reaction would move the nozzle forward. That is how a pneumatic tip jet helicopter works. The gas pressure inside the engine drives a compressor and the compressed air drives the blade forward. I think there was some Greek or Roman by the name of Hero that demonstrated this theory. Werner Von Braun and later on a company called Rocketdyne adapted this principle and sent men to the moon (assuming you believe that). I think Mr. Newton said that for every action there is an equal and opposite reaction. In this case, the expelling of the gas from the combustor creates this differential and it is the force imbalance between the forward wall of the containing vessel and the exit nozzle. The faster you can expel the gas the greater the pressure drop at the nozzle which increases the differential. Since the containing vessel is attached to the conveyance it propels, the conveyence is moved.
I saw on another posting on this thread and it had as picture of a jet engine
with the pressure levels at different points in the engine. If I remember correctly, the internal pressure peak in the combuster was 250 PSIA. As I said previously the internal pressure in an Atlas rocket engine was 600 PSIA and that created 150,000 pounds of thrust. Efficiency in a rocket engine is measured by dividing the thrust by the amount of fuel and oxidizer that is pumped into the engine. It is refered to as ISP or Specific Impulse. If you are right, then Von Braun, Rocketdyne and all of the other rocket scientists are wrong.
Here is a point I should have brought out earlier in this posting. Are you familiar with a pulse jet. If you are old enough you would remember that they did a lot of damage in the UK and parts of Europe. These jets are started up on the ground. Just before start up, you could look through the engine from front to back with minimal restriction. However once the combustion pressure builds louvers at the front of the engine are closed due to the increase of pressure. Under full combustion the pressure builds and the gas is ejected rearward and the the V 1 is shot off a ramp at a very high cyclical rate the internal pressure drops and the louvers open letting in ram air at a fairly high pressure. Ignition takes place and the louvers close allowing the pressure level to raise again propelling the rocket forward. If the louvers did not create a forward wall to allow pressure build up the rocket would have fallen into the water or, not even started. It is that forward wall that takes the reactive force and transfers it to the airframe structure. A more simple system is the ram jet which has no moving parts. However it is the ram effect that creates the forward wall just like the pulse jet louvers. Only in this case the propulsive force is much greater as is the speed of the rocket.
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The Cat
[This message has been edited by Lu Zuckerman (edited 28 October 2000).]
[This message has been edited by Lu Zuckerman (edited 28 October 2000).]
I saw on another posting on this thread and it had as picture of a jet engine
with the pressure levels at different points in the engine. If I remember correctly, the internal pressure peak in the combuster was 250 PSIA. As I said previously the internal pressure in an Atlas rocket engine was 600 PSIA and that created 150,000 pounds of thrust. Efficiency in a rocket engine is measured by dividing the thrust by the amount of fuel and oxidizer that is pumped into the engine. It is refered to as ISP or Specific Impulse. If you are right, then Von Braun, Rocketdyne and all of the other rocket scientists are wrong.
Here is a point I should have brought out earlier in this posting. Are you familiar with a pulse jet. If you are old enough you would remember that they did a lot of damage in the UK and parts of Europe. These jets are started up on the ground. Just before start up, you could look through the engine from front to back with minimal restriction. However once the combustion pressure builds louvers at the front of the engine are closed due to the increase of pressure. Under full combustion the pressure builds and the gas is ejected rearward and the the V 1 is shot off a ramp at a very high cyclical rate the internal pressure drops and the louvers open letting in ram air at a fairly high pressure. Ignition takes place and the louvers close allowing the pressure level to raise again propelling the rocket forward. If the louvers did not create a forward wall to allow pressure build up the rocket would have fallen into the water or, not even started. It is that forward wall that takes the reactive force and transfers it to the airframe structure. A more simple system is the ram jet which has no moving parts. However it is the ram effect that creates the forward wall just like the pulse jet louvers. Only in this case the propulsive force is much greater as is the speed of the rocket.
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The Cat
[This message has been edited by Lu Zuckerman (edited 28 October 2000).]
[This message has been edited by Lu Zuckerman (edited 28 October 2000).]
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Just read the text books. Don't get confused between compressible and incompressible. It alters things a bit, but not the fundamentals. These engines/motors are momentum change devices, and they rely on Newton's Laws of motion.
Also, if the hose were in a straight line, the force would still be there, but the hose would be restrained by the body of the hose itself!
All the basic equations of thrust, as shown in previous threads, relate to mass flow, and the exhaust velocity and the forward velocity, and the density, and things that affect density, like temperature (remember the thread about water injection - it's a bit like an intercooler in a car, it cools the air after compression and raises the density. It also drops the pre-combustion temperature, which increases the change in temperature pre- and post-combustion. This is a fundamental determinant of thermodynmaic efficiency).
Don't take my word for it, read a physics text book, or "The Jet Engine" by Rolls-Royce.
Also, in terms of the original question, am I right in thinking that the combustor of an RR Trent 700 or the like is working at a rate of something like 170MW (I'm not so sure about this - I read it on the back of a matchbox) - try relating that to a power station or the power consumption of a town.
Also, if the hose were in a straight line, the force would still be there, but the hose would be restrained by the body of the hose itself!
All the basic equations of thrust, as shown in previous threads, relate to mass flow, and the exhaust velocity and the forward velocity, and the density, and things that affect density, like temperature (remember the thread about water injection - it's a bit like an intercooler in a car, it cools the air after compression and raises the density. It also drops the pre-combustion temperature, which increases the change in temperature pre- and post-combustion. This is a fundamental determinant of thermodynmaic efficiency).
Don't take my word for it, read a physics text book, or "The Jet Engine" by Rolls-Royce.
Also, in terms of the original question, am I right in thinking that the combustor of an RR Trent 700 or the like is working at a rate of something like 170MW (I'm not so sure about this - I read it on the back of a matchbox) - try relating that to a power station or the power consumption of a town.
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You ask if I am familiar with a pulse-jet, with its spring-loaded louvres. I am, but are you familiar with a ramjet, which has no forward walls at all? How do you explain that?
The engine designer uses a subtle blend of pressure, velocity and temperature distributions, something along the line of Bernoulli's theorem. The highest pressure, is actually just aft of the compressor, and before the combustor cans or rings, not in the combustion chamber.
Speeding gas up drops its pressure, while slowing it down increases it. That is why there are components with names like nozzles and diffusers. Diffusers expand the gas, slow it down, and drop the static pressure. Nozzles tend to do the opposite, and direct the flow in the desired direction.
In spite of the general acceleration of gas in all of these engines which creates a forward force, there is an element in rocket engines and afterburners where the acceleration of the gas is not the only force. There is also an element, especially in rocket engines, where the gas exerts pressure on the divergent part of a convergent/divergent nozzle which als propels it forwards.
Of course I believe in Herr von Braun, and of course I recognise the use of rockets. I'm not so sure that you have interpreted them correctly.
Again, I refer you to "The Jet Engine" by Rolls-Royce, especially the Chapters on "Basic Mechanics", "Working Cycle and Airflow", "Afterburning" and "Performance".
I have only relayed what they say, so if you have a better idea, then let them know
This is a good discussion, because everyone truly seems to have a different interpretation. Even the folks who make the things have good arguments about it from time to time, but don't let that put you off flying!
The engine designer uses a subtle blend of pressure, velocity and temperature distributions, something along the line of Bernoulli's theorem. The highest pressure, is actually just aft of the compressor, and before the combustor cans or rings, not in the combustion chamber.
Speeding gas up drops its pressure, while slowing it down increases it. That is why there are components with names like nozzles and diffusers. Diffusers expand the gas, slow it down, and drop the static pressure. Nozzles tend to do the opposite, and direct the flow in the desired direction.
In spite of the general acceleration of gas in all of these engines which creates a forward force, there is an element in rocket engines and afterburners where the acceleration of the gas is not the only force. There is also an element, especially in rocket engines, where the gas exerts pressure on the divergent part of a convergent/divergent nozzle which als propels it forwards.
Of course I believe in Herr von Braun, and of course I recognise the use of rockets. I'm not so sure that you have interpreted them correctly.
Again, I refer you to "The Jet Engine" by Rolls-Royce, especially the Chapters on "Basic Mechanics", "Working Cycle and Airflow", "Afterburning" and "Performance".
I have only relayed what they say, so if you have a better idea, then let them know
This is a good discussion, because everyone truly seems to have a different interpretation. Even the folks who make the things have good arguments about it from time to time, but don't let that put you off flying!
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To: SRR99
If you will refer to several threads above I stated that pressure is exerted at 90 degrees to the containing vessel and I also referenced a ram jet engine. Regarding the pressure acting at 90 degrees to the containing vessel this applies also to the DeLavall nozzle used on a rocket engine.
Some of the force is exerted at a vector in relation to the center line of the rocket engine so that some of the forward thrust is exerted by the expanding gas in the nozzle itself. However, the greatest forward thrust is exerted on the forward wall of the rocket engine or to be more specific at the fuel and oxidizer injection plate.
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The Cat
If you will refer to several threads above I stated that pressure is exerted at 90 degrees to the containing vessel and I also referenced a ram jet engine. Regarding the pressure acting at 90 degrees to the containing vessel this applies also to the DeLavall nozzle used on a rocket engine.
Some of the force is exerted at a vector in relation to the center line of the rocket engine so that some of the forward thrust is exerted by the expanding gas in the nozzle itself. However, the greatest forward thrust is exerted on the forward wall of the rocket engine or to be more specific at the fuel and oxidizer injection plate.
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The Cat
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Lu,
I refer you again to the chapters in "The Jet Engine" by Rolls-Royce. Are they wrong?
They also seem to think that they know something about rockets, although they don't seem to have the knowledge that you do.
Both GE and Rolls-Royce books refer specifically to the mass flow and acceleration.
Perhaps you could argue that as the fluid naturally goes from an area of high pressure to one of low pressure then it accelerates. You can calculate the thrust or drag of an aircraft engine in each section (inlet, compressor etc), but the overall engine works on the acceleration of gas. Even turbine stages rotate not only by changing the direction of flow, but also by presenting a converging path to the gas which accelerates the gas and forces the rotor in the opposite direction.
I don't know your experience and you don't know mine, but like I say, read the book, and then tell me where you agree or it's wrong.
I refer you again to the chapters in "The Jet Engine" by Rolls-Royce. Are they wrong?
They also seem to think that they know something about rockets, although they don't seem to have the knowledge that you do.
Both GE and Rolls-Royce books refer specifically to the mass flow and acceleration.
Perhaps you could argue that as the fluid naturally goes from an area of high pressure to one of low pressure then it accelerates. You can calculate the thrust or drag of an aircraft engine in each section (inlet, compressor etc), but the overall engine works on the acceleration of gas. Even turbine stages rotate not only by changing the direction of flow, but also by presenting a converging path to the gas which accelerates the gas and forces the rotor in the opposite direction.
I don't know your experience and you don't know mine, but like I say, read the book, and then tell me where you agree or it's wrong.
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I seem to vaguely remember hearing once that a 747 engine expels something like about 7 tonnes of air in its take off run, or per minute, or something like that. I can't remember exact details. Is that
true, or just a load of hot air?
Help me out here.....
true, or just a load of hot air?
Help me out here.....