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Series Of Turbofan-Turbojet Engine Questions  
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Posted (10 years 2 months 1 week 3 days 20 hours ago) and read 5807 times:


  1. But what takes over as a plane approaches its max, speed of thrust or force of thrust? I.e. which will have the greatest impact if increased?

  2. Is the engines max thrust speed the flight speed of the engine had it been given wings to fly on it's own?

  3. I have heard (and read) of turbofan after-burning engines. I can understand turbojets with thrust max's out near Mach 3 as possible, but bypass flows are subsonic and no convergent nozzle can accelerate flow fast enough to match the core of the turbine. The exhaust of the engine now has fast gas moving at the center and a doughnut shaped slow moving air mass surrounding it. How fast is the thrust moving and will greater by-pass ratios kill off the thrust's speed and/or thrust's force?

  4. Speaking of which, are the afterburner injectors located just aft of the turbine section? I am assuming the core of any turbofan is a turbojet, so then is a turbofan afterburner just a turbojet afterburner with a fan in front or does it include the colder by-pass air mass in the secondary burn?

  5. With regards to the former HCST, is a 330 kN turbojet possible considering that the core thrust of a GE90-115b is less than 100 kN? How would it have been a better engine choice in that specific application?

  6. How much more complex is a turbofan afterburner when compared to a turbojet version?




The meaning of life is curiosity; we were put on this planet to explore opportunities.
24 replies: All unread, jump to last
 
User currently offlineGrandTheftAero From United States of America, joined Nov 2003, 254 posts, RR: 5
Reply 1, posted (10 years 2 months 1 week 3 days 19 hours ago) and read 5554 times:

1. I'm not sure I understand this question. What the heck is "speed of thrust"?

2. There is no such thing as max speed for an engine. The engine will fly at whatever speed you need it to as long as the inlet is designed properly, i.e. the inlet properly slows the air from freestream velocity to a speed that can be handled by the engine.

3. The speed of the bypass air flow is not important. The only thing that matters is that pressure at the exit of the bypass duct is equal to the pressure at the exhaust of the core. If this condition is satisfied, then flow from the core will not back up into the bypass duct, or vice versa. Large bypass-ratios are possible but the above condition must be satisfied. In my college senior design course I designed an afterburning turbofan around a GE90 core with a bypass ratio of 7.6 (!!!) for the AIAA's subsonic ultra-heavy lift aircraft RFP. The only way to satisfy the equal-pressure condition was to have a two-stage fan to generate a large enough fan exhaust pressure.

4. You are correct. The afterburner injectors are aft of the last turbine. And an afterburning turbofan in either arrangement is possible. You can build one where the bypass air does not enter the afterburner duct or you can build one where the two flows combine and enter the afterburner.

5. How do you know what the core thrust of the a GE90 is? Anyway, the HSCT (I assume you mean the Boeing 2707) was to be powered by the GE4 turbojet. It was a HUGE, I mean HUGE, BIGNORMOUS turbojet! The GE90 core, if used as a turbojet, would be dwarfed if compared side by side. In addition to just the difference in shear size, the GE90 core was not meant to be a stand alone propulsion system. It was intended to take air from the fan module, add enthalpy, and then push it out to a set of low-pressure turbines. It would not make a good propulsion system if used as a stand alone device... although I suppose it could be done with the appropriate modifications. I don't understand the second part of this question, please elaborate.

6. Not that much more complex... but the equal-pressure condition must be satisfied (see #3).

Hope this helps.

--Shane


User currently offlineGrandTheftAero From United States of America, joined Nov 2003, 254 posts, RR: 5
Reply 2, posted (10 years 2 months 1 week 3 days 18 hours ago) and read 5541 times:

I have to retract a previous statement I made.

"And an afterburning turbofan in either arrangement is possible. You can build one where the bypass air does not enter the afterburner duct or you can build one where the two flows combine and enter the afterburner."

While it is theoretically possible to build an afterburning turbofan where the bypass air does not enter the afterburner duct, I don't think I've ever seen it done in practice. Furthermore I don't know why you would want to do something like this. I don't see any obvious benefit over the other arrangement where the two flows combine and enter the afterburner.

Sorry for any confusion.

--Shane


User currently offlineB2707SST From United States of America, joined Apr 2003, 1369 posts, RR: 59
Reply 3, posted (10 years 2 months 1 week 3 days 15 hours ago) and read 5525 times:

Some technical info, not sure if this will help:

The HSCT's engines were going to be low-bypass turbofans in order to clear noise restrictions and achieve greater subsonic efficiency. The bypass ratio was not released, but published Japanese studies recommend a BPR of 0.7-1.0 for a Mach 2.0 aircraft and 0.4-0.5 for Mach 2.4. The engine would not have incorporated an afterburner. According to NASA, the inlet-engine-nozzle assembly would have been 50 feet long and weighed 8-9 tons.

The GE4 was a single-shaft turbojet with nine compressor and two turbine stages. The GE4/J5P variant for the Boeing SST was 25 feet 8 inches long, had a diameter of 74.2 inches, and weighed 10,500 lbs. (unclear if this includes nozzle assembly). Maximum afterburning thrust was 63,200 lbs. initially, with anticipated growth to over 75,000 lbs. Mass flow was 620 lbs./sec. Fuel consumption at Mach 2.7 cruise averaged about 22,000 lbs. per engine per hour. Using the Breguet range equation to estimate SFC gives 1.52, so cruise thrust was about 14,475 lbs.

Prototype GE4s attained over 55,000 lbs. of thrust, making them the largest turbojets ever built (AFAIK). Late in the program, GE and Boeing decided to drop the afterburner to cut complexity and noise. They planned to grow the engine diameter by two inches and increase mass flow to compensate, to between 720 and 815 lbs./sec. This changed would have raised the 2707-300's takeoff weight by 40,000 lbs. to almost 800,000 lbs. No revised GE4s were built before the program was axed. Incidentally, the Concorde B's uprated Olympus 593s also would have been burner-less.

--B2707SST



Keynes is dead and we are living in his long run.
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 4, posted (10 years 2 months 1 week 3 days 4 hours ago) and read 5451 times:

" I'm not sure I understand this question. What the heck is "speed of thrust"?"

How about exhaust velocity, does that make better sense? Doesn't the by-pass ratio have an effect on exhaust velocity considering fan-thrust will always be subsonic? Another technical error on my part as when refering to the former HCST, I meant the recently cancelled (spring 1999) Boeing/NASA plane with the bus-sized square-exhaust mufflers. As B2707SST stated with GE4 engine performance numbers for a turbojet, I can imagine that the NASA/Boeing HCST engine had to have been massive, it must have been able to dwarf the GE4...

The second part of #5 asked for the difference between installing a turbofan AB rather than a turbojet AB on a large sonic transport.

"The engine will fly at whatever speed you need it to as long as the inlet is designed properly, i.e. the inlet properly slows the air from freestream velocity to a speed that can be handled by the engine."

Hold on, are you saying that provided the inlet issues were dealt with properly, then ANY jet engine can make a aircraft fly at ANY speed?

"While it is theoretically possible to build an afterburning turbofan where the bypass air does not enter the afterburner duct"

As you have stated for #3, I now would have suspected a serious pressure differentiation once the afterburner is lit, even if it was balanced, it won't be once the burner is off. Still, couldn't noise reduction be one benefit? That way, regardless of AB operation, there is a large by-pass keeping the noise down. Now that I think about it, if the main by-pass fan had variable pitch, as much of a technical nightmare it could have beeen, would that be sufficent to deal with the difference in pressure levels with AB engine arangement separate from the by-pass flow?


Thank you for all your responses, they did a lot, more to come though.  Smile.


[Edited 2004-02-14 23:12:16]


The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineGrandTheftAero From United States of America, joined Nov 2003, 254 posts, RR: 5
Reply 5, posted (10 years 2 months 1 week 2 days 18 hours ago) and read 5404 times:

"Hold on, are you saying that provided the inlet issues were dealt with properly, then ANY jet engine can make a aircraft fly at ANY speed?"

Yes, a gas turbine engine doesn't "care" what it's installed in (test cell, airplane, boat, powerplant, etc) or how fast it happens to be moving. The only thing that matters is that the air being fed into the front of the engine is conditioned to the speed that the engine is "expecting". This is usually close to stagnation, i.e. zero velocity.

Here's an important explanation that will lead into a better answer to your question... thrust is the product of two quantities: the velocity of the engine exhaust and the mass flow rate of the engine exhaust (this is simply Netwon's Second Law). You can increase thrust by increasing either quantity. Turbojets generate thrust with high-velocity, low mass flow rate exhaust whereas turbofans generate thrust with low-velocity, high mass flow rate exhaust. The key to efficiency is having higher mass flow rate with lower velocity exhaust. That's why turbofans win in the efficiency game.

Now, lets go back to our "any jet engine in any airplane" scenario. Lets say we have a twin-engine supersonic fighter and a twin-engine subsonic transport that both happen to require engines that produce the same amount of thrust. According to my argument in the first paragraph of this post we could theoretically design one engine type to hang on both airplanes but we know designers don't do this in real life. In reality a turbojet (or more likely a low-bypass turbofan) with a small frontal area would be designed for the supersonic fighter for aerodynamic and stealth reasons. And a high-bypass turbofan would be designed for the subsonic transport for efficiency reasons. It is for these reasons and more that designers don't use just any engine for any plane at any speed even though it could be done with extremely inefficient results.

Hope that helps.

--Shane

P.S. Now that you have the fundamental equation for thrust (the product of mass flow rate and velocity) maybe you can go back and try to figure out some of your questions on your own. If you need help, I, and others, will be here =)


User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 6, posted (10 years 2 months 1 day 8 hours ago) and read 5248 times:

Sorry I have not been around, I do not have internet at home anymore.

anyway, the replies you gave are very useful and the reason I asked about the thrust-speed thing was this: in rocket engines, there is no intake, everything is based on propellants. They have a mass flow rate, exhaust pressure over an area, and an exhaust velocity. According to NASA, the max thrust velocity of the space shuttle's main engines in around 4000 m/s, but orbital velocity is closer to 8000 m/s, how the hell does that work out?!

I'm sure it has the force to accelerate, but are the propellents coming out fast enough to maintain a velocity higher than the engine itself?

This was why I asked, I wondered if the engine's Vout would be the theoretical outright max possible speed of a vehicle attached to an enigne if there was no drag on the vehicle.

MY POINT: In other words, can an increase in the exhaust mass flow rate increase the exhaust Vout?




The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineQantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 26
Reply 7, posted (10 years 2 months 1 day 5 hours ago) and read 5232 times:

According to NASA, the max thrust velocity of the space shuttle's main engines in around 4000 m/s, but orbital velocity is closer to 8000 m/s, how the hell does that work out?!


While the shuttle is in orbit, it's engines aren't doing the propulsive work. They're there to get the shuttle out of the atmosphere, but while it's in orbit the gravity of the earth is inducing it's huge elliptical motion, not the engines. During orbit the engines are only used to make small directional corrections.

Cheers,
QantasA332



User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 8, posted (10 years 2 months 1 day 4 hours ago) and read 5217 times:

If you accelerate something, anything, towards the rear of the vehicle, the vehicle will accelerate forward.

Picture that you are in a train moving at 50 km/h and stand on a skateboard. As you step off the skateboard to the rear of the train, you won't be moving at anywhere near 50, but the skateboard willl still move forward as you do. Imagine a train at 100 km/h and you sitting in a cart with a bowling ball. Throw the bowling ball towards the rear of the train. You will not throw it at over 100 km/h, but the cart will still accelerate forward.

The problem with a jet engine is that the air coming in is at a speed equal to the airspeed. The engine is only able to produce a limited exhaust speed. When the speed of the incoming air approaches that of the air exhausted, the acceleration of the air approaches zero. No acceleration of the air, no acceleration of the aircraft.

Put yourself in a cart rolling by a line of bowling balls. You pick up balls and throw them to the rear. When the speed of the cart is equal to that of the maximum speed at which you can throw a ball, acceleratin the ball to the speed of the cart when picking it upl will slow you down exactly as much as throwing it speeds you up.

A rocket brings it's own pile of bowling balls.

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineLMP737 From , joined Dec 1969, posts, RR:
Reply 9, posted (10 years 2 months 1 day 3 hours ago) and read 5215 times:

Here's an interesting little bit of info regarding afterbuners. When afterbuner is selected you will see a small drop in your TIT. This is due to the increase in airflow.

User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 10, posted (10 years 2 months 1 day 3 hours ago) and read 5215 times:

"While the shuttle is in orbit, it's engines aren't doing the propulsive work"

Please correct me if I'm wrong QantasA332, but it's the launch sequence of the space shuttle using main engines and 2 SRB's until they burn out within 2 minutes and the rest of the 9 minute journey is driven soley by the main engines via main fuel tank. It is those engines, the 3 at the shuttle's rear, that have a Vout max of 4000 m/s. My inquiry was with regards to wondering how the shuttle gets up into orbit in the first place if what comes out of the engines is no where near the orbital speed.

Technically, I assume that in order for those Apollo missions to have worked, one of those stages has got to have had a velocity exhaust beyond Earth's escape velocity of 11,100 m/s.

Or does speed have nothing to do with it and F=ma suddenly takes over??? what about transition?

Heck I read that Concorde's engines were actually efficient up to Mach 3, doesn't that mean the outright max thrust velocity of the engine exceed 2000 mi/h?

"You will not throw it at over 100 km/h, but the cart will still accelerate forward."

FredT, you're impling a different reference frame; using that example, can throwing the ball accelerate the train as a whole? If not, I rest my case.

"No acceleration of the air, no acceleration of the aircraft."

So an extra burst of mass flow by dumping an unecassary amount of fuel is not going to give you any extra boost thereby making the plane's speed faster than the gas out the back?


I still don't get it, WTF......?



The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineKlaus From Germany, joined Jul 2001, 21353 posts, RR: 54
Reply 11, posted (10 years 2 months 1 day 3 hours ago) and read 5207 times:

The exhaust velocity is not a limiting factor because it´s relative to the current speed of the vehicle. The accelerating impulse would remain unchanged (when disregarding drag and relativistic effects);

The only problem is that a large part of the total fuel must be spend to accelerate the remaining fuel and the engine to that current speed...


User currently offlineQantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 26
Reply 12, posted (10 years 2 months 20 hours ago) and read 5163 times:

Technically, I assume that in order for those Apollo missions to have worked, one of those stages has got to have had a velocity exhaust beyond Earth's escape velocity of 11,100 m/s.

You must understand that escape velocity is the initial speed of something from a brief thrust, after which no force is provided. A body can escape the Earth at any speed greater than zero provided its engines supply thrust for a long enough period of time. If a rockets engines burnout close to earth, speeds of 11.2 km/s would be necessary to escape, but if their power is prolonged, speeds less than that are acceptable.

Sorry I'm late replying,
QantasA332


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 13, posted (10 years 2 months 9 hours ago) and read 5112 times:

Lehpron,
exactly, much better example. If you sit in a train moving at 100 km/h and throw a bowling ball to the rear of the train, you will accelerate the train even if you do not throw the ball at anywhere near 100 km/h.

If you dumped a lot of fuel (or water, any mass will do) into the air stream, it would be accelerated rearwards from zero velocity in the aircraft's reference frame, as opposed to accelerated from whatever the airspeed is. Alas, acceleration... IF you can achieve it without decelerating the rest of the air going through the engine.

Conservation of momentum, all the way.

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 14, posted (10 years 2 months 6 hours ago) and read 5062 times:

My replies have been tardier. I try to discriminate the different references in my mind:

  • A rocket engine operating in space


  • A rocket engine operating in the earth's atmosphere


  • An axial flow engine operating in the earth's atmosphere


  • An axial flow engine operating in space (magnetic-ionic comes to mind.)


  • Momentum; I do believe that I may have forgotten the definition of which. What was it again, mass times velocity? Force per unit time? Isn't that an impulse, like a burst or a pulsed detonation? Technically in any of those four scenarios, momentum should be conserved.

    "...you will accelerate the train even if you do not throw the ball at anywhere near 100 km/h. "

    LAMO! Are you freaking kidding me? Dude, if you've got thrust leaving slower than it came in, that sounds like a drag engine. You're not accelerating, you're DEaccelerating! ...wait, is that what you mean? Deceleration is just negative acceleration, but it is still an acceleration...so the train will slow down. Even if I throw balls at an infinite rate, I’ll never keep the train from slowing down, but that's if balls were going into the train at the front.

    Yeah that is a good example and I'm really trying hard to think about it, maybe that is what I shouldn't do, I guess I need to differentiate where the atmosphere ends to get this properly; in my mind, in any of those scenarios mentioned above, I can't balance drag. The speeds don't make direct sense [to me just yet].

    " A body can escape the Earth at any speed greater than zero provided its engines supply thrust for a long enough period of time"

    I understand this point as a matter of being farther from a planetary body the gravity drops and as long as the ship as more thrust that weight, as you go farther both gravity and the escape and orbital speeds drop (out near the moon its a mile a second the orbit i think), but wouldn't you still technically need a thrust speed greater than...something...the exit mass flow has to be moving. Against what I don't know. In the atmosphere it’s against the air[speed].


    I guess I can't design engines yet.  Wink/being sarcastic



    The meaning of life is curiosity; we were put on this planet to explore opportunities.
    User currently offlineMender From United Kingdom, joined Feb 2004, 237 posts, RR: 0
    Reply 15, posted (10 years 2 months 6 hours ago) and read 5058 times:

    It's worth going back to basics here as the principle of thrust can be confusing

    I find the easiest way to describe the principle of thrust is to think of a child's balloon. When you inflate the balloon and pinch the neck you have equal pressure in every possible direction inside the balloon. Because each line of force (pressure) has an equal and opposite force/reaction the balloon will not move. Newton's law. OK gravity makes the balloon fall but you know what I mean.

    When you let go of the neck of the balloon the air rushing out does not oppose the pressure pushing on the front surface of the balloon, thus the balloon goes forward. This principle applies to toy balloons, turbine engines and the rocket motors on the Shuttle. Isaac was a clever bloke when you think about it.

    In all turbine engines the air between the front of the first compressor blade and the last turbine blade MUST be below the local supersonic speed. A good intake duct can control the intake velocity and keep it below supersonic speeds. The exhaust nozzle can increase the velocity of the exhaust beyond the speed of sound but it cannot be above the speed of sound when the gas flows across the last turbine blade.


    User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
    Reply 16, posted (10 years 2 months 6 hours ago) and read 5051 times:

    Lehpron,
    tell me when you have stopped laughing, and I might feel like trying to explain just where you did go wrong in your thinking.

    /Fred



    I thought I was doing good trying to avoid those airport hotels... and look at me now.
    User currently offlineKlaus From Germany, joined Jul 2001, 21353 posts, RR: 54
    Reply 17, posted (10 years 2 months 5 hours ago) and read 5037 times:

    Lehpron: Momentum; I do believe that I may have forgotten the definition of which. What was it again, mass times velocity? Force per unit time? Isn't that an impulse, like a burst or a pulsed detonation? Technically in any of those four scenarios, momentum should be conserved.

    Unless, that is, you´re introducing a force (with a rocket, by means of a chemical/thermic reaction).

    Conservation of momentum is based on a force-less situation.


    Lehpron: LAMO! Are you freaking kidding me? Dude, if you've got thrust leaving slower than it came in, that sounds like a drag engine. You're not accelerating, you're DEaccelerating!

    No, actually not.

    Your problem is that you´re thinking from the external reference system. But what´s happening is that in "throwing the first ball" you´re exerting a force coming from chemical energy (converted in your arm muscles) to accelerate the whole vehicle, including the unburnt fuel / balls that haven´t been thrown yet.

    When you´re picking up the next ball, it is already moving along with the vehicle from the first acceleration (very slowly, probably.  Wink/being sarcastic)

    And when you´re throwing it backwards, you´re converting more chemical energy into an impulse to the vehicle (and to the ball you´ve thrown, that´s where action/reaction comes in). You´ll have no more energy or no more balls to throw, at some point, but as long as you´ve got some of both left, you can accelerate indefinitely (ahem  Wink/being sarcastic).


    The trick is that you´re pumping some of the energy from your muscles into the forward momentum by pushing the "exhaust mass" backwards. None of those factors has any dependency on how fast you´re already going relative to the ground - it only depends on:

    a) how much mass you´re throwing (the mass flow rate)

    b) the acceleration of that mass

    That´s why you can have the same effect by accelerating a large mass at a slower rate or a smaller mass at a higher rate to achieve the same net effect... basically what a rocket engine distinguishes from an ion engine. The rocket engine uses its fuel for both energy source and exhaust mass, the ion engine uses electrical energy from solar cells (thereby eliminating the need to carry the energy source with it) and very small amounts of a chemically inert, ionized gas as exhaust mass (the only really exhaustable resource).

    Both could get you to any speed if there was only enough energy and mass (and no drag or relativistic effects). The only reference system is the ship; The earth´s reference system (ideally) just doesn´t come into the equation any more when you´re moving through the vacuum of space. (close to the planet, you´ll still have drag and gravitation to overcome, of course.)

    The only relevant factor is the impulse generated by the acceleration of mass. The relative speeds are basically irrelevant.


    Aircraft engines, by contrast, constantly interact with the earth´s inertial reference system because they´re hitting the air (drag) and using it for mass flow (different here, because the air is standing still, not moving along with the plane), injecting more mass and the energy through the chemical fuel (which at least moves at the speed of the plane, already, although that doesn´t make a big difference here due to the massive amount of drag, by comparison).

    (minor correction)

    [Edited 2004-02-24 22:28:00]

    User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
    Reply 18, posted (10 years 2 months 5 hours ago) and read 5039 times:

    Sorry FredT, that sounded like a backward thought, it still does. Tell me through this example:


    An empty convergent nozzle, by itself in motion. the air it runs into subjects a force in the opposite direction of motion - drag. Yes the convergent nozzle is speeding up the airflow but not all the energy is completely transfered. The exit air is moving slower than the surrounding and the intake. it's pure delta-v drag. Even if a venturi tube was sent into motion to reacelerate the flow on the other end, there would be a loss of energy such that the whole system is being slowed.

    Yeah like you said if a fluid mass was suctioned by the drop in pressure at the throat of the tube, then there should be an added mass flow to the system. I do not know the condistion after that and will not comment.



    The meaning of life is curiosity; we were put on this planet to explore opportunities.
    User currently offlineMender From United Kingdom, joined Feb 2004, 237 posts, RR: 0
    Reply 19, posted (10 years 1 month 4 weeks 1 day 5 hours ago) and read 4948 times:

    Just in case anyone missed an important point in my earlier post.

    "In all turbine engines the air between the front of the first compressor blade and the last turbine blade MUST be below the local supersonic speed"

    By the phrase LOCAL I didn't mean the speed of sound outside McDonald's I meant the supersonic speed of the exhaust gas. This being rather hot is good deal faster than the airframe Mach 1 speed.


    User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
    Reply 20, posted (10 years 1 month 4 weeks 23 hours ago) and read 4923 times:

    whoever just emailed, do it again, my spamblocked knocked it off.


    The meaning of life is curiosity; we were put on this planet to explore opportunities.
    User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
    Reply 21, posted (10 years 1 month 2 weeks 2 days 6 hours ago) and read 4795 times:

    F = mass * acceleration
    = (mass flow) * (flow velocity)
    = (mass flow) * (delta velocity)
    = (air mass flow in + fuel mass flow out) * (velocity out - velocity in) + (exhaust pressure * exhaust area)

    etc, etc, so on and so forth.

    Could I take a glider (like a Cirrus), attach small rockets to the side (without comprimising area rule) and make it go Mach, provided the thrust was not moving past Mach and the there was enough force to do it?


    If I don't get it by now...forget it, I do not see how this could work.



    The meaning of life is curiosity; we were put on this planet to explore opportunities.
    User currently offlineKlaus From Germany, joined Jul 2001, 21353 posts, RR: 54
    Reply 22, posted (10 years 1 month 2 weeks 2 days 5 hours ago) and read 4777 times:

    As said above, with rockets there´s no "speed limit"; The exhaust velocity is always relative to the already moving craft and will always exert an accelerating force, regardless of how fast you´re going already. Within the atmosphere drag will be the only thing to keep you from accelerating indefinitely. That and lack of fuel.  Wink/being sarcastic

    With air-breathing engines, the larger part of the the mass to be pushed out of the engine isn´t carried with he plane but already has a high speed relative to the aircraft, so it gets progressively more difficult to accelerate this air even faster than it´s going already. The drag incurred by interacting with the moving air at all (fan/compressor/turbine blades cutting through the air/gas stream) is growing with speed, while the net gain (the additional acceleration fo the air) is decreasing.

    A rocket engine has it easier: All the mass it´s pushing out is at a standstill relative to the craft before entering the combustion chamber. So (ideally) there´s no "engine drag" (due to no interaction with any outside air) and a very high acceleration component independent of airspeed (which - ideally - doesn´t enter the equation at all as far as the engine is concerned; Also the reason why it still works beyond the atmosphere). Too bad it has to carry all the expelled mass with it to pay for these advantages...  Wink/being sarcastic


    User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
    Reply 23, posted (10 years 1 month 2 weeks 1 day 1 hour ago) and read 4714 times:

    So wait, say an airbreathing turbofan had a small liquid oxygen spray (reactant mass not moving relative to the plane - like a rocket) in it's combustion chamber, would that provide an increase in plane's motion due to increased force despite relative motion of exiting mass flow?




    The meaning of life is curiosity; we were put on this planet to explore opportunities.
    User currently offlineKlaus From Germany, joined Jul 2001, 21353 posts, RR: 54
    Reply 24, posted (10 years 1 month 2 weeks 1 day ago) and read 4700 times:

    Yes, it would increase thrust a little, at the (prohibitive) cost of carrying it around in the first place. Basically similar to reheat/afterburners, although those get you more thrust, as long as you´re using the atmospheric oxygen anyway.

    If you´d rely exclusively on onboard oxygen, you´d have a rocket engine again. With the unlimited acceleration advantage and the corresponding efficiency disadvantage.

    Subsonically, the drag component in a turbine engine is still low enough to rather use a big fan for a higher mass flow instead.


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