Naked Science Forum
Non Life Sciences => Technology => Topic started by: Mike Maas on 04/04/2011 21:30:02
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Mike Maas asked the Naked Scientists:
Which part of a ramjet transmits the thrust to the airframe?
What do you think?
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A simple ramjet engine has three main components. There is an intake (1), which compresses the air and decelerates it to a subsonic velocity. In the combustor (2), the fuel is injected into the air stream, ignited, and then burned. Finally, the hot exhaust stream is accelerated through a nozzle (3).
All parts of a ramjet engine contribute to thrust generation -- even the intake. The sum of the force vectors from the intake through the nozzle is collectively known as "net thrust". However, when considering where to attach a motor mount then the ramjet designer must consider the discrete force acting on each component. The greatest gross thrust force is usually generated by the nozzle, and so a motor mount is often attached to a thick structure at the interface between the combustor and the nozzle. A second mount is then placed forward on the engine, to maintain the desired thrust alignment with respect to the overall vehicle.
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Is it really accurate to say that the intake contributes to the thrust? Thrust is produced when the air/fuel mixture burns and accelerates through the nozzle. Surely some of the energy released by the combustion of the fuel has to be consumed in compressing the air in the intake. If that were not the case, the thrust would be even greater.
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Excellent question! [:)]
So, let's look at what happens at each component of a ramjet engine. Imagine that you have a supersonic airplane and it has a spare pylon. First, we'll hang a simple duct from that pylon. Let's shape the duct so that it could also serve as the intake for a ramjet.
When the airplane is flying at a supersonic speed, the duct on the pylon will behave almost exactly the same as it would on a ramjet engine. It will compress the air from the low atmospheric pressure to a much higher value, while decelerating the air stream to a subsonic condition. I said "almost exactly" the same because the duct will not experience the back-pressure that it would get from a ramjet combustor.
Now let's add a nozzle section to the duct on the pylon. Without a combustor in between the intake and the nozzle, the best that we can hope for is to return the air stream to the original (atmospheric) state. There will be some friction losses, and also some pressure losses, and so the "exhaust" will never quite return to the original pressure and velocity. As a result, our supersonic airplane will experience a small net drag due to the intake duct and nozzle that are hanging on the pylon.
For the next step, add a combustor section between the intake duct and the nozzle (don't add any fuel just yet). Simply due to inserting this section, we will lose a little more pressure and we will also add some more friction losses. Our supersonic airplane will feel a little more drag while the object hanging on the pylon behaves as an unfueled ramjet engine.
What happens when we add fuel to the combustor, and then ignite it? Well, our ramjet should start to make net positive thrust (instead of net drag). The fuel doesn't do anything to help compress the air in the intake. In fact, if you inject the fuel in the intake then the heat that is released will reduce the efficiency of the compression process.
What happens if we change the design of intake duct on our engine? Well, if it is less effective at compressing the air stream then the engine will generate less thrust. Conversely, a "better" intake design will help the engine generate more thrust.
OK, now we will make one final change. Let's replace the combustor with a new device that simply adds heat to the airflow (maybe using an electric arc). If it adds the same amount of heat as the fueled combustor, the engine thrust performance will be pretty much unchanged. The pilot of our supersonic airplane will not know that anything has changed. So, fuel is not required to help the intake compress the air.
Bringing it all home ...
- A ramjet intake will compress air just as well with or without an attached combustor.
- A ramjet combustor needs the compression from an intake to function.
- A ramjet combustor also needs a nozzle to function. In fact, without a nozzle then the internal pressure will be too low to sustain combustion.
- All three components are required for a ramjet engine to generate thrust.
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Er, I still don't get it [:D]
Compressing air requires quite a bit of work to be done on the air, and that work is a consequence of the fact that the aircraft is in motion. The only reason it is in motion is because it is burning fuel, so the work to compress the air is being done by oxidising the fuel.
Like they say, there is no free lunch.
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Not ramjets but... Screamjets, Scram?
" “What you have is a flying engine,” says McClinton, technology manager for the Hyper-X. When the supersonic speed of the airplane rams oncoming air into the combustion chamber, the engine is called a ramjet. But that works only at very high speeds. The SR-71 Blackbird, for example, has turbojets with afterburners to accelerate and cruise at supersonic speed. When the aircraft is flying fast enough to compress the air on its own, its engines remake themselves. Ducts direct part of the airstream around the rotating compressors, and the engines become ramjets that propel the airplane to Mach 3 or higher.
But ramjets, like turbojets, slow the air to subsonic speed before combining it with fuel. This minimizes formation of troublesome shock waves, but it works only up to about Mach 6. Any faster and the engine begins to melt from the effort to slow all the air down. That’s when a scramjet becomes the only option. As the air races through the engine, it is moving at supersonic speeds and generating shock waves. But a scramjet uses the shock waves to advantage.
Picture a river with banks lined with concrete. The banks reflect waves and boat wakes in the water. A scramjet combustion chamber does much the same with shock waves. On the ground, you could peer into it and it would look like an empty pipe. But once moving beyond the speed of sound, its internal design and shape orient shock waves into a precise pattern. It directs, compresses, and focuses the airflow, creating the right temperatures and pressures for combustion. A scramjet fashions the essential components of a jet engine from the air currents racing through it. Joel Sitz, NASA’s X-43A project manager at Dryden, shrugs and says: “You’re training the air to do tricks.”
The X-43A’s combustion chamber measures about three feet in length, so at a top speed of 7,000 mph, air whips through it in roughly .001 second. That’s not much time for fuel to mix with oxygen, ignite, and burn. Engineers liken the challenge to keeping a match lit in a tornado—but more difficult. If fuel burns too fast, the airflow inside the engine reverses, causing power loss—an “unstart,” in jet jargon. If it burns too slowly, it’s as if your car’s gasoline were igniting a block behind you. All its energy goes to waste.
But in the mad rush of air, the shock waves form a kind of shelter, as you would with your hands to light a candle in the wind. Into the eye of this storm, the X-43A’s engine injects hydrogen fuel and ignites it with silane, a silicon gas that instantly burns on contact with air."
From Air & Space Magazine, July 01, 2005. (http://www.airspacemag.com/flight-today/hyper.html) And this one is seriously c00l. (http://www.airspacemag.com/space-exploration/Outback_Scramjet.html)
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Well, right. As long as they don't try to compress the air without doing work, I'm sure they operate just fine.
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It's going to be a weird world.
Imagine looking at that engine, without any moving parts, and then hear someone tell you. "Of course it will fly.." :)
I still think my idea with perforated toilet paper, always getting ripped off everywhere, except where it was meant too, holds up reading this?
Should I tip of NASA?
Don't need no sinking airframe, just a lot of perforated , ah, ***.
Shhh, not a word more.. :)
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Bear in mind that ramjets and scramjets really do function. [:0]
See if this helps ... change your frame of reference.
1st frame: The ramjet engine is compressing the air and then generating thrust, pulling itself through the air at supersonic velocity.
2nd frame: The air is moving at supersonic velocity, and some is compressed by the intake.
Whatever your frame of reference, a converging tube (an intake) will compress a supersonic airstream. Yes, the intake does work upon the air. Or,the air does work upon the intake. Take your pick.
The rest of the ramjet engine is (simply?) the means for conducting work upon the incoming airstream at a constant level (i.e. staying at a constant flight speed).
If you're still confused, are you ready to look into a good engine textbook?
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All I care about is that we don't think compressing the air is a freebie [;D]. Quite a bit of fuel is spent doing that. If that process was not responsible for creating drag, there would be no need to spend the extra fuel.
BTW - if that's not true, I'm sure we can make some money explaining to RR and GE that they don't need to waste fuel driving all those compressor stages in their engines.
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There is no such thing as a free lunch in the propulsion business. Cheap, yes. Free, no. So, you and 364 of your closest friends can fly from LAX to SYD for less than $1000 in one hop. Your share of the fuel load is about 130 gallons (these specs are for a Boeing 777-300ER).
Since you've mentioned turbine engines ... the main difference (as compared to a ramjet engine) is that there is a mechanical compressor downstream of the intake. The purpose of that compressor is to raise the pressure of the airstream to a level higher than what could be attained via aerodynamic means alone. And, combustion is more efficient at higher pressure levels.
Does the method of running the compressor make a difference to the air that is being compressed? Nope. Run it from a belt-driven motor or from a gas turbine and you get the same effect. Some energy source is required, but it doesn't have to be jet fuel. As it turns out, turbine engines have a nice high-pressure exhaust stream available and so we can extract some work to run the compressor. Ramjets work on thrust alone ...
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it doesn't have to be jet fuel. As it turns out, turbine engines have a nice high-pressure exhaust stream available and so we can extract some work to run the compressor.
Yes, but it is jet fuel that produces the exhaust, and that's the same exhaust that produces the thrust, so the thrust would be greater if we didn't have to use some of the exhaust to compress the air supply. Alternatively, the fuel consumption could be reduced for a given amount of thrust if we didn't have to use some of the fuel to compress the air supply.
No matter what you do, it takes work to compress the air supply, and one way or another, that work is done by burning fuel.
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You don't quite have it. It does NOT take any fuel to compress the air. It DOES take fuel (or some source of heat), applied to the compressed air, to produce positive net thrust. And, if you don't compress the air then your ramjet engine will not generate any thrust. The sum of the forces on the combustor and nozzle would be very negative (or, drag), regardless of how much fuel you dumped into the air stream.
Let's try one more thought experiment. Climb into that supersonic jet from earlier in the thread, and fly up to a nice high altitude. Cruise at something like Mach 4 at 100,000 ft. Waaaay up there!
Now, you'll have an OH NO moment when you realize that someone didn't tighten all of the bolts and your ramjet test engine just fell off. While you are busy trying to fly straight, your errant engine will fall away. No fuel! For a while, the engine will actually fall at a contant velocity. The incoming air still gets compressed. But,there's no source of heat so the engine makes negative net thrust (or drag). The acceleration due to gravity can match the deceleration due to aerodynamic drag.
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It does NOT take any fuel to compress the air.
"In a conventional turbine, up to half the generated power is used driving the compressor."
http://en.wikipedia.org/wiki/Gas_turbine#Compressed_air_energy_storage
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For a while, the engine will actually fall at a contant velocity. The incoming air still gets compressed. But,there's no source of heat so the engine makes negative net thrust (or drag). The acceleration due to gravity can match the deceleration due to aerodynamic drag.
What's gravity got to do with anything, the same could be said for any object at terminal velocity. I think what Geezer is saying is that it is the fuel that produces the thrust that produces the velocity that enables the compression of the air. No fuel, and your compression will just be drag.
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There was I guy who lived near me who used to drive around with a propeller attached to the top of his car. I used to see it quite often.
There was an article in the newspaper about it. The prop was driving a dynamo and the dynamo was charging an extra battery in the car. The gent that owned the car was quite convinced that it was "free energy" that was charging the battery.
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Perhaps if his employer paid for his gas it was free energy [::)]
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Heh heh -- I know some folks like the guy with the propeller on his car. I also enjoy the flying refrigerator concepts, where the exhaust is supposed to feed the intake for "maximum efficiency".
Yeah, in a turbine engine you extract power via the turbine to run the compressor. If it didn't make sense to do so, you wouldn't use that design approach for efficient propulsion and power generation.
But this thread started with a question about ramjet engines. In those devices, there's no physical, fluidic, or thermal linkage to transfer work between the exhaust and the intake. Of course, if the engine doesn't make enough thrust then it will decelerate and eventually there will be no compression.
→ The purpose of the terminal velocity thought experiment was simply to illustrate how a ramjet engine intake will compress air even when there's no fuel, while the engine maintains a steady velocity.
Close out this thread?
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there's no physical, fluidic, or thermal linkage to transfer work between the exhaust and the intake.
There is a physical linkage. The intake is mechanically attached to the exhaust.
The air intake is designed to compress air (which creates a lot of drag), so plenty of work is being done on the air, and that work is a consequence of burning fuel.
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Geezer, I think we're coming at this from completely different points of reference. I could run some numbers, but I don't think that would help.
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Pops, I think where you are going wrong is that you believe it's possible to compress air without doing work. If you can produce numbers that that prove you can compress air without doing work, you have either made an incredible scientific breakthrough, or you've screwed up the calculation somewhere.
Given that's it's impossible to compress air without doing work, and the only source of energy capable of doing work is the fuel, some of the fuel is being spent on compressing the air. This is not rocket science, although, it's not too distant from rocket science [;D]
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Compress air without doing work? Never said that! Didn't mean to imply it, either. [:I]
Yes, you need to expend energy to do work. In some way, you must convert stored energy to kinetic energy. But ... you don't need to burn fuel to compress air in a ramjet (check out SLAM-PLUTO). BTW, every meteor that falls through the sky compresses the air in front of it.
Regarding practical ramjet engines: You can say that fuel must be burned to do the work to generate thrust and compress the air. I can say that the air must be compressed to enable fuel-air combustion. We simply draw our free-body diagrams differently, with the same net results.
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I can say that the air must be compressed to enable fuel-air combustion.
I would say that too. But I'd also say that if you don't account for the energy expended on compressing the air, you're in for a nasty surprise [:D]