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Jet Engine

Ram Air Driven, Jet Engine
(+2, -2)
  [vote for,

I propose that it may be possible to create a ram jet engine, that can operate at very low air speeds. The basic design would consist of all the same components that a typical jet engine has (Compressor, Combustor/Burner and Turbine) with the exception that the turbine would be driven by outside/ram air. Note: the turbine would "not" be driven by combustion/exhaust air coming out of the combustion chamber.

One example of this configuration could look something like a turbo prop engine, with the propeller blades at a negative pitch angle and no internal turbine section. Hence in operation, once the engine started moving through the air (>~20mph), the propeller would start to turn, which would drive the compressor fan, which would then pressurize the combustion chamber where the fuel would be burnt and lastly ejected through the exhaust nozzle. Thereby creating thrust and propelling the engine forward/ continuing the process.

Another example would be to have the engine configuration that looks like a high bypass fan jet engine. With the exception that the Bypass part (outer part of the fan) would be angled such that it would spin or drive inner part of the fan. Then (same as the first example), the fan would pressurize the combustion chamber where the fuel would be burnt and lastly ejected through the exhaust nozzle. Thereby creating thrust and propelling the engine forward/continuing the process.

The obvious advantage of this would be the simplicity of design. No hot turbine section, and no cooling of "hot" moving parts as with traditional jet engine designs. Granted it would not be quite as simple as a true ram jet, but it would not need an initial mach 2+ speed either. I think a more manageable speed of 20-150mph, would be an obtainable design point for the operation of such an engine.

Lastly, I don't think it would be very practical for most types of aircraft. However if such a design could be made to work, it could used for some UAVs, radio controlled aircraft, ultra lights and sport planes. At the very least worth some further baking..

What do you think??

Kevin Ryan E-2D Hawkeye- Electronics Liquid Cooling System Engineer Northrop-Grumman (Integrated Systems) 925 South Oyster Bay Rd M/S Y16-015 Bethpage, NY 11714-3580 Work ( Fax ( e-mail: kevin.ryan2@ngc.com

Rhino1903, Feb 01 2006


       I think you should recategorize and go read the FAQ.
migennes, Feb 01 2006

       Oohurmmm... <sighs> Why do new bakers always post in either "Other: General" or "Halfbakery: Idea" HB: idea is for ideas pertaining to the posting of ideas.   

       I'm no jet tech, we'll just wait for Vernon to notice this.   

       Suggest mods move to Product: Engine?
DesertFox, Feb 01 2006

       I, too, am not enough of a petrol head to comment on the idea, but could I ask for a more informative title, please?
st3f, Feb 01 2006

       Does anyone know what pressure levels are needed to effectively pressurize a hot section?   

       I know that in, say, a 420shp Allison turbine, the actual output of the hot turbine section is roughly 1100hp but the compressor eats 600+ of the available horsepower leaving the remaining 420shp rated output. That's a LOT of energy needed for the compressor.   

       It's hard for me to imagine successfully replacing all that compressor power with a windmilling device, ducted or open.
bristolz, Feb 01 2006

MikeOxbig, Feb 02 2006

       [21 Quest] is not entirely right, about the perpetual motion thing. Some of what he wrote, in expressing his objections, could be better phrased. Let's pretend you have built one of these engines, and you get it started as planned, at low forward velocity. You ARE aware of the velocity of the exhaust gasses, right? Remember that without appropriate quantities of air coming in from the front of the engine, the force that is pushing exhaust gas out the rear nozzle is ALSO going to try to push hot gas out of the intake. Well, your low forward velocity CANNOT let you obtain that much counter-pressure, to keep all the combustion product going out the rear nozzle. Not unless you have windmill-sized propellers to catch the breeze, and a huge gear ratio on the compressor. All this Idea really does is get its compressor power from the forward motion of the plane through air, instead of from the rearward motion of the hot exhaust gas. The fast gas lets a small turbine gather enough compressor energy. To get the same energy from slower gas (plane moving through air), means you need a much larger interaction surface with the air. Like a windmill. NOT efficient OR simple. Especially if you eventually want to go FAST with this engine. Your windmill blades would have to be telescoping (shrink them to reduce "centrifugal" stresses, and to prevent blade tips from exceeding speed-of-sound), featherable (another way to catch less air as the plane goes faster), and attached to a wide-range transmission (only a fairly constant compression is needed in the engine).
Vernon, Feb 02 2006

       Vernon, Thanks for the clarification. But as for the "windmill" sized propellers. I'm not so sure they would have to be that big. The size would greatly depend on the target velocity you were shooting for. Example, if it was 20mph, you're correct, it would have to be huge in order to extract the necessary energy to drive the compressor/fan. However if the target velocity was say 120mph. The fan could be significantly/ exponentially smaller and still be able to extract the same power from the ram air.   

       In addition, it is my understanding that a large fan, accelerating a large air mass, to a small velocity, is more efficient than a small fan, accelerating a small air mass, to a large velocity (thereby creating the same thrust). Correct?   

       If so, I hypothesis that a large ram air driven turbine, decelerating a large air mass, by a small velocity, is more efficient than a small turbine, decelerating a small air mass (exhaust gas), by a large velocity (thereby creating the same power to drive the compressor). This configuration could potentially be more efficient..
Rhino1903, Feb 02 2006

       21 Quest, Where does a Ram Jet get ram air? The answer is the same.
Rhino1903, Feb 02 2006

       I wouldn't like to say whether it would work or not, but I would like to ask: how do you taxi and take off?
Ling, Feb 02 2006

       Ling, Not sure? Perhaps a starter motor or toe plane?? Remember, I never said it would be practical for most applications. Maybe just for a very specialized projects..
Rhino1903, Feb 02 2006

       21 Quest, haha! You sound like my coworkers! Perhaps you/they are correct..   

       But on the other hand, I have fairly good idea of how a conventional ram jet works. However last I checked, this site is for the UNconventional. Meaning different in some way. Hence I didn't propose a conventional ram jet engine. I proposed a different type of ram jet engine (think outside the box). A ram jet engine that uses some additional parts not found on a typical ram jet engine. Those additional parts (external turbine & internal compressor) are there in order to reduce the minimum operating speeds from mach2+ down to a more reasonable speed.   

       I'm simply saying that such an engine "may" be possible to build and run. I could be wrong, but so far I haven't read any responses that prove such an engine violates any laws of physics..   

       Best Regards
Rhino1903, Feb 02 2006

       Ram jets use its present velocity in place of a compressor. Thus, you must run them at high enough speeds to act as a compressor. Hence, 120 mph is not nearly high enough speeds, even if you take a huge scoop and funnel it in really tight. then, wind resistance cancels out any effect your engine might have.   

       It is better written than many newbie ideas around here though. It's a good sign. Just come up with another idea, grit your teeth, and hope for the best.
DesertFox, Feb 02 2006

       DesertFox, Thanks for the encouragement.   

       In answer to your statement, the engine I'm proposing does exactly as you said. With the exception in that the engine's velocity through the air, would spin a large external fan (i.e.. big windmill or wind turbine). This fan would spin a conventional type compressor fan. The compressor fan would pressurize a combustion chamber. The combustion chamber would force air through a nozzle at high velocity. Thereby creating thrust. This thrust would propel the engine through the air/ repeating the process continuously.   

       The speed would be dependant largely on the wind turbine size. Not ram air compressibility at mach speeds.
Rhino1903, Feb 02 2006


       I'm with Vernon on this. I think he has it right   

Willlie, Nov 29 2006

       I cannot provide any objection to this idea. It compresses air, feeds it to a combustor, expands the hot gases through a nozzle and extracts energy, which is then used to compress air. The basic cycle is completely sound.   

       It's all very well to wave one's arms and say "the propellor in my head looks like a big windmill, so it would have to be as big as a windmill" but I don't see anyone backing that up with real numbers.   

       As for trying to vary the speeds to maintain constant compression, I don't see that on a modern turbofan. Higher speed needs higher mass flow, so to maintain pressure in the engine, more power is required by the compressor.   

       Below a certain forward velocity it would stall, granted. But isn't that what a starter is for? To spin the engine up to a self-sustaining condition? What if this were a cruise engine on something suitably large?   

       And how fast is FAST? 600mph? Big engines put more energy through their fans, up to ten feet or so in diameter and travelling at 600mph, than the compressors use. By analogy therefore, a fan that can put about 60,000kW into a 600mph airstream can be redesigned to take a fair amount out of said airstream. This should be workable. Perhaps not terribly efficient unless the exhaust nozzle is reeeally good, but it would certainly work.
david_scothern, May 28 2007

       Does anyone recall the Unducted Fan that was tested on an MD80?? That's the sort of thing to use to extract energy from the air; nice curved blades. The problem of low speed operation needs to be dealt with by using a compressor drive motor (e.g. diesel or petrol engine driving the shaft from the front) to supplement the power input from the UDF (i.e all the power at low speeds, less or none in the cruise).   

       Thrust derived purely from the combustion gases is noisy and inefficient, which is why turbofans are used for commercial airliners. That will always require a turbine to extract the energy.   

       If I had to make a change to the original proposal, it would be along the lines of having a single turbine assembly to drive a turbofan, while perversely having a UDF in the airflow to extract energy (efficiently) from the airflow and a supplementary compressor drive engine.
mightjustwork, Jun 21 2007


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