h a l f b a k e r yWarm and Fussy
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I have not thought of many(and the one I did think was for a product here on HB) uses for this, but bear with me.
Alright, jet engines are basically air compressors on the front end and a fuel injector/combustion chamber on the back end, correct? They have an intake and an exhaust, and for most practical
purposes, there shouldn't be a need to have a the air-flow direction reversed. Yes, most jet planes do have some form of "reverse thrust" provided by a vent that diverts the exhaust gasses coming off the back and having them do a u-turn; but these are only for landings.
My idea would be for a craft that would be able to use the jets as a tank would use its treads. If it needs to rotate very quickly, reverse the flow of gasses in one side, and have a reverse thrust to have counter-directional impulse and have the craft rotate.
basically, my idea is to have a jet engine with the combustion chamber in the center of the device. on either side of it will be two concentric ducts. The outer duct will have the rotating compressor vanes, and the inner duct will be the exhaust/compressor driver. There are also valves that will cover the inner and outer ducts on command.
At any one time, one compressor duct and one exhaust duct will be open to allow for the operation of the jet. However, when the need arises, the pilot will close those ducts and open the opposing set so that gasses will now flow in that direction.
Another design augmentation I thought was to do away with separate ducts and just change the pitch of the blades in the engine.
In either case, I envisioned a common drive shaft for all the components, for ease of maintenance and to keep the parts count low. (thus the single direction blade rotation part).
Harrier diagrams
http://www.technolo.../culture1/harr1.htm
nice animations [xenzag, Sep 11 2006]
Jet Airship
Jetairship
reminded me of this [theircompetitor, Sep 11 2006]
[link]
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It's an interesting idea. However, you would have to reverse the direction of combustion each time. The combustor is designed to allow a flame to burn in a 150m/s wind. It needs that wind to be coming from upstream. Reverse it and, quite simply, the engine will go out. |
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Keep in mind that you've only got one set of combustors, with a continuous rapid airflow through them. If you were to require a change in direction, you would have to slow, stop and reverse the gas passing through the combustor, while reorienting the combustor for reversed flow. Alternatively you would require a second set of combustors for reverse, kept hot by wastefully burning extra fuel. |
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In effect, an engine capable of reversing itself would a) be phenomenally complex and inefficient, and b) take too long to reverse to assist in a high-speed manouevre. [-], but interesting anyway. |
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Where is this a benefit over a system like what is in a Harrier which uses Vectored thrust to do all the things you indicate and its been in service for more than 20 years. |
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take a look at the Harrier jump jet's engine
pods which have swivelling nozzles -
enabling it to stand still and spin around -
very handy for a quick get away ! |
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From what I understand, your bi-directional jet engine would be heavier, more voluminous, more maintenance-intensive and more expensive than a standard jet engine that produces a similar amount of thrust. These disadvantages make it an unnattractive option on a fighter jet. Fighter jets gain an advantage by having high thrust-to-weight ratios, which is something your engine could not replicate as easily as a standard turbojet or turbofan for the reasons listed above. The implementation of thrust-vectoring technology on aircraft such as the F-22 and the Su-37 increases their agility without needing such a heavy, complicated engine as your bi-directional jet engine. It is a novel idea, but one that likely wouldn't provide an advantage in aircraft manuvering, all things considered. |
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CUrrent control surfaces on aircraft, combined with airframe rigidity, and thrust vectoring allow maneuvers in excess of 15 G's. The pilot is the weak point. |
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// If it needs to rotate very quickly, // |
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Just how quickly do you want to rotate? |
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Those are some nice Harrier Diagrams [xenzag]. |
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Good for a setup where ram induction is not possibe but I am not clear on why it is easier to reverse the jet than just flipping it over using a pivot. An example application maybe? |
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Being able to swivel a tank quickly is probably useful, since a tank's traction will generally allow it to change its direction of motion as quickly as it can change its heading. |
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Swiveling a plane quickly is apt to be less useful. If a plane's heading suddenly changes 90 degrees to starboard before its direction of motion has changed much, the plane will longer have air flowing front-to-back over its wings, but rather side-to-side. If the plane is not banked to starboard, there will be no lift. If it is banked, that will convert the sideways airflow to lift, but the plane will still need forward thrust to get going in the right direction. Having an engine thrusting backward would not be helpful. |
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Aeroplanes can fly sraight and level while yawed a fair bit; I've done so in a glider under the instructor's direction to experience yaw, pitch and roll independently for a better understanding of how to combine them in a turn. |
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I suppose this could be useful in military 'planes with a fixed front cannon in order to momentarily aim a little to the side. Or to yaw by pi for backwards flight. |
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//an engine capable of reversing itself would a) be phenomenally complex and inefficient//
Bun. |
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As of 1st April this year, the Harrier will have been in service for 40 years. |
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