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Electric Ducted Fan Jets Discuss electric ducted fan jets here including setup tips, power systems, flying techniques, etc.

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Old 02-11-2013, 02:31 PM   #1
Reynaldo13
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Default Ask about EDF Model Rocket

Hi guys I have a plan to build a green model rocket that powered by EDF not the solid or liquid propellant like always. is it possible to make?

I've planed to use 70 mm Aloy DPS 2200 KV from HobbyKing. It has 2.45 kgf of static thrust and the model rocket that I plan to build only has 1.5 kg of mass, 7.5 cm of diameter and 80 cm of height, so the the thrust weight ratio will be 1.66 and I think it's quite enough to accelerate this model rocket on vertical direction about 16.6 m/s^2. But I still a little bit confused about the inlet configuration, where should I put the inlet area in the body of this model rocket which is not giving me more drag and not reducing the aerodynamics of the model rocket?

When I tell my friend about this idea, he said that I have to overcome the twisting effect that caused by EDF first. My question is, Does EDF really will bring such effect?
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Old 02-11-2013, 05:47 PM   #2
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For every action, there is an equal and opposite reaction (Newton's 3rd law).

Yes, it will twist at an absolutely insane, uncontrollable rate.
I also think your thrust to weight ratio is missing something.... batteries.

Next, part of the effeciency of the EDF comes from inlet pressure once it is under way. If you are not shoving air in, it cant accelerate it on the way out.

At liftoff, you will need more vacuum power than you will generate thrust, and without an inlet 3x the size of the fan, you will be in trouble.

Consider the fact that NASA never strapped jet engines onto rockets, they probably figured it out long ago.

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Old 02-11-2013, 06:48 PM   #3
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it will twist? really? can you explain to me please why it will twist and uncontrollable? and where does that twist come from? so is it still possible to make?

3 times larger than the fan? but the rule thumb of EDF said the inlet must be minimum 120% larger?
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Old 02-11-2013, 07:20 PM   #4
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It will twist from the torque of the motor. As stated above "every action has an equal and opposite reaction". This is much the same as how a gun has recoil when fired. In this case it would be centrifugal force(rotational), which will cause the rocket to twist. The only way to avoid this would be to counter the effect with wings. At that point you would have an airplane, rather than a rocket.
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Old 02-11-2013, 07:24 PM   #5
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Maybe you could make something like this work: http://en.wikipedia.org/wiki/Lifting_body
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Old 02-11-2013, 07:57 PM   #6
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Most RC EDF aren't really EDF, they are a small multi-blade prop in a can. They aren't nearly as efficient as a regular old (larger) prop.

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Old 02-11-2013, 08:20 PM   #7
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You would need some way to couteract the torque for sure.

It may be possible to use two small counter rotating motors in a shround to get a very similar effect. There were actually some experiments with verticle take off and landing "airplanes" that had a body like a rocket with two props counter rotating at the nose, and then small control surfaces to steer them. With gyro's and stability manigement systems, it should be possible to build a "guided rocket" firly easilly, but either the control surfaces or speed would be a major factor.


To make it resemble rocket fins, it would have to accelerate very quickly to have any type of control over it, as well as a low center of gravity. But overall, you would be re-inventing the twin bladed helicopter.
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Old 02-11-2013, 08:23 PM   #8
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Reminds me of the Da Vinci design(and it's limitations).

http://www.aerospaceweb.org/design/h.../history.shtml
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Old 02-11-2013, 08:45 PM   #9
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Actually EDF's generally don't have much reaction torque. That's because they have stators in the duct which take the swirl out of the airflow and in so doing balance the reaction torque from the motor, rather like contra rotating props do in some prop planes. The last EDF I made actually had slight reaction torque in the opposite direction to what you would expect from the fan rotation, which really puzzled me for a while.

An EDF rocket would be a big challenge even if the torque proves to be minimal. First EDFs are really inefficient so you struggle to achieve high thrust/weight ratios due to the heavy motor battery etc that you need. Next unlike a true rocket EDFs (being just a prop in a tube) suffer from ever decreasing thrust as speed increases. I think you would have real problems getting up enough speed to make the rocket stable.

Your acceleration calculation is also wrong because you forgot to deduct the force of gravity pulling the rocket down. A 1.66 thrust/weight even if you could achieve it, would only get you 6.5m/s^2 acceleration. You would need a 2:1 thrust/weight ratio to accelerate at 9.81m/s^2. To get 16.6m/s^2 you would need close to 3:1, which is 'pie in the sky' for an EDF. Plus acceleration would drop off rapidly as speed increased.

I'm not sure how fast a rocket would have to go to be stable but if you used a 2m long guide rod to launch the rocket off and based on a 1.66:1 thrust rato it would only be going 11mph by the time it reached the end of the rod.. not anywhere near fast enough i'm pretty sure.

Possibly if you used a catapult to get it going?

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Old 02-12-2013, 02:33 AM   #10
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I just realized, you want spin on your rocket for stabilization, so if the edf did spin your rocket, good.
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Old 02-12-2013, 03:39 AM   #11
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It Will work, But you will need a few JATO rockets straped to it to get it off the Ground


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Old 02-12-2013, 04:09 AM   #12
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I was actually thinking more like this
http://en.wikipedia.org/wiki/Lockheed_XFV

The biggest problem with counter rotating props is that you would need a gear box to keep them perfectly sinced(sp?) Or you would have to have a variable control or trim to keep it straight. The second prop would also effect the airflow making one prop quite a bit more efficient then the other. So you would have to run different props or something.

And even with very little torque, you would still need a huge control surface or fin to over come the torque if you wanted any hope of steering it. If you wanted it to spin like an arrow, then torque wouldn't be a big deal.
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Old 02-12-2013, 05:32 AM   #13
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I've been firing off model rockets for years, and even put rocket assist on one of my scratchbuilt planes, but the the power to weight ratio is the big deal with rocketry. My factory rockets, and home built ones only weigh mere ounces, with the solid rocket fuel being the heaviest component. EDF is great fun, but easily the most inefficient way of propelling anything ( apart from using the motor out of a harley). Another thing is height; 400 metres put you at legal height, and most model rockets only have enough burn to reach that height, which is a matter of seconds, unless you have an "official" range and are multi staging. A bit off the dial, but have a look at the Luft '46 website, and find the Heinkel "Lerche", which was a VTOL rocket with an external rotating prop arrangement; another idea never hurt...good luck
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