EDF Rocketry

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Richard Dierking

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(Edit on 10-20-21: In the following posts Kv or KV is used to describe brushless EDF motors. It is not kilovolt. The Kv rating is the ratio of the motor's unloaded RPM to the peak voltage on the wires connected to the coils.)

I'm building a rocket using a 70 mm Electric Ducted Fan (EDF). Others have used EDF's but mostly to achieve controlled landings like a Falcon 9. My rocket will recover using a parachute. My objective is close to 2:1 thrust to mass but this will be difficult. The 70 mm EDF is capable of about a 1200 gram thrust at 100% throttle, but I still need to run static tests for thrust and several batteries. Anyway, I'm being careful to manage mass for this project.

Please note, I called the thread EDF Rocketry because I'm interested in what others are doing with EDF's in amateur rockets. Another example of what I'm doing is using a EDF on a lifting body aircraft deployed from an aeroshell on a rocket.

The design and use of EDF's is very interesting and different that using typical rocket motors. I'm even considering using a mix of solid rocket motors and a EDF. My following posts will probably be more images than anything else. Here's the first.
IMG_3849.jpg
 
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Rob Campbell

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Interesting project. Which fan are you using? Wemotec is the defecto standard and performs well. FMS makes some great fans as well. I have a 70mm E-flite Viper and an FMS 70mm Avanti. Both are excellent flyers and the fans develop near 1:1 thrust on 6S lipos.
 

Richard Dierking

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Interesting project. Which fan are you using?
I'm using a Powerfun through Amazon. Not expensive ($41.27) and I've had good results using this brand with different sizes. Balance is good at high RPM and thrust is close to advertised. I purchased two so I would have one for static testing and the other to fly.

Here's some details for the current power system plan:
Powerfun 70 mm EDF, 12 blade, 4S 3400KV, 75 Amp
E-flite 80 amp ESC
Ovonic 1300 mAh, 4S (14.8 v), 80C LiPo battery (typically used for RC helicopters)

I'm currently waiting for a 5Kg load cell for the test stand and will post some info on testing when it's received.
 

jqavins

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The numbers confuse me.

Thrust - 2:1 T:W is less than acceptable when regular rocket motors are used, so why is this OK.

Power - I must be misunderstanding something.
  • I know what "4S" would mean for a battery, but not for a motor.
  • Leaving that aside, for the fan, I'm shocked at 3400 kV, but I guess the speed controller has a huge step-up from the battery? 3.5 MV from a 14.8 V battery, so 230,000:1!
    • OK, if 3400KV is a misprint, actually 3400 V or 3.4 kV, then that's only 230:1 step-up, which is more believable.
  • 3400 V at 75 A would be 255 kW. Youch! How is that not bursting into flames?
  • For a 14.8 V battery, with no loss in the speed controller, you'd need about 17¼ kA, or about 13,250C.
    • Which means the battery is discharged in 1/13250 hours, or about a quarter of a second.
  • Now, let's suppose that the "4S" in the fan spec means that everything I just went through is off by a factor or four. Then you'd need "only" about 64 kW and the battery would last a whole second.
So, as I said, I must be missing something.
 

Richard Dierking

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Yeah, I'll get back to you on the 2:1 thing, but unfortunately, RC motors use Kv (correct abbreviation) for determining RPM per volt without any loading. So, free turning RPM. I think the Kv is for constant velocity or something like that. So, not kilovolts.
Does that help?
So the motor is capable of crazy RPM if you consider the 3400 Kv at 14 volts.
I'm working on this project today and will post some photos later.
 

Rob Campbell

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So the motor is capable of crazy RPM if you consider the 3400 Kv at 14 volts.
I'm working on this project today and will post some photos later.
Exactly. The term I've seen used is "motor constant" and it defines the ideal number of rpms a motor will turn per volt applied. A 3400 kv motor will, theoretically, turn 47,600 rpms with 14 volts applied, which is in the range of needed rpms for a ducted fan to generate useful thrust.
 

jqavins

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Well, yes, that explains a lot. I do know that either KV is used to describe motors, the speed to voltage ratio, now that you mention it. 'K' and 'k' often being used for various constants, and this the voltage constant. Then there's also KI for the torque to current ratio.

So I should have realized that might be it when realizing that something was very wrong if it couldn't be kilovolts.

That's what I get for being an EE who works on air brakes. I'm really rusty in way too many things.
 

Richard Dierking

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Again, kind of unfortunate that Kv is used for the motors. I thought the same thing btw the first time I saw it.

Here's some photos from today's work on the project.
The motor housing required some sanding to fit into the Apogee Components 3" thin wall tube.
I installed a thrust ring in the tube and the 3 motor wires pass through the airframe wall and will run up the side to the ESC.
Feels kind of weird cutting large holes in the airframe, but there will be lots of air that will need to go through the EDF. The idea is to cut enough out for air intake but still look like a rocket. The nose will also have slots and I need to add some more airframe with more openings.
Presently, the fin can with the EDF is 295 grams. Not very encouraging to be honest. The airframe with the fins was 125 grams before cutting the 4 openings and installing the thrust ring. The mass of the EDF is kind of fixed. I didn't want to remove the housing because at high RPM the motor/fan have to be secure.

I need to 3D print an exhaust reduction fitting to keep the air velocity up.

Perhaps we can discuss thrust to mass after I complete the static testing. Then, I will have some good numbers.
 

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jqavins

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Again, kind of unfortunate that Kv is used for the motors. I thought the same thing btw the first time I saw it.
What's unfortunate is that so many people don't bother to use subscripts. The symbol is not Kv, it's KV. I'm not blaming you; I know it's a pain to type K{sub}V{/sub} (substitute square brackets for set braces), but in published work like motor specs there's no excuse.

I installed a thrust ring in the tube and the 3 motor wires pass through the airframe wall and will run up the side to the ESC.
Ooh, ooh. You could tack the wires straight up the side, then enclose them with some sort of half-tube to look like the external conduits on a "real" rocket. Maybe 3D print something that's a half cylinder with quarter sphere ends that covers the entire wire run and the holes.
 

Richard Dierking

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You're right of course, but I'm going to admit that I probably will not use the subscripts. And, the motor manufacturers are probably not going to use them too. For example, on all the RC motors I have, KV or Kv is used on the motor case. For the Powerfun cut sheet, they use KV.
 
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Richard Dierking

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You could tack the wires straight up the side, then enclose them with some sort of half-tube to look like the external conduits on a "real" rocket. Maybe 3D print something that's a half cylinder with quarter sphere ends that covers the entire wire run and the holes.
Very good. Yes, I had something like that in mind. I have small model rocket tube and nose to cut lengthwise. Then shape to fit the 3" airframe. Probably will not make much difference because the velocity of the rocket is not going to be great, but yeah, if it's moving slow might as well be looking good. ;-)
 

Richard Dierking

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The fin can with the EDF and ESC is complete (photo) and mass is presently 427 grams. Hum, not very encouraging because my mass budget for 2:1 was 600 grams. Currently, the rest of the planned parts are 622 grams that would put it way over. So. I'm going to have to get a bit creative with the rest of the build.

For anyone trying something like this in the future, the housing on the EDF added mass and you could probably just get the fan and motor and create your own support for the EDF. Also, because of the roll torque this rocket will probably have, actually two EDF rotating in opposite directions would probably be a good plan.

Alas, I grow a bit weary of this project but will continue to endeavor. Even if I fail, hopefully some of this info will help others.

IMG_3854.jpg
 

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Richard Dierking

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Not sure if anyone is even following this, but perhaps still worthwhile for a person looking for info about a project like this.
The rocket is done. It took a lot of thought/work to keep the total mass down. The rocket mass is 706 grams currently and I only need to add a small BP charge and the 15" thin mill parachute I ordered.
I'm also still waiting on the 5 Kg load cell, but I ran a quick test using the 50 Kg load cell I have and the thrust was about 1.1 Kg at full throttle. A bit disappointing because I believe the 70 mm EDF is capable of more. However, there's a bunch of stuff inside the airframe and this is probably causing the intake air to be "dirty."
Now, I'm thinking that a 64 mm EDF might be better because the mass of the EDF is less and I can use a smaller/lighter ESC and battery. Lower thrust, but the thrust to weight may be better.

IMG_3862.jpg
 

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jqavins

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Yes, still watching.

Brainstorming a bit here.

First is probably a question with an answer that's obvious to anybody who knows the second thing about ducted fans, but I only know the first thing. So, could some sort of nozzle shape at the aft end where the air exits help increase the thrust?

In this picture, it looks like it might help to glue the wiring neatly along the inside of the tube to keep it out of the airflow.
1634674992311.png

Then, in this picture, it looks like it might even be a good idea to place the battery and electronics module in outboard pods.
1634675065975.png

Could the battery enclosure be any smaller (or flatter)? Could it be lighter?

Did you need nose weight? (It looks like the nose cone is slightly translucent, and I think I see the tip filled up and opaque.) Is there room to move the battery and electronics up, obviating the need for that?
 
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Richard Dierking

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All good questions and perhaps a good idea moving some components to outside the airframe, or perhaps a larger diameter airframe with ducting down the center or going directly to the intake.

I need to keep the wiring as short as possible. I can cheat just a bit on sizing the connectors to save mass, but not on the wire gauge. It's 12 Ga silicone wire from the battery to the ESC and three 14 Ga from the ESC to the motor. I just did some testing on wire sizing and smaller gauge doesn't adequately carry the current and gets hot quickly. So, the wiring is pretty tight with the components positioned forward as much as possible. One thing that would be cool is to have raw conductors attached to the inside of the airframe. This would help reduce the air disturbance.

The openings you see on the nose is the battery compartment (photo) I'm still thinking of ways to reduce all the masses including the battery mount. But, I made a mount to accommodate different sized batteries. There's another compartment at the tip of the nose for a FW GPS, but I'm going to see how the maiden flight goes first. ;-)

Currently, the margin of stability in Rocksim is about 1.5, and the CG is 17.5" from the nose on a rocket 30" long. So, not too bad presently. Btw, when I get some reliable numbers with the new load cell, I'm going to create a RASP file for the EDF to run with Rocksim.

There is an exhaust reduction at the aft end that is 1" long (photo). I used an online calculator to find the outlet diameter. I think I might try one a bit longer. The idea is to keep the velocity up.
One thing that will be different for the thrust on this EDF rocket is that the thrust will decrease with velocity. It will be interesting, because the rocket can't go at higher velocity than the exhaust of course, but eventually, the thrust will balance with drag and gravity and the rocket will just be cruising. At least, that's what I hope!

IMG_3867.jpg
 

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georgegassaway

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Very interesting project.

I wonder if you could use a smaller 4S battery, like 500-600 mAh? Would lighten the rocket. Although it might not deliver as much current. I even wondered for a moment about going to a 5S (5 cell) battery, and only 400-500 mAh, but that could cause the motor to burn out. After all, this only has to run for a few seconds, using only a small amount of the total capacity (I presume the electronics system shuts off the EDF at some point before ejection)

Definitely losing thrust with those openings like that. Could consider some air scoops.

A major design change that would change the look a lot, would be to make a new body where the tail section with EDF looks like a booster stage, short. With a nice aerodynamic lip from the front end of the tube to the EDF for good airflow. Then the main body would have an upside down nose cone (tailcone) on it. Join the two assemblies with four struts. Have the motor wiring run along one of those struts. That would be close to ideal for the air intake, but certainly different for the appearance.
 

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How do you keep the rocket from spinning the opposite way of the edf?
 

Richard Dierking

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How do you keep the rocket from spinning the opposite way of the edf?
Roll torque is a great question and a big concern of mine. I've actually held the rocket at full throttle (which is kind of fun btw) and you can really feel the forces. Initially when the motor goes up to full thrust, the roll torque is evident. It then subsides. So, with the fins it will be interesting to see how much it spins.
The best solution and one that may be tried is two EDF rotating in opposite directions. This could also substantially increase the exhaust velocity and thrust.
 

Richard Dierking

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Currently, I'm thinking through some of the comments and options for batteries and motors. I'm looking for a good combination of motor, ESC, and battery. Keep in mind that manufacturers don't make batteries for EDF rockets. The closest I can get is helicopter batteries. Looks like 4S may be the best. The 6S are all pretty large/heavy; they don't make small capacity 6C. There are some 4S like a 850 mAh 75C that weighs 101 grams.

For EDF, the 64 mm 3500 Kv 3s/4s looks good. Motor mass is 102 grams and max advertised thrust is 1460 grams at 16.8V and 52 amps.
 

David Schwantz

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effluxrc

there are a lot better fans out there. Also could have used NACA ducts for the inlets.
 

David Schwantz

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Gens Ace are some of the best LiPos. And Castle Creations are the best ESC made.
 

David Schwantz

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Best prices will be from Big Sky Hobbies in Eagan,MN. Ask for Jeremy, you can tell him I sent you. They aare the worlds largest Futaba dealer.
 

jqavins

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I need to keep the wiring as short as possible... It's 12 Ga silicone wire from the battery to the ESC and three 14 Ga from the ESC to the motor. I just did some testing on wire sizing and smaller gauge doesn't adequately carry the current and gets hot quickly. So, the wiring is pretty tight with the components positioned forward as much as possible. One thing that would be cool is to have raw conductors attached to the inside of the airframe. This would help reduce the air disturbance.
Did you test the wire sizing with a meter (it sounds like you did) or just with a "digital" thermometer, i.e. testing temperature with your digits? If a couple of inches more or less makes an important difference in the wire resistance, and you're only one size up from getting hot to the touch, then I suggest that the wiring is undersized even as is. If that's the case, then it may well be that a few more inches of larger gauge wire nicely dressed could improve your thrust to weight ratio by improving motor performance and free air flow at the same time, while increasing the weight only slightly.
The openings you see on the nose is the battery compartment (photo)
So, this is the battery?
1634731136279.png
I mistook the large item near the top of the tube for the battery. Would you identify some components to help discussion?
1634732862919.png
There's another compartment at the tip of the nose for a FW GPS...
So, is that the opaque bit in the tip? Not nose weight?
1634731488530.png

If that's not there for stability then all of my comments about how to raise the CG are moot.
There is an exhaust reduction at the aft end that is 1" long (photo).
View attachment 486355
Ah. That ends up forming a nice boat tail too.

A major design change that would change the look a lot, would be to make a new body where the tail section with EDF looks like a booster stage, short. With a nice aerodynamic lip from the front end of the tube to the EDF for good airflow. Then the main body would have an upside down nose cone (tailcone) on it. Join the two assemblies with four struts. Have the motor wiring run along one of those struts.
That sound good from where I sit.
That would be close to ideal for the air intake, but certainly different for the appearance.
Hey, form follows function.

Roll torque is a great question and a big concern of mine. I've actually held the rocket at full throttle (which is kind of fun btw) and you can really feel the forces. Initially when the motor goes up to full thrust, the roll torque is evident. It then subsides.
Maybe you could devise a launch clamp that keeps the vehicle (can't really call it a rocket) on the pad during spin-up. Then it's held still during the worst of the roll torque. It would also mean the fan is spun up to full thrust before the first movement, which will help a little with rail exit speed. Use two launch buttons, the first starting the fan ("turbines to speed") and the second releasing the clamp.
1634734395573.png
 

DAllen

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How do you keep the rocket from spinning the opposite way of the edf?
Would having the rocket spin be a bad thing? They spin up sounding rockets all the time to increase stability. With the 2:1 thrust to weight ratio here I wonder if maybe that might help...?
 

neil_w

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Would having the rocket spin be a bad thing? They spin up sounding rockets all the time to increase stability. With the 2:1 thrust to weight ratio here I wonder if maybe that might help...?
I think the main problem would be that it is wasting rotational energy that could otherwise be turning the propeller faster, generating more thrust. Important because it looks like there is really very little thrust to spare here.
 

DAllen

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I think the main problem would be that it is wasting rotational energy that could otherwise be turning the propeller faster, generating more thrust. Important because it looks like there is really very little thrust to spare here.
Ahhhh ok that makes sense...Counter rotating blades? I know adding another fan is adding weight with more batteries and related stuff...hmmmmmm
 

JoePfeiffer

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I'd be very concerned about flying something with such a low thrust to weight ratio without some sort of active stability.
 
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