Does this look like coning, if so how to mitigate?

mpitfield said:

This is one thing I am hoping that comes out of this discussion, some guidelines on where to place a camera. From the threads I have read the decision is pretty much based on what video perspective you want to capture and any thought about the effects on the flight are guesswork if at all. Like we have to CP and CG calibers which the hobby seems to have a good grasp even taking into consideration extreme mach flights, it would be nice to develop a basic rule/guidelines/best practices.

Such as where to place the camera, possible in relation to CP, CG or a combination of the two, like a formula. Other concerns I have had is, like CP/CG at different velocities, will the formula hold up and what adjustments should be made, if any, from one end of the recommendation to the other. At what velocity does having a camera mounted externally a bad idea as I am sure at some point this will come up. What size rocket, tube diameter is the effect if any null.

I am not discounting a basic CP/CG issue, it is still on the table, however regardless of the point of the thread I would like to better understand the whole external camera topic in better detail.

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I would say that the best spot for the shroud would be at the rockets C.G. By placing it there, any asymmetric drag forces produced will impart little to no rotational torque that could angle the rocket and begin the coning oscillations.

Also, in regards to your flight, don't forget that the upper level winds that weekend were very, very strong despite low winds on the ground. To me, the rocket suddenly experiencing a strong wind gust could be plenty to angle the rocket mid-flight, which on such a long skinny rocket will momentarily dramatically lower the stability, and lead to the back and forth wiggling for the rest of the flight.

I think when I checked the winds that day on https://www.aviationweather.gov/products/nws/winds it was showing 20-30 mph above around 3-4K.

DizWolf said:

I think when I checked the winds that day on https://www.aviationweather.gov/products/nws/winds it was showing 20-30 mph above around 3-4K.

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Cool site, thanks! That sounds about right - even my drogueless rockets falling very fast were drifting a good amount. A good test I would think would be to fly the same rocket/motor combo on a day with both calm lower and upper level winds, and see if the coning is still present.

Michael, do launch this rocket with similar motor propellant weights or differing? The reason I'm asking . Would it make sense to have 2 different nose cones 1 weighted to fly a big motor, 1 no weight to fly smaller - or a removable weight system ?

In planning an L motor cluster flight but normally fly mid - high K with this rocket. so if I go for 1 caliber for the L flight - I expect nose will be too heavy for the mid K flights (3-4 caliber overstable) and possiblity coning. So I'm contemplating either 2 noses or removable weight.

Nick@JET said:

Would it make sense to have 2 different nose cones 1 weighted to fly a big motor, 1 no weight to fly smaller - or a removable weight system ?

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I have thought about this approach in the past however I decided to build based on having some engines in mind up front. For example my Tomach was designed for the Loki 54\2800 hardware.

Having said that I completely messed up this Darkstar. I just went over my OR and my measurements are off, my mass was off and my CG was off...I must of had too many glasses of wine that evening. I just mocked up the rocket, minus an engine and with eh AT K700. Attached are two screen shots of stability vs time and roll characteristics. Also it is important to note that I added a 1.5 thick fin 4" i length, with the same profiles as the camera to simulate it's effect on the rocket. The first set of screen shots are with the camera, the second two are without.

View attachment 272977View attachment 272976

Without camera

View attachment 272978View attachment 272979

I am not sure what this is telling me except that the rocket is pretty over-stable.

I also took some time to review the video footage frame by frame, and for the first second of the flight there is very little roll, almost none, then the rocket seems to almost change it's flight path and that is when the roll starts and it increases in frequency to the point when the motor burns out then it appears to reduce in roll frequency.

RocketHunter said:

I would say that the best spot for the shroud would be at the rockets C.G. By placing it there, any asymmetric drag forces produced will impart little to no rotational torque that could angle the rocket and begin the coning oscillations.

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I don't think this is strictly correct, as the radial distance from the CG remains the same regardless of where it is placed longitudinally on the rocket.

the further in actual distance from the CG, the more leverage the force will have.

DizWolf said:

the further in actual distance from the CG, the more leverage the force will have.

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It is also dependent on the direction of the force vector.

The torque is proportional to the normal distance from the line of force to the lever arm. In this instance, the line of force is essentially parallel to the central axis of the rocket, so the normal distance is equal to the distance from the central axis. Therefore, the torque remains nearly the same at any point along a constant thickness body tube.

raptor22 said:

It is also dependent on the direction of the force vector.

The torque is proportional to the normal distance from the line of force to the lever arm. In this instance, the line of force is essentially parallel to the central axis of the rocket, so the normal distance is equal to the distance from the central axis. Therefore, the torque remains nearly the same at any point along a constant thickness body tube.

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Can I get this translated into Inglish please :cyclops:

It is also dependent on the direction of the force vector, eh?

The torque is proportional to the normal distance from the line of force to the lever arm, eh? In this instance, the line of force is essentially parallel to the central axis of the rocket, eh? so the normal distance is equal to the distance from the central axis, eh? . Therefore, the torque remains nearly the same at any point along a constant thickness body tube, eh?


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Dunno if it helped you, I'm still slightly lost

DizWolf said:

It is also dependent on the direction of the force vector, eh?

The torque is proportional to the normal distance from the line of force to the lever arm, eh? In this instance, the line of force is essentially parallel to the central axis of the rocket, eh? so the normal distance is equal to the distance from the central axis, eh? . Therefore, the torque remains nearly the same at any point along a constant thickness body tube, eh?


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Dunno if it helped you, I'm still slightly lost

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Sure pick on the old Canadian eh! ...yep still lost but I appreciate your efforts Dave.

mpitfield said:

Sure pick on the old Canadian eh! ...yep still lost but I appreciate your efforts Dave.

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I drank two pounders of Blue light..... Still not making sense, I may need to convert them to half liters first...

DizWolf said:

I drank two pounders of Blue light..... Still not making sense, I may need to convert them to half liters first...

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Well that at least partially explains it :wink:

I probably didn't put that very clearly.

If you imagine the force of the air on the camera shroud as an arrow showing its direction, you can draw a line through that arrow going off forever in both directions. We will call this the "line of action."

You can then draw a right angle from that line of action to whatever location you are applying the force to (in this case the CG). The distance of that right-angle line is called the "normal distance".

The torque is equal to the normal distance multiplied by the amount of force. Anywhere a force acts on that line of action will have the same normal distance. Therefore, you can apply a force of the same strength and direction anywhere on that line and expect the same effect.

In our case, the line of action is nearly parallel to the sides of the rocket. So, moving that force up and down the rocket basically moves it up and down the line of action. Therefore, the torque remains virtually the same.

raptor22 said:

Maybe I could have explained it better.

If you imagine the force of the air on the camera shroud as an arrow showing its direction, you can draw a line through that arrow going off forever in both directions. We will call this the "line of action."

You can then draw a right angle from that line of action to whatever location you are applying the force to (in this case the CG). The distance of that right-angle line is called the "normal distance".

The torque is equal to the normal distance multiplied by the amount of force. Anywhere a force acts on that line of action will have the same normal distance. Therefore, you can apply a force of the same strength and direction anywhere on that line and expect the same effect.

In our case, the line of action is nearly parallel to the sides of the rocket. So, moving that force up and down the rocket basically moves it up and down the line of action. Therefore, the torque remains virtually the same.

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Hey raptor22

Thanks for the explanation.

While I started to read you post I thought to myself, a picture would help, then I saw the picture next, but nope I am still not getting it. I provided some additional updates last night, see post 35.

Essentially I am hoping to gain two insights on this thread.

Fist understand, with confidence, why my rocket had the flight anomaly.

Secondly because I am enjoying the video side of rocketry I would like to use it on other builds. However I would prefer to place the camera not just where I will get the best picture but also where it will have the least effect on the performance and stability of the rocket.

The latter is something I believe many will benefit from. I have seen and read many build threads with cameras mounted and it would seem that in most cases the camera is placed on the air frame with very little understanding of how this will effect the flight.

As I posed earlier, having a rough guideline like what we have with the old 1 caliber rule, with exceptions on supersonic flights, would be ideal.

there has to be some research on this someplace.

DizWolf said:

there has to be some research on this someplace.

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Dave I thought the same thing but my search did not yield any results, opinions yes but not what I would call a guideline for locating an external camera.

mpitfield said:

Dave I thought the same thing but my search did not yield any results, opinions yes but not what I would call a guideline for locating an external camera.

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Saturn V had all kinds of crap hanging off it. I doubt NASA was like "yea...right there...that looks about right" I just can't find the proper search string.

DizWolf said:

Saturn V had all kinds of crap hanging off it. I doubt NASA was like "yea...right there...that looks about right" I just can't find the proper search string.

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Luckily, for the rest of humanity, we don't work for NASA.

DizWolf said:

the further in actual distance from the CG, the more leverage the force will have.

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I thought this made perfect sense,,
till I read Raptors next post...
Raptor you are quite impressive and you explain things well to boot..
I hope you guy's don't mind,,
but I read Raptors post as explaining why the large over-stability had more of an effect in producing the coning then the camera shroud did....
This has become an extremely interesting conversation I am super pleased to be a part of...
There is certainly one easy way to prove all of this...
Do it...lol...

Teddy

onebadhawk said:

I thought this made perfect sense,,
till I read Raptors next post...
Raptor you are quite impressive and you explain things well to boot..
I hope you guy's don't mind,,
but I read Raptors post as explaining why the large over-stability had more of an effect in producing the coning then the camera shroud did....
This has become an extremely interesting conversation I am super pleased to be a part of...
There is certainly one easy way to prove all of this...
Do it...lol...

Teddy

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Thank you, Teddy for the compliments.

I don't have a lot of experience with dynamic stability questions, but intuitively I think Bob is right that the coning is due to asymmetric aerodynamic forces and an off axis CG. It is likely these were at least partially caused by the camera shroud.

I was just trying to say why I think the camera shroud's effect on the coning would not be lessened by placing it near the CG, but rather that the effect would remain the same.

I think the easiest and most straightforward solution is simply to add another camera shroud filled with ballast on the opposite side of the rocket. This would balance out the forces so everything is symmetrical again.

raptor22 said:

I probably didn't put that very clearly.

If you imagine the force of the air on the camera shroud as an arrow showing its direction, you can draw a line through that arrow going off forever in both directions. We will call this the "line of action."

You can then draw a right angle from that line of action to whatever location you are applying the force to (in this case the CG). The distance of that right-angle line is called the "normal distance".

The torque is equal to the normal distance multiplied by the amount of force. Anywhere a force acts on that line of action will have the same normal distance. Therefore, you can apply a force of the same strength and direction anywhere on that line and expect the same effect.

In our case, the line of action is nearly parallel to the sides of the rocket. So, moving that force up and down the rocket basically moves it up and down the line of action. Therefore, the torque remains virtually the same.

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While I see and agree with your analysis here, also consider that the torque force will not always be pointed like that - say if a wind gust causes the rocket to angle and then correct itself, if the shroud is oriented 90* to the direction the rocket angles, the sides of he shroud will catch the wind as it tilts, in which case its position from the CG does make a difference in the torque/effect on the rocket.

Also, imagine the shroud oriented 90* to the direction of a cross-breeze - again the sides of the shroud catch the oncoming breeze, causing the rocket to both spin and possibly angle away or towards the wind, depending on its location. Its position will not affect the amount the rocket spins, but it will affect how it angles - if its far above the CG it may angle away from the wind, and far below into the wind, whereas if it is at/near the CG it will not rotate the rocket, only spin it.

I have been playing with the numbers a little, however before I chop up the rocket I have a different game plan to try first.

The camera has a remote lens option and the lens on it's own is relatively small compared to the entire camera, both in length as well as width. I purchased this option with the camera but not to use it mainly because I already had a mobius camera shroud and figured everyone else is flying them then why not me. Plus it introduces more prep time in the field. Over the winter I am going to design and build a smaller much more streamline shroud just for the lens. I am going to locate it about 6" lower than it currently is located, but still above my separation point so I can view the apogee event. I will mount the camera inside the tube with an internal mount that I will be able to turn on at the pad.

This will move my CG axis more inline and obviously have less forces on the external bits. I will post pics here to keep it all together.

Now having said that when I play with the numbers on the lower and payload body tube lengths I get three advantages. The over-stability moves down 1 caliber, still over-stable but I also get better performance in velocity and max altitude. So basically if I cut my booster from 44.75" to 40" and my payload from 21.625 to 18" I gain stability and performance. It is just an all around better performing rocket...but I am still not prepared to do that yet.

First thing I did when building mine [see sticky top of page] was cut payload off 4in & fin can 9in. lost .75 lbs . Flies great in slimmer form.Higher & faster than stock. You only need what it takes to hold largest 54 motor in fin can........ & cord & chute in payload

Before I would do anything in your case, I would remove the camera, fly it again & see how it does. If it flies fine, which I suspect it will, you have other issues. Then figure out how & where to mount it.

Did I see somewhere in here you swapped out the fiberglass cone for a plastic one?

blackjack2564 said:

First thing I did when building mine [see sticky top of page] was cut payload off 4in & fin can 9in. lost .75 lbs . Flies great in slimmer form.Higher & faster than stock. You only need what it takes to hold largest 54 motor in fin can........ & cord & chute in payload

Before I would do anything in your case, I would remove the camera, fly it again & see how it does. If it flies fine, which I suspect it will, you have other issues. Then figure out how & where to mount it.

Did I see somewhere in here you swapped out the fiberglass cone for a plastic one?

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Hi Jim,

Practically it is difficult for me to make small adjustments test, make another adjustment then test again, and so on. This is because of my accessibility to the closest launch field, and personal commitments in life; small business owner and a young family. Based on this I have to make some assumptions and adjustments, although not ideal from a troubleshooting perspective, it is more practical in my case.

That being said, mounting the camera internally is something I have wanted to do from the beginning, which is why I purchased the external cable for the lens with the camera. For the reasons mentioned in a previous post I opted not to pursue this, but winter is the perfect time for me to pursue it as I will be repainting the rocket.

The fact that you already modified and built this rocket is inspiring me to make this change now as well. I don't see any downside to it. Like you said so long as it is long enough to fly the motors and get you want to pack into it.

The nosecone I used was not quite a plastic one. I opted not to use the stock one because I felt it was too heavy and I wanted to mitigate the metal in the nosecone/tracker bay. The nosecone I used is the CTI "3" Fiberglass/Polycarbonate with phenolic tip" that Wildman sells and is included in the 3" Punisher, which I believe you have.

Thanks for your input.

It just dawned on me. We do t need to do test flights. Every e and thief brother has a 3" dark star. Time to waste and hour on YouTube.

DizWolf said:

It just dawned on me. We do t need to do test flights. Every e and thief brother has a 3" dark star. Time to waste and hour on YouTube.

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Dave, time for a new keyboard...you seem to missing a few letters. Or I am just missing the hidden message.

mpitfield said:

Dave, time for a new keyboard...you seem to missing a few letters. Or I am just missing the hidden message.

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You didn't get the secret club decoder ring?

DizWolf said:

You didn't get the secret club decoder ring?

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I think only BoD members got the decoder ring. I even tried reading every other letter, only caps, single letters and all I could come up with is "IWETUT" and "tea"...still not getting it.

Bob, I've seen this a lot with my Pershing 1A models, which if you believe conservative CP/CG calculators are usually built overstable...as I've moved the CG back, I've seen a reduction in Coning to little or none....so these fit into the too small fins bucket, but I seem to see a reduction in coning as the overstability is reduced...

Frank

bobkrech said:

Also rockets with marginal stability or fins that are too small have an enhanced chance of coning because the damping factors are lower when the stability margin is reduced.

Bob

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