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

I launched my 3” Dark Star on a K700 and as the burn progressed it appears that the rocket coned. I can’t recall the actual technical term for this effect however I would like someone’s interpretation of the video. As I understand this it may be a result of my CG moving forward due to the propellant being expelled. One other thing about the rocket, which you can see in the still @ 5:06, it has a Mobius action cam mounted on the payload section of the upper airframe. I was also wondering if this contributed to the issue.

Thanks to Coleman, the launch video is here and I have it queued up at my Dark Star https://www.youtube.com/watch?v=pc4m1xoCLTM&feature=youtu.be&t=283

what was the stability before flight? and what did the simulation predict it would shift to? shouldn't the CG should move forward as the propellant burns? did the on board video show a lot of spinning? on the way up?

RKeller said:

what was the stability before flight? and what did the simulation predict it would shift to? shouldn't the CG should move forward as the propellant burns?

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Yes my bad, of course it will move forward. I had the weight down to the gram and the CG measured at the launch wrote it down on a piece of paper and by the time I got home I lost it. So I don't know what it was and this was the first launch so I have nothing to compare it to. I will have to mock up the flight again and load it back into the sims.

I need to edit my post and fix the CG comment.

RKeller said:

did the on board video show a lot of spinning? on the way up?

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Yes it does spin, so I checked my fin alignment and they are very straight, I wouldn't claim perfect but they are very much aligned. Once I figure out how to use the video editing software I will upload the flight. For an IT guy I am feeling pretty inadequate with this multimedia stuff.

I have watched the video 3 times and I am not seeing a DarkStar flight? Am I missing something?????

GRIFFIN said:

I have watched the video 3 times and I am not seeing a DarkStar flight? Am I missing something?????

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Sorry Griffin, my 3" Dark Star, currently black with a red nosecone and red tipped fins, starts around 4:42 and has the Aerotech K700-W splash on the right hands side of the screen. That is the same flight right up to about 5:06ish. It is 3 camera angles and a couple of stills all amazingly put together by Coleman. The best shot of the flight is from the camera that Coleman was holding from the flight line, which starts around 5:00 and you can clearly see that as the rocket ascends the coning appears to develop, or what I am interpreting as coning.

The link I provided in the original post should be queued right at the launch, if not just ffwd to 4:42.

Interesting enough my flight in the video is followed by an amazing flight of someone else's much larger rocket on an CTI O3400 Imax, a nice loud launch, and it also appears to do the same thing as the flight progresses.

How tight was your coupler to the payload section?
A loose fitting coupler can cause the same problem.

JD

I know it's seriously painful to even think it, but use iMovie. I think the current version sucks, and miss the original, but it's still easy to use and edit with.

The Darkstar is pretty overstable, heavy as balls, and yea... that kinda looks like coning. But a K700 is a fairly decent punch. Weird, but it was windy that day.... like, no joke kinda windy. I've put my DS up twice on a K805 and K1100. Both times little wind, good boosts. longer burns seem more prone to coning. I'm guessing as the rear got lighter, the wind gave it a nudge.

My DS is about ten pounds less motor, and 13 pounds or so with an AT 1706 loaded. That fat, any kinda wind is gonna make for some wiggles with a long burn. I bet it'd love to eat that L1400

JDcluster said:

How tight was your coupler to the payload section?
A loose fitting coupler can cause the same problem.

JD

Click to expand...

His rockets are tighter than my wife going over my motor budget for the year...

DizWolf said:

His rockets are tighter than my wife going over my motor budget for the year...

Click to expand...

Hahahhahhahhahahha....
Oh man.....
That just ain't right.....lol.....

Michael is the Dark Star built stock ???
Coning is when you're very over stable and the aft end of the rocket moves in a circle
Leaving the nose in the same position pointed up making the shape of an upside down cone..
I had a Wildman 3" Intimidator that I put a longer payload section on so I'd have more room for the main chute and harness.. When you increase length with all else the same you increase stability..
When you get to far over stable the rocket cones. ..
Notice in the flight with all of the propellant in the motor case the rocket didn't cone. It flew beautifully. .
Then when there was no more mass in the motor case the Coning started...
The coning started when the rocket got further over stable......
The coning got worse as velocity decreased. .. ( after the point of motor burnout )...

Teddy

Coning is the manifestation of undamped roll-pitch coupling. It can be introduced by asymmetric aerodynamic forces, off-axis CG and/or off-axis motor thrust.

If you observe a cork screw smoke trail as your rocket climbs to apogee, it usually is indicative of coning. If you have on-board video, as your rocket spins, you will see the horizon rising and falling at a fixed rate and the amplitude of the oscillation increasing with time.

The minimization of off-axis thrust vectors, off-axis X-Y CG, and off-axis aerodynamic asymmetries in unguided rockets minimizes coning. After this has been done, the classic way to eliminate or minimize coning is to spin a rocket at a sufficiently high rate to average out the aerodynamic and/or off-axis thrust/CG effects asymmetries.

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

bobkrech said:

Coning is the manifestation of undamped roll-pitch coupling. It can be introduced by asymmetric aerodynamic forces, off-axis CG and/or off-axis motor thrust.

If you observe a cork screw smoke trail as your rocket climbs to apogee, it usually is indicative of coning. If you have on-board video, as your rocket spins, you will see the horizon rising and falling at a fixed rate and the amplitude of the oscillation increasing with time.

The minimization of off-axis thrust vectors, off-axis X-Y CG, and off-axis aerodynamic asymmetries in unguided rockets minimizes coning. After this has been done, the classic way to eliminate or minimize coning is to spin a rocket at a sufficiently high rate to average out the aerodynamic and/or off-axis thrust/CG effects asymmetries.

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

Click to expand...

Thank you Bob,,,
I always get answers from you that I can work with,, make a change...
I think increasing stability will get rid of this Michael...

Teddy

JDcluster said:

How tight was your coupler to the payload section?
A loose fitting coupler can cause the same problem.

Click to expand...

Hi JD, Thanks for your input. The rocket as most matched composite rockets do, pretty much like a glove and no wiggle.

DizWolf said:

I know it's seriously painful to even think it, but use iMovie. I think the current version sucks, and miss the original, but it's still easy to use and edit with.

Click to expand...

Well it took me a few days but here is my first go at iMovie https://www.youtube.com/watch?v=N4L0TyW7xV8

DizWolf said:

The Darkstar is pretty overstable, heavy as balls, and yea... that kinda looks like coning. But a K700 is a fairly decent punch. Weird, but it was windy that day.... like, no joke kinda windy. I've put my DS up twice on a K805 and K1100. Both times little wind, good boosts. longer burns seem more prone to coning. I'm guessing as the rear got lighter, the wind gave it a nudge.

My DS is about ten pounds less motor, and 13 pounds or so with an AT 1706 loaded. That fat, any kinda wind is gonna make for some wiggles with a long burn. I bet it'd love to eat that L1400

Click to expand...

My OR file, see below, that I dumped the final numbers into from the shows a different story. The problem I have is that I do not know if the weight and CG override numbers in the file are the right numbers. Bottom line is that I will have to load it up and measure again to confirm, otherwise I am just guessing.

View attachment 272669

DizWolf said:

His rockets are tighter than my wife going over my motor budget for the year...

Click to expand...

lol for a second there I thought you were going to get biblical…sounds like you need a WDK account (Wife Doesn't Know).

onebadhawk said:

Michael is the Dark Star built stock ??? Teddy

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This is the original 3” Darkstar that was with the heavier FW/FG and came with a pretty heavy nosecone with an aluminum tip. In an effort ro remove as much metal from the trackers bay (nosecone) I purchased the new CTI poly-carbonate nosecone with the phenolic tip and used it instead. So there is less weight that the stock.

onebadhawk said:

Coning is when you're very over stable and the aft end of the rocket moves in a circle
Leaving the nose in the same position pointed up making the shape of an upside down cone..
I had a Wildman 3" Intimidator that I put a longer payload section on so I'd have more room for the main chute and harness.. When you increase length with all else the same you increase stability..
When you get to far over stable the rocket cones. ..
Notice in the flight with all of the propellant in the motor case the rocket didn't cone. It flew beautifully. .
Then when there was no more mass in the motor case the Coning started...
The coning started when the rocket got further over stable......
The coning got worse as velocity decreased. .. ( after the point of motor burnout )...

Click to expand...

Yes I noticed the same thing, as the flight progressed the pitch and roll became exaggerated

bobkrech said:

Coning is the manifestation of undamped roll-pitch coupling. It can be introduced by asymmetric aerodynamic forces, off-axis CG and/or off-axis motor thrust.

If you observe a cork screw smoke trail as your rocket climbs to apogee, it usually is indicative of coning. If you have on-board video, as your rocket spins, you will see the horizon rising and falling at a fixed rate and the amplitude of the oscillation increasing with time.

The minimization of off-axis thrust vectors, off-axis X-Y CG, and off-axis aerodynamic asymmetries in unguided rockets minimizes coning. After this has been done, the classic way to eliminate or minimize coning is to spin a rocket at a sufficiently high rate to average out the aerodynamic and/or off-axis thrust/CG effects asymmetries.

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.

Click to expand...

Thanks for the detailed response Bob, very much appreciated. I think I need to go back to the proverbial drawing board on this one, throw the rocket back together in a flight configuration, take my measurements and go from there.

Hey guys,,
I'm going to Potter this weekend for the launch...
( OK,, my wife says I'm going for the food / picnic )..
I have a 1/8" thick X 4 ft roll of lead..
If I remember I'll slice off a piece and bring it up for you guys...
I guess leaving it in the trailer is the best bet..
Michael,,
Cut a piece out with a razor knife and screw it to the bottom centering ring so it doesn't interfere with motor retention or anything....
Then check the stability #'s again..
It should reduce some of that over stability,,
If the #'s look good put that rocket up again...
I'm betting the rocket won't cone anymore...

Teddy

That flight had a ton of spin for a rocket the the fins are pretty straight. I've seen a few videos like this and I wonder if it's because you have the camera mounted pretty close to the nose and inline with one of the fin sets.

ChrisAttebery said:

That flight had a ton of spin for a rocket the the fins are pretty straight. I've seen a few videos like this and I wonder if it's because you have the camera mounted pretty close to the nose and inline with one of the fin sets.

Click to expand...

Hey Chris,

Thanks for the reply.

I was wondering what effect the camera mount and where it is located on the air frame, would have on the flight. In OR I tried to simulate it by adding two fins side by side. Unfortunately OR does not allow anything thicker than I think 5/8th (can't recall exactly) so I improvised and made two fins the same length and profile as the camera mount then placed them side by side. However it does not simulate the profile exactly as they are following the radius of the rocket. As these mounts gain popularity it would be nice to see someone develop a sim method for them in OR.

See if the fun slots are perfectly straight with the axis of the rocket. Take a piece of angle aluminum or steel and place it at the fun roots. This manufacturer and others have had issues in the past with this.

patelldp said:

fun slots

Click to expand...

Where are you buying your rockets

patelldp said:

See if the fun slots are perfectly straight with the axis of the rocket. Take a piece of angle aluminum or steel and place it at the fun roots. This manufacturer and others have had issues in the past with this.

Click to expand...

I will have a look this evening and confirm, if so my bad for not noticing.

mpitfield said:

Where are you buying your rockets

Click to expand...

That's the iPhone talking!

patelldp said:

See if the fin slots are perfectly straight with the axis of the rocket. Take a piece of angle aluminum or steel and place it at the fin roots. This manufacturer and others have had issues in the past with this.

Click to expand...

I had a good look at the booster this evening. I took a piece of 1" aluminum angle that I use for marking tubes on their axis and inspected the fins for alignment on the axis. Whoever was cutting the slots that day been paying attention as the fins look as perfect as you could get without precise tools. I double checked the split fin alignment and they are perfectly in-line with each other. Finally I checked the fin alignment of each fin while resting on the other two fins, so perpendicular to the floor. Again they are bang on.

The fin alignment on this rocket is one of the best I have done, so at this stage I am confident that fin alignment is not a major contributing factor in the issue.

Are the bevels symmetric?

RKeller said:

Are the bevels symmetric?

Click to expand...

I am assuming the leading edge, if so yes they are also very accurate looking. Realistically none of this is exact or measured precisely but to the eye everything about the fin section including the fillets looks, symmetrical, lined up, the right angle, etc.

I keep going back to what Chris said re. the camera located on the upper air-frame, which is something I have had in the back of my mind since locating it there. Ideally i would fly the rocket again, on a K700 but without the camera loaded, as it is removable. But I don't have the cycles to get to a launch anytime soon and this rocket is due for a repaint, as that paint job was temporary.

The reasoning behind locating the camera there was to watch the apogee event, but if it's problematic then now is the time to re-locate it. I would prefer to keep it where it is so I am going to investigate further. This weekend I am going to reload all the recovery gear and electronics, assemble it, without the motor and take some accurate measurements. Then I will throw those numbers back into OR and RS and see what they reveal.

I am hoping it is a simple fix by moving some weight around and I can leave everything as is, but I suspect not.

Wow,,
You guy's are all together on this eh,, lol...
Would you all please watch this vid...

https://onebadhawk.com/red-glare-4112015--rad-rocks-launch-of-4182015.html

The same rocket goes up 3 times in this vid...
At 42 seconds,, at 1 min 15 seconds,,, and at 4 min 7 seconds...

Tell me if you think these fins are on straight and if everything on the rocket is symmetrical
as you are all saying is responsible for the coning of Michael Pit's Dark Star....

Decrease the stability of the rocket and fly it again.......

I fully expect a beer after this one,,, lol...

Teddy

Oh,, I'm getting ready to go up to Potter,,
I may not reply to this,, till later..

onebadhawk said:

Wow,,
You guy's are all together on this eh,, lol...
Would you all please watch this vid...

https://onebadhawk.com/red-glare-4112015--rad-rocks-launch-of-4182015.html

The same rocket goes up 3 times in this vid...
At 42 seconds,, at 1 min 15 seconds,,, and at 4 min 7 seconds...

Tell me if you think these fins are on straight and if everything on the rocket is symmetrical
as you are all saying is responsible for the coning of Michael Pit's Dark Star....

Decrease the stability of the rocket and fly it again.......

I fully expect a beer after this one,,, lol...

Teddy

Oh,, I'm getting ready to go up to Potter,,
I may not reply to this,, till later..

Click to expand...

Nice flights Teddy. I don't mind the questions re. construction. For me this is an investigation and I want to learn something from it and make corrections. So going over the details is helpful in narrowing down a probable cause. Yes stability due to CG is still on the list but in my mind so is the potential CD and asymmetrical weight the camera may have is also on that list.

I will buy you a beer regardless.

mpitfield said:

Nice flights Teddy. I don't mind the questions re. construction. For me this is an investigation and I want to learn something from it and make corrections. So going over the details is helpful in narrowing down a probable cause. Yes stability due to CG is still on the list but in my mind so is the potential CD and asymmetrical weight the camera may have is also on that list.

I will buy you a beer regardless.

Click to expand...

I'm not saying Teddy isn't right, but stability doesn't cause spin. Stability will change the affect the spin has. hopefully we can grab a beer this weekend Teddy!

Uhh uhhh,,, nope,,,
neither one of you..
I'm not saying your build straightness shouldn't be questioned,, no biggie at all..
And please look closely at the two different vids of the flight..
I am trying to get out right now,
but if I remember correctly it wasn't spinning,,
it was coning..

Decrease stability and fly it again and we'll see..

I'd drive 300 miles to have a beer and a glass of red with you guy's...
But unfortunately the red is going to have to wait... lol...

Teddy

Going back to Bob's technical description of coning, and for the sake of this post let's assume what we see is in fact coning, "Coning is the manifestation of undamped roll-pitch coupling. It can be introduced by asymmetric aerodynamic forces, off-axis CG and/or off-axis motor thrust."

asymmetric forces - yes there is a proportionately large camera sticking off one side of the rocket
off-axis CG - yes the camera, shroud, bolts, screws, washers, nuts and epoxy, but I am not sure if the 70ish grams are enough to make a difference by itself
off-axis motor thrust - the motor was a K700 which is a pretty aggressive K with a center core and a regressive burn, so unless it had an issue I cannot see this having an off-axis CG

id be interested to see if the camera is doing it. Mine is pretty close to CG so effects wouldn't be drastic, but if you watch my videos you'll see some butt wiggles going on, and pretty quick corrections by the fins.


I've got a camera and a pair of J820s for my villain.....

DizWolf said:

id be interested to see if the camera is doing it. Mine is pretty close to CG so effects wouldn't be drastic, but if you watch my videos you'll see some butt wiggles going on, and pretty quick corrections by the fins.


I've got a camera and a pair of J820s for my villain.....

Click to expand...

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.

If the rocket is overstable enough just about anything will start the coning effect...

Teddy