Spinning, Not Spinning?

[dhkaiser]

Yesterday I launched my DX3 for the second time with a Mobius camera. Both times the rocket spun early in the flight only to stabilize and fly straight. Any ideas why? Here is a video from yesterday...

https://www.flickr.com/photos/dhkaiser/24316694808

 

[qquake2k]

Weird. It looked like it stabilized as soon as the thrust cut out.

 

[LithosphereRocketry]

Something off with the nozzle/MMT then?

That's very weird. I've seen rockets coast to a stop after burnout in terms of roll but that stopped on a dime. It also didn't start right away.

Hmmm... How did you pack your parachute? If something was untwisting inside you might get that kind of effect- never seen that though.

 

[dhkaiser]

The chute is packed snug enough that I don't think anything is shifting inside the rocket.

OK the strange just got more strange. Looking at both videos I see the launch on a G80 spun in one direction and the launch with the H115 spun in the other direction. This video is at .25 speed...

https://www.flickr.com/photos/dhkaiser/37460750314

 

[qquake2k]

If it was rotating in only one direction, I would suspect motor mount misalignment. But since it rotates in both directions, I have no idea. In the first 1/4 speed video, you can clearly see the thrust smoke in a corkscrew pattern. Very strange.

 

[Rex R]

I think it is a property of 3-fin rockets, a roll instability caused by wind.
Rex

 

[Handeman]

I was thinking it had to do with motor nozzle misalignment. The hole off center, flash in the hole causing a slight side thrust, etc. Hobby rocket nozzles are not high precision manufactured parts.

After reading Rex R post, he might be on to something too.

Guess you'll just have to keep taking video and writing down all the wind and other environmental conditions along with any observations on motor parts or assembly. Another half dozen flights and you should be able to narrow it down.

You do know you can't be a rocket scientist if you don't take notes. :wink:

 

[mpitfield]

What was the stability margin of the rocket with that motor?

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[AeroAggie]

I think it is a property of 3-fin rockets, a roll instability caused by wind.
Rex

I don't think it's that. It snapped straight when thrust cut off, so I'd rule out aerodynamics.

My educated guess - It's the internal flow. A perfect nozzle and a perfectly symmetric burn would give you perfectly symmetric thrust. But your ignitor touches one side of the core so you start burning there, pressure and heat build up until you have a nice burn all around and start making thrust. That internal flow can still be cork-screwy or lopsided and result in very uneven pressure distribution. Even if you have a perfect nozzle you'll get some variation in the flow just due to the differences in burn rates from top to bottom. And sparkies eject a lot of large heavy particles that just kinda bounce their way out, which cause pulses in the pressure that you can see in the smoke. You can build a perfectly straight rocket and still never fly perfectly straight.

I'd say don't sweat it. If you're really curious, try different motor formulations and take notes.

 

[afadeev]

Both times the rocket spun early in the flight only to stabilize and fly straight. Any ideas why?

To summarize:
- rocket launches stable, flies straight off the rod
- ~1.0 seconds into flight and 100+ ft in the air it starts rapid counter-clock-wise spin
- ~3.0 seconds into flight it abruptly stops spinning after motor cuts out
- I only saw it spinning in one direction: clock-wise from the position of the camera / counter-clock wise from the ground.


If it's a one-off, then I would suspect nozzle manufacturing imperfection, or debris accumulation in nozzle throat/diverging section during burn. Especially with a sparky engine.
What motor did you fly on, and how did it look after landing (have pics)?

If the problem repeats itself reliably (not the end of the world, BTW), then I would suspect motor mount misalignment.
Did you build the rocket yourself? Have any reservations about how the motor mount centering rings were glued to the MMT?

Either way, congratulations on perfectly aligned fins!
Once the thrust cuts out, they stabilize the spin in a hurry, and it proceeds flying perfectly straight.

a

 

[jadebox]

I had that happen once with my EZI-65/Upscale Cineroc. It never happened during any other flight, so I have to blame the motor. It is amazing how quickly the rocket stops spinning at burnout.

[video=youtube;B5ALJlULV14]https://www.youtube.com/watch?feature=youtu.be&v=B5ALJlULV14[/video]

 

[ksaves2]

My gosh! That's something. I don't think anyone could fake that! Looks like someone grabbed the fin to stop it and it just kept going up. Wow. Kurt

 

[dhkaiser]

....

 

[crossfire]

why did you have nozzle material when drilling out the delay? Must be a very long delay adjustment tool. makes no sense to me.

 

[AeroAggie]

The nozzle seems to be the issue. Both motors have adjustable delays which are done with a tool that drills out the delay. When doing so there was some of the nozzle material in the drill bit. So the fault seems to be with the drill tool. I did not remember this until the nozzle issue was mentioned above.

How did you damage the nozzle by drilling the delay? They're on opposite ends of the motor. Are you drilling from the nozzle end?

 

[dhkaiser]

You are right, I was thinking ass backwards. You have heard of senior moments, this was a senior minute. Back to ground zero.

 

[neil_w]

My gosh! That's something. I don't think anyone could fake that! Looks like someone grabbed the fin to stop it and it just kept going up. Wow. Kurt

Ditto. That is sort of mind boggling.

 

[Lowpuller]

Coning

 

[RoyAtl]

Roger, what was the motor in that? Was it a C-slot motor?

 

[mpitfield]

Coning

I see that I am not the only one; again what was the caliber with the config?

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[jadebox]

Roger, what was the motor in that? Was it a C-slot motor?

It was an Aerotech I266.

 

[dhkaiser]

I see that I am not the only one; again what was the caliber with the config?

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The flight with the G80 was 1.77 and the other with the H115 was 1.55.

 

[mpitfield]

The flight with the G80 was 1.77 and the other with the H115 was 1.55.

Well those numbers certainly look okay. Where I was going with this was, I was wondering if the rocket was on the edge of being over-stable and roll pitch coupling was induced by the Mobious camera creating drag on one side. Looking at the video the exhaust does have that familiar coning look to it however typically this effect increases as the CG moves forward due to the propellant burning. I wouldn't completely discount this as part of the effect but there is obviously something more at play here.

 

[RoyAtl]

It was an Aerotech I266.

hmm.. Ok, never heard of it, but I'll assume it's a Bates grain for a reloadable. Only thing I can think of is if something got caught in the nozzle, deflecting the thrust.

 

[Dave A]

I agree with Rex R. My 9" Patriot ( I spent a lot of time aligning fins when installed) rotated only 200 deg from ground to Apogee.
Others spin like a mad man, same care taken to align the fins.

I will say I noticed something interesting watching the vidoe closely.
You can see the sections moving (due to very little slop on the coupler fits. It is obvious the drag and wind forces "slapping" it around.

[video=youtube;Riv201JYoEY]https://www.youtube.com/watch?v=Riv201JYoEY[/video]

 

[dhkaiser]

How about this idea. The rocket has a 38mm motor mount and I was using a Aero Pack 29mm adapter. Could a small misalignment in the adapter cause the rotating effect, even though it was in opposite directions on the two flights? It did stop once the motors burnt out. See... https://www.flickr.com/photos/dhkaiser/37460750314

 

[Tonimus]

DHK, I think you might be on to something there about the adapter. How snug does the fore adapter ring fit into the mmt?

 

[dhkaiser]

I just tested the adapter. There is a little play, not much but a little.

 

[Tonimus]

If it isn't bad, shouldn't be enough for that kind of spin. Perhaps cumulative effects of that and asymmetrical thrust?

 

[cobra1336]

Wild guess, fins are to small for the thrust or fin flutter?? Would explain rotation in both ways??