Corkscrewing Rockets

[DanielLW]

This probably applies to MP and HP also, but the video I'm linking shows LP launches.

Anyhow, I was doing some tests with some old Aerotech B and C reloads I have lying around. (That's right, B and C reloads!) But that's a different subject which I intend to post on in the coming days. The heart of the matter is I also recently learned that my phone will do 120fps video, so I decided to take a piece of plexiglass, put it over the phone, and record the launches from just beside the pad. Here are the results:

[YOUTUBE]YbqrFVm5Xvw[/YOUTUBE]

I was suprised to see that my Patriot and Hi Flyer models had significant amount of corkscrew to them that wasn't evident when watching from the standard viewing area. The one rocket that was straight was my Athena, which comes with molded fins. And the corkscrew, of course, was very screwy by design.

I didn't use a fin jig in the construction of these. The fins seem to be straight, but looks like it doesn't take much to throw the flight off a bit.

Enjoy!

 

[mpitfield]

This could be misaligned or canted fins, but it could also be coning.

Coning is the term we using in rocketry and usually increases as the CG moves forward. So as you burns through your propellant the aft end of the rocket gets lighter, moving your CG forward. If your rocket is over stable to begin with, the CG being too far forward of your CP then you have get coning. Someone who know much more about this than me said it was called roll pitch coupling, but I can't recall if they were talking about another phenomenon or just to trying provide me with the proper technical term for it.

Possibly someone with more knowledge on the subject will pop into your post with more details.

The point being it may not be your fins or just your fins.

Here is a link to one of my flights on a HPR and you can clearly see coning. The rocket was launched on a K700 so the CG shift was relatively high. I cut 5 inches off the rocket over the winter to help mitigate the effect. Interestingly enough if you watch the flight after mine it also appears to have a coning issue and that was on an O3400.

[video]https://youtu.be/pc4m1xoCLTM?t=283[/video]

 

[FredA]

misaligned launch lugs...known as the "Gambler effect" in this neck of the woods....

 

[d11rok]

Looking closely at the launch rod,it appears misshapen especially at the distal end. Thus may impart some coarse rifling action to the launch lugs, with an effect augmented as FredA posited with launch lugs. This may eexplain why the Athena had less corkscrew compared to the others.

It also appears breezy, and especially the launch of the patriot, the breeze appears significant. This would also misshape the distal end of the launch rod more compared to the proximal end. I wonder how your flights would be with a straighter launch rod with the same amount of wind.

 

[bill2654]

Here is a really cool corkscrewing rocket. Usually brings it to Airfest every year. I believe it's an 12X Upscaled Drake
[video=youtube;k67pYfLa46U]https://www.youtube.com/watch?v=k67pYfLa46U[/video]

 

[DanielLW]

At first I thought it was the plexiglass causing distortion that made the launch rod look bent. But I checked both my rods today, and they are indeed both a little bent. Looks like I need to buy a new rod the next time I'm at the hardware store!

 

[The_Lone_Beagle]

It will be interesting to see if a new rod solves the problem. On the one hand, a new straight rod should solve most/all of the problem. On the other hand, unless the rod is really thick, it still may bend/sag a little depending on how long it is.

People have started using rails for smaller rockets, but that can be overkill for a lot of people.

 

[Steve Shannon]

If your rocket is trying to corkscrew immediately that might be what's bending your rod.

 

[ksaves2]

Here is a really cool corkscrewing rocket. Usually brings it to Airfest every year. I believe it's an 12X Upscaled Drake

I believe that is achieved by an off center mounted motor mount tube. Look at then end of the rocket and it's akin to a moon burner motor. The off center motor leads to that nice cork screwing. I believe it's been to Midwest Power or
somebody brought something like it there.
Kurt