Why a Video Rocket might spin, or might not.

Fin Alignment... nothing more. You want to win an altitude contest... make your fins straight. Want no spin in "down looking video"... make your fins straight.

Doug

Hi Doug:

Of course fin alignment is the most obvious and most important factor in achieving spin-free boosts.

But apparently, it is not the only factor. I'm sure you've seen the down-looking videos that spin first in one direction, then the other. I don't think this phenomenon can be explained by fin alignment alone. I believe wind shear effects play a complex secondary role.

I have also noticed that in many videos, the spinning stops at propellant burnout. I have been wondering if there is some spinning of the gasses inside the combustion chamber, not coming out exactly in an axial fashion, but more like water going down a drain. This would cause the airframe to want to rotate in the opposite direction.

Just a hypothesis...

Bill the Geek

uneven nozzle erosion perhaps? shockie B)

The mirror shroud also needs to be as straight as possible.

In the case of the I-roc, the rocket has this hitch hiker on the side.

In the Brusier, the camera is just out their in the wind, bolted on the side of the payload bay, looking down at a slight angle. the shroud was left off, as the rocket would not be going super sonic. On a K660, the cam did not even move on it's axis mount.

I had it angled slightly out from stright down., to avoid seeing the payload coupler after apogee, like in pervious Videos I had done.

I posted a small picture of it on a.b.m.r.

so those two things are different between both the rockets.

Acceleration decreases very rapidly after MECO.

I have been wondering if there is some spinning of the gasses

Cool. I just made up a "hitchhiker" like that to put a PerfectFlight mini-alt onto my smallest camera rockets. I can easily swap it from one rocket to another as needed. I made it out of an aluminum cigar case and an old balsa nosecone.

Next comes an L800 for the B6-4 milk run.

Probably wont be till aug till a field around here opens up for that.

Around October, I hope to put the new CTI M560 long burn in it for the C6-7 it's gone flight.

Grain geometry has a lot to do with spin. I suspect you'll see quite a bit of spin with a bit of corkscrew with the M560 since it is either a c-slot or moonburner....can't remember which.

Mike Fisher

Good point. Yes.

I have a great photo shot of a Big Brute on a F20 c-clot that really shows the flame offset.

It will be an interesting test.

The rocket is only 24lbs sans motor.

Well, there's dumb luck, fin alignment and ** how many fins you are using **.

In a 3-fin rocket, there is coupling between angular momentum about the long axis and that purpendicular to it. Thus, as the fins correct the rocket's path, they turn the rocket about its axis.

In a 4-fin rocket, there is no such coupling - at least not through fin corrections. That doesn't mean the rocket won't spin, but it should not turn because of stability corrections.

Regards,

-Larry Curcio

Interesting. so far, the rockets I've used are 3FNC.

I do understand the ideal of the four fin units since you mention it, and will have to look further into setting up a 4FNC unit to test.