Corkscrewing rocket

Hello!

I designed a rocket based on the Little Joe 2 design, and it flies well on a C engine. However, it corkscrews and spins vertically after launch and through the ascent. Any ideas why? It is nose heavy but stable during the stability "spin" test.

https://www.info-central.org/?article=124

The last paragraph is probably what you are experiencing.

FC

And here I was just going to suggest that his fins were mis-aligned...

Are we talking about this kind of corkscrewing...


(not my images)

Which is a result of one or more fins out of alignment (or problems with the motor's alignment)

or this (spiraling)?


(again, not my image)

Which is caused by CP/CG problems.

It could just be too slow. Would you describe the lift-off as slow? If your rocket isn't leaving the launch guide (rod) fast enough, your fins aren't able to stabilize your rocket. If your rocket goes straight, then suddenly starts to corkscrew, then maybe a gust of wind higher than ground level could have knocked it and caused some weathercocking. If flying too slow the fins won't be able to correct it.

Run a sim and see what speed it leaves the pad at.

Jerome

K'Tesh said:

Are we talking about this kind of corkscrewing...

(not my images)

Which is a result of one or more fins out of alignment (or problems with the motor's alignment)

Click to expand...

How far out of alignment do they have to be for that?

lackland1982 said:

Hello!

I designed a rocket based on the Little Joe 2 design, and it flies well on a C engine. However, it corkscrews and spins vertically after launch and through the ascent. Any ideas why? It is nose heavy but stable during the stability "spin" test.

Click to expand...

"flys well on a C engine" and "corkscrews and spins vertically after launch" seem to be mutually exclusive...so, it flys ok for a second and then has problems?

It is the spiral....sounds like the fins. CG checked okay when I did a CG/CP check and a swing test...

It climbs well and seems stable along the horizontal axis...it begins to spiral after leaving the pad...

lackland1982 said:

It is the spiral....sounds like the fins. CG checked okay when I did a CG/CP check and a swing test...

Click to expand...

Did you do this check and test with the motor installed?

How much does this rocket weigh without as motor?


Jerome

All it takes is a slight cant on a fin and they will do that. Another thing is airfoiling the fin. If you try to streamline the fin and get the fin tapered too much to one side you have a wing that wants to lift. The only direction it can go is in a circle.

One thing I've got away from is trying to center a fin on a line. The thicker the fin the more it can obscure the line. The top right corner can be sitting in the line and so can the lower left corner but you have built in 1/8th" of cant into a fin. I set the fin next to the line, making sure all the fins are placed the same way.

It could be the accumulation of small things that added up cause the spiral. Let me confuse things a bit, was it spinning as it was spiraling?

Yes, it was doing both -- the rocket spins and spirals as it climbs vertically. It looks as if there is more than one issue, but the fins seem to be the main culprit. I designed and built another rocket which is much more stable. I may fly it again, or I may just strip it for spare parts and start over again. I don't think the weight is a factor as it comes in at about 3 ounces.

Well, all that activity keeps the altitude down...and it is entertaining...as long as no one loses an eye...heh

lackland1982 said:

Yes, it was doing both -- the rocket spins and spirals as it climbs vertically. It looks as if there is more than one issue, but the fins seem to be the main culprit. I designed and built another rocket which is much more stable. I may fly it again, or I may just strip it for spare parts and start over again. I don't think the weight is a factor as it comes in at about 3 ounces.

Click to expand...

it sounds like you have more than one problem. A 3oz (dry weight) + a motor is pushing the maximum lift-off weight for a "C" motor. So all the spinning and spiraling will bleed off more critical speed exacerbating all of your issues. Standard CP calculations are not the same on stubby rockets that are less than 10:1 length to diameter. Even sim programs have problems with this. More than likely your rocket is far over-stable.

I'd say start over.


Jerome

dave carver said:

One thing I've got away from is trying to center a fin on a line. The thicker the fin the more it can obscure the line. The top right corner can be sitting in the line and so can the lower left corner but you have built in 1/8th" of cant into a fin. I set the fin next to the line, making sure all the fins are placed the same way.

Click to expand...

I draw two lines and use them to tape metal angles to the body with a gap in which the fin just fits. Almost impossible to put the fins on canted then since the angles align themselves on the long axis of the tube.

Zebedee said:

I draw two lines and use them to tape metal angles to the body with a gap in which the fin just fits. Almost impossible to put the fins on canted then since the angles align themselves on the long axis of the tube.

Click to expand...

I like that idea. Easy to get the two lines in the right place, too, using the trusty Estes marking guide. Just make your first set of marks then twist the tube around a bit for the second line. I don't have any problem with them sticking straight out but I still might give it a try, I got the angle aluminum.

K'Tesh said:

Are we talking about this kind of corkscrewing...

(not my images)

Which is a result of one or more fins out of alignment (or problems with the motor's alignment)

or this (spiraling)?

(again, not my image)

Which is caused by CP/CG problems.

Click to expand...

Oh, I want one of those, mama..:jaw:..get me one of those...

Can you teach me how to do THAT? I want one that will fly THAT WAY!>>>> :lol: LOL!

BTW this particular Trident spiraling as it exited the water has nothing to do with cp/cg placement. It was caused by a structural failure of a thrust vector actuator.

I have a Vulcan (cone shaped rocket for those unfamiliar with it) that I put too much noseweight in, and it corkscrewed off the rod just a bit every time until I refitted it with a launch lug that goes straight through the airframe. I suspect that it's the excessive noseweight combined with the slightly off-kilter rod exit that did it. I do wish I had tried it on a longer rod.

A very slightly misaligned fin AND a heavy nose might result in the same thing.

My daughter had a Screamin' Demon that corkscrewed fast on every flight for no reason anyone could find. Simple 3FNC with TTW fins, about the diameter of an Alpha but almost twice as long, and the fins looked perfectly straight.

The "Gambler Effect" -- rail buttons are not aligned with the center axis of the body.....