Stability Question

[mkmilion]

I was out lauching a bit today and found that one of my rockets kept doing loops. I built it exactly per instructions. So why would it do that? It definately wasn't weather-cocking. BTW the kit is the Starlight Rockets: Spike. I might I add due to this episode I quite disappointed.
I hope you all can help me out. Thanks.

Ed

 

[shreadvector]

I am unfamiliar with that rocket. Please post a link to a website with info and/or photos of the rocket.

Stability problems can be caused by the design of the rocket or even by the motor.

The following is cut & pasted from my standard document on the subject:

Why would a normally stable rocket fly unstable when using a motor that it flew stable with before?
Did you look at the nozzle? We have had several VERY scary "flights" where the rocket had little thrust and/or veered into cruise missile mode. After crashing and putting out the brush fire, we examine the nozzle and find that it is either too wide (wider than normal at the throat) or it is eroded asymmetrically. The asymmetric erosion is bad and you can clearly see the exhaust residue all over the missing area of the nozzle indicating that it disappeared at ignition or shortly thereafter.

All unstable flights with Estes motors from years "A" and "B" and maybe "C" need to be inspected and if the motor/nozzle is the cause, a M.E.S.S. form filled out and the manufacturer notified. The least that will happen is a package of replacement motors and a kit. The most that will happen is an improvement in materials used in manufacturing and a product that performs like we remember for decades and decades.

I hope this info helps folks.

-Fred Shecter NAR 20117

Originally posted by mkmilion
I was out lauching a bit today and found that one of my rockets kept doing loops. I built it exactly per instructions. So why would it do that? It definately wasn't weather-cocking. BTW the kit is the Starlight Rockets: Spike. I might I add due to this episode I quite disappointed.
I hope you all can help me out. Thanks.

Ed

 

[radiO]

very interesting!

 

[mkmilion]

I know it definately that the motors weren't an issue due to the fact that yes I did check them pre- and post flight. Not to mention that I launched it 4 times. Until it finally got injured. I had the same results every time. Loops, loops, loops.

As for that link: https://www.jonrocket.com/Starlight.html
It's on this page. Just a reminder it's the Spike. The one w/ dowel fins.

 

[jflis]

well, several things

possible CG/CP problem, which can be helped with nose weight.

Concern about the dowel fins. If they aren't straight it would cause all sorts of problems.

Cool design though

 

[shreadvector]

How many Gigabytes is that website. And folks complain about Estes' site being slow to load. Geez.

OK a half an hour later it has loaded enough of the JPEGs for me to see and read the Spike. It looks like it is similar to the Custom rocket that is also quite unstable straight from the package.

You see, dowels can vary in density. Just because the prototype worked for the designer, does not mean it will work for all production kits. AND if the end user mis-aligns a dowel even a tiny bit, the de-stabilizing force is EXTREME. These kits should be skill level 4 with many safety warnings. Quite a fire hazard.

Anything on emrr about these?

Originally posted by mkmilion
I know it definately that the motors weren't an issue due to the fact that yes I did check them pre- and post flight. Not to mention that I launched it 4 times. Until it finally got injured. I had the same results every time. Loops, loops, loops.

As for that link: https://www.jonrocket.com/Starlight.html
It's on this page. Just a reminder it's the Spike. The one w/ dowel fins.

 

[JStarStar]

Ahh, does this bring back memories, to the very first rocket I ever built, an original-design balsa fin Astron X-Ray, back in the early spring of 1969, when I was a bright-eyed lad of 10.

I read all sorts of good stuff on airplanes and rockets, and I read how important it was that you sand the fins to an 'airfoil shape.'

So I did. Built the rocket, sanded the fins, glued 'em on, launched the rocket. It takes off, goes up about 50 feet, and immediately launches into a huge loop-de-loop. About the third loop around, whammo, into the ground.

I go over and pick up the pieces. Wha'hoppen? Well, I didn't know what the heck I was doing at the time, but I did figure it out - the "airfoil shape" I had sanded the fins into was not a symmetrical airfoil - it was a lifting airfoil, the kind you see on airplane wings - rounded on the top, flat on the bottom.

I had glued the fins on in the exact sequence that had two fins lifting in one direction, and the other two lifting at a 90 degree angle. So, once the rocket picked up any air speed, of course it did a loop.

So I know now what went wrong. What this has to do with your problems I'm not sure, other than I'd guess that you probably do have some kind of situation where your fins were generating lift (or, probably, disproportionate drag), which threw the rocket into a loop.

 

[teflonrocketry1]

What motors did you use when you observed this unstability? It would happen just to be all C's would it?

Send or post the exact dimmensions of this model and I will generate a RockSim version 5 file for that can be viewed with the demo version.

Bruce S. Levison, NAR #69055

 

[mkmilion]

I agree manufacturers should do very extensive testing on there models before any mass production takes place. I can say the alignment is as close to perfect as you can get with the naked eye. If it's of it's only a few thousandths. I guess in this case it created a butterfly effect.
As for the engines they were all C6-7's.
The dimensions are: 11 7/8" for length 3/4" for width. The dowels are 6 x 1/8" and the main BT w/ NC is 6 7/8".

Shread,
Sorry I didn't warn sooner about that site. BTW nothing on EMMR.