TARC challenge instability

[nate_dog747]

Hello everyone!
I am new to this forum, so please excuse any ignorance. Our TARC team launched recently with an Ecnojet F20-7W, and we made it to 927 feet at apogee. The flight was an overall success.

However, there was concerning stability issues off the launch rod (96 inches). The Rocket spiraled off the launch rod. I have attached a video of the launch to help visualize the issue. This spiraling was not an issue for our past launch of Estes F15-6 (an apogee of 704 feet, the video is attached second). I am not sure if this is a stability issue (the stability is at 1.2-1.3 based on open rocket with the F15-6 motor, I haven’t checked the stability on the F20-7W). My current theory is that the motor mount is not parallel with the rocket.
View attachment IMG_5144.MOV


View attachment IMG_5155.MOV
Let me know what you think the problem is, and how you would go about fixing it. If you have any questions, please let me know, I am unsure of what is needed to diagnose the problem. For reference, a picture of the rocket is attached third. However, the rocket has been changed to have smaller fins.

Thank you in advance for your help!

 

[Zeus-cat]

Sorry, I don't have an answer to your question. That second launch was great.

I would launch again using another F20 to see if the problem repeats.

 

[waltr]

My GUESS is that the F20 is heaver therefore moved the CG back and reduced stability.
The F15 is 60gram
the F20 is 80gram
So 10-20gram of weight in the nose should fix it.

Always check the CG position with the rocket fully loaded with motor, chute, etc before launching.
I like putting a small mark on the BT for the CP (from Open Rocket sim) and expected CG range.

 

[Egoldee]

I would recommend checking the stability and because you are almost 100 ft higher than you need to be you can add all the whigt to the nose to help with the stability.
Please don't fly it untill you check the stability, rockets can go sideways.

 

[boatgeek]

Unless you really larded up the aft end of the rocket with weight, I don't think it's unstable. Especially with 2 eggs in the nose, most TARC rockets are very stable. Your fin size seems reasonable, though it's always worth looking at the simulation.

You might look at whether the launch pad can wobble. We had issues with unpredictably squirrely flights until we staked out the launch rail to hold it in place better. Our launch pad is similar in design to yours.

 

[Zeta]

consider the possibility of a mass inside the rocket that was shifting. our tarc team had a flight like that last year because the clay weight that was added was not secure. not sure if this is possible with your design, but something to check

 

[tim cubbedge]

Our TARC team has similar problem last Saturday. They were using E30-7 and both flights the rocket went up for the most part but was spinning pretty wildly. Never thought about their cargo being loose.

 

[boatgeek]

Our TARC team has similar problem last Saturday. They were using E30-7 and both flights the rocket went up for the most part but was spinning pretty wildly. Never thought about their cargo being loose.

I've also seen that where ballast was attached to the shock cord. Hopefully, the eggs aren't swinging around in the body tube.

 

[tab28682]

Once in a blue moon you can get a nozzle anomaly with a motor that can cause a spiral under power.

 

[aerostadt]

The stability in the second video looks much better. I tend to think wobbly TARC rockets can tend to be unreliable on consistently meeting the TARC target altitude. I think that there might be a tendency to make TARC rockets that just meet the minimum length. This seems to be an obvious design concept, because shorter length should weigh less. However, the cardboard tubing is pretty light. I have seen some TARC teams make longer rockets that I thought flew better and more consistently and the weight criteria was still met. In fact lead shot could be added to reach the target altitude.

 

[Andrew Brown]

A potential issue is the rail guides. Having one rail guide on the booster and one on the payload tube could be problematic. If there is any torquing, the payload tube is free to rotate and the two guides don't stay colinear. This would cause them to bind on the rail. That might be your initial shift in direction off the pad, followed by oscillations as the rocket is correcting through it's natural stability. This would probably show up as a random issue regardless of what motor you select.

 

[lcorinth]

I tried to look this up in a couple of my books, but skimming through them, I wasn't able to locate what I was looking for.

But to me, this looks a little like what I think is called coning. An oscillation in one axis and an oscillation in another axis kind of amplify each other, resulting in a kind of corkscrew motion.

Notice, the rocket never actually became unstable. It stayed nose up. But I think excessive coning can cause a rocket to go unstable.

It may be a one-off fluke, or may happen again. I'd maybe check fin alignment to see if something is off there. Or maybe it got a bad start off the rail, due do some corrosion or a misaligned rail button, or something else.

But I'd be interested to see the rocket's profile to see how large the fins are. Perhaps it is actually marginal. Can you post a picture?

 

[Andrew Brown]

My GUESS is that the F20 is heaver therefore moved the CG back and reduced stability.
The F15 is 60gram
the F20 is 80gram
So 10-20gram of weight in the nose should fix it.

Always check the CG position with the rocket fully loaded with motor, chute, etc before launching.
I like putting a small mark on the BT for the CP (from Open Rocket sim) and expected CG range.

Both OpenRocket and thrustcurve have the F15 at 102 g, not 60 g. That puts the F15 at lower initial stability than the F20.

 

[BEC]

Yeah...F15 has 60g of propellant in it.

Lots of good things to check already listed in this thread.

 

[waltr]

Both OpenRocket and thrustcurve have the F15 at 102 g, not 60 g. That puts the F15 at lower initial stability than the F20.

My bad, I read propellant weight for the F15 instead of total weight.
Therefore my guess is wrong.

Maybe then the instability is due to some of the other reasons listed above.