Stability Issues/Help

Hi all,

I had a non-optimal first flight with my wildman vindicator mini last weekend (July 5th) and I'm still not sure what went wrong so I'd like some ideas from y'all about where to look next.

Attached and below are in open rocket sim file and some photos of the actual rocket. It's modified slightly with a bulk head inside the airframe to create a payload bay, altimeter in the nose cone and a cable cutter setup to do dual deploy.

Total lift off weight with motor was 23.5oz including a 3g case with 2 spacers and the CG of the rocket matches well with the simulation. I used a 29mm 1G CTI F36 Bluestreak motor - powerful enough to handle the liftoff weight but total altitude predicted at 1000ft so I could see all the events (this was my first altimeter flight and dual deploy flight). I had the RRC2+ set to fire "Drogue" at apogee to separate the nosecone and "main" at 500ft to activate the cable cutter.

The flight went as follows:

1) Slight shimmy off the rail
2) ~100ft of straight up
3) Rocket goes unstable, flips end for end and starts skywriting.
4) Altimeter fires both Drogue and main.
5) Rocket descends under chute.

As far as I can tell (2), (4) and (5) were optimal (someone can confirm if my assumption about the RRC2+ that firing the drogue at under 500ft also triggered the main charge) I'm concerned about (3) and possibly (1).

OR predicts that the rocket has 2.2 calibers stability so my current thoughts about the bad flight are:

1) Some error in the construction of the rocket - I notice three things (photos below).
One fin set is slightly off perpendicular to the airframe (each pair seem perfectly in line and aligned well front to back).
There is a slight lip between the nosecone and the airframe.

2) Off center drag from the rail buttons (photo attached)

3) Something wrong with the motor (I checked it post flight - it looks fine no burn through or anything)

4) Something wrong with the sim. I'm not sure about this either - the CG is well up the rocket and matches with the actual rocket - the CP looks reasonable compared to other sims and rockets I have flown.

5) Too much nose weight in a skinny rocket with small-ish fins?

Any other ideas?

I have an extra piece of airframe and a coupler (from the DD option wildman offers) so I could make the rocket longer quite easily - I could also lose some weight from the nose (replacing a beefy eyebolt with something else.

Thanks for any input

Zeb



Post flight layout showing Chute, Cable Cutter, 2 starters (1 for main charge which sits on the bulk head inside the airframe under the chute and the other for the CC)

Slight gap and lip on NC

Tail with small fillets and airfoil buttons

Inside the NC - the altimeter mount slides onto the rods

Motor - post flight. I removed the ejection charge and sealed it with epoxy and a penny

Nozzle - nothing wrong here

One canted fin - it's not quite as bad as it looks in this photo but it is a few degrees off

Open rocket file

View attachment Wildman_Vindicator_Mini_2.ork

Can't look at your ORK at work, but subscriber so I can see what others have to say.

I get about a 4.8:1 thrust to weight. And a burnout speed of 52.8 m/s with burnout at 122 feet. peak altitude at 543 feet.

Try a 3 grain motor and see what it does.

Interesting - my OR is showing burnout at 185 and peak altitude at 800+ - what did you simulate with?

"rocket calculator" an app for the iPhone. It's a generic sim and only uses BT size, weight and motor data, but I've found it more accurate than Rocksim. (until you dial in the drag numbers of the sim with actual flight data).

Any wind that day? I've noticed that rockets with small fins could be very stable at zero angle of attack, but the CP can move forward very quickly as AOA increases, say, when there's a gust of wind as the rocket leaves the rod. In OpenRocket you can simulate this using the Component Analysis tab...check to see what happens to the CG/CP relationship at varying angles of attack.

Some wind but not a lot (5mph maybe be) the rocket landed within 100ft of the pad.

Well, I'm out of ideas except that it sounds like some kind of dynamic instability. You described a shimmy, and maybe it increased in intensity as speed increased until the fins couldn't dampen it anymore? Hopefully someone with a better grasp of the physics can weigh in

Simple.

Fins do not stick out far enough into the airstream. They are sitting in the boundary layer and they do not do very much until the rocket pivots as it flies and they are then exposed to the free airstream and they push the aft end of the rocket back. This will repeat back and forth like a pendulum (and it can be a 3-D pendulum which is "coning") where the aft end just keeps swinging out and around. If it swings back and forth enough the fins may not be able to correct it and it will just go unstable or the nose will fall off and it will really go unstable.

TARC rockets with tiny fins ("because the simulation said it was "overstable"....") do this all the time. When the fins stick way out into the free airstream, the rocket goes as straight as a laser beam. The only reason an overstable rocket does not go straight is if it leaves the launch rod or rail at a very low speed compared to a cross wind. If there is no wind and if the rocket leaves the rod at very high speed, there is no weathercocking.

shreadvector said:

Simple.

Fins do not stick out far enough into the airstream. They are sitting in the boundary layer and they do not do very much until the rocket pivots as it flies and they are then exposed to the free airstream and they push the aft end of the rocket back. This will repeat back and forth like a pendulum (and it can be a 3-D pendulum which is "coning") where the aft end just keeps swinging out and around. If it swings back and forth enough the fins may not be able to correct it and it will just go unstable or the nose will fall off and it will really go unstable.

TARC rockets with tiny fins ("because the simulation said it was "overstable"....") do this all the time. When the fins stick way out into the free airstream, the rocket goes as straight as a laser beam. The only reason an overstable rocket does not go straight is if it leaves the launch rod or rail at a very low speed compared to a cross wind. If there is no wind and if the rocket leaves the rod at very high speed, there is no weathercocking.

Click to expand...

Interesting. My assumption would be that smaller fins push the CP forward, but if he has an acceptable CG/CP ratio, he should be able to get a stable flight. Obviously Wild Man believes this rocket is stable or they wouldn't sell it. I've seen finless rockets fly straight.

But I don't have a better answer.

shreadvector said:

Simple.

Fins do not stick out far enough into the airstream. They are sitting in the boundary layer and they do not do very much until the rocket pivots as it flies and they are then exposed to the free airstream and they push the aft end of the rocket back. This will repeat back and forth like a pendulum (and it can be a 3-D pendulum which is "coning") where the aft end just keeps swinging out and around. If it swings back and forth enough the fins may not be able to correct it and it will just go unstable or the nose will fall off and it will really go unstable.

TARC rockets with tiny fins ("because the simulation said it was "overstable"....") do this all the time. When the fins stick way out into the free airstream, the rocket goes as straight as a laser beam. The only reason an overstable rocket does not go straight is if it leaves the launch rod or rail at a very low speed compared to a cross wind. If there is no wind and if the rocket leaves the rod at very high speed, there is no weathercocking.

Click to expand...

I think Fred's got it...I read this and remembered what G. Harry Stine said about coning in the Handbook.

The slight discontinuity between your nose cone and the tube mouth may be tripping the boundary layer and exacerbating the problem...the fins may be in turbulent air most of the time, setting up small oscillations that become bigger with increased energy/airspeed.

shreadvector said:

Simple.

Fins do not stick out far enough into the airstream.

Click to expand...

How big do you think the fins on this kit should be to avoid this issue?

Zebedee said:

How big do you think the fins on this kit should be to avoid this issue?

Click to expand...

Maybe you should contact Wildman for advice...this is probably not the first time this has come up. There must be a solution besides enlarging the fins?

Maybe the answer is to extend the airframe tube since you already have the part from the DD kit. If Fred's theory is correct, then lengthening the airframe could accomplish 2 things: 1) give the boundary layer more time to reattach to the airframe before reaching the fins, and 2) increase the length of the lever of the body; if the fins are not in clean air, then a longer body means that a smaller angle of attack will put one or more of the fins into the airstream. That would both slow down the speed of oscillation, and reduce the amplitude.

But seriously, try contacting Wildman as they may already have a solution.

It stayed cool til burnout...then flipped yes? I really think it just wasn't enough motor... It kind of lobbed, then didn't have enough speed after burnout and knuckleballed. Put something faster in there and see what it does. I'm betting it likes it.

No - it was still going when it flipped. I'm happy to try a larger motor - the RSO might not be though

I also have an 8" piece of airframe and coupler I can use to make it longer (and maybe give the airflow more chance to settle near the fins).

Just to rule things out, is there any possibility that the rocket encountered resistance on the rail?

Beyond that, if you pop the numbers into thrustcurve.org the speed off the rail indicates too slow at end of the rail @47 ft/s. If you maintain a 4:1 forward to wind speed as recommended by NAR your wind speed could be 8mph. This is pretty calm actually so you may not have had the needed speed. And with smaller fins, I would guess your more apt to suffer from the CP moving forward with greater angle of attack.

As for the RSO, I would talk to him about it. The more you are familiar with what might have gone wrong, the more likely they are to give another chance. I would expect the flight to be called heads up at any rate.