A stability problem

I have question regarding a recent launch failure.

Last weekend I decided to launch a rocket. It is the largest one I've built, 6'6" high. It was windy last weekend, measured wind speed around 12mph with gusts up to 20mph.

I always have problems with recovery in wind and usually launch a small pilot rocket to test the conditions. In this case it was a small 15" rocket. This was the second rocket I built, designed using OpenRocket and accurate to the design in terms of dimension and C.o.G.
I've flown the rocket five times before using both C and B motors. On all previous launches the flights were straight, stable, and visually consistent with the design.

On this occasion however things were different. I launched the rocket with an Estes C motor. The launch site was about 20' in front of an 8' hedge. The wind direction was exactly normal to the back of the hedge.
The rocket ignited well, the igniter remained on the ground attached to the clips.
The rocket rose cleanly from the rod and flew approximately 50' vertically into the air before turning 90 degrees then spiralling downward to land in the top of a ditch (in front of the hedge) about 70' from the launch position. The trail of smoke marked it's path visibly.
After a couple of seconds following it's landing in the ditch, the ejection charge audibly and visibly fired.
On inspection the rocket was undamaged, the nose cone and parachute had separated, the shock cord was intact. There was no apparent reason for why the rocket had become unstable.

For the first time I'd decided to video the launch. In stop frame the video confirmed a clean launch off the rod. I do not believe that any instability occurred at that level.
I was able to capture the rocket at a few points in the spiralling phase. Initially the rocket appeared normal in shape (though it should be said, very blurred on the image). On later images however, there appeared to be two distinct but indeterminate shapes. One conclusion I've drawn is that the nose cone separated during the latter part of the flight. This was following the initial unstable and spiralling flight.

The condition of the spent motor was normal in all respects.

I know that the separation was not initiated by the ejection charge as it had not ignited in the air. I can only conclude that the separation resulted from the unstable conditions rather than it causing the unstable condition.

In short and following the previously successful launches, I haven't got a clue why this one failed.

Can anyone help with a reason? I'm really keen to know why this one went wrong so that I'm aware of whatever risks there may be in larger rocket behaviour.
Needless to say I still haven't launched my 6'6" rocket.

Knowing why this happened is obviously important in making sure it doesn't happen again. There is however a far more important lesson to be learnt.

Because of the wind direction my grandson and I had moved to the far side of the field that we usually launch in. As such the usual mob of hecklers and forced spectators (my family) were watching from the far side of the field. The landing position of the unstable rocket may well have been where my family might normally have been standing!
Having had 100% success in all of our launches to date! it's clearly easy to become over confident and complacent.
Even low powered rocketry is very dangerous and carries risk.
There is much to learn by failure and much to gain from experience. For the time being I'll stall may ambitions to "fire higher" until I fully understand what can go wrong at a low level. Your help to explain will be appreciated.

As a footnote, I read an old thread last week from a guy that wanted to launch a car battery as a rocket payload. At the time I smiled at the comments and reminders of the Top Gear episode when they launched a Reliant Robin car.
In hindsight the launch suggestions in that thread were clearly horrific!

SO.

before anyone can give an accurate post failure opinion we would need to SEE the model or Design involved.
Just reacting to your post I'd say the model more then likely has fins of insufficient size to keep the model stable in all but near Ideal flying conditions. This may be the fin design itself or just the number or Square Inch Area. Impossible to tell without actually seeing the model involved.

Most Simulation programs do not automatically take into account wind gusts at high angles of attack. I have little experience with Open Rocket as it just will not load on my machine, but have been using Roc-sim and SpaceCad for some time. I've noticed they both tend to allow fins and static margins that are IMHO to small for stable flight in all but near ideal flying conditions (winds under 10mph).

Upload a photo of the model so we can better analyze the design. CP/CG locations would also be helpful.

concur that a pic of the rocket would be helpful.
your 20 mph wind gusts certainly didn't help the situation, and in combination with a marginally unstable rocket may be the answer to you question.
I wonder if that 8 foot hedge in combination with 20 mph gusts might create some pretty weird vortexes, although you said rocket was well above it ("50 feet") when it went squirrelly.

NAR safety code:
9.Launch Site. I will launch my rocket ..... in safe weather conditions with wind speeds no greater than 20 miles per hour.
https://www.nar.org/NARmrsc.html

Smart rocketeers know when to launch. REEEEAAALLLY Smart rocketeers know when NOT to launch.

how heavy is the nose? how tight a fit is/was the nose and body tube?
Rex

My friend has an Estes Majestic. He's launched it multiple times on an Estes black powder F-15 with no problems. Then one windy day it got about 50 feet up and started doing loops while still under thrust. It hit the ground at an angle and a short while later the ejection charge popped the chute out.

Everyone's reaction was "wind."

He' launched it since on days with lighter wind with no problems.

There's something about three finned rockets that makes them unstable in wind gusts.

Rex R said:

how heavy is the nose? how tight a fit is/was the nose and body tube?
Rex

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My thought as well, sounds like drag separation.

You took some video - can you post it?

That might give us some clues.

This was the 15" rocket on a C motor? Can you give a more detailed description? 15 inches, and it looks like a standard Estes Alpha, or it looks like a Big Daddy?

About how much does it weigh with the motor installed?

a few comments, we shouldn't jump all over the OP for not following the NAR code, he lives in the UK. I do agree that 20 mph winds are a bit high to be flying. We don't know how many fins the rocket has...
a loose nose might have moved forward by a sudden change in the rockets attitude, or by the motor burnout (or both). however most of this thread is just speculation.
Rex

Rex R said:

a few comments, we shouldn't jump all over the OP for not following the NAR code, he lives in the UK.

Click to expand...

... where we also have a safety code. We're not supposed to launch into winds over 20mph either.

We don't know how many fins the rocket has...
a loose nose might have moved forward by a sudden change in the rockets attitude, or by the motor burnout (or both). however most of this thread is just speculation.

Click to expand...

Agreed. We need to see pictures of the rocket, and preferably the video, to make any informed guesses about what happened this time. In particular, does the video show the rocket in two pieces before or after the spiralling starts? That would settle the question of whether a loose nose cone caused the unstable flight.

Even if the nose was not fully separated, if the instability started about 1.5 seconds after launch (which is when the thrust of a C6 burns out and the rocket starts to decelerate), the nose may have shifted part way out of the body tube and made the rocket unstable. Then during the spiralling flight the nose could have completely separated. But without seeing the video, this too is pure speculation.

Rex R said:

a few comments, we shouldn't jump all over the OP for not following the NAR code, he lives in the UK. I do agree that 20 mph winds are a bit high to be flying. We don't know how many fins the rocket has...
a loose nose might have moved forward by a sudden change in the rockets attitude, or by the motor burnout (or both). however most of this thread is just speculation.
Rex

Click to expand...

Do note folks WE ARE allowed to fly in winds Up to 20mph pre item #9 of our Safety Code.

As stated in my original post we really can't give any kind of concrete response until we see the Vehicle involved particularly the size and shape of the fins.

Assuming this rocket has flown just fine in the past and the failure was not caused by a motor issue.

If it nosed with the wind it was probably some event near the nose causing the Cp to abruptly move forward. If it nosed into the wind and did not correct itself it is mosts likely an incorrect dampening moment coefficient.

If you want to learn more about this, Tim from Apogee wrote a nice newsletter about it.
https://www.apogeerockets.com/downloads/Newsletter195.pdf

If you want to dig deeper, MIT press has a good book on advanced amateur rocketry.


Sent from my iPad using Rocketry Forum

Thanks all for your helpful and interesting replies.

I will try and send the OR design.

Regarding the wind speed, I would repeat that the average wind speed was 12mph. Though possible it was improbable that a gust of 20mph hit at the point of launch.

SO.

I don't think wind at ground level is an issue in this situation. At this time of year, it is normal for significant wind shear to be present at relatively low altitudes. These are both velocity and directional changes. Gusting in the spring and fall is partly a result of vertical changes in the plane of the shear. The sudden deviation would seem to indicate the rocket crossed a shear boundary. It wouldn't take a significant directional delta between levels to dislodge the nose, which would alter the rocket's stability.

Space Oddity said:

Thanks all for your helpful and interesting replies.

I will try and send the OR design.

Regarding the wind speed, I would repeat that the average wind speed was 12mph. Though possible it was improbable that a gust of 20mph hit at the point of launch.

SO.

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I think I may have found an answer which appears to be nothing to do with wind.

I tried two more launches with the same motor last evening. The first with a B6-4 motor, the second with a C6-7.

The rocket was prepared in exactly the same way for each flight. I did add a layer of masking tape to the nose cone for a tighter fit. The wind was negligible (<4mph) with no gusting.

The first flight (with the B6-4) flew perfectly. Clean launch, vertical flight, good separation, and predictable landing.

The second flight (with the C6-7) launched perfectly. Then at around 200' veered at about 45 degrees then started to corkscrew downward again. I saw the nose cone separate at about 100'. The ejection charge fire at approx. 50'.

Once again, no damage to the rocket.

I can only conclude that the problem is with the use of a C motor in it. Maybe the wind affected the height at which instability occurred, but was probably not to blame for the basic problem.

If any of you are still interested I will post the OR design but conclude the rocket is too risky to launch further. It's now consigned to the back of the shed.
I still don't understand why I've previously had good flights on a C motor.

SO.

None of this is inconsistent with the previous comments about wind. Again, what's happening on the ground with respect to wind is no indication of what's happening at 50 or 200 or 5000 feet.

Space Oddity said:

I think I may have found an answer which appears to be nothing to do with wind.

I tried two more launches with the same motor last evening. The first with a B6-4 motor, the second with a C6-7.

The rocket was prepared in exactly the same way for each flight. I did add a layer of masking tape to the nose cone for a tighter fit. The wind was negligible (<4mph) with no gusting.

The first flight (with the B6-4) flew perfectly. Clean launch, vertical flight, good separation, and predictable landing.

The second flight (with the C6-7) launched perfectly. Then at around 200' veered at about 45 degrees then started to corkscrew downward again. I saw the nose cone separate at about 100'. The ejection charge fire at approx. 50'.

Once again, no damage to the rocket.

I can only conclude that the problem is with the use of a C motor in it. Maybe the wind affected the height at which instability occurred, but was probably not to blame for the basic problem.

If any of you are still interested I will post the OR design but conclude the rocket is too risky to launch further. It's now consigned to the back of the shed.
I still don't understand why I've previously had good flights on a C motor.

SO.

Click to expand...

Again this is all 98% speculation, but I'll throw in my two cents worth that it sounds as if the model is dealing with wind shear -- you are encountering different wind conditions from flight to flight which affect the performance of the rocket with inconsistent and seemingly illogical results.

The thrust profiles of the C6-7 vs that of the B6-4 are essentially identical with the difference the B6 burns out at 0.86 seconds while the C6 continues to burn for approximately 0.9 more seconds.

Both motors feature a high initial thrust spike peaking at 12 newtons at 0.2 seconds intended to kick the rocket off the pad and up to stable flying speed. (We know from practical experience, onboard vids and also flight sims that many rockets, in fact, have not left the launch rod before this spike phase ends.)

After the initial spike, thrust drops and levels off at approximately 4 newtons for sustaining thrust which lasts about 0.5 seconds for the B6 and 1.5 seconds for the C6. Rate of acceleration should be roughly constant throughout that period so IMO it is unlikely that any sudden change in direction of flight during that sustaining phase would be due to the function of the motor.

https://www.nar.org/SandT/pdf/Estes/B6.pdf
https://nar.org/SandT/pdf/Estes/C6.pdf

As a completely speculative guess I would suspect that on the C6-7 flight, the rocket experienced a significant wind shear almost coincidental with the moment of burnout. Traveling upward in unpowered coast phase, the rocket would naturally tend to nose into the direction of any sudden lateral wind gusts, thus accounting for the 45-degree veer.

As others have mentioned, a 20-mph wind, or even a 12-mph wind, will certainly have noticeable effects on most rocket flights.

A flight simulation or video would be helpful but 200 feet is also a pretty rough ballpark estimate of what the rocket's altitude would be at the moment of burnout of the C6-7.

My guess is this rocket is still probably OK to launch but I would probably confine it to conditions with very light winds.

View attachment Andromeda II Mk.2.ork#2.orkSorry it's taken so long but here is the OR file for the flawed rocket design.

Just to repeat, the second two launches were in very low, almost negligible wind.

SO

What is the mass object at the motor?

The motor is shown flush with the back of the motor tube. Moving it back to indicate standard overhang moves CG back slightly, but there is still 1.25 cal stability. So this does not explain the stabilitiy issue.

Random Flying Object said:

If it nosed with the wind it was probably some event near the nose causing the Cp to abruptly move forward. If it nosed into the wind and did not correct itself it is mosts likely an incorrect dampening moment coefficient.

If you want to learn more about this, Tim from Apogee wrote a nice newsletter about it.
https://www.apogeerockets.com/downloads/Newsletter195.pdf

Click to expand...

Thank you for the pointer, this is relevant to my interests (a rocket that is stable, but tends to oscillate all the way to apogee).

I would add a 7g mass to the nose cone, just to play it safe.
Rex

No wind at what altitude? You seem to be refusing to consider wind shear and this is exactly the time of year when it is most pronounced.

Space Oddity said:

Just to repeat, the second two launches were in very low, almost negligible wind.

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At first glance, I'm thinking the rocket is too small for those fins. I.e. turbulence will prevent the fins from being efficient enough to keep the rocket on it's original path in certain circumstances. If you extend the fins' span a half-inch, maybe even just a quarter-inch, you may have no more trouble.

That design scaled up to 2" dia. would probably be fine; to 4" would be dandy.

The simulators do not take turbulence into account, and the smaller the rocket, the more turbulence will be evident.

RoyAtl said:

At first glance, I'm thinking the rocket is too small for those fins. I.e. turbulence will prevent the fins from being efficient enough to keep the rocket on it's original path in certain circumstances. If you extend the fins' span a half-inch, maybe even just a quarter-inch, you may have no more trouble.

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My Petrel is about the same size and has slightly smaller fins, though it has six of them. The clutter around the rear, intended as an approximate scale representation of the real thing, should create more turbulence than a smooth body tube, but this model has flown perfectly in all sorts of conditions, albeit never on anything bigger than an A.

@Space Oddity: where is the real, measured CG? I trust computer programs such as Open Rocket for the CP but not for the CG. Also, even if Open Rocket does have the CG correct, see what happens to it if you move the parachute right back. When the rocket accelerates suddenly at launch, the parachute may very well end up at the bottom of the body (along with the wadding, which is not included in Open Rocket's simulation). With a C6-7, the calculated CG ends up rather close to the CP. With a B6-6, there's still plenty of separation between them. As for wind, what are the clouds doing? They will show more clearly what the wind is doing higher up. From your original report, you're launching near a large hedge, which will probably affect how the wind feels at ground level. You could also launch a more reliable rocket first and see how that drifts, which should give you some idea of the true wind at altitude.

adrian said:

My Petrel is about the same size and has slightly smaller fins, though it has six of them. The clutter around the rear, intended as an approximate scale representation of the real thing, should create more turbulence than a smooth body tube, but this model has flown perfectly in all sorts of conditions, albeit never on anything bigger than an A.

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The addition of two more fins may have added the area needed. The turbulence forced at the rear also changes things. The turbulence I was talking about is the flow around the entire rocket, broken at the nose/body joint, which affects the flow that is available for the fins. See the airflow illustration in https://www.apogeerockets.com/technical_publication_16 for what I'm talking about (and the article, while dealing with other issues, may shed some light as well). And then you say it has flown on no more than an A, which may not burn long enough for the rocket to run into the situation that was mentioned by @Space Oddity, so basically we can't compare these situations at all.