Is this Rocket stable or not?

I created that rocket just by playing arround with the fins.

I used the zenith angle to determine if it is pointing upwards (is stable).
It looks like it is very stable, but the caliber value is below 1,0... I thought the value should be above 1,0 for a rocket to be stable.

The azimuth angle is jumping arround very crazy... is this an error in the simulation?

Pleeze help a noob :-/

Best regards

Above 0 is stable, however, wind speed, off the rail speed, and velocity can change the stability margin. Going through transonic can cause a forward CP shift that if the rocket is to close to 0 stability it the rocket can become unstable.

If OR is telling you 6000 ft then it thinks it's stable.

OR will generally give you the benefit of the doubt on marginally stable designs, but I would certainly not fly a rocket like that with .5 caliber stability (and I'm guessing the RSO wouldn't either, if you told him). Just eyeballing it, those fins look very small in proportion to the rocket (and also quite slender and break-off-on-landing-y). Also, you have the motor hanging waaaaay out the back of the rocket, which is pulling your CG back and cutting into your stability. If I move the motor back into a reasonable position (I gave it 1" overhang) you're stability creeps up to .75 calibers. I still wouldn't fly it like that. Need larger fins.

I'm a little curious, you're calling yourself a noob but that is a high-powered rocket. May I ask what your rocket building experience is (that is, if you're actually thinking about building this).

Usually I can tell just by looking at it. Please post a jpg so I can see it. I don't require OR or Rock Sim for stability. That's why I don't have them.

my thoughts are that the fins have to little semi-span...however that may be a moot point since the motor isn't going to stay with the rocket.
Rex

The fins are too small, resulting in a too far forward CoP. The CoP is typically, just slightly forward of the leading edge of the fins.

Also, you have only added a mass component for the parachute, so the sim will not deploy anything at, or after, apogee. You need to deliberately add a parachute component that deploys at apogee, otherwise your sim will result in a ballistic return. At the moment your ground hit velocity is 68 m/s.

I'm a little concerned about the motor overhang, too. You have used an engine block for your motor mount tube - you need to use an inner tube for this and select it in the Motor & Configuration tab.

You need to use the correct components in your sim otherwise your results will be erroneous.

Thank you guys for the reply.

I try to answer all questions and give more information.

Prequel:
My experience in rocketry started years ago as a child. But now I started to play around with rockets again at the age of 33.
I have little to some experience with commercial motors but I decided to climb up to experimental rocketry and build my own motor. I adapted some plans from nakka-rocketry for a motor and gathered some ideas from him. Thanks if you can read this!!
My background is a lot of experience in mechatronics, especially electronic and mechanics (automotive industry) and semi professional fireworks.
But no great experience in rocketry. That's why I call myself a noob.... and that's what I am
I know.... not a good idea to start with such a huge and complex project.... milling, machining, static tests, electronics, telemetry... and a lot of danger.
I took that all into account and as i am a familiar with the safety requirements of fireworks and stuff, I will be very very careful.
And buy the way... I quit with my wife and have lots of time now :-D

Current Rocket / Parts of it:
This is the very first try to get a feeling of the dimensions. The weight of each component is just an estimation except of the motor. I has 985 g (~2,17 lbs) loaded with fuel and with everything attached.



Fins:
I wanted the rocket to be very robust even with winds of 5-6 m/s (~20 fps). That's why i chose the fins that small. And I just played around with shapes and so on and found this one very well performing IN THE SIMULATION. And that's the point. I have no real experience. And I don't trust the sim. That's why I wanted to ask you guys for help. I don't want my 1000+ Euro Project to go up in flames on the first run. :-D

I used the azimuth and zenith angle as indicator if the rocket stays on track. With Winds of 5 m/s this is better with smaller fins. So I thought it would be good to chose as small as possible fins. Of course this lags in calibers and thus stability.

What are your azimuth and zenith angles with your rockets in simulation?
What are your rule of thumb to design the fins? And what are your experiences?


Motor:
The Motor has a 7 cm (~2,75 inch) nozzle witch overhangs at the end. I found no better way to model this. Thanks for the advice on how to model the rocket the right way. I will try to do so and post the new version here. I saw that I can import thrust curves... this i will do after a few static tests... but where can I set the weight of the motor itself?

Parachute:
I have no idea at the moment, on how to solve the problem with the parachute. As I am a skydiver too, I have access to some old canopies and try to tailor one that fits my rocket.
For now I try to find some in the sim tool that works well. This would even give me an idea witch size to tailor :-D

Thank you guys - you are great.

From a simplistic trace I drew in Autocad, I'll have to side with Voyager1 on this. The fins appear to be too small. No matter what size rocket this is. I enlarged them by 1.5 times and that size looks doable. 2 times their size would be better, but may not be necessary.

Try this model file.

Thanks a lot. Looks great! And the azimuth is steady now.

But the zenith angle is still 3 degrees off the ideal 90 ° with only 3 m/s wind. Is there any possibility to get it more robust against wind without reducing the size of the fins?

Stomper said:

Thanks a lot. Looks great! And the azimuth is steady now.

But the zenith angle is still 3 degrees off the ideal 90 ° with only 3 m/s wind. Is there any possibility to get it more robust against wind without reducing the size of the fins?
View attachment 360994

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Not really. Plus, it’s doubtful that wind speeds will remain 3 m/s or even the same direction at all altitudes throughout the flight. I can happen, but it’s rare.

Higher velocities off the pad reduce the off vertical deviation. Greater margins of stability typically increase it, but without enough of a stability margin you could have very unpredictable flight. Stay as close to 1 to 1.5 caliber for you stability margin, launch fast, use a very stiff launch platform, launch on a still day, and don’t have unrealistic expectations. 3° off vertical is very small.

Great... I getting into it more and more.

For sure the wind will not be steady throughout the whole flight.
My launch region will be a mesa in the middle of a forest. I recorded the winds the whole summer and even on still days i have gusts of about 1-2 m/s wind there.

Thanks for the statement " 3° off vertical is very small." I have no clue about the quantities of my simulation...
But as the simulation shows it is an fading oscillation. So everything looks good i think.

Special thanks to voyager1 who has sent me the new design in open rocket sim.

I will come back to you guys when I have my first thrust curve recorded in a static test.
But much work to get there :-D I expect static tests in late October or November.

Best regards
Stomper

Gary Byrum said:

Usually I can tell just by looking at it. Please post a jpg so I can see it. I don't require OR or Rock Sim for stability. That's why I don't have them.

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LOL.

Can you tell by looking at a rocket jpeg the CG location, wind speed, rail length, thrust profile, and launch velocity?

I have been designing and building rockets since the late 60's, I developed a mind sim. Granted, it may not give me things like "thrust profile & launch velocity", (which btw, is rather irrelevant to me) I'm talking about stability. NOTHING ELSE!!! Presently I have over 150 rockets that I designed myself and I can honestly say, 8-10 of them were off the mark and a couple that wouldn't fly at all. So YES, I can visualize stability. I also know quite a few others that are like that also.

Stomper said:

Thanks for the statement " 3° off vertical is very small." I have no clue about the quantities of my simulation...

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Simple trig. At three degrees off vertical, the rocket will gain 99.86 m altitude for every 100 m of straight travel. And it will travel 5.23 m uprange. These numbers are well within the range known as "who cares?"

You also used a very short launch rod length resulting in a launch velocity of about 27 m/s. Increase it to at least 2m to give you a launch velocity of about 42 m/s. That will help the initial stability off the rod.

If you desire better (less) than 3 degrees off zenith, then you will probably require a sophisticated active vertical trajectory system that provides vector control of the motor or via correcting canard fins. That 3 degrees is a very small error for a passive fin-stabilized rocket, particularly considering the simplicity of your design.

As the respondents above have suggested, the REAL winds throughout the rockets ascent will have a far greater effect on the zenith error. Don't be too obsessed by this one parameter, there are many other things that can go wrong.

Voyager1 said:

You also used a very short launch rod length resulting in a launch velocity of about 27 m/s. Increase it to at least 2m to give you a launch velocity of about 42 m/s. That will help the initial stability off the rod.

If you desire better (less) than 3 degrees off zenith, then you will probably require a sophisticated active vertical trajectory system that provides vector control of the motor or via correcting canard fins. That 3 degrees is a very small error for a passive fin-stabilized rocket, particularly considering the simplicity of your design.

As the respondents above have suggested, the REAL winds throughout the rockets ascent will have a far greater effect on the zenith error. Don't be too obsessed by this one parameter, there are many other things that can go wrong.

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Just a note that 27 m/s is 88 feet per second. That’s about twice as fast as is usually quoted as necessary for aerodynamic stability.

Steve Shannon said:

Just a note that 27 m/s is 88 feet per second. That’s about twice as fast as is usually quoted as necessary for aerodynamic stability.

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Good point Steve. I was deliberately going for overkill here, though. I generally aim for about 15-20 m/s.

Gary Byrum said:

I have been designing and building rockets since the late 60's, I developed a mind sim. Granted, it may not give me things like "thrust profile & launch velocity", (which btw, is rather irrelevant to me) I'm talking about stability. NOTHING ELSE!!! Presently I have over 150 rockets that I designed myself and I can honestly say, 8-10 of them were off the mark and a couple that wouldn't fly at all. So YES, I can visualize stability. I also know quite a few others that are like that also.

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Perhaps you confuse "center of pressure" with "stability." CP can be estimated by a good eye. Stability includes CP and all the other things I mentioned (and more) that impact the rocket's flight profile and restoring force relationships. Stability cannot be determined from a photograph alone.

Buckeye said:

Stability cannot be determined from a photograph alone.

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When I draw a design, I look at and determine if it looks stable or not. If it doesn't, I tweak it until it does. 95% of the time I'm right. Now and then I'll have to add a little nose weight but it can be just that simple.

Gary and several others can estimate static stability by eye pretty darn well. I can't, but I can tell if a design is probably way out of bed or is worth better computation. And static stability, enough and not too much, is enough to know whether it'll go up or go wild.

Detailed performance also depemds on things like moments of inertia, damping ratio, etc., etc. which are, I presume, a lot harder to eyeball or mindsim. (Though if someome with lots and lots of experience says he can do it, I won't be the one who says "No you can't.")

jqavins said:

Detailed performance also depemds on things like moments of inertia, damping ratio, etc., etc. which are, I presume, a lot harder to eyeball or mindsim. (Though if someome with lots and lots of experience says he can do it, I won't be the one who says "No you can't.")

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The society of mind simmers (though they are aging out of existence) would like to thank you for your support.

Steve Shannon said:

Just a note that 27 m/s is 88 feet per second. That’s about twice as fast as is usually quoted as necessary for aerodynamic stability.

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Thanks for the hint. I think I can say more about the REAL take off velocity when i have recorded the first static test thrust curve
Hopefully I got the motor up running fast on the right chamber pressure without blowing it up ^^

Best regards
Stomper

Buckeye said:

Perhaps you confuse "center of pressure" with "stability." CP can be estimated by a good eye. Stability includes CP and all the other things I mentioned (and more) that impact the rocket's flight profile and restoring force relationships. Stability cannot be determined from a photograph alone.

Click to expand...

Also being a member of the Ancient Fraternal Order of Mindsimmers, I understand what Gary is saying. Like a mechanic that can listen to a car and diagnose its issues, or a sailor who looks at the sky and can predict the weather, after many years of doing scratch designs, one develops a kind of holistic understanding of the stability of a design. Not just because one can predict the position of the CP, but because of familiarity with the materials, and the relative proportions of the components, one gets a sense of the probable location of the CG as well, and consequently can assess stability is one swell foop. This is especially true of the rather ubiquitous 3 or 4 Fins and Nose cone designs that get pumped out by sim program users.

As for flight profile and restoring forces, as Gary mentioned before, those are generally irrelevant many scratch builders who grew weary of 3FNC in their childhoods. You see, since our focus is on the rocket planform - the search for the completely unique silhouette, we seldom care a great deal about performance. We just want to confirm that our one-of-a-kind design flies successfully (albeit, a somewhat subjective assessment).

LW Bercini said:

You see, since our focus is on the rocket planform - the search for the completely unique silhouette, we are seldom care a great deal about performance. We just want to confirm that our one-of-a-kind design flies successfully (albeit, a somewhat subjective assessment).

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I’m not a mind-simmer (mostly), but I just wanted to say that this little bit of prose pretty eloquently sums up my participation in this hobby.

On wind instability of high aspect ratio (length/diameter) airframes:

What Barrowman Left Out

https://www.apogeerockets.com/education/downloads/Newsletter470.pdf

LW Bercini said:

Also being a member of the Ancient Fraternal Order of Mindsimmers, I understand what Gary is saying. Like a mechanic that can listen to a car and diagnose its issues, or a sailor who looks at the sky and can predict the weather, after many years of doing scratch designs, one develops a kind of holistic understanding of the stability of a design. Not just because one can predict the position of the CP, but because of familiarity with the materials, and the relative proportions of the components, one gets a sense of the probable location of the CG as well, and consequently can assess stability is one swell foop. This is especially true of the rather ubiquitous 3 or 4 Fins and Nose cone designs that get pumped out by sim program users.

As for flight profile and restoring forces, as Gary mentioned before, those are generally irrelevant many scratch builders who grew weary of 3FNC in their childhoods. You see, since our focus is on the rocket planform - the search for the completely unique silhouette, we seldom care a great deal about performance. We just want to confirm that our one-of-a-kind design flies successfully (albeit, a somewhat subjective assessment).

Click to expand...

WHAT HE SAID!!!
And for what it's worth, I have never read the Handbook Of Model Rocketry.