Fineness and Stability Margin in OpenRocket

[atestani]

I have a tall/thin rocket with a fineness (aka aspect ratio or length to diameter ratio) of ~26 (80” long 3.1” diameter). As currently configured, it has a stability margin of 3.7/14.6% in OpenRocket.

I am trying to answer the age-old question is if this rocket is over-stable. The only “rule of thumb” I have heard for tall/thin rockets is 8%-18%. At 14.6%, I am roughly at the center of the range but I don’t know if that means anything. I’m simulating at various wind speeds looking at angle of attack, side profiles, and lateral orientations but am having a hard time understanding what it all means.

As an aside, I see that Rocksim has a flight profile function that claims to show what the flight looks like but I don’t trust Rocksim as it seriously overstates stability if using straight Barroman and even more so when using its own CP equations. In another case, for example, OpenRocket shows CP at 40.8” and RockSim (Barroman) is 42.1 and RockSim (Rocksim method) shows 44.5. Weight and CG come out the same in RockSim and OpenRocket, so I believe the models are correct in each program.

So how do I use OpenRocket to check the results of specific stability, i.e. answer the question if the rocket is over-stable (at various wind speeds)?

 

[kjhambrick]

@atestani --

There is an older post here on TRF: Disadvantage of overstability of a rocket ... check it out !

There is a lot of good info and advice in that thread ( especially the jokes about calibers -vs- calipers ) but maybe the best advice in that thread is to fly the rocket with enough thrust to minimize the initial angle of attack off the rail which will reduce weathercocking and gravity turning before apogee.

EDIT: Another very useful tidbit is avoid angling your launch rail into the wind -- point it straight up or even a little downwind.

EDIT: to answer your 'how to' model stability in OR question:

Goto [Flight Simulations] , Click [Plot/Export], change [Preset plot configurations] to [Stability -vs- Time]:


Then Click [Plot] and zoom in from launch to apogee:


All the 'rules of thumb' say this rocket is over stable but it sure flies good with the right motors

HTH

-- kjh

TL;DR

I am a recent RE-BAR and at this point in time, all my flyable HP Rockets are Vulcanite Scale Models.

I chose to model this rocket for AltAcc flight data collection back in the 1990's because it flies so dang well

My real LOC Vulcanite ( "Spock's Johnson" ) is 54 inches long and 2.26 inches in diameter -- a fineness ratio of about 24.

Without a motor OpenRocket reports 6.28 calibers or 26.4% stability margin at Mach 0.3 but that is without a motor.

The next time I can fly I'll send it with an old I357 which brings the stability margin down to 4.2 cal / 17.4% -- I've flown this motor in SJ a few times and it flies beautifully on it.

I've also flown SJ four times on the RMS 38/1080 J570W to about 8800 ft / Mach 1.2 and with that motor, the stability margin is 3.4 cal or 14.3%

I have not flown a J510 yet ( I don't recall any RMS 38/1320 motors back before Y2K ), but that motor will fit SJ's fin can and will send to about 10,000 ft at Mach 1.3 with a stability margin of 3.5 cal or 14.7% and I wouldn't hesitate to fly it at the right site.

My advice is to fly your rocket with a good thrust to weight ratio so it never has a chance to weathercock.

 

[atestani]

@kjh- Thanks for the information. I have seen the "calipers/calibers" thread and many other. I find that simulating stability vs time gives you what the stability is doing (increasing) but doesn't help to see what the effect is.

I am trying to figure out what how the side profiles and lateral orientation functions of OpenRocket work.

 

[neil_w]

I am trying to figure out what how the side profiles and lateral orientation functions of OpenRocket work.

When plotting a simulation, you can add that as a Y axis type. I can’t remember exactly what it’s called but it’s towards the end of the list of available Y axes. I can go look up the exact name tomorrow.

 

[Alan15578]

@atestani --

There is an older post here on TRF: Disadvantage of overstability of a rocket ... check it out !

There is a lot of good info and advice in that thread ( especially the jokes about calibers -vs- calipers ) but maybe the best advice in that thread is to fly the rocket with enough thrust to minimize the initial angle of attack off the rail which will reduce weathercocking and gravity turning before apogee.

EDIT: Another very useful tidbit is avoid angling your launch rail into the wind -- point it straight up or even a little downwind.

EDIT: to answer your 'how to' model stability in OR question:

Goto [Flight Simulations] , Click [Plot/Export], change [Preset plot configurations] to [Stability -vs- Time]:
View attachment 629394

Then Click [Plot] and zoom in from launch to apogee:
View attachment 629395

All the 'rules of thumb' say this rocket is over stable but it sure flies good with the right motors

HTH

-- kjh

TL;DR

I am a recent RE-BAR and at this point in time, all my flyable HP Rockets are Vulcanite Scale Models.

I chose to model this rocket for AltAcc flight data collection back in the 1990's because it flies so dang well

My real LOC Vulcanite ( "Spock's Johnson" ) is 54 inches long and 2.26 inches in diameter -- a fineness ratio of about 24.

Without a motor OpenRocket reports 6.28 calibers or 26.4% stability margin at Mach 0.3 but that is without a motor.

The next time I can fly I'll send it with an old I357 which brings the stability margin down to 4.2 cal / 17.4% -- I've flown this motor in SJ a few times and it flies beautifully on it.

I've also flown SJ four times on the RMS 38/1080 J570W to about 8800 ft / Mach 1.2 and with that motor, the stability margin is 3.4 cal or 14.3%

I have not flown a J510 yet ( I don't recall any RMS 38/1320 motors back before Y2K ), but that motor will fit SJ's fin can and will send to about 10,000 ft at Mach 1.3 with a stability margin of 3.5 cal or 14.7% and I wouldn't hesitate to fly it at the right site.

My advice is to fly your rocket with a good thrust to weight ratio so it never has a chance to weathercock.

Unless your rocket is truly rigid, like the sims assume, you may need extra static margin to maintain stability through high q flight.

 

[Buckeye]

I have never seen OR and RS Barrowman differ by more than a couple millimeters, up to 15 L/D or so. Sounds like the difference gets greater the longer the rocket.

You may find this useful. It adds AOA to Barrowman. More stability margin needed on long/skinny rockets.

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

I don't think there is a thing as too much stability. I agree with @kjhambrick though. Give the rocket a good punchy motor so that there is minimal AOA off the rail.

 

[neil_w]

When plotting a simulation, you can add that as a Y axis type. I can’t remember exactly what it’s called but it’s towards the end of the list of available Y axes. I can go look up the exact name tomorrow.

This is what I was referring to:


Honestly, trying to figure out worst-case weathercocking is not easy.

 

[kjhambrick]

+1 !

Thanks @neil_w !

I need to study that plot.

-- kjh

 

[atestani]

I had looked at vertical orientation but wasn't sure how to interpret the plots. I now see that it is a view as seen from above the launch pad looking down at it. I believe the plots show how off from vertical the rocket is flying.

Attached is a set of plots for 0, 5, and 20mph wind from the East,, launch angle is vertical, and a 60" effective rail. There are 3 static margin cases. The rocket is 80" tall, 3.1" dia. and weighs 123oz. loaded with an I195J. I wish there was a way to set the scale of axes (like Excel) and better resolution of the plots themselves (screen shots are good but importing them into Word to make the table wasn't). I marked the vertical scale and the angle at 7sec into the flight on each plot. Note: If I do this again, I'm going to export data and plot in Excel!

In any case, the attached shows roughly what happens. I actually think I am close to the sweet spot at 5mph. I ran one with 10% margin (from the 1-2 caliber/10% "rule-of-thumb") and it looked under-stable there.

I *think* I now understand how to use OpenRocket to assess this. Please let me know if I am misunderstanding.

EDIT: Updated PDF

 

[atestani]

So now the next chapter...

The attached is at 5mph wind. Which is better in terms of vertical orientation at various stability margins?

 

[atestani]

So now the next chapter...

The attached is at 5mph wind. Which is better in terms of vertical orientation at various stability margins?

My own answer to the question is to design for 12% - 13%. In that vane, here is what I plan to use for my L2 cert flight next Saturday. This is a somewhat different rocket (63.5" vs 80", 3.1" diameter, ~10% less fin area, large nose weight = 32oz. ) with a J350, but the fineness is still over 20. Here is the plot (wind = 5mph):

 

[kjhambrick]

Thanks for the interesting Qs, @atestani !

I've got no answers for you but you've got me looking at OR sim data -vs- Blue Raven data for an actual flight.

These are raw gyro and integrated tilt and roll angles for Spock's Johnson on an AT I225FJ:

This is the same Rocket Sim'd in OR:


The actual wind speed was 8 mph and a little gusty -- set the same for the OR Sim ...

Note that the tilt angles are complementary between OR and the Blue Raven ( ? why ? ) but the shapes of the curves are VERY similar ...

One interesting thing about the real flight is that there was a 'roll reversal' during the coast phase at about 7 seconds ... why ?

I've definitely got some studying to do but this does look interesting !

Thanks again !

-- kjh

 

[kjhambrick]

Here is another one where I can compare Sim -to- Actual.

This is "Nocturnal Missions" a 2.95 inch Glass Vulcanite Upscale.

Note that according to the rules-of-thumb, this rocket is overstable at 5.14 calibers / 21.6% when loaded with an AT RMS 38/720 J350W.

This is the OR vertical orientation plot for the Motor:


I did my Level 2 cert last November with NM on a J350 and my payload was a Blue Raven set up for dual deployment.

It was pretty windy, maybe 12 mph and gusty ( you can hear the wind in Stephen's video below ).

These are plots of the raw and integrated gyro data:

Once again, the shape of the OR sim and the actual tilt data for the flight are very similar when you factor in the complementary angles.

My level 2 cert was actually a recert. This rocket has flown four times on an AT RMS 54/2560 K700 to 11k ft and as fast as Mach 1.4, depending on the tempperature.

And all theory aside, the Vulcanite model flies so pretty !
View attachment konrad_hambrick_l2_recert_aarg_hutto_2023-11-04.mov
Thanks to @bad_idea for the .mov file !

Still studying ... Thanks again !

-- kjh

 

[atestani]

I agree, it is interesting. I think the complementary angles are simply a reference difference. It sure looks like OR is doing a great job simulating the tilt angle. Unfortunately, tilt angle isn't being recorded by my Eggtimer Proton. I need to look into that.

 

[kjhambrick]

@atestani --

Yes, I am trying to understand the choice of references for the angles ... is one better than the other ?

Probably not.

I am not ( yet ) familiar with Eggtimer Avionics.

Does the Proton have a gyro or a magnetic sensor ?

-- kjh

EDIT: I must add this: YES ! OR does a VERY good job with their sims !

 

[atestani]

Does the Proton have a gyro or a magnetic sensor ?

No, but it does have a baro sensor and accelerometer. I'm looking at this post to try to calculate the tilt angle from data I have from other flights:
https://www.rocketryforum.com/threads/tilt-angle-comparison-from-flight-data.179803/

 

[kjhambrick]

No, but it does have a baro sensor and accelerometer. I'm looking at this post to try to calculate the tilt from data I have from other flights:
https://www.rocketryforum.com/threads/tilt-angle-comparison-from-flight-data.179803/

Yes, Larry Curcio / @ihbarddx has done some very interesting work deriving tilt angles from baromertic -vs- inertial altitudes.

The data he's shared looks very promising and he also flies a Blue Raven and his results have compared very well !

I believe he was working on a paper for NAR ?

Larry ?

-- kjh

 

[neil_w]

Personally I prefer tilt angle (or “angle off vertical”)