Minimum Stability Coefficient?

What is the minimum stability coefficient you're willing to accept on an OpenRocket design?

In the words of my old IBM DB2 instructor, "it all depends." Stability margin varies by overall length, fin design (ratio of span to chord,) fin surface location (forward of the main fins) and polar moment of inertia. I've had stable 0.5 caliber and unstable 2.0 caliber rockets.

Unstable 2.0 doesn't seem like it'd fly at all. I flew an unstable rocket once. It was scary. I was 11. I forgot to check stability. It needed nose weight bad. Top of the rod , it flipped and flopped in the air until it crashed to the ground and burned out.

So I have a stability of 0.565 and that ought to fly in reality? That is a chief by tripling the recommended clay weight. If I leave it the factor clay amount, the stability is 0.061 but I think there's a drag factor being ignored by the program. It's fin abilities are a little lacking for this rocket.

There's a feature in OpenRocket that I really like...you can adjust the Angle of Attack (which would be zero with no wind and increases with wind speed) and watch what it does to the calculated CP position. This can alert you to situations where the rocket is stable in zero wind but could go neutral or negative with an unexpected gust.

One check is described in one of the newsletters from Apogee, have forgotten which one. Setup a simulation that applies a step disturbance to your rocket during flight. If you rocket recovers you are most likely in good shape. Setup your simulation to launch straight up (90 degree normal to the earth), fly in no wind to about 500ft, at 500ft set the wind for a constant 20mph. This is obviously going to disturb the flight path of the rocket. Besides some of the interesting numeric data that can be derived from this simulation. If the rocket recovers with minimal AOA oscillation and keeps going up, this is a good sign that your rocket will fly with out any major failures.

Two things to consider:

1. The above does not account for poor design resulting in low rail speed or high high AOA at rail departure.

2. Your numbers in the simulation model must be correct to lend any reasonable amount of reliability to the simulation itself.



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My current Launch Field is only about 140 Yards wide, so I only fly when there is no wind, but require that my Rockets fly as near perfectly straight up as possible.

Unstable 2.0 doesn't seem like it'd fly at all.

Click to expand...

I think he meant OR showed 2.0 stable and yet in real life it was not adequate.

A thing to consider is calibers of stability is determining a length in units of the rocket's width (diameter). Width is one measure of the rocket's size but not the only one. It does affect stability but possibly only 1/10 cal. of margin is caused by width, the rest is caused by length. So a rocket that is relatively fat doesn't need as large a margin as a portion of its width, but a longer one compared to its length. If you somehow added something to the rocket that didn't affect the stability at all, but increased what OR considered to be the diameter, that magnitude of the stability in inches would be the same but in calibers the magnitude would be reduced.

One unrelated reason for using calibers is, if you know the CP, and can balance the rocket for CG, it's easy to eyeball about how many caliber units that is in the field.

mrichhcirm said:

There's a feature in OpenRocket that I really like...you can adjust the Angle of Attack (which would be zero with no wind and increases with wind speed) and watch what it does to the calculated CP position. This can alert you to situations where the rocket is stable in zero wind but could go neutral or negative with an unexpected gust.

Click to expand...

Yes, this is important. It can help verify a shorter rocket is more stable than you might have thought. On the other hand, with some long rockets you have to compromise between high stability margin at low AoA and becoming unstable at not that much higher AoA.

There are some cases, mainly saucer-style rockets, where base drag is considered a stabilizing force. I am skeptical that it is significant with other rockets. As always, the limit of what should be tried depends slightly on how large and dangerous a rocket ship it is.

It depends, as stated above. See https://argoshpr.ch/joomla1/articles/pdf/sentinel39-galejs.pdf for more information.

A long, narrow rocket may be unstable even if CP calculations show it to be stable, while a short, stubby rocket may be fine with .5 caliber of safety.