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Open rocket stability number?

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Miles Goodson

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Hi all,
I just got Open Rocket to simulate an F powered rocket that I'm building, and for some reason, I can't get the stability number past 0.6! I've tried everything. Any ideas on what to fix?
 

dhbarr

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Did you build the sim yourself, or download it?

Did you apply any overrides?
 

CalebJ

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Add a bit of nose weight and your stability increases immediately. 3 ounces shows it at 1.27.
 

Nytrunner

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Check your fin materials and nosecone materials. OR defaults to cardboard which puts undue weight at the back.

That being said, you have a heavy motor and large fins at the aft end which draws your CG back. If you want to fly this exact shape, you'll need a fair amount of nose weight in the cone, which will add weight, and probably require a stronger motor.

Lengthening the body will put the nose and parachute further forward which can also draw your CG forewards
 

neil_w

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Check your fin materials and nosecone materials. OR defaults to cardboard which puts undue weight at the back.

That being said, you have a heavy motor and large fins at the aft end which draws your CG back. If you want to fly this exact shape, you'll need a fair amount of nose weight in the cone, which will add weight, and probably require a stronger motor.

Lengthening the body will put the nose and parachute further forward which can also draw your CG forewards
All this is correct. I'll tack on just a bit.

1) Garbage in, garbage out: you must first enter your data accurately before you can evaluate what OR is telling you. In fact, *all* your materials are set to cardboard, the body tube wall thickness is probably way off (I could be wrong on this; just a guess). A better way to start would be to use presets from the database for nose cone and body tube and other parts where possible. For body tubes, start with a preset and then adjust the length as needed. For the fins you're on your own to make sure everything is set properly (materials and thickness).

So before you start adding additional nose weight, get everything else correct, *and* make sure you include the weight of the altimeter and camera, appropriately positioned up front.

2) That is indeed a very large and heavy motor to be putting in a very small rocket. Are you sure that's what you want? Also the rocket is short and stubby, which tends to make it a bit harder to stabilize without nose weight. As Nytrunner says, longer rockets tend to be easier to stabilize.

3) The fin shape is not doing you any favors. They have a very long root and are not very high. You can gain quiet a bit of stability just by adjusting the shape a bit, for instance (no other changes here but fin shape):
upload_2020-2-17_15-13-27.png


4) In your original post, you said "I can't get the stability number past 0.6! I've tried everything." I'm genuinely curious what you tried.
 

Miles Goodson

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I tried changing the density of the fins, changing the shape of the fins, and changing the length of the body tube.
 

retortec

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No worries. Good thing you are here asking. All of the above advice is great. In the end to get the most out of your sim weigh all the components during the build. Override the given values as needed. Alter the sim with any mods during the build as well. Many builders make the mistake thinking your done with the sim once assembly starts. The sim modifications will give you the best shot at getting as close as possible to the true stability.
 

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