Re: Over stability?

> Hi,

> > > > I am building a rocket that is overstable by 3 to 4 caliblers. Rocsim > > says that it will still go about 1,700 feet, but I wasnt sure if there > > are any problems that could arise. I forgot to mention that it is a 7 > > motor cluster I'm doing, so Im not sure if that will cause any > > problems besides weather cocking ( regardless to being a cluster ). It > > is a scale model I'm building ( Porthos II ) so I cant change make the > > fins larger or anything like that, I just wanted to know If I > > can/should fly it the way it is now being so overstable. > > > > Thanks, > > > > Tom Sak > > An overstable rocket will tend to weathercock more and will probably > have a little bit of spiral to its flight path. Overstable basically > means too much restoring force so if it gets kicked a little bit > sideways when it's leaving the rod or by a gust it will tend to > overcorrect and swing back and forth a little. It's especially a > problem with long, thin rockets, which yours probably isn't if it's a > 7 motor cluster. > > At 3 or 4 calibers you'll probably notice a little bit of those > effects, but that's not enough for it to be really severe. > > Be careful, though, if your rocket has a transition from a narrow > airframe to a wider airframe. There is a bug in Rocksim where it will > always use the narrowest airframe for its "Static margin" calculation. > So if you've got a short bit of narrow airframe up front, the "Static > margin" will be based on that narrow airframe instead of the main > airframe and will be completely misleading. You really need to look > at the actual distance between the Cp and the Cg and compare that to > your main airframe diameter. Or look at the flight graph and the "2D > flight profile" > > Erik Ebert, L3 > TRA# 09105 > NAR# 79868 >
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Hi Erik,

The rocket I'm building does have a transition like you metntioned (

4" dia airframe to a 6" diameter "hipped booster"). I looked at the 2D flight profile and set the wind to 20 MPH, and the rocket weathercocked a lot. The static margin with all 7 motors is about 3.05 to 3.14, and so the rocket is overstable. I do not know how to do the CP calculations with pencil and paper, so do you have any other suggestions on determining the stability of the rocket? This is a problem I wasnt aware of so I am glad that you told me about this!

Thanks alot,

Tom Sak

NAR# 82966

Reply to
TJ
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The Cp and Cg locations that Rocksim calculates are correct (well the Cg depends on how accurately you enter the weights of all the components, but you can always get the Cg by balancing the rocket on something).

What Rocksim messes up on in this case is the "static margin", which is just the distance between the Cg and Cp divided by the diameter of the rocket, because it uses the first diameter it sees, which may not be the main diameter of the rocket.

So just use the Cg and Cp numbers you've got, and the static margin is just (Cg - Cp) / (main diameter of the rocket).

-- Erik

Reply to
Erik Ebert

Over stabilized rockets tend to wx-c*ck. Cluster engine rockets tend to veer off course do to engine performance/ignition problems. I never had much success with cluster engine rockets. Good luck.........

Reply to
robot

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