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 (

Thanks alot,

Tom Sak

NAR# 82966