Overstable...What???

[Paul]

Hey everyone,
I am working on a scratch built design (The same one I have been workng on for years-It got "delayed"[but that is a story for another time]) I ran the rocket through Rocksim and it came up as saying "Overstable". Overstable! How can it be overstable" I checked the sizes and all -still over stable. I added nose weights -still over stable. What is going on? How can I fix Overstabibility? Thanks for the help.

-Paul

PS. Even though my rocket has been delayed and I still havent given the pics I promised months ago, they are coming. As soon as I fix the overstabibility and put on the long put off last coat of paint. ;-)

 

[Jase]

I'm sure someone more knowledgeable than I about this will chime in, but my understanding is this:

Overstability is where the CG is further in front of the CP than the standard 1 BT diameter. It is not always harmful (unless it's wayyyyy out there) as most of my rockets are a little overstable as well. better to be a little overstable than understable.

:Jason's thinking/warning: As long as the CG is no more than 2 diameters in front of the CP, you should be fine :end Jason's thinking:

Take out the added noseweight and if need be, add a little weight to the aft end to bring the CG back farther.

Most of all......

DON'T PANIC!!

 

[sandman]

I will try to describe this in words as best I can.

Overstable is perfectly fine as long as there is no wind. Like dead calm.

Any wind at all and the over stable rocket will head right into it, especially if the models speed at the end of the launch rod is less then the wind speed.

Stability at or just over one caliber (body tube diameter) will cause a rocket to remain vertical (go straight up), but just shift over a bit with the wind.

I hope that makes sense.

Good luck

sandman

 

[rstaff3]

Overstability occurs, as others have pointed out, when the rocket has over 'one margin' of stability. The primary effect is that the rocket will weathercock, and possibly go unstable. I wish I could remember a better technical description.

There are some other things to point out however.

The '1' margin you hear about is based on the Barrowman approximations. I forget where these start to break down, but they don't really hold for short/stubby or 'longneck' rockets. For instance, you don't need one margin of stability for stubby rockets (~<10:1 length to width ratio).

Also, don't think that if you get 2 calibers this is bad, or three, or....

My Mean Machine has around 6 calibers of stability (best I recollect) and it flies nicely on D12s in moderate winds. Again, I don't know/remember rules of thumb, but the long 'superocs' are some that can fall prey to overstability.

If I run across better info I'll point you to it.

 

[Paul]

OK sounds resonable and makes some sense;-) I wont worry about it for now.

-Paul

 

[kgholloway]

Paul,

One thing I've noticed about overstable rockets is that they seem to have no trouble if they are launched from a rail instead of a rod. I theorize that the problems encountered with overstable designs are caused by sideways forces on the launch rod at low speeds. Since a rail is resistant to such forces, and is usually longer than a rod, an overstable rocket flies very well when launched from one.

Ken Holloway

 

[jflis]

Just to chime in, if the concern is still there...

The problem with over-stability is launching in wind or with a low initial thrust motor.

Using a long launch rod, high initial thrust motor and/or a rail, and launching in low or no wind and this won't be a problem.

To help correct (assuming you don't WANT it over-stablie), don't ADD weight to the nose, remove weight from the nose (eg: knock the CG back closer to the CP), or reduce the size of the fins.

By way of example, egg lofters are notoriously overstabile... that's why you see them launched from rails or long rods and they have such small fins (for competition anyway).

hope this helps,
jim