Correcting Overstability

[Mike]

I'm building a MMX Amraam missle and having simmed it all out its got 5.58 calibers of stability, which is classed overstable. I know that overstability makes the model prone to weathercocks but seeing as its only getting about 30m altitude would it be severe?

Lets imagine it would be and I need to make the model less stable. I can't shrink the fins as its scale but would it be alright to add weight to the rear to bring the CG back? Bearing in mind this is a 13mm tube so space is limited, where would I put it?

Thanks for your help

 

[jetra2]

Did you check you stability with the motor installed? That usually changes the caliber a little bit.

You could do some extra heavy fillets or something like that...

Usually, rockets with forward fins like the Amraam are overstable, and you should be ok with what you've got, especially since you'd be using short burn, high thrust motors like the MMX 1 or 2, and the A10 or A3. I would think that you'll be OK with what you've got.

Also, most programs seem to overcalculate the CP a bit, so it might be a tad lower than 5.58! Let's hope so, ok?

Jason

 

[powderburner]

jetra2 raises a good point: are you SURE you have that much stability margin? (That's kinda hard to do most of the time, especially in low-power)

If you have not yet built your rocket, you can substitute some other aft fin materials like basswood, or sheet plastic. Or soak the balsa fins in epoxy and laminate them with cardstock.

Does your scale model have the wiring conduit? On the real AIM-120, there is an external channel that runs along the rear 2/3 of the missile, carrying wires past the motor casing to the controls at the back end. You could model this using a double-up paper clip or piece of copper wire for weight, covered with a rolled-n-glued piece of cardstock.

If your construction is finished, you can still add a bit of weight by cutting a small strip of paper, rolling it tightly to fit inside the front of the BT, and sliding it all the way back against the motor mount before letting it unroll into place (cut the strip short enough so there is still a hole through the center of the rolled-up paper). Secure it with a couple drops of CA. This will not only add weight in the rear, but will also give your BT a little protection from ejection gases.

 

[powderburner]

Forgot one---

If you are really planning to use MMX motors in a 13mm BT, you can add considerable weight in the rear end. Center your motor tube by rolling a strip of paper around the outside of the motor tube until you build up a layer large enough to be snug inside the BT.