ever seen those single staged rockets with 2 sets of fins?

[rocketsonly]

Hey all. Have you all seen those military rockets (or missiles?) with a set of fins near the bottom of the rocket, and another set of fins near the top? I'll find a picture and upload it soon, can't do it now because dial-up takes forever, most likely tomorrow. Well, why two sets of fins and not just one?
Thanks.

 

[Bill]

Originally posted by rocketsonly
Hey all. Have you all seen those military rockets (or missiles?) with a set of fins near the bottom of the rocket, and another set of fins near the top? I'll find a picture and upload it soon, can't do it now because dial-up takes forever, most likely tomorrow. Well, why two sets of fins and not just one?
Thanks.

A lot of those are air-to-air missiles. The designer goes for neutral to marginal stability so that the guidance system can "turn it on a dime" when in pursuit of the target.


Bill

 

[powderburner]

A lot of times, one set of fins will be "fixed" or non-moving, and function like wings to help the missile fly a little farther. Strakes located at the mid-body also serve this purpose.

Maneuver fins are usually set on a single pivot spindle (per fin) and can be identified from the exterior of the missile. These will deflect to steer the missile toward a target.

One of the new French anti-missile missiles has banks of maneuver rocket motors pointed outward from the middle of the missile, enclosed within four large strakes.

rocketsonly, is there any missile in particular that you are wondering about?

 

[rocketsonly]

Not really. I've just seen many military rockets with 2 sets of fins and just wondered what they did. I remember once when I launched my first D engine in an Estes Executioner that my dad suggested me to put another pair of fins near the top of the rocket. I tried explaining to him the CG/CP relationships and how fins near the top of the rocket will move the CP towards the nose. If I recall correctly, he said the fins near the top of the rocket will make the rocket fly straighter? I forget. I think his way of thought was that if the wind pushed on the lower set of fins and made it weather cock, then a pair of fins near the top will balance it out and therefore not weather cock as much.

 

[Jacques]

A second set of fins near the nose of a model rocket makes it less stable even unstable. The Estes Aim-9 Sidewinder scale model had canted lower fins to induce a roll and make the model more stable.

 

[Jacques]

Actually I should have said a spin along the longitudinal axis of the rocket!

 

[wyldbill]

Originally posted by rocketsonly
I tried explaining to him the CG/CP relationships and how fins near the top of the rocket will move the CP towards the nose.

You're right, your Dad's wrong, but don't rub it in

 

[Justy]

Originally posted by wyldbill
You're right, your Dad's wrong, but don't rub it in

They're both right, aren't they? Adding fins near the nose will shift the CP closer to the CG, which will reduce weathercocking. Of course if you make the front fins too large, the CP goes too far forward, you get negative stability, and the rocket goes anywhere but up.

 

[fehskens]

>Adding fins near the nose will shift the CP closer to the CG, which will reduce weathercocking.

Not necessarily. You also have to consider the (pitch and yaw) moments of inertia. I have several designs with very large stability margins, but they also have high moments of inertia and they rarely weathercock.

You get a high moment of inertia by putting the model's mass as far from the CG as possible. These models have heavy motor mount/tail assemblies and even heavier nose cones. They fly like arrows.

len.