Fin area

I’ve never seen the formula, but if the fin semi-span is less than the airframe diameter, the root has to go up a LOT to counterbalance reductions in semi-span. It is NOT linear.

W

Most, but not all, of the equations are based on a physics MIT graduate student named James Barowman who, with others, came up with them in the late 1960’s. Estes an Centuri produced booklets with graphs for you to design you own. I still have my copies.

There is also the cardboard cutout method. Very conservative but if you’re stranded on an island and you want to build a rocket and all you have is cardboard and scissors… But now there are programs, like Rocsim, to do it for you. However, Rocsim has their own method of calculating stability and in the general section of the design you can select which method to use.

Please note that these equations, like all the methods, are based on assumptions and will not fit every design. I haven’t seen the assumptions that Rocsim uses but I’m sure they are there. A typical example about the Barrowman method is that the fins are “thin flat plates”. The calculations do not work for a stick attached to a motor like a bottle rocket (which we know is stable). This is especially of concern for me when I have a camera cowling sticking off the side of the rocket. I still treat it as a fin but in case you wonder why my rockets have over sized fins… However, for most typical rocket designs they work great.

Now if you’re really into the math you may wish to check out “Topics in Advanced Model Rocketry” by Mandell, Caporaso and Bengen published from MIT Press in 1973. It has many of the equations and methods used by the simulation programs. It is available from Amazon and you also may be able to find it in your local library. It was my bible for rocketry unlike Stine’s Handbook…

Doug

Thanks Doug. The rocket im building now it stable in Rocksim. Yet my SSSS was stable in rocksim but never flew straight, everytime it did cartwheeles. Im assuming its because i just didnt have enough stability from such small fins. My new set of fins have a span equal to the rockets. But this time my root is longer, almost 60% longer. So im curious to know beofre hand if that is enough.

Another cause of cartwheels that almost nobody ever talks about is a lateral mass offset. If the center of mass is a little bit away from the centerline, then the motor thrust will make your rocket pitch over. I had at least one rocket that had plenty of fin area to make it stable, but a mass offset caused it to cartwheel all the way to the ground. If your rocket will roll smoothly on a table with the fins hanging off the edge, it will fly o.k. But if it wants to settle to a particular orientation, watch out. Even if it has enough fins to be stable, there’s a good chance it will take off at an angle or have some nasty coning.

I flew the same small-finned rocket twice, first without any correction to the CG. It took off at about a 30-40 degree flight angle, had some nasty coning and I was lucky to recover it. Then I flew it again with a counterbalance weight to center the CG, and it flew beautifully.