Would this be stable?

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Well-Known Member
Jan 26, 2009
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This is a very scratch design on something I might make... Main tube od is 2.6"bt80 , the outside "fin" tube is 3.6", mmt 24mm... Rocsim said it was unstable, but i dont think it took into affect that the outer tube would be a "fin" if you would.

Anyone ever built something like this before?


Check the Apogee web site for the writeup on how to sim tube and ring fins. IIRC divide the circumference of the ring fin by
6 and treat as six additional flat fins of a square profile for the purpose of the simulation. I have built simular rockets and simed
then with RocSim { ring fins, offset ring fins, two tube fins, etc)
and most have been sucessful as predicted with the simulation.
That looks too cool. :cool: What size mmt are you putting in it?

hard to say, the design looks based on old bombs dropped from world war 2 air planes. they were always mid weighted and nose heavy. if the weight in the tube and nose are overly heavy i do believe that it would infact be unstable. if the nose is pretty light and tube doesnt have excessive weight, and u add some weight to the tail. id think that itd be more stable. but i dont think ur gonna get much of a flight out of it, be lucky to break 1000 with a f or g. i dont need rocksim to tell me a fat body and skinny hide is unstable ; )
Originally posted by Const Star
if the weight in the tube and nose are overly heavy i do believe that it would infact be unstable. if the nose is pretty light and tube doesnt have excessive weight, and u add some weight to the tail. id think that itd be more stable.

Sorry to come down hard on this one, Const Star, but that is just plain wrong. Feel free to PM or email me if you want more info.

Rabid, I think you already know the c.g. needs to be forward of the c.p. You may have a stable design but you will have to do some calcs to begin to find out. One thing you have going for you is the 'fat' rocket effect where static stability margins can be safely decreased for low length/diam configurations.
Hard to say. Built only one rocket before with a ringtail.

Make the fins as light as possible (balsa?) and make the rocket nosecone as heavy as possible. PERIOD

This is when you'd probably want to string test it. If worse comes to worse, then you can just add more noseweight. There are plenty of 24mm motors (G37 EM) that should be able to lift that thing.

If you could, you could send me some dimensions and I'll put it into VCP for another opinion.

This should be a stable design; post or PM me the RockSim file and I will touch it up for you ( I wrote the articles about how to simulate tube and ring tail fins using RockSim visit: https://www.apogeerockets.com/education/downloads/Newsletter119.pdf ) Stl1951 has it correct!

You don't need all five ring struts one will do, if the rocket is built correctly. You could also use two three or four struts.

Bruce S. Levison, NAR #69055
welp, i aint the smartest rocket head here obviously but i just have a hard time believing itll be atable with how fat it is and fighting aerodynamics. id be willing to put money that if a slow burning motor was used it wouldnt be stable at all and barely go anywhere, id say definatly go with a fast burning motor, nice boost of thrust to get her moving quick. wouldnt be my first choice as a rocket to build but i do like the way it looks. definate modeling peice
weight in the rear is no good for stability. think of it like balancing a broomstick, if you turn the broomstick so the brush part is on your palm, its really hard to balance...however, if you turn it up, so the brush part is up, its WAY easier to balance. its the same thing in rocketry. the brush acts as the nose weight. and the broom handle is the rocket body. just something to think about.
Well, if you do build this, don't forget to give her a good 'ol swing test.

yes i know the balancing broomstick theory, but it isnt the same when the broomstick is as fat as a water melon and way shorter.
actually its exactly the same....try this one...take a really stable rocket and balance it on its nose cone...wont be so easy, now turn it right side up and balance it on its tail, it'll be a lot easier. even try balancing the nose cone on your hand with TWO fingers, and then balancing it upside down with ONE. its a WHOLE lot easier.

I created the attached RockSim version 5 file for your design. Needs some nose weight (2.1 ounces or 60 grams). It should fly great on C11-3, D12-3 and E9-4 motors.

Bruce S. Levison, NAR #69055
thats a nice theory but ur talking about balancing a some what flat surface compared to a rounded one. thats like trying to ballance an egg on its tip versus smashing one side in. the wide body of the object compared to its short length differs it from other objects in comparisson with a long body and small width. my main concern with the design is the tail end where it forms a bottle neck. a slight gust of wind in any direction aerodynamically can cause it to take a quick turn. and most of the fins rely on the aerodynamics of the wind breaking vertically through the rocket due to their short length. so them keeping a straight path and keeping this fat rocket stable is quite honestly a wonder in performance. im quite curious to how itll do in actual flights.
Hey sheeep, the more I see this design, the more I like it. Would you mind if I built it sometime in the future?

then balance it on the tip of the fin. if it's going to blow over because of the wind speed, then its unstable because the CP is forward of the CG. and thats a bad thing....does rocksim check the CP for you? or do you have to do it manually...?
The design will work. Build it, balance it and ADD NOSE weight for added stability. Broom sticks and water melons not withstanding, it a simple fact that you dont add weight to the tail of this design to make it stable. Just look at an egg lofter... They fly fine with essentially the same design concept. The Estes funky Cluster Bomb is almost identical but not as cool looking as this design. Turn off your computer and use the force!
i was worried that the airflow would almost create a vacuum where the actual fins are, making it more of a "finless" model

i guess ill just string test a few times and launch it in an empty park
I built a very similar rocket using BT60 and a Big Bertha nose cone. Imagine your design, without the tail cone, just a sharp cut off at the bottom of the body tube. The motor mount only extends about 1.5" beyond the end of the tube. There are three tube fins, 1.5" long, made from BT60 tube, glued to the motor mount.

The tube fins did extend beyond the edge of the BT60 tube, though. If your lower ring fin is the same diameter as your forward tube, I still think you'd be OK.

Unfortunately, I built it with a 10.5mm motor mount and flew it on Apgoee B2-3 motors. The rocket's now a decoration because I've run out of B2-3 motors.

It flew great and was very stable. It gave really cool flight every time and handle wind well, despite the slow burning motor.

I keep meaning to make a version with a 13mm motor mount. Or I could simply make a 2.6" diameter version with a 24mm motor mount.