AboutJames Bell Expertise I am a member of Tripoli Rocketry Association, and am certified to fly hi power rockets. I can answer questions anyone may have about design, materials selection, stability calculations, motor selection, altitude and performance predictions, and parachute and recovery options.
Experience I make my rockets from existing kits or parts readily made through commercial channels. I do NOT make my own motor propellants (they are challenging enough even for the companies that do this for a living), and I do not advocate making rockets using "typical household item" such as paper towel rolls.
I am 10 years old in 4th grade. This year, all of the 4th graders will have a contest to see who can build a cardboard-tube bungee rocket that goes the farthest. I am having a little trouble with that because I don't know 1) how many fins to have, 2) how big/ what shape should they be, and 3) where to put them on the rocket.
If you are wondering, the diameter of this cardboard rocket is 3 1/4 inches. Also, the circumference is 10 1/4 inches. The bungee rocket is thirty-six inches long - but I can cut it if I want to.
Also, I was wondering if a shark fins would help. One big one on the top, two medium sized ones on bottom, one fin in the back, and two tiny ones in between the center and the back... Or would that cause too much drag? I know sharks can swim really fast but I wasn't sure if it would help my rocket to be fast too?
For the cone in the front, should it be a perfect cone, a square pyramid, or a half-sphere? Or something else?
I saw the pictures of the Saturn V on your website. Should my rocket be shaped like that getting gradually narrower at the top and then having a needle for the tip?
One last question, should I cut the tube?
Since I am only in 4th grade, I haven't learned everything about rockets yet so I hope you can help me. Thank you for taking my questions into consideration,
Isabelle K.
Answer Isabelle –
I must say, in all the years I have been answering questions, most are sent to me in a format similar to one teenager “texting” another. There is no punctuation or capitalization, and sentence structure and grammar appears to have disappeared from the English language. It is very refreshing and encouraging to receive a question from someone of your age who still knows how to formulate a proper sentence and ask lucid, well thought out questions. I am more than happy to answer your questions.
First let’s talk about the Saturn V and sharks. The Saturn V was a project I started with high hopes of making an awesome model. But, as I worked more on the project, I realized I just couldn’t make the model stable. A rocket needs to “weathervane” into the wind as it comes off the launch rod. The guide fins on the Saturn V were so small; they didn’t provide enough guidance to make the rocket stable. Also realize that model rockets have no guidance systems. The actual Saturn V had gimbled rocket engines that stabilized and guided the rocket into space.
A rocket is most stable when it has descent sized fins on the bottom end. That allows it to do the weathervaning I mentioned earlier. In technical terms, the center of pressure is behind (aft) the center of gravity. If you add fins to the middle or front, or make the rear fins smaller than any of the others, the rocket becomes more unstable because it can’t weathervane. The same goes for your shark fin idea. Just like the Saturn V, a shark changes the angle of its fins relative to the water flow by twisting its body and rocking back and forth. A rigid rocket can’t do that.
Your rocket will be most stable with either three or four fins. Try and make the width of the fins equal to the diameter of the rocket body. In your case, that would be 3 ¼”. Keep the shape simple, straight lines are fine. Just have a good taper back from the leading edge at roughly 60 degrees, then you can take them straight back when you get to the 3 ¼” width. If you look at the Estes Big Bertha fin design, that’s one of my favorites. I actually “up-scaled” a home built design based on that model to a 4” tube size. It flies great.
For a nose cone, you can go with a round nose – again like the Big Bertha – but I prefer a pointed spherical, just like modern rockets and missiles use. I would NOT suggest a pointed end like my Saturn V. That rocket was never finished and it would have had a tip resembling the escape tower of the real thing. It's never a good idea to have a pointed end that can injure someone. As far as the length of the rocket, I think having the rocket three feet long should work good.
If you have any other questions, don’t hesitate to ask, it is a pleasure answering your questions. Good luck on the bungee contest and be sure to let me know how it goes.
