AboutDave Landgraf Expertise I will be happy to take on questions about low-power rocketry in general, including rocket design, operating characteristics of blackpowder motors, low-power motor selection, rocket construction, aerodynamics, parachute and streamer duration competition, boost glide (and a little rocket glide), egg loft, altitude competition, and scale competition. I can help you with Barrowman stability calculations, but I am not a rocksim user. I know the basics of mid-power rocketry and small composite motors but I don't fly them much (I'd rather stretch my money by sticking with low-power). I believe I can explain many concepts in a way that just about anyone can follow (and maybe even over-explain?). I absolutely do not mind "newbie" questions one bit; the only bad question is the one you don't ask!
Experience I have flown model rockets for some 40 years, including a few early years of NAR competition. I have been active at the local club level as well as in the NAR. I enjoy designing and developing unconventional designs (the kind of thing you look at and say "it won't fly!").
Organizations National Association of Rocketry
Publications Model Rocketry (magazine)
Education/Credentials I am an aerospace engineer who has worked professionally in the field for over 30 years. I have worked often with local school, church, and scout groups to lead introductory build-n-fly sessions.
Question How does the fin location of a bottle rocket affect the altitude of an one?
Thank You For Your Time
Answer Hi, Dev,
I am guessing that when you say "bottle" rocket you mean the kind made from a big plastic soda bottle, that squirts water, and not the kind of bottle rocket that they sell at fireworks stands. The fireworks kind of bottle rocket is stabilized by a long stick, and does not have any fins.
If you mean the kind of rockets propelled by water, then I normally don't try to answer those questions (my experience is with model rockets that burn blackpowder motors, not with water rockets). However, your fin question is directly applicable to both kinds, so let's back way up and start at the beginning.
Fins are attached to rockets in order to help keep them pointed in one direction. Once a rocket can be controlled in a stable flight path, then you can point it upward to attain maximum altitude and you can start tweaking other design features to increase that altitude a little.
Fins work to stabilize a rocket by creating a more effective aerodynamic correcting force on the rear of the rocket. If the rocket tries to turn in flight, and the tail "sticks out" a little, then the fins work to push the tail back into line. To do this, they have to be located on the rear of the rocket; specifically, they have to be located behind the rocket's center of mass.
You can locate the center of mass (some people call it the center of gravity, and that's OK) by loading your rocket for flight. Include all the pieces that the rocket has on it during flight (any parachute, etc) and include any water that would be loaded into the rocket bottle. Hold the rocket horizontally on the side of your finger and find the balance point along the rocket's length. That is the center of mass.
For the rocket to be stable, the total aerodynamic effects (called the center of pressure) must act on the rocket at a point located behind the center of mass. This means that fins must be large enough and located far enough toward the rear that the center of pressure is properly located behind the center of mass by a large enough margin to provide the necessary stability. If you are making water bottle rockets you should follow your instructions that show where to place the fins.
If you place the fins too far forward the bottle rocket may become unstable and will simply tumble in the air (and won't go very high at all).
If you place the fins a little bit forward from where the instructions say you may end up with a design that is theoretically stable but does not have enough actual stability margin. It might fly strait, or it might wander around the sky. Either way, it will not go any higher.
If you place the fins where the instructions indicate, your bottle rocket should fly reasonably well (assuming that the person that designed the water bottle rocket and wrote the instructions knew what they were doing?).
If you place the fins a little bit aft from where the instructions say you will have slightly increased stability and the bottle rocket should follow a strait path. However, this will theoretically only fly as high as the basic design...it will just look "better" while it is flying.
If you place the fins too far aft (and have excessive stability) the bottle rocket might fly a slightly more strait path on days with no wind but on windy days you might also run into "weathercocking" problems. The bottle rocket flight path will bend over into the wind and the rocket will not fly as high.
Once you find the right combination of fin size, location, and number of fins, if you attach the fins further aft they can be made a little smaller and still provide the necessary stability without creating excessive stability that leads to weathercocking. This is a little more advanced stuff than you might want to plow through (send another question if you want to really dig in?). The main advantage of attempting this would be to reduce the aerodynamic drag (slightly) of the smaller fins, but I think the resulting altitude increase would not be very much. If you attempt these sort of design changes I think you run the risk of cutting the fins too small and not having enough stability.
That was a lot of information to throw at you; I hope some part of it answered your question. Have fun with your bottle rockets!
