Q: In science class we are using 1.5 liter bottles for bottle rockets. We have to add on to the bottle (nose cone, fins, ect.) to make it stay in the air longer. Any suggestions? And what kind of material should each thing be built out of?
Also, how much water should the bottle be filled with?
Answer
This is fun. I just helped my son with his bottle rocket for science. Two critical areas of the rocket for success are the nose cone/parachute and the stability (getting the center of mass well forward of the center of aerodynamic drag. There is an easy test you can perform to check drag. Of course, fins are one method of moving the aerodynamic drag back and the nose cone moves the center of balance forward.
The biggest advance we found was using a compression plate for the nose cone. We used a cottage cheese container lid glued onto the base of the bottle to set the nose cone into. My son had concerns about the edge of the lip of the lid catching the air and pressurizing the nose cone as the rocket ascended, so we made exhaust holes to match up with the relief between the bumps on the bottom of the bottle. This makes use of the venturi effect to keep the nose cone firmly locked on until the rocket reaches its apex.
The nose cone has to be constructed so that it will tip off at less than a 45 degree tip angle. W placed 6 pennies (hot glued into the lid of the nose cone for additional mass forward and to help tip the nose cone off. His competition required that the nose cone, parachute and all parts of the rocket remain together is less than linear ft of length when deployed. So the nose cone also has a line tethering it to the rocket, as does the parachute. But we used an elastic shock cord to reduced damage to the rocket and parachute. This shock cord is tied through the compression plate.
For the parachute we used the thinnest clear trash bag we could find. Cut a 42" circle with a 2" circular vent in the center. This helps the efficiency o the parachute by keeping air flowing through the parachute but with lots of drag. If you don't have the vent, you can get a pillow of stale air stuck under the parachute and the whole thing comes down too quickly.
For stability, first you want to find the center of mass. Tape the nose cone with the parachute onto the rocket. Take a string and tie it around your rocket. Slide the string toward the nose or tail until the balance point is found and the rocket hangs horizontal to the ground. Mark this point with a permanent marker +M. Now to locate the aerodynamic center of drag. This is a bit more difficult. The rocket needs to be held horizontal over a pivot point. A short piece of wood trim with a 2-3 ft. dowel put in the middle of it forming an upside-down "T" works well. Hang the trim piece by a short string at the end of the dowel. Now you need a large fan. Using scotch tape, tape the rocket body horizontally attached to the trip piece with the location of your best guess of the aerodynamic center of drag located at the dowel point. now suspend the rocket in front of the airflow of the fan 5-8 ft from the fan. The rocket will likely tent to turn in the airflow. If the nose turns to the fan, then reposition the rocket a bit further forward on the pivot. If the tail turns toward the fan, reposition the rocket further back on the pivot. When the rocket is neutral in the air flow and has no tendency to turn toward or away, or just spins freely in the airflow then you have located the center of aerodynamic drag.
This point can be marked with a "+A" The +M should be at least 3-4" forward of the +A for good stability. Adding fins will
move the aerodynamic center of drag to the rear of the rocket. You can now do a quick check of the stability of your rocket. tie the same string you used for the center of mass test at the center of mass so the rocket hangs horizontal. Then put the rocket in front of the same fan you used previously. The rocket should point into the fan in a very stable fashion.
You will likely need to add fins. Be careful a this point. There should be constraints on where the fins can be. Check with your teacher. Thin plastic,as used in these cheap disposable cutting boards works well. Or if you can get thin balsa wood at a craft store you can cut any shape of fin that you want. Keep in mind that you want as much of the area of the fins as far back as possible. Unfortunately, the curve of the bottle near the neck makes mounting the fins challenging. That is why sloped or angled fins may work best. They will mount just forward of the curve of the bottle, but extend well to the rear of the curved part of the bottle all the way to the mouth of the bottle. Again, be careful of the constraints for clearances for the launch pad and pressure fitting. We had to keep out of the curved area of the bottle within the cylinder defined by the rest of the bottle, and the fins could not extend beyond the opening of the bottle.
Adhesive for the fins is critical. You cannot use hot glues or any solvent glues that molecularly bond with the plastic of the bottle. This could compromise the strength of the bottle and cause an explosion hazard on the launch pad when the bottle gets pressurized.
Good luck and have fun!
