Rocket Stability
Basics of aerodynamic stability
A stable rocket requires that
the Center of Pressure (where the aerodynamic forces balance; #2 in the diagram)
is behind
the Center of Mass (where the mass balances; #3 in the diagram)
MagentaGreen CC-BY-SA 4.0 via Wikimedia Commons
Soda bottle instability
Unfortunately, soda bottle rockets with their lightweight blow-molded front-end and their heavy threaded back-end get this backwards. Left to their own devices, they will flip over, and the momentum will continue them tumbling.
The fix
We're going to play fast and loose with physics terms here. For our purposes:
Fins move the Center of Pressure backward
More precisely, fins add relatively more surface area than mass, so they pull the Center of Pressure more than the Center of Mass. If you put the fins at the back, it pulls the Center of Pressure rearward more than the Center of Mass => more stability
Payloads move the Center of Mass forward
More precisely, payloads add relatively more mass than surface area, so they pull the Center of Mass more than the Center of Pressure. If you put a payload at the front, it pulls the Center of Mass forward more than the Center of Pressure => more stability
Play with Fins and Payloads until the rocket is stable
When stable, it will point the direction of travel, into the wind. Like a windvane.
Now you know more than I did for my first two years of this project while I was preoccupied with getting the launcher to work. Let's discuss Fins and Payloads in the next sections.
Explanation videos
Please see the videos below for primers.