Long before we had personal computers, a lot of smart folks were looking at drag and its effects on rocket flight. The math is fairly complicated - at a basic college calculus level - but even back in the 60's and 70's, advanced rocketeers were producing tables and formulae to estimate the amount of drag a model rocket would experience in flight. A oversimplification would characterize the drag by a single dimensionless number, Cd, called the drag coefficient. A flat surface has a drag coefficient well above 1, whereas a sphere has a Cd around 0.45. Most of my rockets seem to have drag coefficients around 0.75 - at least that's what I get when I try to match the simulations to the altitude profile given by the altimeters. The deal with drag is that you want to keep the airflow smooth around the rocket - any disruption, whether it is caused by rail buttons, base drag at the rear of the model, rough finish, or improper match of the nose cone to the body tube, increases the drag coefficient and reduces the altitude. TARC rockets need rail buttons or launch lugs (unless you want to spend the bucks to build a launch tower), so not much can be done there. However, every rocket can have a smooth finish - all that takes is additional work.
|Rocksim screen capture showing Cd override panel (Click to enlarge).|
|Effect of varying the Cd.|
Back in 1970, Estes produced Technical Report TR-11 "Aerodynamic Drag of Model Rockets". I very much recommend that all TARC teams and serious rocketeers read this, even though it is filled with math and stuff. At a very minimum, they should read pages 39-47, where there are some simple guidelines for drag reduction and flight test data for various configurations and finishes for an Estes Alpha. By airfoiling the fins, adding a boat tail, and putting a smooth finish on the model, they were able to improve the Alpha's measured altitudes from 319 to 446 feet, which is huge! This is a increase of about 30%, which is in line with the Rocksim simulations I performed for the Over Easy.
In case you can't locate a copy of TR-11 online (Google "Estes TR-11 drag"), here are the 5 rules for drag reduction:
- Rule 1: USE GOOD WORKMANSHIP
- Rule 2: ALIGN FINS AND LAUNCH LUGS PROPERLY
- Rule 3: PUT A SMOOTH FINISH ON THE MODEL
- Rule 4: FILLET THE FINS
- Rule 5: BOAT-TAIL WHENEVER POSSIBLE
The moral of this post? A slick rocket is a good thing.