Excerpt from January 2001 Newsletter Vol. 8 No. 1
Tips for Competition Rocketry
Although sport kit rockets are permissible in competition rocketry, the best performance is usually achieved with a scratch-built rocket specifically designed for the event being flown. Basically, the key to parachute and streamer duration events is to design a rocket that will go as high as possible on the engine class being flown, then pack the largest possible parachute or streamer that will fit into this rocket. Designing for maximum altitude and for largest recovery system can be at odds with each other, however.
When designing for maximum altitude, the key is to reduce weight and drag as much as possible. The typical competition rocket is the standard three-fins-and-a-nose-cone design, with the body diameter as small as possible and the length of the rocket no longer than needed to hold the recovery system and maintain stability (Superroc events being the exception). Use a parabolic nose cone and clipped delta or eliptical shaped fins to minimize drag. A boat-tail added to the bottom of the rocket, if possible, will also reduce drag; if a boat-tail isn't possible because the body tube is the same diameter as the motor, at least make sure the bottom edges of the fins are slightly forward of the bottom edge of the body tube and engine. Weight and drag can also be reduced by using the thinnest fin material possible (1/32" plywood -- available at hobby stores -- is commonly used). Drag can be reduce by sanding the leading edges of the fins -- not quite rounded, not quite pointed, but somewhere inbetween), tapering the trailing edge, and sanding the outer edge the same as the leading edge ("rounded point") or leaving it square.
Then there is the recovery system. Forget those parachutes and streamers that come with sport kits; the material they are made from make them too heavy and bulky. Make your own parachutes and streamers instead. For parachutes, try mylar or thin plastic such as dry cleaner bag material (you can find drop cloths made of even thinner plastic than dry cleaner bags at hardware and paint stores), and use thread for shroud lines instead of the thicker string that comes with sport parachutes. For streamers, some competitors use crepe paper (although it tears easily and is not the thinnest material available), but many experienced competitors use tracing paper (available where you buy office supplies). Again, use thread or some other thin material (such as fishing line) to attach the streamer instead of using common string.
In addition to the design of the parachute, there is the method of attaching the parachute to the rocket and protecting that parachute from the ejection gases. First of all, don't use the braided elastic strap that is found in sport kits; it's too bulky. Use thin elastic cord, or a short length of thin elastic cord tied to a length of stout thread or fishing line. Don't attach the shock cord to the inside of the body tube with a folded wad of paper such as sport kits use; it reduces the inside diameter of the tube (which reduces the potential size of your parachute or streamer) and creates a bottleneck within the tube that your parachute or steamer might get caught on instead of being blown out of the tube. Instead, tie or glue your shock cord to the top of the motor mount
Next, think about how you protect that parachute or streamer. Estes wadding might be good enough for sport kits, but it has to be loosely crumpled to work and therefore takes up a lot of tube space. In contest rockets, this extra tube space means added weight, so try alternative materials -- such as the finely shredded paper wadding that looks like insulation, a piece of fireproof foam the same diameter as the inside of the body tube, or a protective shroud wrapped around the parachute or streamer
Recovery systems present the need for additional trade-offs in high-performance design. Some rocketeers opt to use a larger body tube in order to accomodate a larger parachute or streamer, hoping that the slower rate of descent of the larger parachute or streamer will make up for the loss of altitude due to the increase weight and drag of the larger tube. Other rocketeers choose a minimum-diameter tube and a smaller parachute or streamer, hoping the extra altitude will make up for the faster rate of descent. In parachute design, the more shroud lines used the better the parachute traps air under it and the slower the descent, but additional shroud lines also add weight and bulk and increase the risk of a tangle. Experimenting with alternative wadding materials also means taking a risk with successful deployment.
Another option available to the competition rocket designer is to use materials specifically designed for competition rockets. For example, Apogee Components sells competition engines in smaller-than-standard casings, with lower and higher average thrusts than Estes or Quest engines in the same engine class (A,B, C, etc.) and Totally Tubular sells thin-walled body tubes as well as smaller-diameter body tubes to fit the Apogee motors.
It is the design challenge, the spirit of trial and error and experimentation, that makes competition rocketry so much fun. Because of all the trade-offs involved in high performance design, there is no single "perfect" or "best" design. Luck also plays a little bit of a role as well; a superior design may also be more complex and be more at risk of something going wrong, while a lesser design could catch a strong thermal on the way down and stay aloft much longer than all those sleeker models. If something does go wrong, there will always be an opportunity to try that design again the next time that contest event is scheduled. Finally, good sportsmanship is common; the vast majority of participants in competition rocketry are more than willing to share their design ideas and help each other out (this article is a prime example). Give it a try; you might be surprised at how addicting it can become :-) .
(Note: many of the recommendations on body and fin shape to reduce drag are taken from the Handbook of Model Rocketry, by Harry Stine. This is a valuable reference book for all rocketeers who want to understand how rockets work and how to make them work better, whether a casual sport flyer or a hard-core rocketeer.)