Kevin Wickart's Tech Tips #5

The Handbook of

Model Rocket Competition

by Kevin Paul Wickart

THE R.S.V.P. PRINCIPLE

AUTHOR’S NOTE: This article was one of the core pieces of a series of seminars on beginning competition that the author delivered at NARCON's ’97 through ’99.

In my opinion, the primary goal of a beginning competitor should be simply this: to qualify. Qualification means no more—and no less—than avoiding a disqualification for whatever reason. Simple qualification in every event is no easy task; many expert competitors fail to do this.

In the course of preparing for competition I performed some statistical analysis on the percentage of each type of disqualification experienced in each event at a national contest. This led me to realize that there are certain factors that one could build into one’s contest models in order to virtually eliminate the chance of disqualification.

These factors are Reliability, Stability, Predictability, and Visibility, and they form the heart of the R.S.V.P. principle.

RELIABILITY: A reliable model is one that functions properly in the manner for which it was built. This covers a lot of territory, from simple reliable ignition to proper operation of on-board electronics. The common earmarks of a reliable model are: staging that works, recovery systems that deploy fully and remain attached to the rocket, payload sections that properly secure and cushion the payload, fins that stay on, body tubes that don’t crimp, and motors that stay in the rocket.

In my analysis reliability problems accounted for more disqualifications than the other three factors combined. Nonetheless reliability takes a beating at every contest because more trade-offs are made on it in the name of performance.

To achieve good reliability, do your best work when building your contest models. Get in the habit of building carefully and using familiar techniques. If you want to try a different technique, make it familiar before using it on a contest model.

Don’t trust to lighter but weaker materials. Those few milligrams of mass saved amount to nothing if your model comes apart in the air.

Avoid complex systems as much as possible, for they are the breeding ground for gremlins and Murphy’s Law. Sure, your projections look real good for that E-powered sliding-pod flop-wing rocket glider. But if your tests have shown that the slide pod works two-thirds of the time and the flop-wings work right half the time, then the odds are that your model will work entirely correctly once out of every three flights. This is the wrong kind of model to bring to a contest. Why put all that work into a model that is more likely to crash on its first flight?

Let’s face it—if you aren’t sure your rocket will work like it’s supposed to because that’s the way you built it, you’re just burning motors.

STABILITY: This means knowing that your rocket will fly in a safe and stable manner under the conditions in which it is to be flown. I emphasize the last part of this because I have seen—up close under power—several models that were designed with extremely marginal stability in order to reduce mass and drag.

If you don’t know how to calculate the stability of a model, then learn. The math isn’t hard and can be done with a five-function calculator and a piece of scratch paper, or you can invest in one of the model rocket design programs available. They’re worth the money. But be warned: that high-performance model you designed with a stability margin of two millimeters will certainly fly just fine—if there is virtually no wind, if the rod doesn’t whip at all, if your launch angle is perfectly vertical, if there is no upper-altitude winds hear, if the model doesn’t hit a layer of dense air, if the motor thrust is absolutely centered, if you got the fins on perfectly straight, if…well, you get the idea, I hope. There are very good reasons why the rule of thumb for stability margin is one caliber.

I cannot stress enough that the contest range is no place for experimentation. It is imperative that you determine the stability characteristics of your models. If you can’t learn how to do this, then limit your flying to proven models. Otherwise, you’re just burning motors.

VISIBILITY: Can your model be seen during its flight? If the trackers or timers can’t see it, they can’t return results. If you can’t see it, you may not be able to return it if needed.

As a veteran timer and tracker I can testify that a minimum-diameter unpainted rocket with waferglass fins—and there are a lot of these being flown—becomes invisible at an altitude of between one hundred and two hundred feet. And this is from the timer’s point of view; imagine trying to follow it from a tracking station three hundred meters away.

To improve your model’s visibility and thus your chances of qualification, put some color on the rocket. A simple contrasting pattern of black and a bright color works very well in the air and on the ground. You do not need to paint the model and add "all that weight." Trim Monokote™ is easy to use, comes in a wide range of colors (including fluorescents), and gives the rocket a relatively low-drag finish at the cost of a very little mass. Another popular method is to color the model with felt-tip markers.

Instead of flying a minimum-diameter model, build your rocket one or two tube sizes larger and boat-tail it down to the motor tube. Not only is visibility increased, but also you have more room for the recovery system thus improving reliability. And the truth is that a well-built boat-tailed rocket is capable of outperforming an average minimum-diameter model, as well as being easier to see.

Again, one very important key to qualification is the ability of the trackers and timers to see your models. If they can’t, then—all together, now—you’re just burning motors.

PREDICTABILITY: this is not so much a design and construction issue as it is a practice issue. Predictability means knowing your model’s flight profile and how it will perform. This covers not only being aware of the altitude or duration it will achieve, but also how it handles wind, thermals and other variables. Flying a high-performance contest model for the first time can be a startling experience—so startling, in fact, that you are likely to lose the rocket.

Now, I confess that this is the one aspect where I take my own advice the least. I am nearly famous for flying reliable, stable, visible models that perform so well I never see them again. If I took the time to learn how they fly, I’d likely get more of them back (and do better in the standings!). I’m so bad at this that two months after thermalling a boost-glider away at NARAM-40, I thermalled an identical glider away on half the impulse at a WOOSH Regional. And did it again at NARAM-42. I figure I’ll learn my lesson by NARAM-50, and rig a dethermalizer or de-trim the glider slightly.

It’s predictability. If you don’t know how your rocket will behave, then you are just ___________ ___________ .(fill in the blanks)

PUTTING IT ALL TOGETHER: I have an 18mm Altitude model that serves to illustrate all aspects of the R.S.V.P. principle. It’s designed for good performance, with an elliptical nose and fins (with a great airfoil) and mid-body ejection.

Reliability was achieved primarily by designing in a sufficient stability margin so it would fly straight in even moderate wind, and by using braided Kevlar™ for the shock line. The fins are made from 1/16" hard balsa attached with double glue joints and small epoxy fillets. Although the model is fairly large in diameter for B Altitude (which is what I designed it for) there is room for tracking powder and a small streamer without jamming it all in.

I assured stability during the design phase by performing the Barrowman calculations to determine the model’s center of pressure. Comparing this to the model’s center of mass, I knew it had one-caliber stability and would fly well under just about any conditions.

Good visibility was attained not only by using a larger-than-minimum body tube but also by wrapping the tube with fluorescent orange trim Monokote™. The fins, nose and upper body are painted gloss black with an orange accent stripe near the nose. The model is capable of carrying quite a lot of tracking powder in the tube above the mid-body separation point. The rocket is quite easy to see on the ground and in the air.

This is the most predictable model I’ve ever flown. Prior to NARAM-40 I’d flown it six times. I knew its altitude capabilities, what delay to use, how it would weathercock, and how far I’d have to hike to retrieve it. I was able to inform the trackers that it would go about 275 meters directly over the rangehead. In spite of fairly tricky winds it performed exactly as advertised, posted a closed 285-meter altitude on a single flight, and I had no trouble returning it.

Sure, I didn’t place anywhere near the top in that event, but I did qualify and still had time to prep and fly the second event of the day while others were burning motors over and over in an attempt to get a closed track.

Reliability…Stability…Visibility…Predictability…Accept no compromises and you should have no trouble qualifying. Then you can work on improving performance.