Issue #42 (03/04/01)

Every now and then, I get a letter that reads something like this:

"I just designed a rocket using your RockSim software. When I flew it this weekend, it went unstable. RockSim told me that my design was stable; so what went wrong?"

First off, let me say that "generally speaking" I get this question most often when the modeler is really pushing the performance of the model. That is, they are trying to reduce everything to a bare minimum so that the rocket achieves the highest altitude. Let me say that I can relate to this 100 percent. To me, this is engineering at its best. I like to see modelers doing this type of thing, and asking these types of questions. It tells me that they have a genuine desire to learn and become better modelers.

The answer to the question, however, is that there could have been a zillion things that could have happened. Since I wasn't there and didn't get to see the flight, it is hard for me to give you the reason.

But, let me give a list of things that have happened to some of my rockets. Yep, that's right. I've crashed a lot more rockets than you have. Here are some of the things that I now watch out for.

The first thing I always suspect are the fins. They are the cause to at least 80 percent of the rockets that go unstable.

1. Crooked or canted fins. If you have fins on your rocket that are not perfectly straight, they have the potential to cause unexpected lift forces to be generated.

2. Fins where the airfoils are different. If each fin on your rocket has a different airfoil, this would have the same effect as a crooked or canted fins. It generates non-uniform lift forces. The best airfoil on all the fins would be the teardrop shape (symmetrical). But if it isn't uniform on both sides, you have what is called a "cambered" airfoil. This is the same type of airfoil that is on the wing of an airplane; who's purpose is specifically to generate lift.

3. Forward fins. These are any fins placed on the model in front of the Center-of-Gravity (CG). They are always destabilizing if they generate lift. So it is critically important that they be made as small as possible, and that they be "perfectly straight" on the model. If they aren't, the model is probably going to be unstable.

4. Asymmetrical fin arrangements. The word asymmetrical means "not" symmetrical -- in other words fins that are not placed or spaced equal distances around the tube. It would also include having some fins on the rocket being bigger than others. In either case, what happens is that the lift force on one side of the rocket can be bigger than on the other side. This can cause the model to do loops if it is hit by a sudden gust of wind (on the wrong side of the model).

5. Fins that pop off during flight. When this happens, the result is that the lift forces around the rocket are not uniform. This makes the rocket do loops. And it is pretty easy to figure out after the flight if you're fortunate to find the parts afterward.

6. Loose fins. Even if the fins don't pop off during flight, the reason we don't tolerate loose fins on the rocket is that they can vibrate back and forth. This disrupts the airflow on one side of the model, and can cause it to go unstable. So never tape a fin onto a rocket, or permit someone else to do so. This is just asking for trouble.

7. Fin flutter. This condition is a lot like loose fins. But the difference is that the root edge of the fin is securely attached. Typically, it is caused by fins that are made from very thin material, or material that can flex. During flight, the fins twist. When this happens, the fin tip is at an angle of attack. That generates lift, and can cause the model to go unstable. If you ever hear of a rocket that buzzes as it goes up into the air; this is fin flutter.

8. Protrusions on the side of the rocket that act like fins. It doesn't have to look like a fin to act like one. Anything on the side of the rocket body tube can generate lift or drag forces when the model is at an angle of attack. It may be a canopy on a model that looks like a jet; or maybe a parasite glider that is there just to be boosted into the air. These forces are very difficult to predict; which is the main reason that RockSim does not allow things to be strapped to the sides of the tube.

9. Parachute that isn't fully inserted into the rocket, and flutters along-side of the rocket. This disrupts the smooth flow of air over the fins. I've seen this happen to a lot of younger modeler's rockets.

10. Loose nose cones that are canted in the tube. This is similar to the number 8 above, because a canted nose cone can generate more lift forces on one side of the rocket than the other.

11. CG shift during flight. This can happen on really light -- high performance models. On these models, the parachute sliding rearward in the tube can be enough to move the CG behind the Center-of-Pressure. This has happened to me several times during competition.

12. Air being ducted through the rocket, and out one side. This one isn't common, except for rockets that have jet intake scoops. You just have to watch out for the direction of the air coming out the back.

13. Putting the rocket high on the launch rod, instead of near the bottom. I see this one all the time. Most time it is because the launch rod is bent near the base, and you are trying to avoid that area. Or maybe there is crud on the rod. And it happens a lot on gliders, which have long tails that stand on the launch pad. The result is the same -- there isn't enough length of rod for the rocket to travel while it builds up to that critical flight speed. And as we all know, there is a minimum speed the rocket must be traveling at before the fins become effective at keeping it flying straight.

14. Rocket binding on the launch rod. This is similar to the one above. The rocket hangs up for just a moment; decreasing its speed. When it lets go, now it isn't traveling fast enough.

15. Getting entangled in the igniter clips; preventing it from lifting off smoothly. Again, anything that slows the rocket while it is on the rod may be a detriment to the stability of the flight.

16. Piston launchers come with their own problems. When the rocket is traveling upward on the piston and it reaches the stop before popping off is the most common problem. In effect, the rocket comes to rest for a brief instant, and then pops off the piston assembly. Because it is basically starting from rest, the fins of the rocket are not effective at all. So if you are using a piston launcher, just be careful. Better yet, mount the rocket inside a tower launcher too. That way, when it pops off the piston, it has the guidance of the tower while the rocket builds up speed again.

17. Insufficient thrust level. If your rocket is heavy, and you're using a low thrust motor, you need to be extra cautious. Run your RockSim simulations, and see what the lift-off speed is when the rocket clears the rod. If it isn't up to the minimum lift-off speed; try a motor with more initial thrust.

18. A strong gust of wind right when the rocket clears the rod. We blame a lot of unstable flights on this; although it is preventable. Just like number 17 above, run your RockSim simulations; but with a high wind speed. See what happens to the flight.

19. A nose cone that pops off in flight. This one is obvious when it occurs, and usually happens on larger diameter models. And there is a reason nose cones suddenly pop off during high speed flights. It is because the internal pressure inside the rocket is greater than the outside air pressure. So there is a force trying to push the nose cone out of the tube. It is easily solved with a pressure relief hole.

20. Rod whip. We blame this one for a lot of unstable flights too. If there is wind, we can see the rod waving about. This movement adds a velocity component that isn't anticipated. But a lot of times, we use this excuse when there isn't any wind at all. What we think my be occurring is that as the rocket is traveling upward, it is somehow flexing the rod. There needs to be some high-speed movies made to actually determine what is happening, and if this is a legitimate reason for the model to go unstable.

21. Canted thrust line. This may occur when the motor mount is cocked inside the rocket body tube. Because the motor is pushing in a different direction than the rocket wants to move; it causes it to go unstable.

22. Off-axis thrust lines. This is different than a canted thrust line, but the effect can be the same. One kit in particular uses an off-axis thrust line. That is the Estes skill level 5 Space Shuttle. The motor is straight along the length of the model, but is offset from the centerline. This is because the orbiter is creating lift forces that counteract the off axis thrust. But if you tried to fly the External Tank by itself, it will do cartwheels across the sky.

23. CG that is off-axis. This one is rare, unless the CG is way off to one side. It is usually caused by something heavy on one side of the model (maybe a payload inside the rocket that has a heavy battery against one side of the tube).

24. Nozzle Erosion. I have a good friend that thinks this is the cause of a lot of unexplained unstable flights. The theory is that the gases coming out the rocket are eating at the sides of the nozzle. If the erosion of the nozzle isn't uniform, then you'll get a vectored thrust. This would cause the rocket to go unstable. I think there is a lot of legitimacy in this theory, particularly for black powder motors that have clay nozzles. I don't know if it occurs on composite motors that use a phenolic nozzle. The problem can be alleviated by using a longer launch rod. Because if the model is traveling fast enough, the fins can cancel out the effect.

25. Side wall failure. This is actually a motor cato. A tiny pin-hole develops near the base of the motor, which vents hot exhaust gases out one side. It always leaves physical evidence that can be seen after the flight. There isn't anything you can do to prevent it, but it can be used to explain why the flight went unstable.

26. Short Rockets. This is where it gets really tricky. Short rockets are less "dynamically stable." That is, they are more easily disturbed from the flight path, and they take longer to correct back to straight flight. In my opinion, there is something (one of the other causes listed above), that triggers the model onto its course of instability. Figuring out what that is will be difficult because the design of the model makes it tend to be less stable to begin with. See the related article in e-zine newsletter #86.

27. Very long rockets can flex. This means that the nose is flying in one direction, while the tail and motor are going in a different direction. The NAR super-roc competition can really be fun to watch because of this fact.

That's my list of causes of rockets going unstable. As you might guess, most of them are preventable. It takes care and patience during the construction of the model, and setting it up to fly.

If you have any others causes, please send them to me, and I'll add them to this list. By knowing the things that can go wrong, we modelers can try to prevent them from happening in the first place.

Also, there are some things that you can do to enhance the stability of the rocket. Those things are listed in my book "Model Rocket Design and Construction." Check it out, and see if any of those methods might fit your situation.

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Tim Van Milligan is the owner of Apogee Components (http://www.apogeerockets.com) and the new rocketry education web site: http://www.apogeerockets.com/education. He is also the author of the books: "Model Rocket Design & Construction" "69 Simple Science Fair Projects with Model Rockets: Aeronautics" and publisher of the FREE e-zine newsletter about model rockets. You can subscribe to this e-zine at the Apogee Components web site, or sending any message to: apogeerockets-subscribe@listbot.com This article may be reprinted as long as this paragraph is included with the text.

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