Throwing You a Curve

Bonsoir, mes amis. I wish to speak to you this evening about thrust curves. I promise that I will connect this discussion to rocket fins, but let us begin with the inner workings of the rocket engine and the performance of the rocket.

Model rockets are nearly always powered by a solid-fuel engine. The engine consists of a thick, rolled-paper tube (not metal, which would be very dangerous in the event of explosive engine failure), a ceramic nozzle, and a powdered fuel formed into a hard cake. This is the minimum component complement of a rocket engine, but many engines will also have an explosive charge for the deployment of a recovery device, such as a parachute.

The fuel is usually ignited electrically by a nichrome wire coated with a flammable substance. This wire is inserted into the engine’s nozzle until it makes contact with the fuel, and it is secured there via masking tape or other means. When a current is passed through the wire, it heats the coating until it bursts into flame, thus igniting the fuel. The burning fuel creates hot, expanding gases that exit through the nozzle and provide to the rocket the thrust necessary to lift it from its launching pad. The nozzle is designed to shape these expanding gases, direct them downward and maximize their ability to do their job.

As the fuel burns, a cavity forms inside the engine. This not only affects the weight of the rocket, but also changes the rate at which the fuel burns. Additionally, the fuel may be manufactured in such a way that it burns more explosively in the early stages, that is, the fuel closest to the nozzle may be more volatile. In any case, the thrust of such an engine is neither constant nor linear (by this I mean that the change in thrust is not constant; it varies with time, although the time is very short).

The thrust curve looks like this: After ignition, the thrust takes a brief moment to build. It then increases rapidly to a peak, then falls off. Depending upon the particular engine, it may find a point where it levels off for a bit, or it may merely continue to drop. In any case, the fuel will not burn forever, and the thrust must, at last, come to an end. What goes up must come down.

What on earth could all this have to do with fins? I will tell you. The thrust in the first part of the curve can be considerable, especially in engines designed to be boosters for multi-stage rockets. It can be so strong that it may tear the fins completely off the rocket. Yes, yes, I have seen this many times. Additionally, a booster stage becomes quite unstable when the stages separate, and it will turn directly sideways to the direction of flight. I must remind you, mes amis, that the rocket will be moving quite rapidly at this point, and the full force of the atmosphere will come against the fins in the form of drag. If the fins are not firmly attached, they are subject to being torn violently from the rocket. This, too, I have seen with my own eyes.

So! What do we do? During construction, the root edge of the fins should first be tacked to a straight line drawn on the rocket body using either white or cyanoacrylate glue. When this glue has set, then it is time to apply a fillet of white glue along the sides of the fins. Apply it evenly between the fin and the rocket body, keeping the rocket horizontal until the glue sets. It is best to apply these fillets to only one side of one fin and to the other side of the adjacent fin. This helps prevent the glue from dripping, and if done very neatly, it will not only increase the strength of the bond between fin and rocket, it will, additionally, reduce the rocket’s drag, improving its performance.

In any case, we must design the fins to withstand the maximum thrust produced by the engine. The worse the rocket’s performance characteristics, however, the less will be the effect of the thrust. But we desire to make our rocket sleek and lightweight, do we not? We wish to see it go higher that the last one, so our choice of engines will often mean higher thrust. Be careful how you choose and mount your fins, use a good-quality sanding sealer and paint to make them sleek to reduce drag, and above all, my friends, be safe, and have fun.



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