Through a number of sources, we have put together this section of Tips and Tricks for using HyperTEK products. If you require further information or have a suggestion to add to this list, please feel free to contact us.
Clustering Hybrid Motors

With the growing popularity of the HyperTEK hybrid motor system, it surprising that there have been only a few attempts at clustering these motors in high power rockets. Hybrid motors are sometimes perceived to be more complicated than traditional HPR solid motors. They also require different ground support systems but many clubs now provide HyperTEK ground support in addition to solid motor launch systems. The low cost per flight associated with these motors make them attractive for cluster projects.

There is one fundamental requirement with any clustered motor scheme regardless if it's hybrid, solid or liquid. Ignition has to be predictable, simultaneous and reliable. With a solid motor this can be generally be accomplished with a properly matched igniter /initiator system and by selecting a propellant and motor combination that will come up to operation condition as quickly as possible. Hybrid and liquid motors are a little more challenging in this regard. This is mostly due to fact that a number of prerequisite events must occur during the start sequence before the motor or engine can operate at design pressure. High performance liquid engines such as those used on the shuttle are one extreme example. These engines are started at T minus 6 seconds while being monitored and controlled by computers. There are so many variables to control during the start sequence that computers must be employed as no human could respond fast enough given the overwhelming task and the number of decisions that have to be made in such a short period. Only if all parameters have been met by T minus one second are the solids ignited and the hold down clamps released, otherwise the main engines are shutdown and the launch is aborted.

Small hybrids that employ self-pressurizing oxidizers such as nitrous oxide are obviously far simpler to start but none the less employ a start sequence. In the case of HyperTEK propulsion systems, there are two main events that occur prior to the engine reaching full operating pressure. When the launch switch is activated on the ground support panel, gaseous oxygen (GOX) starts to flow inside the motor and an arc is simultaneously produced on the end of the igniter lead inside the polymer motor grain. The polymer begins to burn rapidly in the pure oxygen environment and the resulting flame exits the nozzle. Two nylon tie wraps that pass through the very base of the nozzle restrain the motor and rocket. The flame quickly melts these hold downs and the pressure of the nitrous oxide lifts the motor and rocket a fraction of an inch off of the nitrous fill stem allowing nitrous oxide to flow freely into the combustion chamber. The motor instantly comes up to full pressure and the rocket launches. Some of those who have attempted to fly hybrid clusters have had difficulty for a number of reasons. The most common is having only partial ignition of the cluster. Other problems include melted fill stems or incomplete ignition (warm starts) of the remaining motors. The clustered example described in this article attempts to address these problems by identifying the cause and suggesting solutions that have been used successfully in other hybrid applications. The first and most commonly overlooked detail is a falling fill stem. Most of you who have experience with the small J class motors probably aren't familiar with that term. Those of you who fly L and M Hypetek motors are. The standard J class motors have a fixed fill stem as I mentioned previously. When the hold downs melt away, the nitrous pressure lifts the motor and rocket off of the fill stem. This allows nitrous oxide to flow freely into the combustion chamber. The amount of this force is a function of the sectional hydraulic area of the fill stem versus the tank pressure of the nitrous oxide. Think of it like a small piston in a pressurized cylinder. This works well. When flying a rocket with an L or M or a cluster, the total weight of the tanked rocket may exceed the lift capability of the pressure generated on the fill stem. In a situation such as this the hold-downs will melt away but the nitrous oxide will not flow and the rocket will just sit there burning GOX until the operator aborts. What usually happens is that the inexperienced operator is not aware of the problem and continues firing the GOX. This exposes the fill stem to very high temperatures for an extended period of The fill stem eventually melts and the rocket launches leaving a melted fill stem in its wake.

The fix for this is very simple. Instead of bolting the fill stem solidly to the rail we attach launch lugs or a slider to the fill stem plate so it can actually slide on the rail. The rocket is placed on the rail and lowered to a lug stop. The fill stem is raised into the motor injector and the hold-downs are installed. It is very important that the distance that the fill stem be allowed to drop is kept as short as possible. It only has to fall enough to allow the tip to clear the injector bell. If you provide too much distance, the stem/slider assembly can gain excessive momentum and shear the lugs off. A rubber bumper or mechanical cushion on the assembly or slide stop is also a good idea.

The next area of attention is the igniter leads. If one or more of the motors is slow in coming up on GOX ignition, the flame from the adjacent motor may melt its igniter lead prematurely and the motor may warm start or not start at all. To prevent this from happening, take some silicone rubber fuel tubing and sheath the igniter lead from the base of the motor nozzle to a point away from the blast deflector. Silicone tubing ablates at high temperatures and will protect the igniter wire from the heat of the adjacent motor until it's had a chance to start. Silicone fuel tubing is available at most hobby stores that handle R/C.

Now if you are determined to have the ultimate in reliability and safety, the following is the best insurance policy. You will have to do a bit of work however. This example employs everything we have discussed so far but instead of being dependant on nylon tie wraps as a hold down system, this launcher uses manually activated, pneumatic hold down clamps not unlike real hybrid and liquid rockets. This design example uses two opposing short stroke, medium diameter pneumatic cylinders to restrain the rocket on the rail and the fill stem(s) in the injector. You will also require an electro-mechanical pneumatic valve to actuate them. A release button to activate the valve is integrated or used in conjunction with the ground support control. The operation of the system is identical to the standard system but with one important advantage. The operator has full authority and releases the rocket only when all the motors are fully initiated on GOX. The operator can also abort anytime during startup if there is a problem.

For those who enjoy rocket science and like to tinker, a clustered hybrid rocket can be a very worthwhile and fun project.

 



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This Page Last Updated: April 19, 2001
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