The calculations show that the burn time for the motor would be 1.59 seconds. However, based upon experience with the J/K motors, it was thought the burn time would be closer to 2 seconds. A motor that has a total impulse of 2000Ns and a burn time of 2 seconds has the designation of K1000.

Three K1000 motors were built and tested. The first two were strap down tests to see what problems might occur with a motor of this size. The third motor was tested in a test stand to measure the performance of the motor. The test stand was the same test stand that has been used to test all of the previous PVC motors. In all cases there was only minor erosion of the nozzle, no blow-by, no leaks, no deformation, and no erosion of the interior of the casing.

The new nozzle design has no convergence as the other PVC motor and the inset is exposed directly to the heat of the interior of the combustion chamber. Upon dissection of the motors, the washer was found to have separated from the concrete divergence as a result of the thermal expansion of the insert pushing against the wall of the casing. However, there were no signs of abnormal erosion in the concrete and it is believed this separation occurred after the burn.

Following is the measured thrust curve for the K1000

From this curve it can be said that the goal to achieve a flat thrust curve was achieved. When this curve was first observed it was thought that there had been a problem with the test stand. However, the test stand was carefully disassembled, analyzed and no abnormal operation was found. Further, the analysis of the curve gives a total impulse of 1993Ns, which is within less than 1% of what was predicted. The spike on the leading edge of the curve is the flash of the igniter and ignition initiators. The bumps on the trailing edge are the expulsion of the remains of the lower 4 inhibitor sleeves. This also occurred with the "J" motor, but was not detrimental to the motors.

The results achieved with these tests have provided some insight into the burn characteristics of Sorbitol. First, it is possible to shape the Kn of a grain to compensate for the progressive burn nature of Sorbitol. This would suggest that this odd characteristic is not random as might occur if cracks were opening in the grain. Further, the fact that the inhibitor sleeves were expelled late in the burn suggests that the burn process occurred evenly along the core and not in a conical fashion as though burns rates were faster or erosion was occurring nearer the nozzle. This information may prove useful in the development of future motors because it means that motors can be built without pressure spikes that might otherwise limit their performance.

Internal Retainer versus End Caps

During the 4 years of experimentation with PVC/Sorbitol rocket motors, nearly 50 motors have been built, tested and/or launched including 7 successful consecutive test firings of the "J" and "K" series of motors. There has never been a catastrophic failure of any motor when built to the instructions herein. However, failures have occurred with non-conforming motors that were built either as test subjects or to study new design and building techniques. Following is a description of those motors and observations about the failures.

1. Two "G" motors of the end cap design were built specifically for test purposes to deliberately over pressure and fail. One of these motors was the test subject in the experiment to determine the bursting strength of 1" PVC pipe.

When these motors failed, the end caps shattered as completely as the casing. Sheer lines extended along the length of the casing and through the end caps.

2. An "I" motor was built with an epoxy plug top end closure to test this type of closure technique. Although the epoxy plug is not a PVC retainer, it has the same mechanical properties. The motor had the same grain configuration and Kn as the other "I" motors that have been successfully built using end caps.

This motor failed. The casing failed at the inside edge of the epoxy plug. The casing was completely shattered, but the PVC pipe was still intact around the plug.

3. A "K" motor was built using internal retainers of PVC pipe glued to the inside diameter of the casing to test this type of design. The motor had the same grain configuration and Kn relationships as the other successful "K" motors that used end caps.

This motor failed in the same manner as the "I" motor. There was clear line of failure at the inside edge of the retainer. The casing completely shattered, but the casing/retainer remained intact.

The reason the "I" and "K" motors failed, was the stiffness given to the casing by the epoxy plug and retainer. As pressure builds inside the casing, the casing expands in a radial direction. Because the retainer stiffens the casing, it cannot expand as easily as the rest of the casing. This creates a bend in the wall of the casing at the retainer and if the stress created by the bending is great enough the casing will fail. It is like inflating a long balloon, which is inserted half way into a rigid tube. The portion of the balloon inside the tube will expand only to the inside diameter of the tube while that portion outside the tube will continue to expand unrestricted. As the balloon outside of the tube expands the wall of the balloon at the edge of the tube develops a sharp bend. It is the bending at the edge of the retainer that induces stress that causes the casing to failure. In some cases these stresses may not be great enough to cause failure, but in the larger motors the casings are not strong enough to resist these stresses and the casing fails.

Commercial reloadable hobby motors are designed so that there is no restriction to the expansion of the casing. The casing has either internal threads to accept a threaded male closure or has a grooved ring to accept a snap ring to retain the closure assembly. In either case, the closure assembly is not "attached" to the casing and is allowed to "float" as the casing expands.

What is different about the end caps that allow them to be used successfully when retainers cannot? The answer is not clear, but the way the "G" motors failed may offer some suggestions. The shattered remains of the "G" motors had sheer lines that extended along the casing and through the end caps suggesting that the end caps may not be as strong as the pipe. When the pipe failed the fittings also failed. Manufactures of PVC pipe and fittings may have matched the fittings to the pipe to minimize bending stresses. Fittings are made from a softer, lighter and generally weaker plastic than PVC. As the pipe expands the soft fitting may expand along with the pipe keeping the bending stresses to a minimum. The secret to the success of the PVC motors may be end caps that minimize stresses that would otherwise reduce the inherent strength of the PVC casing.

Rocket Motor Operating Pressure versus PVC Pipe Pressure Rating

PVC pipe is different from metal pipe and bears its load differently. PVC pipe develops cracks if held under pressure for a long time and a failure even at rated pressures can occur after many years. The rating printed on the pipe has a pressure-temperature-time relationship. That is, the rating indicates that the pipe is designed to sustain a static load, at a given temperature and pressure for a certain period of time, usually tens of years. Thus PVC pipe can have a large difference between a one time quick burst pressure and it's rating. Further the rating on the pipe is based on a static load and not a dynamic load that PVC pipe experiences as a rocket motor casing.

These factors make it difficult to compute or even measure the burst pressure of PVC pipe as it applies to use as a rocket motor casing. For example, PVC pipe does not hold up well to sudden changes in pressure. It also does not bear up well under high temperatures. A rocket motor casing experiences both of these conditions. An attempt was made to measure the burst pressure of 1" PVC pipe as it applies to use as a rocket motor casing. The results were given in the article on the "G", "H" and "I" motors and extrapolated to larger size pipe. Although, the success of the PVC motors may suggest that there is some validity to these results, the pressures suggested by this single experiment are for information purposes only and should not be considered as a rating for PVC pipe.

It might also be pointed out that PVC motors using either sucrose or dextrose may be more prone to failure than those motors using Sorbitol. PVC pipe is a visco-elastic material and does not stand up to sudden changes or surges in pressure. Thrust measurements of motors fueled with the various sugar propellants have shown that Sorbitol has a slower pressure build up than either sucrose or dextrose. This may make Sorbitol more suitable for use with PVC pipe than the other sugar based propellants.