Tooling was built and three full up prototype motors were constructed and tested. The first test was only to confirm the integrity of the motor casing, nozzle and top end closure. The second two tests were performed on a test stand to measure thrust with following results.

Static test #1 thrust curve


Static test #2 thrust curve

These curves were sectioned at deflection points along the curve and measured to which calibration factors were applied for time (x) and thrust (y) so that the performance of the motors could be computed. Analysis of the two curves is shown below.

Performance summary #2

Performance curve #2

There is good correlation between performances of the two motors. The burn time and total impulse were nearly equal for the two motors and peak thrust was within 5% of each other.

Compared to the model the actual thrust curves show a progressive burn and a longer than expected fall time for thrust.

Comparison between modeled and actual

The plot compares the model and actual performance curve #1. The total impulse or area under the simulation curve has been adjusted to 69 Newton-seconds to match the measured results of performance curve #1. Also since the simulation does not allow for ignition delay the simulation has been shifted +0.053 seconds so that the leading point of the model matches the first deflection point on the measured curve.

The triangular shape of the actual thrust curve, which is a characteristic of KNSB propellant, has been the subject of several investigations. One study suggest that gasses flowing through the core erodes the propellant from the core and produces a coning affect, which widens the core near the nozzle at a faster rate than at the top end of the grain. The expanding area of the cone produces a greater burn surface than what would be achieved if the core had remained cylindrical, thus increasing thrust at a faster rate. Once the conically expanding core reaches the OD of the grain at the nozzle, the core begins to shorten as the bottom end of the core moves upward toward the top of the motor causing a gradual tail off of thrust.

Another recent study suggests that the spacing between grain segments and the ignition difficulties of KNSB may be factors. It has been speculated that if the spacing between the segments is too tight, the flame spread created by the turbulence of the hot gasses flowing between the segments may not be capable of reaching and igniting all of the hard to ignite end surface. Incomplete ignition of the end surfaces would produce a progressive burn. Experiments by Richard Nakka have shown that increasing the gap between the segments to create a greater volume for the swirling gasses produces a thrust curve with good correlation to the simulated thrust curve.

The F motor as shown here has only minimal spacing between the segments and no primers were used to aid ignition. The igniter for these tests was a standard through the nozzle wire bridge igniter with black powder pyrogen.

The appendix to this article offers an explanation that suggests that neither of these hypotheses alone may be totally adequate to explain the triangular shaped thrust curve. Rather a combination of the two theories might be the best way to explain KNSB's odd characteristics.

Click Here to see a static test videoclip of the F motor with a 11 second delay built for this article.

(2.4MB, AVI file)

Note: A too conservative propellant density factor was used when computing grain size based on a propellant weight. The actual propellant weights for the two prototype motors tested were 65.75 grams and 65.85 grams respectively. Since a propellant weight of 62.5 grams was the primary design goal, the grain was shortened in the final design to bring the total propellant weight closer to the design goal of 62.5 grams.

Top End Closure Delay Grain

O-rings are classified by their ID, thickness and material. The O-rings used for this motor have an ID of 5/8", a thickness of 3/32" and are of a Buna-N material. The O-rings were purchased from allorings.com and are cataloged on their web page as -114 Buna-N O-rings selling for US$6.25 in quantity of 250. During the prototype testing of the three motors, no leakage was observed around the O-rings. This was confirmed by post mortem examination of the motor and by the fact that there was no premature ignition of the black powder ejection charge, which was part of the full-up test configuration of these motors.

The delay grain mix for the F motor is the same delay grain mix described in the construction of the G, H and I motors. The epoxy used in this mix is the same clear, 5-minute epoxy adhesive found in hardware stores, home centers and hobby shops and sold under several different brand names. The epoxy comes in squeezable plastic bottles or dual syringe