This report covers tests of a 2" diameter model of the 7.2" Chemical Rocket to determine its performance and possible means of increasing stability and reducing dispersion.
The rocket was tested with the two original tails, the ring tail designated herein as No. 61 and the ring tail with extended fins designated No. 62. Three other tail designs were tested designated No. 63, No. 67, and No. 68. Of these, No. 67 was the only one that produced results superior to the No. 61 and No. 62 designs. This No. 67 Tail has extended fins similar to Tail No. 62 and projects beyond the nozzle about one diameter. Details of these tails are given in Figure 12.
Tail No. 62 gave a restoring moment 50% greater than Tail No. 61, and Tail No. 67 gave a restoring moment 45% greater than Tail No. 62, both values being for 5° yaw. It is believed that Tail No. 67 represents about the best that can be done in redesigning the tail, as it produced a fairly high moment, a very large center-of-pressure eccentricity, and only one of the five tails tested has a lower drag coefficient.
In this connection it should be noted that all the tails tested gave, without exception, adequate stability to the projectile to insure satisfactory flight after burning is completed. Therefore, the only benefit to be obtained from an increase in the stability above that produced by the original ring tail (No. 61) must come from whatever reduction it might effect in the dispersion occurring during the burning of the propellent.
Calculation of the period of oscillation of the projectile in flight, and the equivalent wave length, makes possible a comparison of projectile performance from the standpoint of dynamic stability It can be shown that, for rockets with long burning times, the shorter the wave length for a given projectile, the less will be the dispersion. Using this measure of dispersion, Tail No. 67 would be expected to produce 1S% less dispersion than Tail No. 62, and Tail No. 62, i8% less than Tail No 61.
This investigation leads to the conclusion that the No. 61, No. 62, and No. 67 Tails will give a high degree of static stability and it is improbable that much more can be accomplished by a redesign of the tail. It is also a fact that the dynamic stability of the projectile cannot be materially improved if its present physical dimensions are to be retained. The conclusion must, therefore, be reached that the most effective means of lowering the dispersion of this rocket is by reducing the malalignment of the jet with the axis of the projectile and eliminating as far as possible asymmetry in the tail assembly
