Advent of Pencil at Kokubunji

From these repeated tests appeared a test rocket Pencil, 1.8 cm in diameter, 23 cm in length and 200 g. As described before, the solid propellant for the Pencil rockets was "double base" (smokeless gunpowder). The main ingredients of double base are nitroglycerin and nitrocellulose, with the addition of moderate amounts of stabilizing and hardening agents. This mixture is stirred and kneaded into something like a ball to be squeezed through an extruding machine.

After confirming an internal burning pressure of 112 atm, a burning duration of 63 msec, a thrust force of 29 kg and so forth in the factory, the horizontal test launch of Pencil was carried out on March 11 at a firing range in Kokubunji, Tokyo, followed by an open firing test on April 12 in the presence of people from government offices concerned and the press corps.

Launched horizontally from a 1.5 m long launcher, the Pencil flew through a series of thin-wired paper screens to pierce the sand pit beyond. The velocity change of Pencil was estimated by measuring the time difference of wire cuttings using an oscillograph. The direction change of tail fins gave the spin rate. Supported by high speed cameras, in addition, basic data for future flight tests were obtained, for example, velocity, acceleration and trajectory dispersion due to changes of the center of gravity and the shapes of tail fins.

This type of horizontal flight test continued for 10 days, and all the results of 29 flights were satisfactory. These pencils were filled with 13 (or 6.5) grams of propellant to produce around 30 kg through 0.1 sec. The tilt angle of the fins was changed to be 0°, 2.5° and 5°, while three kinds of materials ( steel, brass and Duralumin ) were used for the nose cone and the body. The center of gravity was changed in three positions to the front and the rear. Pencil attained its maximum velocity of 110-140 m/s at 5 m from the launcher.

After the tests at Kokubunji, a number of variations were tried, such as (1) 300 mm long Pencil (Pencil 300), (2) two-stage Pencil, and (3) Pencil without tail fins. More orizontal flight tests were attempted to gain experience at a laboratory attached to Tokyo University in Chiba City.

Though the flight tests were carried out in a basement, packed trains passed by outside the concrete wall. So whenever a train approached, the countdown was stopped at a sign from a member of staff sitting on the wall.