When developing the guidance system of a missile, it is necessary to know the resonant frequencies and mode shapes of the missile in flight. Simulating the free vibration condition in the laboratory, however, is a difficult task, since the required supports can affect the test results.
To obtain more accurate data for use in developing the electronics and algorithms in the guidance system, Coleman Aerospace Corp. conducted ground vibration survey (GVS) tests on the missiles it designs and builds. For the GVS test, the missile is supported in a horizontal plane and excited at its aft end. To achieve accuracy, it is necessary to minimize the influence of the supports on the test results.
The missile under test was a short-range air-launched target (SRALT) Hera craft that weighed 21,300 lbs. (9700 kg) and measured 35 ft. (11 m) long. Previous tests used slings from an overhead structure to support the missile. Problems stemming from this setup included continual stretching of the slings and excitation of them under vibration. The stretching required frequent readjustments to maintain the proper load distribution and alignment with the shaker. The responses of the masses of the slings and chain falls, excited by vibration, influenced the test results. To eliminate these problems, the missile was supported on air springs.
Two configurations of the missile were tested, with one representing the ignition (full motor) condition and other the burnout (empty motor) condition. The test setup for the missile is shown in Figure 1. For each configuration, three cradles supported the missile, with a pair of Firestone Airmount® springs in each cradle. Model 22 springs were used for the ignition condition and Model 20-2 springs were used for the burnout condition.
The axial stiffnesses of the air springs were reduced by placing a large air reservoir with each spring. The springs have minimal radial stiffness, so it was necessary to provide restraints to maintain the location of the missile when it was elevated on the inflated air spring. Lateral location was achieved by placing the air springs 30° from vertical. Longitudinal and roll location were achieved by the use of bungee chords attached to the missile near its center of gravity. The cradle configuration is shown in Figure 2, with the saddle supporting the missile raised to its test position one inch (25 mm) above the cradle.
The operation of the air spring system was simple and efficient. The missile could be raised from the resting position to the elevated—testing—position and be maintained there indefinitely. The only influence on the result was the added masses of the saddles supporting the missile, and these could be removed by post-test analytical calculations.