An apparatus has been designed and built for testing the effects, on moderatesized objects, of cushioned decelerations having magnitudes ranging up to several hundred g [where g = normal Earth gravitational acceleration ( ≈9.8 m/s2)]. The apparatus was originally intended for use in assessing the ability of scientific instruments in spacecraft to withstand cushioned impacts of landings on remote planets. Although such landings can have impact velocities of 20 to 50 m/s, the decelerations must not exceed a few hundred g. This requires the deceleration to occur over a distance of as much as 50 cm in a time of tens of milliseconds. This combination of conditions is surprisingly difficult to simulate on the ground. The apparatus could also be used for general impact testing.
The apparatus is simple to build. Relative to drop-tower apparatuses that could produce equivalent impacts, this apparatus is very compact. This apparatus is also relatively safe to operate because its design inherently prevents the object under test or any debris from accidentally striking persons or equipment in the vicinity.
The apparatus (see figure) includes a steel pipe having an inside diameter of 20 cm and a length of 216 cm. A lightweight polyethylene piston carries the object under test. The piston is sealed to the inner wall of the pipe by means of a gasket. One end of the pipe is open; the opposite end of the pipe is closed and connected to a vacuum pump.
At the beginning of a test, the piston is held at the open end of the pipe by use of a latch while the vacuum pump is activated to introduce a partial vacuum [typically, characterized by an absolute pressure of 0.2 bar (20 kPa)] between the piston and the closed end. Once the desired partial vacuum has been reached, the latch is released so that differential pressure (atmospheric minus partial vacuum) accelerates the piston along the pipe toward the closed end. The piston reaches a maximum speed typically between 20 and 40 m/s, the exact value depending on the mass of the object under test and the starting pressure in the partly evacuated volume. As the piston approaches the closed end, it compresses the air ahead of it until, at a distance between 5 and 20 cm from the closed end, the pressure becomes high enough to stop the piston (and bounce it back toward its starting position). An accelerometer on the piston measures the impact deceleration.
The impact deceleration is a strong function of the amount of gas (and, hence, of the pressure) initially in the partially evacuated volume at the closed end of the pipe. Therefore, it is easy to adjust the peak deceleration by adjusting the pressure of the partial vacuum. The maximum speed is not a strong function of the mass of the object under test or of the starting partial-vacuum pressure but can be tailored over a wide range by using pipes of different length
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