This New Ballistics Tester Will Play Catch with Air Force Artillery

Normally, when the military tests an artillery prototype, it uses the direct method—they fire it at a wall using a cannon and record what happens. But that's an expensive and destructive test method. A new method, however, developed by students at Rice University may soon revolutionize how the Air Force tests its big guns.

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The Air Force needed a means to stop high-velocity projectiles without destroying them so that it could study the projectile's deceleration more than once. Rice University's team CADET (Controllable Acceleration-Deceleration Equipment Tester), headed by adviser Andrew Dick, assistant professor of mechanical engineering, devised just such a method—using a fish tank, a slingshot and some lumber.

The device prototype is a 14-foot long track able to sustain the projectile's deceleration for at least 10 milliseconds. A surgical tubing slingshot mounted at one end of the track launches the shot—in this case, a hollow aluminum cylinder with an embedded accelerometer—down its length at about 50MPH. At the far end, a tank of water suspended above the track dumps its contents through a V-shaped funnel. This produces a sheet of water flowing parallel over the track. As the projectile passes under the stream of water (think of a bullet hitting the thin edge of a piece of paper and slicing the sheet in half), friction between the two slow the shot so that it can be captured safely in the receiving unit—in this case, a bucket packed with foam and yoga mats.

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"Nothing is destroyed. You just fill the tank with water again and reload the slingshot," Duncan Eddy, a junior in mechanical engineering and member of the Controllable Acceleration-Deceleration Equipment Tester (CADET) design team, told R&D Mag. The prototype is obviously still in its early stages, however the team is confident that it can be scaled up to accommodate the biggest artillery in the Air Force's arsenal. Military brass is currently mulling over whether to develop the technology further. [Rice University - R&D Mag]

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DISCUSSION

Some thoughts:

1) Whatever the projectile's weight, the rubber band of the slingshot will always (more or less) give out the same force profile through a given length, i.e. the kinetic energy provided will (more or less) remain constant. Launch a peanut with it, it'll fly away so fast you won't see it. Launch a much bigger projectile, not as fast. In any case, even if the projectile used in the example is big & heavy, once launched the only difference it has with the peanut is the fact that it has a bigger cross-section so it'll be a bit more affected by the water. But then, it also suffered from slightly higher drag while accelerating, but since the drag force in water > drag force in air the Al projectile will slow down much more than the peanut. (law of conservation of energy, F=ma, E(k)=SF*dx, etc. ...sorry, can't really type integrals...)

2) Kinetic energies involved in ballistics are much higher, by several orders of magnitude, for which the properties of water (high mass density, relatively high viscosity) become undesirable, so much that firing a projectile with a kinetic energy in those ranges in water won't be much different than firing it at a brick wall.

3) In ballistics, the weight profile & mass distribution of a projectile is directly related to its behavior in flight, if it wobbles, drops too fast, etc. as well as when changing medium (i.e. when it enters a body or construction). In short, you can hardly place an accelerometer in a projectile while keeping the exact same weight distribution, unless spending as much time in R&D designing that dummy projectile as the one you're testing.