WHIPPLE SHIELDS CHARACTERIZED BY A NONDIMENSIONAL GEOMETRY PARAMETER

Impacts onto single-bumper (Whipple) shields were conducted using a te chnique that allows simulation of aluminum impacts onto aluminum shiel ds at velocities up to 21 km/s. The results provided information on ho w the threshold impactor diameter for incipient perforation of the wal l depends on the thickness b of the bumper, the thickness t of the spa cecraft wall, and the spacing S between the bumper and the wall. Exper iments were conducted to test a scaling prediction that these three va riables are described by a single nondimensional parameter, pi(t) = tS (2)/b(3), for aluminum impacts faster than 7 km/s. Experiments at simu lated velocities near 10 and 17 km/s confirmed the prediction. Additio nal tests revealed the effects of rr, on the ballistic limit curve, wh ich expresses the threshold diameter for perforation as a function of impact velocity, For velocities above about 10 km/s, the ballistic lim it curve is nearly flat if pi(t) is less than a few thousand, and has a positive slope for larger values. This result, which differs signifi cantly from existing ballistic limit models, is believed to be due to fractional vaporization of the impactor and bumper at high velocities. An empirical expression was developed that can be used to calculate t he threshold impactor diameter as a function of t, S, and b for impact velocities up to 18 km/s.