Two projectiles connected by a flexible tether dropped in the atmosphere Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/5712m8985

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  • This study investigates the atmospheric flight dynamics of a munition system and the effect of a tether reel resistance mechanism for limiting the impact that the unreeling process has on the munition system. The munition system consisting of two projectiles connected by a tether line is released from an aircraft at altitude and drops toward a target on the ground. Initially the two projectiles are rigidly attached. At a specified time, the projectiles separate and subsequently unreel the tether line. After the tether line is fully payed out, the system settles toward a steady state as it approaches the ground. Two different computation procedures are compared for modeling the tether unreeling process, namely, the pop-out and all-out methods. The all-out method requires significantly higher computation whereas the pop-out method induces spurious vibration into the tether line as line is released. It is shown that while projectile position results converge for a relatively low number of tether line elements, the maximum tether loads require a significantly larger number of elements. Parametric studies indicate that increases in tether stiffness contribute to increases in maximum tether line load and maximum follower projectile acceleration while having very little effect on the range of the lead and follower projectiles. An increase in the drag coefficient ratio increases the maximum tether line load and the maximum acceleration on the follower projectile. However, increasing the drag coefficient ratio also causes a decrease in the speed of the lead projectile, which leads to a decrease in range of the system. A follower projectile equal in weight to the lead projectile results in an increase in tether deployment time while having little effect on the range of the lead projectile. For a low follower-to-lead projectile-mass ratio, the tether line unreeling process is predominantly due to the follower and lead projectile separation. Conversely, for a high follower-to-lead projectile-mass ratio, the tether line tends to billow and subsequently unreels itself independent of lead and follower projectile motion. The parametric studies led to the determination of reel configurations which decrease tether line loads, maximum acceleration on the follower projectile, time to reach a steady state condition, and the terminal miss distance. Reel resistance functions based on feasible mechanisms were determined for various drop speeds, mass ratios, drag coefficient ratios and tether line stiffness. High end, low end, and mid-range optimal resistance functions were chosen from the range of each category. Each selected optimal resistance function was utilized in the above computation procedure for modeling the tether unreeling process to determine and compare the deployment characteristics produced over the entire range of each category. The deployment characteristics produced by each optimal resistance function were used to determine envelopes for successful deployment. Reel resistance based on either line out or line out rate provided two powerful means to reduce line loads. Resistance as a function of line out yields the overall best performance, however, resistance based on line out rate provided suitable performance over a wider band of drop speeds, mass ratios, and drag coefficient ratios.
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