Predictive Model of a Parachute Retraction Soft Landing System

摘要

The U.S. Army Soldier Systems Command's Natick Research, Development andEngineering Center (NRDEC) is currently examining a novel concept for reducing the impact shock sustained by airdropped payloads upon ground impact A device, called a parachute retractor, is placed between the payload and parachute confluence point, and when activated, accelerates the parachute and payload toward each other; slowing the payload prior to ground impact. The goal is to eliminate the cushioning material currently placed under airdrop loads, providing a roll-on/roll-off (RO/RO) capability. The retractor concept consists of a pneumatically driven piston/cylinder mechanism connected by cables to upper and lower pulley blocks to increase the system's overall mechanical advantage. Full scale testing of payload/retractor combinations is considered impractical, given the varied weights of military cargo presently airdropped and the multitude of retractor configurations possible. The need for a computational tool to determine the activation height and to optimize system design parameters, therefore, was recognized early on in the exploratory development effort. This report describes a predictive model, developed in response to that need, which couples a simplified parachute model to a model of the retractor mechanism. This model is able to predict the motion of the piston, payload and parachute confluence point, as well as the forces generated during retraction. This report first reviews the model's underlying theory and method of coupling. Computer program predictions are then compared to behavior observed in an experiment conducted on a instrumented prototype retractor device at Tustin Marine Corps Air Station, Santa Ana, CA in April 1994.