Predictive Model of a Parachute Retraction Soft Landing System.

The U.S. Army Soldier Systems Commands Natick Research, Development and Engineering 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-onroll-off RORO capability. The retractor concept consists of a pneumatically driven pistoncylinder mechanism connected by cables to upper and lower pulley blocks to increase the systems overall mechanical advantage. Full scale testing of payloadretractor 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 models 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.