An analytical technique developed during the Apollo Program for predicting the effect of a drogue parachute on the vehicle’s oscillatory motion was rediscovered and applied during an NESC assessment to the Orion crew module (CM) drogue system with improved prediction results over current methods. One of the prime functions of a drogue parachute is to stabilize and damp vehicle oscillations to provide a stable platform for main parachute deployment. When the drogue parachutes were deployed during the Pad Abort-1 (PA-1) flight, they produced more damping than was predicted by twobody 6 degrees of freedom simulations. To resolve this difference, the Apollo-era technique was resurrected and shown to accurately predict the full-scale motion.
During PA-1, the CM initial orientation at drogue deploy had large oscillation angles and rates, as expected. The combined effect of the vehicle and drogue damping caused the system to damp much faster than preflight simulations had predicted. This faster flight damping was also predicted by wind tunnel tests on scaled models in the LaRC Vertical Spin Tunnel (VST). To explain this anomaly, the legacy tools and techniques used to analyze Apollo drogue flight test motion were resurrected. The legacy hypothesis is that the drogues will align with the total velocity vector at the attach point. This assumption was empirically verified by examining both VST and PA-1 measured relative motion of the drogue with respect to the CM. The legacy tool supports single-body simulations by modeling an effective drogue static and dynamic moment. Using these models, the single-body simulator accurately predicted the PA-1 oscillatory motion. Additional VST testing is scheduled to acquire the necessary data for the legacy method to further validate its applicability and to refine Orion drogue parachute performance predictions.
The basis of the Apollo legacy damping model is the hypothesis that the drogue parachute aligns with the resultant velocity of the attach point on the CM. This resultant velocity is the sum of the free-stream velocity plus the velocity induced by the angular rates of the system about its center of gravity (CG). The parachute damping results from a hysteresis in the moment arm of the parachute as the CM oscillates over a cycle. Simple equations for the equivalent static and dynamic moments about the system CG due to the drogue parachutes were recently derived. These equations provide a powerful preliminary design and analysis tool for use with drogue parachutes. It is noted that the Apollo legacy model is applicable only to smallersized parachutes such as drogue parachutes. The large parachute mass and associated air mass for main parachutes prevent the lateral motions required to align with the attach point velocity.
As a result of the accurate prediction of the PA-1 drogue motion, the Apollo legacy methodology has been adopted by the Orion Multi-Purpose Crew Vehicle Program for their drogue parachute performance predictions. This legacy tool plus recent improvements is now available to the NASA community.
For more information, contact Dr. David Schuster (david.m.schuster@nasa.gov)