Measurement of CPAS Main Parachute Rate of Descent

Abstract

The Crew Exploration Vehicle Parachute Assembly System (CPAS) is being designed to land the Orion Crew Module (CM) at a safe rate of descent at splashdown. Flight test performance must be measured to a high degree of accuracy to ensure this requirement is met with the most efficient design possible. Although the design includes three CPAS Main parachutes, the requirement is that the system must not exceed 33 ft/s under two Main parachutes, should one of the Main parachutes fail. Therefore, several tests were conducted with clusters of two Mains. All of the steady-state rate of descent data are normalized to standard sea level conditions and checked against the limit. As the Orion design gains weight, the system is approaching this limit to within measurement precision. Parachute "breathing," cluster interactions, and atmospheric anomalies can cause the rate of descent to vary widely and lead to challenges in characterizing parachute terminal performance. An early test had contradictory rate of descent results from optical trajectory and Differential Global Positioning Systems (DGPS). A thorough analysis of the data sources and error propagation was conducted to determine the uncertainty in the trajectory. It was discovered that the Time Space Position Information (TSPI) from the optical tracking provided accurate position data. However, the velocity from TPSI must be computed via numerical differentiation, which is prone to large error. DGPS obtains position through pseudo-range calculations from multiple satellites and velocity through Doppler shift of the carrier frequency. Because the velocity from DGPS is a direct measurement, it is more accurate than TSPI velocity. To remedy the situation, a commercial off-the-shelf product that combines GPS and an Inertial Measurement Unit (IMU) was purchased to significantly improve rate of descent measurements. This had the added benefit of solving GPS dropouts during aircraft extraction. Statistical probability distributions for CPAS Main parachute rate of descent and drag coefficient were computed and plotted. Using test data, a terminal rate of descent at splashdown can be estimated as a function of canopy loading.

Improved CPAS Photogrammetric Capabilities for Engineering Development Unit (EDU) Testing

“…3. 11 As shown in Figure 20, losses in projected area cause strong minima in drag coefficient (plotted on the secondary axis) due to a high rate of descent. Unfortunately, the total projected area is often not available during the highest drag periods, because at least one canopy is out of frame when the cluster is furthest apart.…”

Isolating Added Mass Load Components of CPAS Main Clusters

“…The equations for all inflating parachutes are derived from Newton's second law of motion, the general form of which is given in Eq. (1). A simplified free body diagram of a decelerating parachute system is shown in Figure 3.…”

“…The payloads incorporated upward-looking video cameras to observe the inflating shapes and cluster behavior. The avionics system on the payload provided highly accurate system state data, 1 and the Main risers were instrumented for loads. 2 This paper focuses on identifying the drag and momentum terms that make up the total inflation load to develop an alternate inflation model.…”

Numerical Formulation of Parachute Payload Decelerator System and its Animation with Moving Background

“…Namely, deformations in the canopy and elasticity of the suspension lines are neglected. Because the parachute is usually made of very flexible material, this assumption may be different from application of the parachute in real life due to the breathing phenomenon [10][11][12], however, this assumption will greatly simplify the model. This reduced model always correctly reproduces the most relevant features of the trajectory of the decelerator system and has been applied to many parachute trajectory models [3][4][5][6][7][8].…”