CALCULATING AERODYNAMIC COEFFICIENTS FOR A NASA APOLLO BODY USING TELEMETRY DATA FROM FREE FLIGHT RANGE TESTING

The U.S. Army Research Laboratory (ARL) was requested by the National Aeronautics and Space Administration (NASA) Langley Research Center (LaRC) to perform a free-flight experiment with a telemetry (TM) instrumented sub-scaled Apollo shaped reentry vehicle in order to determine its aerodynamic coefficients. ARL has developed a unique flight diagnostic capability for reconstructing flight trajectory and determining aerodynamic coefficients of projectiles by using sensor data telemetered from free flight experiments. A custom launch package was designed for this experiment that included the Apollo shaped projectile, which housed a modular telemetry unit, and a rapid prototyped sabot. The experiment was able to produce estimates for aerodynamic coefficients that were considered accurate and this technique is appealing to NASA for the development of their spacecraft in the future. INTRODUCTION Project Constellation was launched in response to President Bush’s speech at NASA Headquarters on January 14, 2004 in which he set forth an aggressive plan for future space exploration. NASA plans to develop a new fleet of vehicles with extended capabilities in order to travel back to the moon, to Mars, and beyond. One of the main challenges of this bold vision is the development of a new crew exploration vehicle to house the astronauts during their missions. The CEV being developed is similar to the shape and function of the 1966 Apollo (see Figure 1); however it is three times its size and can transport up to four astronauts to the moon at a time. Achieving these ambitious goals will require NASA to focus on new technologies and methodologies. ARL has investigated instrumented developmental munitions with custom TM systems to obtain aerodynamic coefficients for the past forty years. Recently, ARL has partnered with Arrow Tech Associates to develop a custom software program to utilize the telemetry data, along with other information available, to calculate the aerodynamic coefficients of a projectile from measured flight data. This software code, Extending Telemetry Reduction to Aerodynamic Coefficients and Trajectory Reconstruction (EXTRACTR), imports the sensor data, meteorological (MET) data, radar data, and projectile physicals to process, through an iterative algorithm, a solution for the aerodynamic coefficients that would have caused the measured flight response. The code attempts to fit the measured translational and rotational sensor data to the six-degree-of-freedom (6DOF) equations of motion using both the Maximum Likelihood Method and Least Squares arriving at an acceptable solution for a given aerodynamic coefficient (usually within three or four iterations). It was the goal of this program to extend the capability of EXTRACTR to determine the aerodynamic coefficients for a projectile shaped like a NASA CEV, which will experience minimal spin. In particular, the nonlinear coefficients were of great interest to NASA. There are many other ways of estimating aerodynamic coefficients that NASA has available to them. In the past they have used a combination of spark range and wind tunnel testing. Both have their shortcomings. Spark Range testing is taken at an indoor spark range where measurements are made from shadowgraphs placed along the range. The measured translation and orientation of the projectile is recorded and fit using a similar method as described for EXTRACTR. The limitation of spark range data is that it is only taken at a few discrete points along the flight path and the body must fly relatively straight in order to avoid damaging the indoor range. Wind tunnel testing is capable of simulating a wide variety of flow regimes and flight conditions with the ability to sustain the loading environment for as long as desired. However, the interference of sting mounts in the flow regime and reduction in the degrees of freedom make it a less ideal environment for collecting pure flight response data. Figure 1 – NASA Apollo Crew Exploration Vehicle (1966) BODY The experiment was conducted using an extended travel 120mm smooth-bore artillery cannon with a scaled-down Apollo CEV. This provided the quickest and most cost-efficient means to demonstrate ARL’s TM technique to provide the required aerodynamics data needed by NASA. In all, 2 M829A1 slugs, 3 Apollo CEV shaped slugs, and 4 Apollo CEV TM units were shot. The M829A1 and Apollo CEV shaped slugs were fired for charge development and for verification of instrument alignment and triggers. Of the four rounds fired with telemetry, two had sabots with their symmetry axis aligned with the central axis of the gun bore (referred to as 0 degree