Variable Drag System (VDS)
The VDS is an air-braking system designed to deliver a high-powered rocket to precisely one mile AGL. The VDS does this by actuating three aluminum drag blades into the airstream surrounding the rocket, actively changing the vehicle's drag profile on ascent. The system responds to sensor data taken in real-time and ensures that the vehicle has the correct balance of kinetic and gravitational potential energy at all points in the flight to reach one mile AGL exactly. In the NASA Student Launch competition flight, the VDS successfully delivered the launch vehicle to 5,303 ft.: only 23 ft. from its target. The VDS also contributed to the team's Vehicle Design Award. More detail on this system can be found below and in the team's technical documentation.
Simulation
To predict the performance of the VDS and the launch vehicle, the team developed a simulation in Mathworks' Simulink shown to the right. This simulation allows the team to predict mission performance for each launch with unique and customizable weather scenarios, 6DOF equations of motion, and a launch animation. Future improvements include a better aerodynamic model and hardware in the loop capabilities.
Mechanical Analysis
To ensure the VDS would be able to withstand the predicted flight loads, FEA simulations were performed. An FEA simulation for a single drag blade as well as the entire VDS assembly was performed. The FEA simulations were used to justify material selection and determine the critical dimensions of key components.
Embedded Software
The embedded software for the VDS is written in a combination of C and C++, it is organized over five classes, and can currently operate at a speed of 90 Hz. This code is responsible for communicating with multiple sensors to determine the state of the vehicle, processing that data, and actuating the drag blades. The code does all of these things by integrating pressure, accelerometer, gyroscope, and magnetometer readings together with a Kalman filter. More detail on the VDS software can be found on this Github account.
Mission Success
The VDS has been demonstrated to successfully deliver the launch vehicle to one mile AGL +/- 23 ft. The thorough documentation of this system, the attention to detail in simulation, and its successful performance were all cited as contributing factors to the team receiving the Vehicle Design Award at the end of the 2016-2017 season.
Design Overview
The VDS actively changes the drag profile of the launch vehicle on ascent by actuating three aluminum drag blades into the airstream surrounding the vehicle. With the flat faces of these blades perpendicular to the airstream, the VDS is able to increase the projected area of the vehicle by a factor of 1.28 and the coefficient of drag by an estimated factor of 1.35.
Key Components
The three drag blades are driven by a single DC electric motor and can be actuated precisely to any position in its range with feedback provided by a rotary encoder. In this way, the drag blades can be actuated with the power of a DC motor but with the precision of a servo. A Teensy 3.6 microcontroller is responsible for controlling this actuation and for taking readings from a 9DOF sensor and a barometric pressure sensor. It uses this sensor data to determine the state of the vehicle on ascent and to respond with corrective braking accordingly.