After the flight of the Stratos III sounding rocket, a team was instated to investigate the cause of the anomaly that occurred. After a thorough investigation, the team concluded that the rocket suffered from a phenomenon called inertial roll coupling. This coupling is inherent to all flying vehicles with a slender profile, such as sounding rockets.
The resonant motion initiates when “the roll rate of the rocket is equal to the natural frequency of the pitch/yaw rates of the rocket”. This means that the rocket spins as fast as it likes to wobble its nose up and down. Several mitigation techniques are possible, but none of them would ensure the Stratos IV rocket would not suffer the same motion. Thus, in order to deal with this problem it was decided to implement active roll control. Most importantly, the team required a system that was testable on the ground, such that, with certain confidence, the Stratos IV rocket can be launched successfully. The system that was chosen is a “nitrous monopropellant roll control thruster system”. This means that thrusters are being used to push against the spinning motion of the rocket. These thrusters oxide in the tank and convert it into nitrogen and oxygen. This process releases heat that gives the power to counter the spinning of the rocket. Without spin the phenomenon cannot occur! The thruster system is also testable on the ground. The system can be put on a bearing to allow it to rotate, after which the thrusters and control system can be tested.
To control the monopropellant thruster, a brand-new electronics systems was developed: the roll controller. This system will control 4 thrusters that will actively stabilize the rocket’s roll. It is able to measure the roll rate and flight conditions and accordingly actuate the thrusters. Also, the Roll Controller is the interface between the cable channel of the tank and the flight stack, so it will route all data connections to the separation connector.
In order to do all of this in a very small space, we decided to make a Printed Circuit Board (PCB) with all the sensors, processing power, and actuators onboard. The roll rate is measured by 3 MEMS (Micro-Electro-Mechanical Systems) chips that are soldered on the roll controller PCB. A choice algorithm is used to choose the best value of the 3 sensors for a simple bang-bang controller to decide which direction to thrust. Simply, when 2 sensors say the rocket is rotating in one direction it will actuate the thrusters which rotates the rocket back. Each thruster also has a thermocouple inside that will be measured by the PCB to know if the monopropellant successfully decomposed. This picture shows the PCB, after long design, routing, and soldering processes. The coming weeks will see testing of the board and writing of the software to get it test ready. Meanwhile, a full test setup with some dummy thrusters and a rotating jig will be made so we can see if the roll controller succeeds in practice.