Rocket AirBrakes
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In 2024 the Waterloo Rocketry sub team, for the first time, began working on active in flight controls in the form of air brakes. A primary aspect of the competition we take part in is to most accurately hit a target apogee you declare prior to the launch of your rocket. In the past achieving this target was only possible through rigorous simulation and precision manufacturing, which in 2023, allowed us to be within 1500ft of our 30,000ft apogee, resulting in the 4th closest apogee amongst all 32 teams present (best in the hybrid/liquid engine category). This year we decided to try to further improve that result by introducing air brakes on the rocket. With air brakes we would gain the ability to actively control our flight thus more accurately being able to hit our target apogee. For the past year (2023-2024) I have been working as a key member involved on the air brakes subsystem, working the full stack from mechanical, simulations, and firmware.
First I did a lot of design research into what other teams have done in the past, as well as spoke to mechanical engineering professors at the University of Waterloo to come up with a design that me the following criteria:
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Must only use one relatively low power servo motor to drive the mechanism
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All 3 panels of the air brakes must extend simultaneously and ALWAYS be in sync
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Rotational motion of the servo must be linearly proportional to the extension of the panels
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Be as compact and light weight as possible
There are several mechanisms to address the first criteria, however finding one that was linear to meet #3 was challenging. Eventually I settled on the following cam and slot mechanism, however, the profile had been modified from the traditional slot profile to allow for a linear relationship between degrees rotated and linear extension.
Based on this I designed the spinning plate to include 3 of these spirals to move each of the 3 panels, and then built the rest of the design shown below around this.
While designing the mechanical air brakes I was also working along side one other team mate on Ansys computational fluid dynamics (CFD) simulations of the air brake panels to do two things.
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Validate that by adding the air brakes we don't move the rocket's center of pressure (COP) too far in front of the center of gravity (CG) as to cause the rocket to be unstable
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Determine our coefficient of drag (CD) at various different air brake extensions so we could develop a model that would be needed to begin work on the trajectory prediction model
Below you can see some visual representations of the simulations we performed.
Finally, the last thing I was involved in, alongside roughly 10 others on the subteam, was the firmware development for our flight boards designed to control these air brakes. While code does not present nearly as interesting pictures as CFD or CAD, it was nonetheless one of the most challenging and interesting parts of the project. We developed embedded C code on an STM32 microcontroller within a FreeRTOS environment, implementing Kalman filters for state estimation, 4th order Runge-Kutta methods for trajectory prediction, and a PID controller for dynamic control of the fin extension to achieve hitting a target apogee as accurately as possible.
The results of all this work? Well we still haven't launched our rocket yet, so I will update this page once we do!
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