Rocketry Drag Flap System

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Preliminary Design/ Concept

The primary purpose of the drag flap system on a rocket is to slow the rocket during the ascent phase and precisely control its apogee. The need for apogee targeting primarily comes from the points awarded for accuracy in apogee determination in the Spaceport America Cup competition. I worked on this project during my time in the Illinois Tech Rocketry Club, where I began research and development using a smaller-scale, 3-inch-diameter rocket. To successfully test this technology, we designed a capable of deploying three evenly spaced flaps from the surface of the rocket body tube.The system also required closed-loop feedback and an onboard sensor suite. The final objective of the project was to set an apogee target, simulate the launch in Simulink, and have a custom flight control algorithm adjust the flap deflections in real time during flight.

Shuttlecock Lead Screw Design

The shuttlecock drag flap system was designed in SolidWorks, allowing for subsequent CFD analysis to be conducted natively within the software. For the overall construction, 3D printed and fiberglass components were intended to be used. The main sections were printed using carbon fiber PLA material and later reinforced with carbon fiber layups. The fiberglass bulkheads were cut using a CNC router, and the main structure, which allows the unibody to move, rides on aluminum rods. Finally, the main lead screw enables the unibody to traverse the rods using a servo.

Completed Assembly

In addition to the shuttlecock system, an Altitude Heading Reference System (AHRS) was designed to control the flaps. This system not only houses the servo and sensor suite but also a Raspberry Pi Zero, which serves as the main processing unit.

The flap control mechanism uses a continuous high-torque servo paired with a Hall effect sensor for absolute positioning. Other sensors include a barometer, a high-G IMU, a low-G IMU, and a GPS receiver. Together, these sensor readings enable the flaps to be controlled using a custom closed-loop feedback controller.

Build / Development Photos

Initial Brainstorming

Drag Flap Epoxy Sanding

Test Assembly

Finished Paint Job

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First Test Deflection

It is clear that this system is quite dense and complex for such a small 3-inch diameter rocket. It was paramount that all the tolerances for the cutouts and systems fit within the rocket's body tube. In the video above, you can watch the first real actuation of the drag flaps while installed in the body tube.

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CFD Testing

To properly create a flight model and controller for the drag flap system, extensive CFD analysis was conducted in SolidWorks Flow. The main goal of this analysis was to determine the drag coefficients for different drag flap deflections. For rapid design and testing, I decided to use SolidWorks and its built-in CFD solver. Another crucial aspect of this project was ensuring that the drag flaps did not significantly affect the stability of the rocket. This was achieved by clocking the three drag flaps out of phase with the fins.

Flight Day Installation

We decided to launch this rocket on one of the coldest days in Wisconsin. It was truly a challenge to set up everything properly in the cold, as we could barely feel our fingers. However, we persevered and were able to fully assemble the rocket, including all the ejection charges for dual deployment, and successfully connected to the Raspberry Pi using a network it hosts. The rocket was loaded with an AeroTech K1103 solid propellant rocket motor.

Last Minute Checks

Once the rocket had been mounted to the launch rail, we made sure to remove all the pull pin switches. These switches allowed us to power on all the electronics, including all three flight computers: a Raspberry Pi, Easy Mini, and RC3. The final step was to try and start the onboard camera, which you can see me attempting to do in the photo above. However, the camera refused to record for more than a second in the cold. Unfortunately, we did not get any onboard video from this flight.

Family Photo

Pictured here is me in the center, along with two other key contributors to this project: James Marek on the left and Ethan Kuo on the right. This project was primarily the brainchild of Ethan and me. We worked very hard on not only the conceptual design but also the flight software and simulation for over a year.

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T-minus.....

The rocket footage on the left shows how violent and fast the launch actually was. Even though these types of forces were accounted for in the design, I wasn't expecting it to be so intense. Due to the low cloud cover on that day, we lost sight of the rocket very quickly. However, we were able to find it shortly after launch, as it had landed on a frozen pond and was easy to spot.

Once we retrieved the rocket, we were eager to see what kind of data had been collected and to check if the drag flaps had deployed. After some data analysis, we confirmed that the drag flaps had indeed opened during the ascent portion of the flight. The acceleration the rocket experienced was noticeably higher in all instances when the flaps were deployed, which was clearly reflected in the resulting data.

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