Eagle X
Thrust Vector Controlled Model Rocket
Airframe
Description
The airframe of Eagle X was 3D printed in 2 segments (originally 4) and a nosecone, resulting in a total length of 3.75 feet. The airframe’s wall thickness was 0.9 millimeters. The segments were securely joined using three (originally four) M3 screws per segment joint.
Engineering Challenges
Since Eagle X’s airframe was 3D printed (as opposed to cardboard or plastic tubing), substantial mass reductions were necessary while maintaining sufficient strength to withstand vertical compressive loads, high-speed horizontal impacts (landing), support 25.3N of engine thrust through 4 load-bearing joints, and normal handling loads. The initial iteration encountered issues such as intra-layer separation, thermal deformation, and excessive weight. The subsequent solutions included:
- Airframe Thickness Reduction: 2.1mm —> 0.9mm
- Segment Count Reduced To 2 (To Decrease Weight from Segment Joints, Reduce Points of Failure, and Enhance Manufacturing Efficiency)
- 3D Printer Filament Change: PLA —> ABS —> PLA+ (PLA+ Demonstrated Superior Tensile and Yield Strengths)
- Increased Layer Count to Reduce Aerodynamic Surface Friction, Complementing Adjustments to Printing Temperature, Speed, and Other Parameters.
Fun Fact
Just like a real rocket, if you tip this one over too much or happen to touch it wrong, it falls apart :)
Hold Down System
Description
The initial version of the Eagle X Hold Down System comprised 4 counterweighted clamps. Upon release, these clamps would allow the rocket to detach from the pad. Each clamp was individually controlled by 4 servos. The second iteration employed a single ring that secured the rocket to the pad. A gear system controlled the rotation of the ring, enabling alignment and hold-down pins on the rocket to slide through corresponding holes in the ring. The third iteration utilized 3 small, lightweight, and powerful magnets to hold and align the rocket. The fourth and final iteration adopted a pedestal system. The rocket would rest in pre-formed grooves within the pedestal, and its weight would provide the necessary gravitational hold. Upon ignition of the rocket engine, a brief period would elapse during which the engine’s thrust would overcome the static friction of the pedestal, enabling thrust vector guidance to become effective.
Engineering Challenges
The initial iteration encountered reliability concerns with the counterweighted clamps retracting and clearance issues during the rocket’s takeoff. Additionally, it possessed a high level of complexity, a substantial number of components, and extended manufacturing times. Furthermore, the substantial central base of the hold-down system led to warping during the manufacturing process.
The second iteration addressed these issues by simplifying the takeoff process to a single rotational process involving a ring. It also eliminated the high part count and facilitated easy and precise manufacturing. However, the ring exhibited excessive rotational static friction, even in the absence of engine load, and was prone to jamming when subjected to engine thrust. The driving gear system lacked accuracy and reliability, rendering it incapable of rotating the ring to the desired set point.
The third iteration introduced a simple magnetic hold-down system to mitigate mechanical concerns. Its simplicity allowed for a calculated force required for separation from the hold-down system. Despite its mechanical advantages, the system added unnecessary mass to the rocket and necessitated replacement with each new airframe due to the use of glued magnets.
The fourth iteration consisted of a launch pedestal with a 3D-printed alignment pin and an alignment and holding cavity. This design nearly eliminated rocket-side mass from the hold-down system and reduced the number of active processes required for takeoff to zero. The pedestal also simplified manufacturing and assembly, as it was a single 3D-printed component.
Fun Fact
The part count for the first iteration was 30. The final iteration's part count was 4.
Launch Pad
Description
Eagle X’s Launch Pad was constructed using 13 80/20 aluminum extrusions, comprising 12 for the base and 1 for the strong back. The extrusions were covered by 8 3D-printed shells, which were securely fastened to the extrusions. The strongback remained stationary but could be retracted utilizing a pneumatic system integrated into the base of the launchpad. The strong back was equipped with two servo-actuated “pinchers” at its top to ensure the stability of the rocket during its stay on the launchpad, mitigating wind and other disturbances. Additionally, the strong back pinchers served as a catching mechanism during Eagle X's landing (this was BEFORE SpaceX proposed Mechazilla :) ) The rocket interfaced with the launchpad via the pedestal hold-down system.
Engineering Challenges
During the initial 3 iterations of the hold-down system, the pad incorporated a flame trench. This feature proved advantageous when the rocket directly interfaced with the pad (without physical separation). However, when physically separated, it became redundant. Additionally, the launchpad shells presented significant challenges during printing due to warping issues. To address this, adjustments were made to the 3D printer settings to optimize adhesion between the shells and the print bed while preserving a similar surface finish. Initially, the first iteration featured 11 3D-printed shells covering the framing. By the final iteration, this number reduced to 8, which is surprisingly impressive considering the capabilities of the 3D printer.
Initially, the strong back was designed to retract during the rocket’s lift-off. While this concept appeared feasible from a clearance and “coolness” standpoint, it proved to be excessively complex for its intended purpose. The absence of active hold-down systems necessitated continued protection against wind-induced tipping of the rocket pad. Consequently, a shorter strong back was considered, but the substantial cost associated with modifying this single component made it impractical.
Thrust Vector Control System
Description
The thrust vector control mount is a 3D-printed structure composed of a static outer ring and an inner pivoting ring. The inner pivoting ring serves as a pivot point for the motor mount casing, enabling the second axis of movement. The axes are driven by two servos powered by an external Li-Po battery and an I/O pin for control. The flight control software onboard the Eagle X Flight Computer determines the movement of the axes during flight. The rocket motor’s thrust is directed through the pivoting rings and distributed to the four mounting wings. The mount possesses a maximum rotation of 15 degrees on both axes.
To facilitate rapid design iterations and enhance flexibility in the geometry, 3D printing was chosen as the manufacturing method. This approach enabled 3-4 iterations per day and provided unique manufacturing capabilities. PLA+, a slightly stronger version of PLA, was selected as the material due to its favorable balance of thermal and mechanical properties. The entire mount is composed of 3 main components (the outer ring, the inner ring, and the motor mount casing) and 4 smaller components (pushrods and servo horns).
Over time, plastic components experience degradation due to friction, leading to significant drift or “slop” in the mount’s alignment. While this issue could not be completely eliminated, design improvements partially mitigated its effects. Additionally, the thermal characteristics of the mount were a concern for extended flights due to engine heat. However, the mount never operated with long-duration motors, and therefore, this concern was never encountered.
Electrical Systems
Description
The flight computer, responsible for commanding the rocket from pre-takeoff to post-landing, was designed in-house using Altium PCB Design software and manufactured by external services. The flight computer has undergone various versions, ranging from a breadboard to fully surface-mount component PCBs. The optimal design proved to be a majority surface-mounted flight computer. The flight computer incorporated the following subsystems: STM32 microcontroller, wire terminals, a USB port, power electronics (filtering, brownout protection, etc.), sensors (gyroscope, accelerometer, magnetometer), and visual/physical systems (status LEDs, power switches, reset buttons).
Attached to the flight computer are various outputs for servo control, parachute deployment, and other electrical systems. These components, along with the sensors onboard the flight computer, communicate using protocols such as SPI, I2C, and others.
