While the other steps in our process follow each other in order, the mechanical design aspect of this project has been developed in parallel, with the results from each subsequent test affecting our overall design.
The current design puts three rocket motors on hinges and controls the angle of the motors with servos. This way we can get both orientation and thrust control. This downside of this design is that it depends on the three motors igniting at approximately the same time, but we know that the uncertainty in the ignition timing is roughly 60 ms, and we will simulate the worst case scenarios to determine if it is safe to run. The main limitation of this design will be servo speed, which will limit the settling time and accuracy of the orientation and vertical controllers.
Trackstar TS-P12S Servo
From the orientation test we determined the minimum speed and torque requirements for the servos, so we looked for servos that had those specs or better. We chose the Trackstar TS-P12S due to speed and price. Faster servos cost close to double what this costs, and this servo’s specs are even better than what we used for the orientation test.
With screws going through the two holes, this aluminum link connects the horn of the servo to the sleeve that houses the rocket engine. The motor’s range of motion depends on the distance between these holes and the location of the servo. (The gif above does not show the full range of motion.)
This is press fit into the motor sleeve, allowing it to rotate on the rod without friction. We chose a standard skateboard bearing because we already had them available.
The rocket engine fits snugly inside the bottom of this sleeve. There are two exhaust holes to vent the motor’s ejection charge, normally used to shoot out a parachute. The motor sits on a lip inside the sleeve so that the ejection charge has room to vent. The large hole in the center was bored out to press fit the bearing, and the topmost hole is to connect with the linkage. The angle cut on the top was done in order to remove unnecessary weight. This was made out of aluminum due to machinability and ability to withstand high temperatures.
Estes C6-0 Rocket Engine
The landing stage of the rocket will be powered by 3 C6-0 motors.
This part, similar in shape to a 3-jaw-chuck lathe spider, places the motors’ rotation axes on an equilateral triangle. The holes on the outer legs were drilled to fit and mount the rods and servos, with the servo location chosen to maximize range of motion of the motors, as mentioned above. In the vertical controller test, a cable will be used to constrain the rocket to 1 degree of freedom, and this cable will route through the central hole. The remaining three holes will be used to mount a plate above the servos. Our batteries, sensors, and circuitry will be mounted on this top plate. Minimizing the overall weight of the rocket gives us more flexibility with our controllers, so we needed to choose a lightweight but strong material for this piece since it is the largest part. By performing some statics analysis and FEA in Solidworks, we established minimum strength requirements. We chose to make this out of polycarbonate due to its low cost, low weight, machinability, and impact resistance.
We started the mechanical design of our rocket by brainstorming different ways to control thrust and orientation of the model rocket engines.
Putting the motor on a gimbal would give us orientation control but not thrust control. We would need to time the ignition perfectly in order to achieve a soft landing, so that design has a low margin for error. The range of motion is also limited by the spherical bearing to ~10 degrees in any direction.The preliminary design would put the sleeve housing the motor in a spherical bearing to provide two axes of rotation.
Gimballed motor with spherical bearing
We discussed the possibility of routing the motor exhaust through pipes that were controlled by valves. This could give us both thrust and attitude control, but those valves would need to withstand temperatures up to 2000 degrees Fahrenheit.
Valve idea concept sketch
We could also use a combination of solid-fuel motors for vertical thrust and compressed air for orientation, but the components for compressed air are too heavy.
Combined solid-fuel motor and compressed air concept sketch
Lastly, we could use a control moment gyroscope or reaction wheels to control orientation, similar to what is used on satellites and the ISS, with the model rocket motors to provide upward thrust. However this was also deemed to be too heavy.