So where to start building a rocket?
Even though a small rocket such as the one the WBPP didn't have many subsystems, we split up the group so that each one of us could look at the requirement and design his subsystem and then present it to the others. However before going too much into the details of the rocket, the software Open Rocket was used to come up with a first design iteration of our "space" vehicle making sure to meet the requirements given by the safety board. Eventually each subsystem was considered individually and considering load cases, ease of manufacture and systems accessibility before launch design decisions were made.
Open Rocket Estimations
The engine file corresponding to the one that our rocket would have mounted was uploaded to the designing file; this would have granted us to have reasonable prediction of the rocket's flight in absence of wind. Since the team opted for a separation of the rocket in two parts at the moment of the parachute deployment, the rocket was divided in subsystems accordingly.
The picture shows from right to left the engine section, the parachute section and the payload section. The main components were placed on the rocket to have a mass estimation: the bulkheads needed to sustain the rocket, the parachute itself, the egg and the electronics. The mass of each of the component was approximated considering the density of the material and its size. The size and shape of the fins were adjusted so that the static margin was between 1.3 and 2.5 for the all flight duration. Eventually sizing the parachute was one of the most critical part of the design; in the preliminary phase its size was overestimated since the team aimed at reaching a very low touch down speed in order to minimize the probability that the egg could break at touch down.
As the graph shows the vertical deceleration with such parachute would have brought the rocket to decelerate at 90m/s. With such big parachute (0.95m diameter), in fact, the parachute had to be opened up only at a very low altitude in order to respect the maximum flight time of 90 seconds not to escape the safety area. Even if the shock load this parachute would have produced on the rocket would have been really high, it was eventually built and presented to the safety board in combination with elastic chords and a slider.
The shape of the parachute was also a crucial design parameter to consider since it would have affected the drag coefficient; at the same time, since we had to sew the parachute we wanted to come up with a shape allowing us to produce a good quality parachute. As a matter of fact the parameters of a cross parachute were used in the simulation.
Design Choices
Before starting to come up with an initial design of the subsystems some decisions concerning the final layout of the systems had to be made; in particular the team focused on the following:
The material of the rocket's body tube
The system that would have trigger the parachute deployment
The internal structure that would have supported the electronics and the egg compartment
The subsystems
Engine Section
The engine is screwed in the bulkhead on whose the carbon fibre body tube would be attached. In the picture it is also possible to see the ring at which one of the two parachute chords would be attached: when the parachute opens up the stress would be directed on the screw and the ring in tension and on the bulkhead in shear direction. In order to obtain the following bulkhead a top part obtained lathing was combined with an aluminium tube using a very strong glue for aluminium. Because of this type of shear load, it was decided that the aluminium bulkhead could be attached with the alumnium cylinder (its walls) using a strong glue for aluminium. The lower section of the carbon tube covering the engine would be attached to these long walls; on top of this, the middle section of the rocket containing the parachute bay had to slide over them having enough lateral stability during flight.
Nose Cone
At this design stage the final layout of the nosecone was not yet established; two types of configurations were considered as outlined in the Conceptual Design post. Both nose cones would have presented challenges in their manufacturing phase; both of them were light but the carbon nose cone would be of course more sturdy and it would match the outfit of the rocket better than the balsa nose cone. Eventually both nosecone were built. Which one do you think we used?
Upper Section
The idea of a stage structure made by several thin aluminium bulkheads was considered to be the better than making a 3D printed structure to hold the electronics and the egg. In fact the 3D printed structure would have been less stiff and it wouldn't have been easily accessible before launch. In addition the aluminium structure would have been better since it would have allowed the team to build several prototypes during the development of the rocket, easily and quickly, without relying on a 3D printing machine.
It is worth explaining the mechanism that would have allowed to team to easily access the payload and the electronics: the stage structure was an independent section that on the bottom had to be screwed on the top of the parachute bay; this would have meant that the longitudinal load deriving from the rocket's thrust had to be born by the structure. The top of the stage structure was manufactured such that the nosecone and the upper carbon tube section could be screwed on top of it.
At this stage the team decided to start manufacturing the first prototypes of the rocket's parts; this design phase was extremely useful because it gave the team insight in many technical challenges that could not be predicted beforehand. In addition some details of the rocket were not yet defined such as how the on off, the arm switch and the break wire would be placed or how the wiring would have fit in the stage structure.
Even though a small rocket such as the one the WBPP didn't have many subsystems, we split up the group so that each one of us could look at the requirement and design his subsystem and then present it to the others. However before going too much into the details of the rocket, the software Open Rocket was used to come up with a first design iteration of our "space" vehicle making sure to meet the requirements given by the safety board. Eventually each subsystem was considered individually and considering load cases, ease of manufacture and systems accessibility before launch design decisions were made.
Open Rocket Estimations
The engine file corresponding to the one that our rocket would have mounted was uploaded to the designing file; this would have granted us to have reasonable prediction of the rocket's flight in absence of wind. Since the team opted for a separation of the rocket in two parts at the moment of the parachute deployment, the rocket was divided in subsystems accordingly.
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| Preliminary design on Open Rocket |
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| Flight Simulation |
The shape of the parachute was also a crucial design parameter to consider since it would have affected the drag coefficient; at the same time, since we had to sew the parachute we wanted to come up with a shape allowing us to produce a good quality parachute. As a matter of fact the parameters of a cross parachute were used in the simulation.
Design Choices
Before starting to come up with an initial design of the subsystems some decisions concerning the final layout of the systems had to be made; in particular the team focused on the following:
- All members of the team were aiming at building the rocket in carbon fibre; however the manufacturing techniques involved with carbon are complex the probability of failing would be high. Despite these considerations the team discovered that carbon socks could fit perfectly the application for our tube. So here we go we went for a full black carbon rocket!
- Looking at the success rate of the methods to trigger the parachute the team found out that the most reliable and lightweight system to open up the parachute would be using a pyro charge. Any other system such a servo or a pressure gauge would have been to complex and heavy.
- After completing the Open Rocket simulations, the team went deeper in designing the individual subsystems of the rocket; for this reason it was crucial to use Catia to come up with the individual parts.
The subsystems
Engine Section
The engine is screwed in the bulkhead on whose the carbon fibre body tube would be attached. In the picture it is also possible to see the ring at which one of the two parachute chords would be attached: when the parachute opens up the stress would be directed on the screw and the ring in tension and on the bulkhead in shear direction. In order to obtain the following bulkhead a top part obtained lathing was combined with an aluminium tube using a very strong glue for aluminium. Because of this type of shear load, it was decided that the aluminium bulkhead could be attached with the alumnium cylinder (its walls) using a strong glue for aluminium. The lower section of the carbon tube covering the engine would be attached to these long walls; on top of this, the middle section of the rocket containing the parachute bay had to slide over them having enough lateral stability during flight.
Parachute Bay
The next part of the rocket was made by the parachute bay; the rocket has to split in two in this point meaning that the middle tube containing the parachute would have to be able to slide over the bottom bulkhead without being fixed to it permanently. At the top of the parachute bay the pyro charge is placed so that its blast will pull apart the rocket's parts allowing the parachute to come out.
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| Catia Render of Parachute Bay |
Nose Cone
At this design stage the final layout of the nosecone was not yet established; two types of configurations were considered as outlined in the Conceptual Design post. Both nose cones would have presented challenges in their manufacturing phase; both of them were light but the carbon nose cone would be of course more sturdy and it would match the outfit of the rocket better than the balsa nose cone. Eventually both nosecone were built. Which one do you think we used?
 |
| Catia render of nose cone |
The idea of a stage structure made by several thin aluminium bulkheads was considered to be the better than making a 3D printed structure to hold the electronics and the egg. In fact the 3D printed structure would have been less stiff and it wouldn't have been easily accessible before launch. In addition the aluminium structure would have been better since it would have allowed the team to build several prototypes during the development of the rocket, easily and quickly, without relying on a 3D printing machine.
 |
| Catia render of upper section |
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| Stage Structure Manufactured |
It is worth explaining the mechanism that would have allowed to team to easily access the payload and the electronics: the stage structure was an independent section that on the bottom had to be screwed on the top of the parachute bay; this would have meant that the longitudinal load deriving from the rocket's thrust had to be born by the structure. The top of the stage structure was manufactured such that the nosecone and the upper carbon tube section could be screwed on top of it.
At this stage the team decided to start manufacturing the first prototypes of the rocket's parts; this design phase was extremely useful because it gave the team insight in many technical challenges that could not be predicted beforehand. In addition some details of the rocket were not yet defined such as how the on off, the arm switch and the break wire would be placed or how the wiring would have fit in the stage structure.
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