Deciding the number of fins:
Our first decision surrounding our rocket was whether it would have three or four fins. To do this, we made two basic prototypes, one with three fins and the other with four. We took these to an empty park and threw them to observe which appeared more stable (as the fins are mostly for stability). We filmed the throws so we could watch back which was more beneficial. In the end, we concluded four fins was significantly more stable. We also did a decision matrix, as shown below, which also identified four fins as being better for our rocket.
Decision Matrix:
This decision matrix allowed us to make objective decisions about other aspects of the rocket and launcher.
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For example, our nosecone was shown to have been best in a conical shape and we later decided it had to be a blunt end instead of pointed, even if pointed was the most aerodynamic, due to safety.
Next we decided on aspects of the launcher. We found that a bike valve as a stopper would be preferable over a cork. We also found a pvc pipe and bike pump to be better than a hand pump due to availability and ease of use (a hand pump would take more effort and the optimum pressure may not be reached due to this).
Safety Considerations:
A major consideration of this project was the safety surrounding launching. Because the rocket and launching system are fairly basic, there is little control of what will happen and where the rocket will go when launched. Where it lands is affected by so many things, for example the wind direction, that it is near impossible to predict where it will land and there is a possibility that someone could get hit. With this in mind, we made sure our nosecone would be designed so that damage would be minimal if it were to hit one of our team or a sectator during launch. Because our nosecone was designed to disconnect upon descent, it was important it wasn’t pointed and would not cause significant harm if it were to collide with somebody when landing. The nosecone would also have to be hollow for the same reasons.
To slow the descent of our rocket to minimise risk of damage to the rocket or other, we would add a parachute. The idea of the parachute is to slow the landing and allow time for people to get out the way if needed. It is also in place to protect the micro:bit.
In terms of safety surrounding the launcher, we would not have the pressure of the rocket at launch higher than 3bar even with 10bar being the given limit. This is because there is a risk of the bottle exploding and so a great danger of harm.
(Our risk assessment is shown on our homepage, which addresses all of this.)
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Material Selection:
The selection of what materials we would use for the main rocket was significantly centred on the aim of a cheap and easy to replicate design. Re-use of common household materials was a great focus, for example the use of plastic bottles, cardboard, tape, string and a plastic bin-bag – things often found in packaging or cheaply in shops. The one component that did not follow this was the nosecone which was 3D printed. Most people don’t own a 3D printer, so this would be included in the package.
We chose 3D printing because of how thin the nosecone could be. Having the nosecone tightly fitted to the rocket would help a lot with aerodynamics, unlike a foam nosecone we made which needed a thick wall in order to not break apart, so not being anywhere close to a seamless fit once assembled. 3D printing also ensured a level surface with no bumps or defects. The nosecone, however, had to be sanded down for the smooth finish and even then had an uneven surface.
The plastic bin bag was for our parachute and used because of it is lightweight and has a large surface area once opened to really slow the rocket down upon descent. String can be used to attach it, however we used wool because the string we had was was too thick.
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