Step 1: Bottles
as there are many bottles with different shapes and dimensions, I had to select the best bottle shape for a rocket design, this needs to be streamlined and aerodynamically efficient, The second aspect of picking the correct bottle is the capacity of water it can carry because this will affect size an stability as if it‘s too short or too long it will alter the balance and weight distribution. I wanted a bottle that is cylindrical and not square as its easier for air to flow past it.
I chose a 1.5L bottle as I thought this would be a good compromise between length and weight, the length of the bottle on its own is 30cm and weighs 40g, it is cylindrical in chape for the best aero start point.
Step 2: Nose cone
For a nose cone I had a choice between 3 different types of cones, I require some space between the bottle and the cone as I need space for the microbit and a weight for stability.
Type 1: (Green cone)
The green cone would be the most aerodynamic as it’s a cone shape and is shaped around the base of the bottle so no excess would overlap the bottle and effect the aerodynamic efficiency, it is the lightest of the 3 cones at 5 grams as its made from card, because its made from card it will be the weakest and will get damaged after several launches.
Type 2: (Blue cone)
The blue cone would be the second most aerodynamically efficient as it fits the bottle well and has smooth body for air to flow but hasn’t got a tapered nose so it can cause air to hit the cone instead of flowing over it smoothly, as its originally a cup it has 400ml of space for the microbit and nose weight to be stored, its made from a transparent blue plastic and is strong enough to take impacts from launches, but with this strength the blue cone is the heaviest of the 3 at 40 grams.
Type 3: (Orange cone)
The orange cone is the least efficient aerodynamically as it fits the bottle the worse out of the 3 and has a hole in the top so air would be trapped as it is travelling through the air, it is the second heaviest at 20 grams and can hold 200ml so it can’t fit the microbit and the weight.
Step 3: Propulsion
I researched simple water rocket designs and most used PVC pipes and a bike pump for the base and propulsion, but as I didn’t have any adequate piping, I searched for other ideas that can be simple to make using basic equipment.
In the end I had some spare bike valves that would be suitable to use as it can easily be used with any normal bike pump. I first tired using the original cap from the bottle and cut a hole for the valve to go through but ended up using a cork to plug the lid as the original lid couldn’t hold consistent pressure, I attempted to seal the lid with some rubber gloves but still didn’t seal properly, I trimmed down the excess cork with a blade so the bike pump can fully attach to the valve without it being too small and leaking air.
Step 4: Release mechanism
As it states in the assignment brief “There needs to be a mechanism to hold the rocket in place while it is being pressurised and for a period of time after” this mechanism that I used is a pin mechanism that goes through the cork and the bottle to hold in place, I started by drilling a hole off centre of the cork so the valve and the hole drilled for the release mechanism don’t merge as that would cause problems such as the pin catching and not releasing smoothly, I tried some different pins to test which is the better option and found a Allen key fitted the best as it was the right diameter and length, another benefit of the Allen key was its 90 degree bend that I could attach some string to make pulling the pin out safer and easier as I can distance myself when the rocket launches.
The pin material was integral as weaker materials such as a wooden skewer flexed too much and caused air to escape gradually as the pressure exceeded 25Psi in the bottle, I decided stronger materials such as metal would be more suitable as it won’t flex as much as wood and tested different objects such as nails and screwdrivers but found the Allen key suited the criteria best.
I decided the pin would need a length of string attached to it so it can be pulled out from a distance as originally, I needed to pull it out by hand and I found this unsafe.
Step 5: Base
The base consists of 3 different components: the extension on the rocket, rocket legs and base.
The extension on the rocket was used so I could attach legs to the rocket, without this the legs were too short and couldn’t fit the bike pump attachment without falling over, the extension provided a good structure to attach the legs, before the extension the legs needed to be over extended causing them to flex and fail at time as there wasn’t sufficient connection between the bottle and the legs. The extension was made from a thin cardboard and connected to the bottle with duct tape, cardboard was used as it is light and flexible so I could shape it to the bottle, the downside with cardboard on a water rocket is that cardboard doesn’t want to get wet as it can start to damage and deform it as it goes soggy.
The rocket legs are used to create a gap between the floor and the cork so the bike pump can attach fully without hinderance, different types of legs and configurations were tested and found the wooden fork legs was the best choice as they arch out slightly for better stability while not interfering with its aerodynamics, wooden forks were used as they are light (3 grams each) and can withstand getting wet. A triangle was made to support the forks and maximize stability, but the design didn’t work as when the rocket took off after being pressurised the cork would strike the triangle and cause it to come away. I tried using spoons instead of forks and cut slits in the spoons for it to hold each end of the triangle but failed as the wooden material was too fragile and would split.
A base had to be used because there still wasn’t enough room between the floor and the rocket when the bike pump is attached and fully extended, a hole was drilled in the base so the pump can fit without hinderance
