Testing and Modelling

 

 

prototype 1

 

Rocket + Launcher First Prototype report

 

Design

 

The design of this rocket and launcher is intended to be cheap and easy to make, so it is accessible to a large amount of people. To reach this design criteria, we had to ensure all the materials we used were common household items, or available cheaply or free from a wide range of places. It was also important to consider the difficulty of constructing the launcher, and the accessibility of the tools that are required. The tools we used are; a drill (which could be swapped for a screwdriver), a wood saw, a hammer, scissors, a tape measure, and a pen/pencil (for drawing markings to make construction easier).

Materials + costs

 

  • Small wooden pallet – free
  • Can be found at many shops as they are used to transport goods and usually thrown away
  • Pallet was broken down into planks to use to make the frame of the launcher
  • Alternatively, wood can be bought at DIY shops to assemble the frame, varied cost depending on the type of wood

 

  • Bottle for rocket combustion chamber – 17p
  • 2 litre fizzy drinks bottles are available in most food shops and supermarkets, and are usually very cheap
  • Important to use a fizzy drink bottle as they can hold significant pressure (fizzy drinks are usually pressurized at around 14 bar)

 

  • PVC pipe (22mm diameter) – £2.50
  • Available in B&Q or other DIY stores
  • Used to create a relatively tight fit between the opening of the bottle and the pipe, and 22mm was a near perfect fit

 

  • Tent pegs – free
  • Available in most outdoor shops, I used tent pegs I had at home but if you don’t have any available, they are available for under £5
  • Used to hold the bottle in place while it is pressurised and then pulled out to release the bottle

 

  • Waterproof Silicone sealant – £10
  • Available from DIY shops
  • Used to create a seal between a bicycle valve and the PVC pipe. This is how we would transfer pressure into the bottle

 

  • Bicycle inner tube – Free
  • We used an inner tube that was punctured to source the valve, however if you don’t have an inner tube, they can be bought for under £5 in stores like Halfords
  • We cut the valve out from the inner tube with some rubber left around it, to make it possible to use a standard bicycle pump to pressurise the rocket

 

  • Assorted Screws – £2.50
  • Can be bought from any DIY store
  • Used to attach the wooden frame together

 

Results of the first tests

  • General design concept worked well and is likely to be our final design
  • A few improvements we could make have been highlighted

Our first observation of the design highlighted an issue where screws extended from the wood beneath the launcher, where you would instinctively reach to pick it up. This is obviously a danger so our next design must not make this mistake. On our first test launch, the rocket released before we had secured it onto the launcher as we were not expecting it to be pressurised so quickly. When we tested again, the sealant between the pipe and inner tube valve failed. It is clear that it was not a suitable adhesive for the application, so we used a hot glue gun to secure the valve properly. This was not an ideal fix as a hot glue gun is not a common household item and doesn’t fit with our values of creating an easy and accessible rocket design.

After this repair, the rocket launched successfully multiple times, however we noticed it would leak quite a lot of water at high pressure. To combat this, we overfilled the bottle to allow water to leak out and still be at 1:3 water to air ratio when the rocket launched. We also noticed it was quite difficult to release the rocket using the tent peg. The person releasing the rocket would have to hold the launcher firmly and pull the peg out with force, meaning the rocket could not be released from a distance. This means the person would get covered in water when the rocket goes up. On our last launch, the rocket released early without us intending for it to be released. We realised the problem could be due to the PVC pipe not being securely attached to the platform, allowing the bottle neck to bend away from the tent peg securing it in place.

 

Issues and potential fixes

 

  • Screws protruding through the launcher base
  • Can be fixed by selecting shorter nails to attach the launcher together, however it is important the nails are long enough to properly secure the launcher together

 

  • Bottle leaking water under high pressure
  • We need to slightly expand the PVC pipe at the bottle neck, to create a better seal. We tried Using an O-ring, however it was too wide to fit the bottle. We plan to slightly heat a ring of the pipe to expand it a bit.
  • Another solution is also to pump up the bottle quicker. We could use an electric pump or a bicycle pump with a bigger air chamber to pressurise the bottle quicker, therefore the water would have less time to leak out
  • Quite difficult to release the rocket with the tent peg
  • The hole the tent peg is in needs to be slightly enlarged, to allow the peg to slide out easier. This would allow us to attach the peg to a rope, so the rocket can be released from a distance, and the person releasing it doesn’t get wet

 

  • Rocket released without the tent peg being pulled out
  • This issue occurred due to the pipe bending to the left/right of the peg that secured it in place. This can be fixed by securing the pipe in a second location, currently the pipe is only secured at one spot, which can cause flexibility in the pipes orientation.
  • Another fix is to secure the bottle with a tent peg on either side of the bottle neck, however both tent pegs would have to be pulled out simultaneously

 

  • Valve system requiring a hot glue gun to be sealed
  • We need to test/research a different way of sealing the valve to the PVC pipe
  • An alternative way is to buy a pipe connector to fit the end of the PVC pipe, and drill a hole to perfectly fit the valve, however this would make the design more complicated.

 

 

 

 

                                                                            {CFD TESTING-PROJECT RATIONALE}                                                                                                             (Solidworks wizard flow simulation and justification of design language based on CFD comparisons, wind tunnel data and graphs )

                                                                                             By George Diamantopoulos 

 

Testing parameters:

Z-axis velocity: 40 m/s, X-axis velocity: 3 m/s for a duration of 4 s every 4 s, Y-axis velocity: same as X-axis

Air density: 1 atm

Mass: final+850 ml water: 1120.56 grams, prototype + 850 ml water: 1190.67 grams

Final product fin parameters: , prototype fin parameters:

 

  • Pressure analysis (Animation snapshots)

Explanation:

  1. Different fin  and nose cone design on the left, contributed to a smoother pattern of air pressure around the rocket body, therefore greater stability during time of flight.
  2. Using the top half of a second bottle for the nose cone reduced the drag footprint and its concentration is now focused around the bottle neck.

 

 

  • Turbulence length analysis

 

Explanation:

  1. By reducing fin surface area and eliminating their sweaping angles, turbulence length is reduced to <0.002 m (left) instead of >0.002 m (right).
  2. Rear turbulence trailing impact is eliminated, therefore less drag is produced on the finalized product.
  3. The cap nose cone solution (left) creates a larger area of zero turbulence around the rocket body compared to the old design on the right.
  4. Turbulent air footprint is more symmetrical on the left and as a result the final product (left) is more efficient than the prototype (right)

 

 

  • Turbulence length analysis of optimal aerodynamic shape-Discussion

 

 

Explanation:

  1. The above figure presents two different extensions in grey color, in an attempt to minimize drag; created by the flat top of the cap. Both options are compared to the unedited design of the water rocket in the middle.
  2. When using a small nose cone placed on top of the cap (image on the right of the unedited rocket), no significant changes are observed.
  3. When using an exaggerated nose cone (image on the left of the unedited rocket), turbulent air is reduced. This is especially observed around the bottom cap, and behind the rocket(trailing path).
  4. The exaggerated nose cone shows improvements compared to the unedited form. The issue is, that the design may injure the user or a pedestrian when falling. Therefore, the small aerodynamic benefits gained are eliminated by the risks involved.

 

 

  • XYZ shear stress-drag coefficient chart comparison-Conclusion

 

 

 

 

  • Wind tunnel data

 

 

 

  • Wind tunnel setup

 

 

Explanation:

  1. The rocket was tested at 5 different velocities at a pressure of 99.75×10^3 Pa and at a temperature of 21°C.
  2. Maximum speed was set at 25.4 m/s, which indicated a maximum force of 2.35 N.