Amateur Water Rockets Lemonade Bottle Rocket Science. Jamie Bignell. University of Bath, Electronic Engineering Undergraduate

Transcription

1 Amateur Water Rockets Lemonade Bottle Rocket Science Jamie Bignell University of Bath, Electronic Engineering Undergraduate 1 Abstract Over the past 10 years, I have been exploring water rocketry at home as a hobby. Starting from a single washing-up liquid bottle in the garden at a very young age, to implementing electronics learnt in academia into a modular rocket with fall away boosters and a 15 litre+ volume, capable of flying to over 400 feet. The quest for altitude and the ability to record video footage from the rockets at apogee, followed by a safe recovery of the rocket via passive means, has always been the goal. Developing the rocket further has presented many new challenges that have taught me about problem solving and has usually led to new discoveries. Despite being an informal project, water rocketry has led me through the traditional process of analysing a problem and developing a solution from first principles, using knowledge and materials readily available, at a reasonable cost. Always optimizing designs and pushing materials to their limit has fuelled my learning and studies through GCSEs and A-Levels. Failures are always a great way to learn something new. I decided to also document my progress in detail through my water rocketry website so others wouldn't need to make the same mistakes that I have made. The website also became a technical journal that is often referred to for past experiments when working on a new design. 2 Nomenclature Apogee PET PVC Peak of the rockets trajectory Polyethylene terephthalate Polyvinyl chloride 3 Introduction A water rocket is a type of rocket using water as its reaction mass. Such a rocket is typically made from a used plastic soft drink bottle - at least this is the stereotypical view of a water rocket. However, it s clear that water rockets have many practical advantages over their pyrotechnic counterparts with regards to safety, cost and how readily available the construction materials are. In a water rocket the reaction force is generated by the ejection of water by a pressurised gas, typically compressed air. Like all rocket engines, it operates on the principle of Newton's third law of motion. The water thrown out of the nozzle forces the rocket upwards. As more water is lost out of the rocket, the lighter the rocket becomes which accelerates the rocket to an apogee. The physics behind a water rocket is very complex, with many different parameters as well as physical variables needing to be considered to obtain accurate results via calculations. Practical experience and a trial and error approach to engineering a rocket, containing different sub-systems, is what has led to the most recent successes within this venture. Within this paper, the journey over several years will be outlined, explaining the steps undertaken to obtain a successful flight profile of a water rocket. 1

2 Water rocketry will seem abstract and bizarre to discuss from an academic standpoint to most members of the public. This is in part due to retailers such as Maplin selling water rocket kits as toys, which is precisely what started my inquisition into the field. The Maplin kit contained some small plastic rockets with nose-cones and fins, with a small pressure vessel, having very thick walls of PET plastic. The launcher used a female garden hose (Hozelock branded) connector on the base, with male nozzles being screwed onto the rockets themselves. As pictured in Figure 1, Maplin Water Rocket Kit Figure 1, Maplin Water Rocket Kit These nozzles used a specific thread that was identical to a standard lemonade bottle thread. This feature allowed me to take trivial toy rocket to a whole new level of possibilities. The entire rocket launching process, can be broken down into components, each fulfilling a specific role in the timeline of a launch. Each component and sub-system could be redesigned and improved depending on the desired outcome: in general, more altitude. In other cases, modifications would lead to additional capabilities of the rocket system and would require the prototyping of a new component. What s more, it was discovered that a world-wide, online community of water rocketeers would all be willing to help share and develop new ideas. Everyone is after one thing more altitude. 4 Initial Development and Exploration of Fundamentals and Concepts Referring to Figure 1, the major components of water rocketry can be seen. Breaking down the Maplin kit into its constituent systems, the parts that make up the rocket and the systems needed to launch it could be and were improved. Starting with the rocket: it was evident that the volume of the pressure vessel could be increased so that more potential energy could be stored in the form of compressed gas and indeed, the burn time increased by carrying more water. It was clear that lemonade bottles were to be used as a basis for my first generation of rockets, due to the price and ease of accessibility of the building resource. After using a single lemonade bottle as the pressure vessel, the necessary procedures in ensuring stability in the flight path were explored. The centre of gravity and the centre of aerodynamic pressure and their placement were critical in ensuring a stable flight path up to apogee instead of spiralling around fruitlessly after launch leading to some hair-raising situations. The centre of pressure needed to be behind the centre of mass. The centre of pressure is not easily measured, but by using fins made from corrugated plastic, near the back of the rocket of a standard shape, have been used for the duration of the project. Making the rocket body longer also helped maintain stability of the rocket as has putting the fins further back. Because the launches are low cost and frequent, this led to lots informal prototyping and fin development. 2

3 The centre of mass was controlled using weight in the nosecone. Thus, early rockets that used a single lemonade bottle as the pressure vessel, subsequently included new fins and more bottles taped on top to maintain a suitable centre of pressure. Some nose weight in the form of lead fishing weights was also used in some tests. In the early stages, the research and findings of the flights were noted and changes were made on a broad level with regards to the overall rocket, not specialising on one part. These early rockets flew well, once these changes were put into practice, but the altitudes were still relatively low. Hence some direct major improvements in specific subsystems were required. Figure 2 shows an early rocket using the Maplin launcher and a single 3 litre lemonade bottle as the pressure vessel. Figure 2, Early Single Bottle Rocket 5 Improving the Pressure Vessel Figure 2 shows that multiple bottles on top of the pressure vessel, despite being useful for stability could potentially be used as extra volume. Joining multiple bottles in an airtight fashion would further increase the volume. Research was done online regarding different methods of joining bottles together. Examining their advantages and disadvantages as well as looking at the required tools and skills. Splicing bottles appeared to enable large rockets to be produced using a simple step-by-step procedure that could easily be adapted for materials available locally in the UK as the water rocket community is worldwide. There is a large variation in the different bottles available, mainly with respect to the thickness of the bottle wall and the number of threads at the neck. Splicing is essentially, gluing bottles together end to end and then using couplers purchased online to join these spliced bottles together. This makes the rocket modular so if a spliced bottle fails, it can be removed and replaced. The process of splicing involves cutting off the bottom of two bottles. There are two stages to gluing the bottles together: one end is shrunk and slid inside the other, it is then sealed in place with a soft glue that creates an airtight seal. Subsequently a 75mm wide plastic sleeve is glued round the joint and held in place with a brittle but strong glue, making up a hybrid splice. At first, I experienced much difficulty in finding suitable glues with the required properties for both these steps. Many glues simply couldn t withstand the pressure and those that could, led to a non-airtight seal. The splice is now airtight, strong and flexible in a crash. See Figure 3. Figure 3, Cross Section of the Hybrid Splice 3

4 These spliced bottles are then joined together using tornado couplers. This is basically a doubled sided lemonade bottle top that was originally sold to schools to help demonstrate the Coriolis effect in water passing between two lemonade bottles. Figure 4 shows the purchased couplers with their respective O-rings which enhance the seal between the spliced bottle and the coupler. Figure 4, Tornado Couplers with O-rings and Nozzles Once the spliced bottles had been fabricated, some were tested to destruction. It was discovered that the splice was stronger than the remaining PET plastic bottle which deformed the neck of the bottles and would subsequently fail. Thus, additional bottle plastic was used to form a jacket around the ends of both the bottles and then held in place with fibreglass strapping tape. This reduced the deformation and allowed yet higher pressures to be reached safely. Figure 5 shows the Hybrid splice with bottle jackets. This makes up the modular pressure vessel of the rocket. Figure 5, Completed Hybrid Spliced Bottle 6 Developing the Launcher Larger, heavier rockets were unsuitable to be launched using the Maplin launcher, therefore a dedicated launcher was needed that retained the benefits of using the Hozelock system, as well as ensuring the rocket was held upright while being pressurised. Extra safety features and a larger compressor were included. The new launcher (as shown in Figure 6) used more robust plumbing, along with professional airline equipment, and a pressure release valve was added in case a launch should need to be aborted. With a longer airline, the launch could be carried out at a safer distance with the rockets getting larger and at a greater pressure at launch. Figure 6, The New Launcher 4

5 A detachable guide rail, as shown in Figure 7, was used to support the rocket while being pressurised and maintained the correct flight path for the rocket at low take-off speeds, when the fins are most ineffective. A new compressor designed for truck air-horns was purchased from the USA. This reduced the time pressurising the rocket and allowed higher pressures to be reached. Figure 7, Launcher with Guide Rail Larger rockets using splices, were heavier and consequently using existing Hozelock nozzles with an internal diameter of 9mm resulted in a restricted water flow being expelled, preventing the required level of thrust needed for the rocket to take off or at least reach a dignified altitude. Subsequently, a modular modification to the launcher was adopted that allowed larger nozzles to be fitted to larger rockets when needed. An adaptor was fabricated which enabled a larger 15mm nozzle to be used on rockets, as photographed in Figure 8. Depending on the general set-up of the launcher, it could be decided what nozzle to use and the parameters available to change, such as pressure vessel volume, pressure, payload mass and weather conditions would affect this decision. A larger nozzle would lead to a lower burn time, but a greater short burst of thrust. A smaller nozzle would give lower thrust but a more longer sustained burn time, leading to more authentic looking launches. Figure 8, The 15mm Launcher Adaptor with 9 and 15mm Nozzles Mathematically, both nozzles should give the same specific impulse (i.e. the product of the force and time) but it was quickly realised that the actual physics behind the burn time is very complex, with an optimum balance between thrust and burn time leading to the maximum altitude. Delving into the maths, it was quickly established that there is still much discussion within the understanding of the fluid dynamics, over many aspects of a water rocket propulsion. When a larger nozzle is used the water is expelled in less than half a second. Most water is lost near to the ground during the initial acceleration period shortly after launch. Through research in was found that the altitude of the flight could be increased by approximately 20% with the use of what is termed a launch tube. The launch tube goes from the nozzle head up through the body of water inside the rocket into the airspace in the pressure vessel. It is attached to the head of the launcher via friction fit. Upon release of the rocket the initial motion is provided by a piston effect of the compressed air alone. The early acceleration of 5

6 the rocket, over the length of the launch tube, is achieved without losing any water. The new 15mm nozzle addition allowed standard 15mm copper tube to be used as a launch tube. Different lengths could be used depending on the size of the rocket. Figure 9 shows this in practice. 7 Recovering the Rocket from Apogee Figure 9, Launch Tube Fitted to Launcher Within the project so far, a reliable collection of systems had been developed, able to launch large rockets to altitudes of over 80m. However, due to the successes of ensuring the centre of gravity and the centre of pressure were in the most effective places, the rockets tended to nosedive from apogee into the ground, which raised questions of safety and most evidently, rocket reusability. Usually rockets that crashed could be reused, but only for a limited period. Thus, development of a recovery system was the next logical step. Research showed that, many different methods could be used, which ranged from using chemical reactions to deploy a parachute, to using helicopter blades that unfolded from apogee and led to the rocket to fall similar to a sycamore leaf. However, experience from school, led to me looking at electronic circuits able to detect the launch and using an extra mechanism to deploy a parachute. After several experiments with different types of parachutes, fabricated from a variety of common materials including bin liners, a suitably modified umbrella, proved to be ideal to fulfil this function. The physical mechanism was produced from combining ideas online, to form the side-deployment arrangement, that used corrugated plastic to form a v-shaped compartment with a plastic spring. The parachute was held in place by a door which was held shut by a servo motor. This mechanism was reliable and robust, deploying the parachute into the airflow. Figure 10, Early Side-Deployment Mechanism 6

7 Initially, radio control was used to activate the servo and deploy the parachute, as seen in Figure 10, but it was always long-term aim to fully automate this process. Combining the deployment system, with a simple electronic microcontroller could be used to control a servo motor and release the parachute after a small-time delay from launch that was detected by a trigger input to the microcontroller. All trigger options for detecting launch had their advantages and disadvantages, but all functioned in a similar same way. Examples of trigger options included a wire breaking on launch or a G-switch that activated after the rocket underwent a certain acceleration threshold. Once the rocket reaches apogee, ideally the servo motor activates the deployment mechanism, which permits the parachute to be deployed and recover the rocket safely to ground. However, in early flight controller designs, this was after a pre-set time delay that was calculated using a simulator on the web. Figure 11 shows an example an early design of such a controller. Figure 11, Early Flight Controller Figure 12, 3rd Generation Flight Controller PCB Layout Later flight controllers, as shown in Figure 12 became more complex, with multiple trigger options available to use, along with supplementary functionality which included configurable time delays and servo starting and ending positions. Most recently a flight controller that detects when the rocket tips over at apogee has been developed to ensure that the parachute is deployed at the correct moment in the flight path. A compass sensor is used to detect the strength of the earth s magnetic field in three planes. Trigonometry is used to calculate the angle from vertical, of the rocket. Once the rocket passes 90 degrees, the servo motor is activated and the deployment of the parachute commences. The chute is attached to the rocket using a shock cord. The parachute is wrapped in this cord and folded in a very specific way, which means when the chute is deployed, it s still packed when it passes through the fins and then opens behind the rocket. This prevents the chute snagging on the rocket fins and has been another development activity in its own right. Figure 13 shows this system in operation. 7

8 Figure 13, Latest Rocket Recovery System with Apogee detection 8 Payloads With the development of reliable recovery mechanisms, small issues could be ironed out such as parachute size and mechanism dimensions to enable the system to have a good deployment success rate. Thus, potentially fragile payloads could be safely carried up to apogee by the rocket. Payloads such as cameras to record the on-board perspective of the launch, along with a small logging altimeter, which produced a graph of height vs time as shown in Figure 14. These are mounted in the spaces in between the spliced bottles of the rocket, within fairings made from other bottles as pictured in Figure 15. These fairings also contain a hook which attaches to the launcher guide rail to support the rocket at the beginning of the flight, when there is least airflow over the fins. Figure 14, Typical Altitude Plot Figure 15, On-board Camera Fairing 8

9 9 Pressure vs Volume To increase the volume over 15 litres, this would require even larger nozzles, this was found through practical experimentation. This would make using the standard lemonade bottle as the base building resource challenging. In order to further increase altitudes, increasing the launch pressure of the rocket was chosen. This entailed further reinforcement to the spliced bottles using composite materials, allowing the spliced bottles to reach target pressures of 200psi. Other sources had had success with using fibreglass reinforcement, so it was decided to take this further. The process is comparable to splicing, except the outer sleeve was replaced with two wraps of 200gsm fibreglass cloth around the whole bottle, which was then left to cure with epoxy resin worked into the cloth weave as depicted in Figure 16. Disappointingly, the process took a long time, was expensive and fibreglassed bottles themselves were brittle. In the event of a parachute malfunction the necks of the bottles tended to shear off, as opposed to those using the standard lemonade bottle splices, which flexed at the neck and giving them some robustness. There were also safety concerns with the higher pressures being used. This led to the fibreglass reinforcement approach being adandoned, despite a fibreglass rocket (as shown in Figure 17) being used to set the project altitude record of 434 feet. Figure 16, Fabrication of a Fibreglass Spliced Bottle Figure 17, A Fibreglass Rocket Launch In order to increase the altitude, without increasing the pressure significantly, existing rocketry principles were used as a guide. Fall away boosters with a larger nozzle (than the existing 15mm nozzle), would eject all their water in a short time, giving lots of thrust. This would get the main stage up to speed, which would then use a smaller nozzle sustaining the velocity up to apogee. 9

10 The boosters don t use a fitted nozzle, the bottle opening of 22mm is used as the nozzle. The boosters fall away using a simple hook mechanism (see Figure 18) to reduce the weight brought up to apogee. At the end of the short burn, no thrust means they are free to fall away without any complex mechanical release systems. This required the launcher to be modified slightly as shown in Figure 19. A T piece adaptor was used to enable the boosters to be pressurised alongside the main stage. Additional frame working was made out of wood which supported the boosters while being pressurised. These two new systems used 22mm PVC piping with an O-ring, to provide an airtight seal for each booster, either side of the rocket. The boosters were not retained to the pad so that, once the main stage was released, the boosters would be free to launch, being held in place only by the hook mechanism to the main stage. The assembled multi-stage rocket with fall away boosters is pictured in Figure 20. This combination of sub-systems used alongside the boosters still needs refining as the flight path of the rocket is as yet unsatisfactory. Figure 18, Booster Attachment Mechanism Figure 19, Modified Launcher for Rocket with Boosters 10

11 Figure 20, Rocket with Boosters on Launcher 10 Conclusions To conclude, this project has proved to be immensely enjoyable and rewarding hobby away from academic study, but has also been a useful learning tool and has continued to fuel my passion for the applications of electronic engineering and fundamental engineering principles which are central to progressing in any field of engineering endeavour. A child s toy has been taken from a play thing to a high-tech collection of systems combining technologies from different branches of engineering, that flies over 400 feet into the air and recovers itself safely to the ground. The project has guided myself through the classical problem solving process while in my garage and kitchen. By following both my own design direction and combining this with knowledge gained from other practitioners, or through research, I have been able to enhance my own ideas and implement new principles that hadn t been explored by the worldwide water rocket community, which I have contributed to, via my website and specific forums. 11 Acknowledgements I would like to thank subject specific teachers throughout my education at Holyrood Academy including Mr Gitsham and Mr Wilson. I thank my family for being supportive and enthusiastic of this ongoing hobby and venture. I d like to acknowledge Leonardo Helicopters who invited me to talk at a Lunchtime Seminar about my Water Rockets, along with the RAeS Yeovil Branch who also suggested I present my experiences at the local Reggie Brie presentation competition. 12 References Personal Website: Air Command Rockets: