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Hold it! Don't turn off just because it looks as though you have downloaded the wrong page and are in the middle of a Engineering Text. (Of course if you have downloaded the wrong page and you were looking for a Engineering text then you should escape now.) For the rest of you there is no cause for alarm as this is my page where I will go into the details of some remarkably interesting contraptions that can be made around the house one bored Sunday afternoon.

Warning

Why have I called this page "Hydropneumatics" ? Well simply because all these devices rely, either entirely or to a large extent on the use of compressed air and water. This is for a number of very good reasons.

• Convenience: Everyone has access to air and water, or at least everyone who I expect to be reading this. Anyone from another planet has the right to send me an angry email.
• Safety: While it is possible to kill or injure yourself with high pressure air or water, to achieve those pressures with these designs will require a level of recklessness that I won't feel responsible for. I repeat If you manage to injure or kill yourselves with anything you get off these pages, I won't pay you a cent of compensation. This is because I don't live in the USA and I have a lot of lawyers as friends and relatives.
• Environmental responsibility. Air and water are the most environmentally safe materials you could possibly name. Not that I oppose the use of unsafe chemicals if there is a valid technical reason for them, but to ignore the possibilities of harmless ones is just silly.
• Cheapness. Both air and water are, for all practical purposes, free.

So then, without any further ado, I shall move onto the projects themselves. The order in which these projects are arranged is chronological, in the order in which my friends and I devised them ourselves. This also means that they are approximately in order of increasing difficulty.

• Metho Cannons: A lightweight and cheap form of explosive propulsion.
• Blowpipes: a revival of an ancient weapon suitable for todays conditions.
• Rocketry: Simple Hydropneumatic rockets, from 5 minute projects to serious high altitude missiles.
  

  Metho Cannons

  

  This is the first project of this type that I was ever involved in. As such it was back in my school days. The basic concept of these devices is that a canon can be made, a simple muzzle loader usually, which relies not on gunpowder but on the explosive force of a mixture of alcohol vapour and air. The alcohol being in the form of metholated spirits or "metho".

  The basic design of the device is as shown here. A simple tube is used, which is blocked off at one end. At the time when I was involved with these we used a tube made from 3 or 4 softdrink cans joined end to end. Since that time, however, softdrink cans have changed both from steel to aluminium and in shape. So I am not sure that modern cans would suit the construction of such a cannon.

  Down the muzzle of the cannon would be pushed a tight fitting, but light, projectile. For the softdrink can cannon a tennis ball was ideal. This was not pushed all the way to the end but a space was left at the bottom, about the size of one can. Then, through the firing hole, some drops of metholated spirits would be added. When these were evaporated, a lit match was applied to the hole. The metho fumes would explode and the tennis ball would be fired out of the cannon a considerable distance.

  It goes without saying that this design is easily enlarged. The largest version I have seen used a water pipe, with a large drum as the combustion chamber. It was also fitted with remote, electrical ignition, which was necessary as the pressures achieved would send an apple flying with such velocity that it would never be seen again. Eventually, the pressure proved too much and the pipe separated from the drum and the cannon disintegrated.

  The apples were slightly larger than the diameter of the pipe. Pushing them into the muzzle would result in a thin layer of skin being scraped off, and an airtight seal being formed.

  Blowpipes

  These are the simplest of all. Everyone (Or at least everyone who is educated enough to be using a web browser) knows of the natives of Borneo and South America who use blowpipes for hunting and war. They are merely a long straight pipe in which is placed a small, lightweight dart that seals tightly in the bore. Then the pipe is raised to the lips and the user blows through it. The dart is then expelled at high velocity. This is sufficient for hunting small game, and with the addition of poison, for large game. For the purposes of this document I shall ignore poison.

  Obviously the construction of the pipe poses little problem. Well, little problem in a modern, industrialised society. If you were living as a hunter gatherer in the middle of the tropical rain forest you might find it a bit more of a challenge, but the chances are that you are not. So rather than having to select a long straight pole of hardwood and then laboriously boring a smooth cylindrical hole through it, all you need to do is find a piece of plastic pipe, or aluminium pipe, or even a rolled up sheet of paper.

  The darts will prove a bit more of a challenge. But not much more. The favourite of urban blowpipists is to make a cone out of stiff paper, and tape a nail so that it protrudes from the apex as is shown. This is not the only design, and you may wish to copy a more traditional design or develop your own. The only requirements are that it seal in the bore and be stable in flight. I've even seen people construct darts with heavy, lead tips using a .45 caliber bullet, or alternatively, a dart that explodes on impact.
  

  Rocketry

  The area of hydropneumatic rocketry is a wide one, with a wide variety of designs possible once the basic principals have been grasped. These principals are best explained through the design of the simplest of all the rockets.

  Stage One Rocket

  Once again this can be made from a variety of different components but the ones I recommend are a plastic softdrink bottle and a cork.

  Simply, the rocket is assembled as follows.

  • The softdrink bottle is half filled with water.
  • A cork is securely inserted into the neck.
  • The bottle is then turned upside down.
  • A thin, hollow spike is inserted through the cork.
  • Air is then pumped through the spike.
  • When the air pressure inside the bottle is sufficient to force the cork from the bottle, the rocket takes off.

  As far as the thin spikes are concerned, I have used a pneumatic corkscrew, designed to force corks from wine bottles. I have also seen a football pump used, which had the advantage of being faster to pump. It is also possible that a hyperdermic syringe might be usable, if extremely laborious, but as always, when using a syringe, ensure you wear a condom.

  As can be seen from the accompanying diagram, the compressed air forces the water out the neck of the bottle, which is now a nozzle. As a reaction to this the rocket (or bottle) accelerates in the opposite direction.

  While this elementary device is suitable for the demonstration of principal, it is in need of some improvement for actual use. The primary problem that you will notice is that there is a distinct lack of directional stability. As has been found by centuries of rocketeers, this can be overcome by the addition of fins, which only need be cardboard or plastic and held on by tape. The second problem is far more insidious, and sadly, can never be cured, it can, however, be temporarily alleviated. I am referring to here, the need for MORE POWER.

  A number of approaches are available to tackle the problem, as I've listed on my Advanced Rocketry Page.

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