The Copper Tube Launcher

This is the launcher that I have built using parts that I have managed to find in the UK - a drier alternative to getting soaked with hand-held launches. This design does not have a central launch tube and, to be quite frank, I have found that it does not need one although there is nothing to stop you from putting a rod down the pipe in order to act as a guide.

This is the basic launcher - you can attach it to whatever you find convenient (Just to try it out, I first tied it to a SwingBall base - right). Like many launchers, it is loosely based upon the Clark Cable Tie Launcher although this design offers a slighly higher pre-launch force for a given pressure allowing heavier rockets to be launched than other designs.

By using an extrnal seal, it offers 28mm diameter instead of 21mm - an increase in area (and therefore force and thus maximum lift-off weight) of 78% should you ever need it.

The reason for choosing this method of sealing over the internal method is entirely down to the materials that I could find and was not based upon some superior intellectual process - the thought of extra force never entered into the design specification.

Design Considerations

By no means an exhaustive list - it should be . . .

• slim enough to fit inside any sets of fins or tube fins;
• long enough to fit inside any skirt;
• made from materials that are available or have equivalents worldwide;
• easy to make;
• easy to use; and,
• as safe as is reasonably pricaticable.

Parts

These items are for the launcher itself - I eventually tracked down the pieces I wanted at Coussins Homestyle at 271 Blagreaves Lane in Littleover Derby UK (+44 (0)1332 768 177). If you want to design a launcher for yourself, walk around with the neck of a bottle in your pocket and be prepared for some disbelievers.

You decide what you want to attach it to (To try this launcher out the first time, I used the bit of a SwingBall that sticks into the ground and secured it with cable ties. There is nothing to stop you from building a tripod like device, a sledge stand or fixing it to a table as long as it meets your needs of transportability - the flange on the mounting bracket will help you do this).

• Through-Wall Outside Tap Mouting Bracket (essentialy a piece of copper pipe with a useful flange and thread at one end);
• 28 - 15mm End Feed Connector;
• 28mm Straight End Feed Connector;
• 90º ½" BSP - 15mm Compression Tap Connector;
• Woods/Schrader adapter;
• 14 SWG Copper Wire;
• Solder and Flux;
• Plumber's PTFE tape;
• ½" x ¾" Jubilee Clips (also called variously worm or gear-drive hose clamps or screw clips or screw clamps in various parts of the world);
• 200mm Cable Ties (or you can use 20 x 100mm cable ties);
• 3" of 38mm ID plastic sleeving (plastic drain pipe);
• 21mm dia 'O' Ring; and,
• piece of string (about 20 yards or whatever you feel safe with).

Construction

Make a copper ring from the copper wire, clean it so that it can solder and slide it over the through-wall tube (this one will fit near the middle). At this point, you can also slide the two jubilee clips onto the tube but don't worry if you forget this bit as they can be uscrewed and slid over later.

Solder the 28 - 15mm connector to the tube and then the straight end feed connector on to that in order to make the top of the launcher as shown in the diagram.

Find a suitable point on the through-wall tube for the copper ring such that the cable ties may be gripped above and below with the jubilee clips (with the 100mm cable ties, this needs to be around 1cm from the bottom of the 28 - 15mm connector). Clean the copper tubing at this point and sweat the copper ring on. This will form a stable base that will not slip at all.

Slide the plastic sleeving over the end and slide down to the bottom, bringing the jubilee clips to the copper ring. Fit between 4 and 8 cable ties and secure them (oriented as in the diagram) such that they will grip the flange of a pop bottle placed in the end of the tube. If you use the 100mm cable ties, use all 20 and use an elastic band to assist in lining them up - putting the elastic band over the 28mm connector and sliding them through it. Tighten the jubilee clips being careful not to cut the cable ties as you do it.

Slide the plastic sleeve up the tube so that it grips a bottle - you may have to shave a little off the outside of the cable ties so that it only requires a minimum of force to pull the sleeve down on launch so don't force anything at this stage (this launcher (like many others) is designed so that it will fit inside the skirt of a rocket without getting in the way - see photograph). Once the sleeve moves reasonably freely, check to make sure that the ties will hold about 50lbs weight. Make a hole in the sleeve and pass a short piece of string through it to which you can tie a longer piece at the launch site (using a short piece means that when you are assembling the thing in sub zero temperatures, you are not trying to thread a piece of rapidly fraying string through a tiny hole in a piece of plastic while everyone looks at you struggling).

Make another copper ring from copper wire - with a square cross-section (this one will join the Woods/Schrader adapter to the 90º connector). make it so that it fits around the knurl of the adapter and sweat it on. Then sweat it onto the end of the 90º connector. Using plenty (just the right amount) of PTFE tape, screw the 90º connector into the through-wall tube.

Put a rubber 'o' ring onto the bottle so that it sits in the groove. Screw on the flexible hose and the bicycle pump in the usual manner. Fill the launcher and the bottle completely with water, sitting the bottle in position as shown in the diagrams.

Note that you should slide the 'o' ring to the thread end of the groove (a), and when you slide the rocket onto the launcher, the 'o' ring rolls up the groove slightly (b). This means that when the rocket launches, all that the rocket has to do is roll the 'o' ring back and this means that the seal is not sliding over any surface and therefore should last longer.

Now pressurise the bottle to working pressure and check for leaks. If you have done the soldering properly and everything matches up, there should be none. Unscrew the hose connector and the system will depressurise.

Fix the flange on the Tap Mouting Bracket to either a stake, at base that rests on the ground or to a tripod type of stand.

Use

This assumes that you have pressure tested everything.

1. Put the launcher in place (making sure that it launch tube is vertical - non-vertical angles lead to internal waves and unpredictability in flight) and lay out the string (it may pay you to add an extra yard at the rocket end so as to warn you that your kids are going to launch it while you are still pressurising it - giving you enough time to put your foot on it or run. Another alternative is to have a hook and eye about a yard from the launcher - only linking the two when you are ready thus precluding your offspring's attempts at entertainment) passing it through a loop (in the base so that pulling it from a distance pulls it downwards - you can have a pulley or wire loop, just as long as it pulls downwards) making sure that it will be able to move freely (try it a few times just to make sure).
2. Connect the pressure hose and then fill the launch tube with water. Put the correct amount of water in the rocket and quickly mount it on the launch tube making sure that the 'o' ring is in place (if you want, you can overfill the rocket slightly, pump a little bit of air into it and then let out water until it is down to the correct level by unscrewing the pressure fitting) and then slide the plastic sleeving up to lock it in place.
3. Pump air into the bottle until it is at the right pressure (remembering that you will have a slight back pressure on the one-way valves). Remember - do not touch the rocket when it is pressurised.
4. Walk away to the end of the release string.
5. Ensure that everyone who is close enough to need to know what is going to happen next actually knows that you are about to launch and then pull gently on the release string.

Launching from a distance gives things a different perspective to launching by hand.

Variations

As this launcher does not have any launcher parts on the inside of the nozzle such as seals or launch guide tubes and the thread of the bottle is not being used for anything structural, (you can remove it if you really want to see if it makes any difference,) it is possible to insert a threaded device into the nozzle so that you can experiment with defferent sized nozzles.

An insert that screws into a thread that is tapped into the inside of the nozzle is shown in a although it must be remembered that even though this is not a load-bearing part of the bottle, tapping the inside may weaken the neck of the bottle and may provide an opportunity for cracking under pressure. b shows a threaded sleeve that is glued into the neck of the bottle with resin - the sleeve being able to take a number of nozzles for the purposes of experimentation. Possibly a and definately b require some engineering capability such as a lathe.

Another variation is to add some extra hardware so that it is controlled entirely by one compressed air line. Look at the page showing An idea based upon the Copper Tube Launcher.

Note

You may find that with different sized bottles with different shaped flanges and grooves, the 'o' ring slips at very high pressures, moving to the top of the groove and, as a result, water may start to leak out. This can be combatted by using 2 'o' rings instead - the one nearer the flange stopping the first one from shifting.

Esoteric Warning

If you are experimenting with different gasses in order to find the effects of the ratio of specific heat capacities of the gas at constant pressure and constant volume ( Gamma or compressability) on performance and are considering gasses with a low, you should not use Acetylene (it is dangerous enough gas as it is) NEVER PUT ACETYLENE IN CONTACT WITH COPPER. Try looking only at gasses with a high such as Argon.

Thanks to (in alphabetical order) Alistair Bell, Brad Calvert, John Darbyshire and T. Nazim with the international nomenclature and Mark Branigan with the offer of parts for this project.

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