Whoosh! The gleaming projectile hurtles towards the stars, trailing vapour and open mouths.
Well, sort of.
Rockets are popular. Big, powerful and, above all, expensive. Shuttle launches cost billions of dollars a shot, and unmanned launches aren't much cheaper. You can, of course, do it yourself. Burt Rutan has his own company that builds rockets as well as aircraft, and the British Starchaser team are working on their own system for launching tourists and satellites.
Unfortunately, 'real' rocketry is far beyond the scope most pockets, and even modest-sized liquid and solid-fuelled rockets are too expensive for the casual hobbyist. They are more in the realm of the dedicated enthusiast. Toyshops sell kits by companies like Estes, and high-power models are also available online, but they are still a bit of a stretch for the casual pocket, costing up to several tens of pounds per launch. This instructable describes three ways of getting into rocketry on a pocket-money budget.
Step 1: Take a Step Back...
... and think what a rocket actually is. Newton's Third Law of Motion states that, for every action, there is an equal and opposite reaction. That's how a rocket works; push something out the back, the rest of the rocket gets pushed forwards. The faster it gets pushed out, the faster the rocket goes. Here's the important point: nowhere does Newton mention fire. He doesn't say 'throw hot stuff out the back to go forwards'; he just says 'throw stuff out the back to go forwards'. It doesn't have to be a blast of burning gases. Water will do the job just as well. This really is rocket science.
But, Water Rockets?
Yes, water rockets. The easiest way to find out what we're talking about is to try it. Time for an experiment.
Buy a can of cheap fizzy drink (make it as cheap as you can, because you aren't going to drink it), put on old clothes and go outside. Give the can a good shake, then throw it on the ground. Throw it at something pointy, like stones, so that the can bursts. What happens? Fizzy drink goes everywhere, that's what happens (that's why you're wearing those old clothes), but so does the can. When the pressure in the can shoots drink one way, the can goes the other way. That is a rocket. All we need is a more controllable way of putting the pressure in and letting it out again when we want to.
Time to Build a Rocket.
This is important: if you are going to make a rocket, you must be aware of the dangers. Water rockets are fast and can hit hard. You should take all the precautions you see fit. You have been warned.
Building a rocket isn't as hard as it sounds, but we will need some specialist items:
> A fizzy pop bottle (any size, but it must have contained something fizzy - the bottles that hold still drinks are not as strong)
> A cork or rubber bung the right size to fit very snuggly into the neck of the bottle (they can be bought from some brewing supply shops, found in junk shops or scrounged off friendly science teachers)
> A bicycle pump (foot or hand, either is fine)
> A football inflation adaptor (the long spike that connects onto the pump and slides into the football's bladder - they cost a few pence from sports shops)
> Some water
> A couple of bricks
> Safety goggles are also a very good idea, as the plastic bottles can burst, although this is rare. You are more likely to get clipped by a flying bung or a ricocheting rocket
The most difficult part is getting the adaptor through the bung. If you are using a cork, you will have to drill through the cork (use the narrowest drill-bit you can), insert the adaptor and seal around it with silicon sealer. Some people use a rubber bung. To pierce the bung, heat a narrow nail to red heat with a blow torch (holding it with pliers!), push it through the bung and pull it straight back out. Very quickly, push the adaptor through the same hole while the rubber is still hot. After a few moments, the rubber will have cooled and set to make a good seal around the adaptor. Now it's time to test it.
(Alternative method (which I have not tried) is to freeze the rubber bung (24 hours in your icebox), and then drill it with a narrow bit.)
Stand the empty bottle right way up in a bucket, push in the bung and connect your pump. Give several pumps. The bottle should creak and shift slightly as the pressure builds. Stop and listen. Can you hear a hiss? Quiet, high-pitched hisses are fine, because the bottle does not need to hold pressure very long. Louder, deeper hisses are also not usually cause for panic, but if you can't cure them with extra sealant you will have to pump very fast to launch your rocket.
Pump some more. And more. It should not take long, but after about ten to twenty pumps (depending on pump and bottle), the bung will suddenly shoot out of the bottle with a bang. Your basic rocket is finished.
Launching the Water Rocket.
Launching a water rocket is an outdoors activity, as they have been known to go up to 100m high, and this Researcher lost a couple a street or two away from the schoolyard they were launched from.
Half-fill your bottle with water, insert the bung, and prop it up with the bricks (upside down now, so that the water shoots downwards). You should have bricks all round the bottom to stop it slipping, and the bottle should not be tilted more than 30 degrees from the vertical (look at a clock face - the rocket should not launch 'before' 11am or 'after' 1pm), preferably pointing slightly into any wind-blowing.
Attach the pump, and go for it. Do not lean over the bottle! You will see the air bubble into the bottle and feel the pressure building up. It will not take as long as the test because there is less space for the air to fit. When the pressure pushes the bung out, it will also push the water out.
Remember Newton? As the water rushes down and out, the bottle whizzes up and away. Rapidly - that's why you didn't lean over it. Water rockets have been known to reach altitudes of 100 metres (300 feet) - they aren't kiddy toys!
Recovering the Water Rocket
Now that it's empty, the bottle is fairly light, so it doesn't need a parachute, but it can still give people underneath a nasty crack if they aren't paying attention. Watch carefully as it falls, just in case a gust of wind carries it away.
Improvements to the Water Rocket
You will have noticed that your bottle tumbled as it flew. Adding fins made of card or cut from another bottle will help it fly truer and further. Try streamlining it with a nose-cone made of card, or carved from polystyrene. Fix all these with duck tape, epoxy or hot-glue. Remember, you are not allowed to have exposed metal on the outside of any amateur rockets flown in the UK or USA (that's the law).
Can you create a recovery system? Parachutes, rotors and fixed or folding wings have all been used to keep them in the air longer.
You will also find that adjusting the amount of water in your rocket will change the range, but I'll leave that up to you to investigate.