steam turbine car

by Jeff Bindon

We live in a world dominated by machines. If something is not a machine, like a motor car, a refrigerator or a sewing machine, then it is something that is made and processed by a mechanical device, like the clothes we wear and the food we eat. Mechanical Science and Technology is thus important. Exposure to it, at school and at home, is vital for all young people as part of their education and the appreciation of the world around them.
It is also important to learn that science and engineering can be fun and exciting.
The presentation of something mechanical in a school or at home is difficult, particularly when it comes to anything 'hands on' since specialist facilities and skilled instructors are needed. The production of a machine requires powerful machine tools, a wide range of hand tools, skilled technicians and expensive materials.
To translate even some of this into a classroom is expensive and requires a high entry skill level of a participant.
When schools invest in this type of education, it unfortunately classifies participants as inferior to the academic science streams and thus the gifted seldom experience anything mechanical in their school career. To proceed with only theory is dull and uninteresting. Real learning requires that both hand and mind be involved.

Since 1990, the Department of Mechanical Engineering at the University of Natal has been involved in making Mechanical Science as easy as any other school subject. A series of educational kits have been designed which enables mechanical science and technology to enter the classroom and home. With only a pair of pliers and kitchen scissors and the low cost kit of "tin" type materials, working self powered machines can be made by anyone from 8 to 80 years old. No previous experience is needed.

Participants are given an invaluable educational experience and are exposed to every aspect of mechanical science, from the burning of fuel to the balancing of a wheel while also becoming literate in the terminology of the subject.

hero steam turbine

The Heros turbine consists of a beverage can steam boiler resting on a three-sided tin 'fire box'. It is heated from below by a methylated spirits flame burning from a wick pushed into a ZamBuk tin fuel tank. The wick is made from a fluffy mop string inside a drinking straw which is wrapped in aluminium foil to prevent it from melting. On top of the boiler is a rotor made from plastic cotton bud tubes pushed into a fish tank T piece. It spins on a short length of cotton bud tube inserted tightly into the top of the boiler.

Hero of Alexandria (Alexandria was then a Greek state), lived from about AD 20 to AD 100. He was a scientist, mathematician and engineer and wrote many learned books, on geometry and machinery.
An example is "Pneumatica", a two volume description of siphons, fountains, slot machines, fire engines, water driven organs and of course, the steam driven turbine. Whether he invented the turbine that now bears his name, or whether he even made any, are not known. But it appears in his books and perhaps if the dark ages had not put an end to progress, his contributions could have powered the industrial revolution 1500 years earlier.

How the turbine works

The formation of steam

When water is heated it gradually gets hotter (the temperature increases). When it reaches the boiling point (100 degrees Centigrade at sea level air pressure) it begins to turn into bubbles of steam (ie a vapour or a "gas"). The temperature will now stop increasing and will stay at the boiling point. The heat that is now added is called "latent heat", or the "heat of vapourisation" and does not make the water "hotter" but is actually used to convert the liquid into gas.
It is also very important to realise that steam is 1700 time larger than water. Imagine trying to leave the kitchen if eating your breakfast made you swell up until you were 1700 times larger! You would fill the whole room and would find yourself being squeezed out of the doors and windows.
That is what happens to steam, it has to try and get out of the container or boiler. If the lid is open, the steam simply wafts out. But if the lid is closed, the pressure of the steam will push the lid up until it opens just enough for the steam to leak out. If the lid is tightly held down, the pressure will get higher and higher until something gives way, either the lid is blown off or the boiler explodes.

How a jet or a nozzle works

If there is a small hole in the boiler, the pressure will only increase until the steam jets out of the hole just fast enough to exactly balance the amount of steam being boiled by the flame. The smaller the hole, the higher the pressure will become and the faster the jet will be. But why does a higher pressure cause a faster jet ? This is the same as asking why does air come out of your mouth faster when you increase the pressure in your lungs and "blow harder". Lets look at a diagram of a nozzle.

Inside the nozzle there is a high pressure and outside the nozzle there is only the low pressure of the atmosphere. The high pressure means that a large force is exerted on the air or steam on the inside and only a small force is exerted at the other end. The difference between the two forces accelarates the fluid as it moves through the nozzle. Of course, the higher the pressure, the greater the accelaration and speed.

We have seen that the pressure of steam will get higher until it jets out just fast enough to balance the amount of steam that is boiled. But this is not the whole story because as the pressure rises the steam also becomes denser, ie each each cubic millimetre weighs more & more. Thus not only does the nozzle jet faster but the steam becomes heavier as well and it is the combination of the two things that balances the nozzle flow with the amount of steam evaporated.

This is a hero turbine made from a light bulb. Light bulbs make very good boilers.

read about hero of alexandria

tin can tennis ball cannon

Butane gas can easily be ignited in a tin can to explosively fire a tennis ball.
You will need

two large empty beer cans, 450ml or bigger,
a rod shaped spark ignited fire lighter
a butane gas refill aerosol can
a syringe 10ml or larger
a fluffy tennis ball

a box cutter (Stanley knife)
a kitchen scissors or tin cutters
a large nail, 3.5 to 5 mm in diameter
a long nose pliers
a hammer

Making the Cannon

The first can forms the butane gas explosion chamber. The second one, jammed tightly onto the first, contains the tennis ball and forms the barrel of the cannon.
A hole for the gas lighter must first be made in the dish shaped base of the first can. Hammer the nail through the center of the dish. Use a light but rapid strike of the nail to avoid crushing the can. If necessary, start with a thinner nail. Insert the tip of the long nose pliers into the hole and rotate it while pressing hard. The hole will gradually increase in diameter. As it nears the right size, work slowly until the lighter rod just fits through.
Next you have to learn to cut off the ends of the can without damaging yourself or the thin metal. Place the can flat on a cloth or newspaper to prevent it slipping. Poke the point of the box cutter directly downwards through the thin metal 5mm from the edge of the lid so that the blade will not hit the wide inside of the lid. Push the knife downwards to make a small cut 4 or 5mm long. Carefully withdraw the knife, re-insert it and slightly rotate the can so that a second short cut can be made. Repeat this until only a short uncut length remains which is quite difficult to cut. Bend this back and fourth until it breaks off.
The cut end of the can is now jagged and sharp but is easily trimmed if a tin cutters or kitchen scissors is used in the right way. Hold the can in your left hand with the jagged end facing your chest. Then with the scissors in your right hand, cut off a narrow strip of the jagged metal. Cut just once round to leave behind as much as possible of the narrowed down neck which will position the tennis ball. The explosion chamber is now complete.
In the same way, cut the lid off the other can, which will form the barrel. But when trimming the jagged edge, keep on cutting off strips until the narrowed down neck has been completely removed.
The barrel must now be pressed by hand onto the open end of the explosion chamber. To make this possible, the dished end has deliberately not yet been cut off. Press evenly at first. Then press on one side and then the other so that the can goes on unevenly in small jerks. Do not press it on too far. Only 2 or 3mm is needed because the gases and ball cannot apply any force to the barrel to remove it. Finally make sure the two cans are straight. Use a ruler if necessary.
The base of the barrel must finally be cut off using the box cutter technique. The metal near the base is thicker so poke the box knife through 2mm from the edge and slice repeatedly as before and trim the jagged edge off.

Loading and Firing the Cannon

To load and fire the cannon, wet the tennis ball to prevent the hot gas from singing the fur. Push the ball down the barrel as far as it will go until it is resting on the narrowed down neck of the explosion chamber can.
The volume of butane gas needed is the volume of the combustion can divided by 31. For example, most large cans are 450ml. So 450/31 = 14 ml
Use the syringe to measure this volume. Hold the butane refill can vertical, press the nozzle of the syringe against the nozzle of the gas can. Press downwards and the gas will push the piston of the syringe upwards. Discard any excess gas delivered. Slowly inject the gas into the explosion can through the igniter hole. Use a circular motion in order to spread the gas as evenly as possible. Insert the igniter rod and wait 15 or 20 seconds for the gas to diffuse right through the air. Point the cannon appropriately and fire the igniter. There should be a popping sound as the ball is ejected.
To repeat the process, make sure the explosion chamber is flushed of spent combustion gas and replaced with fresh air by pushing the ball in and then blowing it out by mouth 2 or 3 times.

Going Further

The barrel can be made longer by fitting extenders in order to fire the ball further. Cut the lid off another can and trim it until only about 1mm of the neck remains. This will be sufficient to stiffen it and provide an entry for it to be pushed gently into the existing barrel. Apply adhesive tape to reenforce the structure. See if your cannon shoots further and add another if you feel the experiment is worthwhile.

Getting Dangerous

Instead of a tennis ball, the explosion chamber can be used to make a much louder bang and to fire part of a drinks can as a projectile. Cut a can in two and trim the jagged edge of the sealed base half. Force this onto the explosion can in place of the barrel (or make a new explosion can). Add the volume of the projectile to the gas volume calculation and fire away. But be careful of your ears and be sensible about where you fire it and where you aim it. For safety and to prevent damage to the projectile, fix thick string or fishing line to it and the explosion can.

some improvements

A lamination is a plastic strong enough to form a high pressure cannon barrel.

You will need :-
An A3 size lamination. Clear (ie without paper) if possible so you can see the flame
A good quality adhesive, insulation or double sided tape
A 20 ml syringe. Smaller will do but then you will need multiple loadings to get the 15 or so ml fuel needed (volume needed is can volume divided by 31)
A small crumpled ball of aluminum foil to just fit through the mouth of the can
Squeeze the foil and push it into the can to create the shaker mixer
Roll the A3 lamination carefully around two drinks cans and tape down.
If you have double sided tape, cut a piece slightly longer than needed and stick it down along one edge while the lamination is lying flat on the table. Allow the two ends to stick to the table to hold the lamination in place. Now peel off the backing paper and carefully roll the lamination up around the two cans and allow it to be stuck by the tape. Trim off the excess tape with a scissors.
Push out the cans and tape the laminated tube to the open end of the can

To use the Tennis Ball Cannon:

Use the syringe to measure out 20ml butane(the chemically correct amount is 440ml/31 = 14ml but the extra helps to mix it properly with the air.) Squirt it in hole made in the cannon can.
Hold the hole closed with a finger and shake so that the mixer inside stirs the air and gas
Insert the igniter and hold the barrel firmly in the left hand with the base of the can up against the base of your thumb to prevent the kick back hurting your right hand which is holding the igniter. Or you can make a hand grip.

watch it on youtube