Storm in a Teacup: The Physics of Everyday Life(10)
When you look at a steam engine, the main feature is the long cylinder of the “engine” itself, stretching from the cabin to the funnel. I’d never really thought about what was in there, but it’s full of tubes. The tubes are carrying the hot gas from the firebox through the engine, and this is the kettle. Most of the space around the tubes is taken up by water, a giant bath of bubbling, boiling liquid. As this is heated by the tubes, it produces steam, hot water molecules that are zooming about in the space right at the top of the engine at very high speeds. This is what most of the steam engine is: furnace and kettle, producing vast clouds of hot water vapor. This dragon isn’t breathing fire, it’s breathing billions of energetic molecules, all whizzing about at gigantic speeds but trapped in the engine. The temperature of that gas is about 350°F and the pressure in the top of the kettle is about ten times as high as atmospheric pressure. The molecules are thumping hard against the walls of the engine, but they can only escape after being put to work.
We climbed down from the cab and walked up to the front. The towering engine, the half-ton of coal, the giant kettle, and the human teamwork were all in service to what we found there: two cylinders containing pistons, each about 20 inches in diameter and 28 inches long. It’s down here at the front, dwarfed by the dragon above, that the real work is done. The hot, high-pressure steam is fed into one cylinder at a time. The atmospheric pressure on the other side of the piston is no match for the ten atmospheres that the dragon has breathed out. The hammering molecules shove the piston along the cylinder, and then are finally released to the atmosphere with a satisfied “chuff.” This is what you’re hearing when a steam engine’s familiar “chuff, chuff, chuff” comes toward you. It’s the release to the atmosphere of water vapor whose work is done. The piston drives the wheels, and the wheels grip the rails and drag the carriages. We know that steam engines need vast quantities of coal to keep them going, but almost no one talks about the water used on every journey. The half-ton of coal that is shoveled into this engine on each trip is used to convert 1,200 gallons of water to gas, and then that gas pushes on a piston and is lost to the atmosphere one “chuff” at a time.?
Finally it was time to leave the engine and get back in one of the carriages to be carried home. The return journey felt different. The billows of steam whooshing past the windows had made their contribution to our excursion. Instead of appearing loud and intrusive, the engine pulling us along seemed relatively quiet and calm considering what was going on inside it. It would be lovely if someone could make a glass steam locomotive one day, so that we could all see the beast at work.
The steam revolution in the early 1800s was all about using the push of gas molecules to do something useful. All you need is a surface with gas molecules hitting one side harder than on the other. That push could lift the lid of a pan as you cook, or it could be used to transport food and fuel and people, but it comes from the same basic principles. We don’t use steam engines anymore, but we do still use that push. A steam engine is technically an “external combustion engine” because the furnace is separate from the kettle. In a car engine, the burning happens in the cylinder—gasoline burns right next to the piston and the burning itself produces hot gas to shove the piston along. That’s classed as an internal combustion engine. Every time you get into a car or a bus, you’re being carried along by the push of gas molecules.
It’s easy to play with the effects of pressure and volume, especially if you can find a wide-necked bottle and a hard-boiled egg with its shell taken off. The neck of the bottle needs to be just a bit narrower than the egg, so that the egg will perch on top of it without falling in. Light some paper, drop it into the bottle, let it burn for a few seconds, and then put the egg back on top. After a while, you’ll see the egg squeeze itself down inside the bottle. That’s a bit weird, and it’s inconvenient that now you have an egg in a bottle and it won’t come out. There are a few solutions, but one of them is to turn the bottle upside-down so that the egg is sitting in the neck, and then run the bottle under a hot tap. After a while, the egg will come whooshing out.
The game here is that you have a fixed mass of gas (in the bottle) and a way to tell whether the pressure inside is higher or lower than the pressure of the atmosphere. If the egg is blocking the neck, the volume of gas inside is fixed. If you increase the temperature by setting fire to something, the pressure inside will increase and air will escape around the sides of the egg (if the egg is sitting on top). When it cools down again, the pressure inside will decrease (since the volume is fixed) and the egg will be pushed inside, because the push from outside is now greater than the push back from inside. You can get the egg to move just using the heating and cooling of air in a container with a fixed volume.
The high pressures in a steam engine are controlled and stable, ideal for pushing on pistons and making wheels turn. But that’s not the end of it. Why waste energy on intermediate stages between the gas and the wheels? Why not just let the hot high-pressure gases shove your vehicle forward directly? That’s how guns, cannons, and fireworks have always worked, although the early ones were all notoriously unreliable. But by the early 1900s, technology and ambition had moved on. Along came the rocket, the most extreme form of direct propulsion ever invented.
It wasn’t until after the First World War that the necessary technology reached any degree of reliability, but by the 1930s you could launch a rocket that would probably go in the right direction and probably wouldn’t kill anyone. Most of the time. As with many new technologies, inventors made it work before anyone knew what to do with it. And out of the fertile pond of enthusiastic human inventiveness came something very new and modern-sounding and utterly doomed: rocket post.