I want steam powered dirigibles in my world around the year 1700. As this is an alternate world, a new gas with the same essential properties as Hydrogen, apart from the whole exploding aspect seems doable. I still need these things powered though. It seems easy enough to have someone come up with a Watt like compressor earlier, but does anyone have any thoughts on how to make a vacuum steam engine plausible? In my world, Europe is altered. Russia is the great power, challenged by France and Germany.
Vacuum engines are kind of puny. They can use 1 atmosphere pressure maximum to work. The main piece of tech needed for a sweet steam engine is metallurgical tech capable of making a boiler and containing the steam pressure and letting it out to do work. Many atmospheres of pressure accumulate in the boiler and you can use that to do serious work, like powering a train. But it is metal, so the boiler is heavy and thus less than ideal for an airship.
In your world the people can make a film that is super strong and very light. Also fireproof. Maybe it is made from the bodies of sky jellyfish that fall to earth. Of course my shorts are made of that stuff because it is also more or less see-through and my underwear has very funny prints. But you can make your sky vessels from it.
Here is le plus awesome: you use the float balloon as the boiler! Deflated, the balloon is full of water. Heat it up and it fills with steam. At maximal size it occupies more volume than the same volume of air, and so is buoyant. But you can keep heating it. It won't get bigger because this film doesn't stretch, but the steam gets hotter and hotter and the pressure rises higher and higher.
You do not need a new element besides hydrogen; @AlexP will stop grimacing. All you need is some unobtanium film and then you can play with physics.
Benefits 1: Steam engine tech. You can release high pressure steam from the balloon to do work on your airship. Propellers of course, or some other powered means of propulsion you dream up. Also steam cannons, and fast-cooking stew, and you can press your uniforms to look snazzy. Also, if you really crank the pressure up, you can release a burst of steam and propel the ship like a rocket engine. Fold up the propellors before you do that because they will slow you down.
2: If you run out of water your balloon will still work with air. You will be more buoyant because air is lighter but you have become a hot air balloon. You can still release the hot air to do work, like steam.
3; Balloon spinnaker. If you have a lot of pressure and suddenly inflate a second balloon you will suddenly get a lot of lift. Your airship balloons can launch like helicopters or surge higher into the sky to avoid anti-aircraft guns.
4: The film is transparent, so if you (are evil) and want to put people in the balloon you will be able to see them swimming around in there. This could be useful for the mwa-ha-ha factor because you will have to figure out some way to get the captives out before it gets too hot in there.