>>12055>okay, suppose I want to build one of your engines in my backyard, how thick should the metal-sphere be?
It just has to be strong enough to hold a vacuum inside, while being really hot.>can I use recycled steel cans?
In principle yes, just consider that most cans are made from aluminum these days.
I'm not sure if you meant this request in earnest, but in principle this can be miniaturized, and build like a steampunk fusion reactor. Except for the small fusion bombs that essentially are the fuel, those get more complicated to make the smaller the reactor gets.
The fusion reaction is set off by a fission reaction that needs a critical mass of fissionable materials. You would need exotic materials to make that happen in a small size. For example Californium-252 reaches critical mass at 1.8 grams, and that would allow you to make a really tiny fusion bomblet for a small reactor. You have to store it in cold conditions below -15°C / 5°F to keep stable. So a really reliable freezer is a must-have to store your fuel igniter-caps.
You would place the ball into a tank filled with salt. Purge the tank with nitrogen gas before adding salt to avoid corrosion problems. You run radiator pipes through the salt and if you put water into one end of the pipe it will come out as steam on the other end. The salt is a cheap way of storing lots of heat energy in a smallish space, it's not critical and you can also use something else like quarts-sand or graphite-powder.
Even if you could manage to get Californium, i don't recommend building this as the most ambitious diy project in history, because if your vacuum seal fails it will vaporize your yard and a bunch more.