Technically it's a pulse jet, but let's call a fart machine a fart machine.
British inventor Colin Furze is building a gigantic fart machine and fake butt to put it behind so that he can point it at France. Why? I've reached out in an email asking exactly that, but have not had a response yet. For now let's just work under the assumption that it's because farts are funny, so giant farts should be even funnier.