Thanks for that explanation, Боб! In English, "going to" is possible in every example you gave, but in such cases it expresses a предсказание (prediction) about something that will happen in the future, rather than a conscious plan/intention:
"In about 5 billion years, the Sun is going to expand and completely swallow the Earth."
"Run for your lives -- the dam is gonna break!"
"Oh, Jesus, amannah puke -- I shouldn't have followed beer with vodka..."
I would assume that in Russian, a construction with собираться is impossible in all these cases -- in the first two sentences, because there's no conscious agent as the subject; in the third one because vomiting is usually not a matter of free will.
ЗЫ Although in the case of bulemic supermodels who throw up on purpose, I would guess it's possible to say "Я собираюсь вызвать рвоту" (lit., "I'm preparing to induce vomiting") or something like that? However, in the context of a person who's seasick or drunk, AFAIK you would normally use an impersonal verb in the 3rd-person-singular with the "logical subject" in the accusative: Господи, меня/её/нас сейчас вырвет -- принести ведро, скорее! ("Ohmygod, I'm/she's/we're about to throw up -- quick, bring a bucket!")