Not sure if you are joking or not..What does ETOPS have to do with Spaceships? C'mon mate, you know you are just being silly. Anyway the principal applies to aircraft too, regardless of ETOPS, you carry what you need, that includes contingency fuel.
Your 777 has just a *little* more endurance than my bird- our average mission length was 1.6 hours. You forget that you can never have too much fuel, according to the old maxim. We would tank, after takeoff, to a gross greater than max takeoff weight.
And your bird has two engines. I had only one, and once my generator failed. I was about 200 NM short of Stornoway, so, it was one of those experiences that the PvP type guys so treasure. Hottest approach I've ever flown.
Someday, you whippersnappers, with all your flight data and comms on the same physical net, and with only two engines, is going to follow the computer into the drink.
Oh. Wait. The Air France bird that went down off Brazil, because y'all forgot how to fly manually. Or the 777's with battery fires, Or, (insert maniacal told you so laughter)...