Engineering
Delta IV and Delta IV Heavy sets itself on fire right before takeoff. This is expected behaviour and looks alarming but it's OK, they know it's going to happen and they've made sure the flames won't damage anything.
The usual explanation is that Delta IV runs hydrogen through the engines early on. The rocket has liquid hydrogen and liquid oxygen on board and they don't want excess oxygen to build up in the engines because oxygen is very chemically reactive and could damage the engine. So they run excess hydrogen through the rocket, especially early on. This hydrogen burns in a dramatic looking (but not actually very dangerous) fireball.
But then why don't other rockets set themselves on fire? The Shuttle, Ariane 5, Japan's H-II and China's Long March 5 are all using hydrogen on their first stages but don't set themselves on fire. I get that rockets using kerosene won't have the same fireball issue but why don't all hydrogen fueled rockets have a fireball?
I feel like it's been explained a hundred times but with only half an explanation. What does Ariane 5 do differently to Delta IV that means one has a fireball and the other doesn't?
