Whilst cobbling together an answer for this question, I found myself thinking about the effects of hot rocket exhaust on a rocket launching from a confined space. I assumed it would be generally considered to be a Bad Thing.
So I found myself watching this video of a Minuteman launch, and it looks to me like hot exhaust is shooting straight up the silo and the rocket is launching through it... there seems to be a good couple of seconds where it would be toasted, and I see no indication of exhaust being vented anywhere else. I check another video (which starts with a Titan II launch with obvious venting away from the rocket, which I had assumed to be essential and standard on silos) and I can see a moment where a Minuteman, clear of the silo, is apparently wreathed in flames.
Am I interpreting this correctly? Was the Minuteman made specially flame retardant, or am I a) overestimating the hazards of rocket exhaust and b) underestimating the toughness of a solid-fuelled ICBM?