e.g. was just reading about some cold war era missiles that from launch command needed 30 minutes to be able to be launched.
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short answer: limits of engineering and fuel technology.
a lot of the early ICBMs were built using liquid fuel systems, and specifically relied on Liquid Oxygen (LOX). They were effectively outgrowths of the German V2 missiles from ww2.
Oxygen is only a liquid below -180 Celsius (roughly -300F). Keeping it this cold requires large refrigeration systems, far too big to install in the missile, so it is impractical to keep the LOX in the missile until you need to use it.
Thus, it was kept in a insulated tank system in the silo, until the ready order was given, when it was pumped into the missile. this is what caused the intended delay between order and launch, often referred to as “fuelling” (in many designs, their are other cryogenic fuels that need to be added alongside the LOX as well.)
as the cold war went on, new rocket fuels were discovered that avoided these issues, being either liquids that were stable at room temperature, or solid fuels. Most modern IBCMs are of these newer types, which is why they can react to a launch order much quicker. However, these new fuels often came with their own handling issues (some of the Russian liquid fuels were very toxic and dangerous to the crews that worked on them, others are safe to handle but less powerful and limit range and payload, etc).
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