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What does delay element mean?
Delay elements are special slow-burning propellants found in rocket fuel that continue to burn after the initial propellant that launched the rocket is used up. Sometimes they are designed to ignite a number of seconds after the first store of propellant is depleted. Most of the time, when the rocket is launched, both of these propellants are lit at the same time. One of the propellants burns very quickly to produce the big upward force a rocket needs to get into the air, and this runs out quickly. When this is gone, the only propellant left is the delay element, which burns slowly for a longer time and allows the rocket to coast through the air.