|An Atlas ICBM uses 2
propellants, RP-1 and Liquid Oxygen (LOX). The RP-1 Kerosene type fuel
is aboard all time and then just prior to launch the LOX is loaded while
the rocket is still in the silo protected from the outside elements.
Once loaded with LOX the silo doors are rapidly thrown open using high pressure nitrogen (Gn2) gas to push the hydraulic fluid from the accumulators to the door cylinders. The rocket is then elevated at high speed to above ground level for launching. Loading of LOX and Countdown to Launch took less then 15 minutes.
The above view is an Atlas F just after a Simulated Launch Countdown has
been aborted and the Oxygen Boil Off Valve has opened to Vent Pressure
from the Liquid Oxygen Tank before being lowered back into the silo.