The Titan I was a multi-stage intercontinental ballistic missile whose stages separated after their engines ran out of fuel, reducing weight in flight so it could reach a longer target range and carry a heavier payload. Its first stage used two Aerojet LR87-AJ-3 rocket engines at liftoff, delivering 300,000 pounds of thrust and burning for 134 seconds. After that stage disconnected, a single Aerojet LR91-AJ-3 rocket engine powered the second stage, producing 80,000 pounds of thrust and burning for 156 seconds to boost the missile. When the second stage fell away, two small vernier engines made final corrections to its trajectory before burning out. The re-entry vehicle carrying the warhead reached an altitude of 541 miles above the Earth's surface at apogee before falling to its target. The engines used liquid oxygen and kerosene as propellants, but these were dangerous and difficult to handle, and because the liquid oxygen had to be kept refrigerated at -195 degrees C and both fuels could only be pumped aboard just before launch, the time from launch command to liftoff was twenty minutes.