The Titan was a huge leap in the US ICBM program. The Atlas, like the Soviet "SAPWOOD", was a 1 and 1/2 stage ICBM: All the engines were lit when they launched. When it was time to get rid of empty tanks and un-needed engines, the Atlas dropped two engines, the Soviet missile jettisoned four strap-ons. The advantage was no need to light the second stage high up in the air after it rode up there on the first stage. The TItan was a straight 2 stage rocket, stage 2 got lit after stage 1 was dropped. Now everyone expects it, but what a big deal to do it the first time. I'm really glad nobody used these things as they were intended to be used. The Titan 1 was a kerosine and LOX liquid fueled rocket, very powerful, very efficient, but it had to be stored empty and fueled VERY quickly, just before it would be launched. Just like the Atlas. The Titan II was fueled with nitrogen tetroxide and Aerozine 50 (a 50/50 mix of hydrazine and UDMH), two toxic, exotic and dangerous liquids, which could be stored in the missile, without refrigeration. They were also "hypergoilic": they ignited on contact.

Plenty more at: en.wikipedia.org/wiki/Titan_%28rocket_family%29