The SM-68A/HGM-25A Titan I missile was this country’s first multi-stage ICBM, deployed in the early 1960s and retired in 1965. The Titan I was unique among the Titan models in that it used RP-1 (Rocket Propellant 1) and liquid oxygen (LOX) for the fuel and oxidizer, respectively. All subsequent versions used storable propellants instead. Originally designed as a backup in case development of the Air Force's SM-65 Atlas missile ran into problems, the Titan was deployed anyway even though Atlas proved successful. This was done to rapidly increase the number of missiles on alert and because the silo-based Titans were more survivable than the early Atlas systems. In May 1955, Air Materiel Command invited contractors to submit proposals and bids for the Titan I missile, formally beginning the program. In September 1955, The Glenn L. Martin Company (which became The Martin Company in 1957) secured the contract and in early October the Air Force's Western Development Division was ordered to start work. The Titan I was initially designated as a bomber aircraft (B-68), but was later designated SM-68, and finally HGM-25A in 1962. Although the Titan I was stored in an underground silo, its propellants had to be loaded just prior to launch and the missile had to be lifted by an elevator to grade level for launching. A Titan I complex contained an entry portal, control center, powerhouse, terminal room, two antenna silos for guidance radar antennas, and three launchers in close proximity, each composed of: three equipment terminals, three propellant terminals, and three missile silos--all connected by an extensive network of tunnels. Both antenna terminals and all three launchers were isolated with double door blast locks, the doors of which could not be open at the same time. This was to ensure that if there was an explosion in a missile launcher or the site was under attack, only the exposed antenna and/or missile silo would be damaged. The launch crew was composed of a missile combat crew commander, missile launch officer (MLO), guidance electronics officer (GEO), ballistic missile analyst technician (BMAT), and two electrical power production technicians (EPPT). There were also a cook and two Air Police. During normal duty hours there was also a site commander, site maintenance officer, site chief, job controller/expediter, tool crib operator, power house chief, three pad chiefs, three assistant pad chiefs, another cook, and more air police. Additionally, there could be a number of electricians, plumbers, power production technicians, air conditioning technicians, and other specialists when maintenance was being performed. These were by far the most complex, extensive, and expensive missile launch facilities ever deployed by the USAF. They were likened to subterranean cities. Three such complexes, at least 17 miles apart but usually 20-30 miles, constituted a squadron (nine missiles total). Following launch of the first missile from a complex, the other two could reportedly be fired at 7-1/2 minute intervals. The Titan I missile utilized radio-inertial command guidance. The missile was controlled by an autopilot which was informed of the missile's attitude by a rate gyro assembly. During the first minute or two of flight, a pitch programmer put the missile on the correct path. From that point, a guidance radar tracked a transmitter on the missile. The guidance radar fed missile position data to the missile guidance computer in the Launch Control Center, and the guidance computer used the tracking data to generate instructions which were encoded and transmitted to the missile. Guidance commands continued for the stage 1 burn, the stage 2 burn, and the vernier burn, ensuring the missile was on the correct trajectory and terminating the vernier burn at the desired velocity. The last thing the guidance system did was determine if the missile was on the right trajectory and pre-arm the warhead which then separated from the second stage. The “business end” of the Titan I was an AVCO Mk 4 re-entry vehicle containing a W38 thermonuclear warhead. The Mk 4 RV also deployed penetration aids in the form of mylar balloons which replicated the radar signature of the Mk 4 RV. The complexity of the Titan I system, combined with its relatively slow reaction time–fifteen minutes to load propellants, followed by the time required to raise and launch the first missile–plus its vulnerability just prior to liftoff, led to its replacement by the Titan II missile, also built by The Martin Company.