safe featured a separate lock for each operator, who unlocked it using a combination known only to themself. The safe contained a number of paper envelopes with two letters on the front. Embedded in the thirty-five letter code sent from HQ was a seven-letter sub-code. The first two letters of the sub-code indicated which envelope to open. Inside was a plastic "cookie", with five more letters written on it. If the cookie matched the remaining five digits in the sub-code, the launch order was authenticated. The message also contained a six-letter code that unlocked the missile. This code was entered on a separate system that opened a butterfly valve on one of the oxidizer lines on the missile engines. Once unlocked, the missile was ready to launch. Other portions of the message contained a launch time, which might be immediate or might be any time in the future. When that time was reached, the two operators inserted keys into their respective control panels and turned them to launch. The keys had to be turned within two seconds of each other, and had to be held for five seconds. The consoles were too far apart for one person to turn them both within the required timing. Successfully turning the keys would start the missile launch sequence. First, the Titan II's batteries would be charged up completely and the missile would disconnect itself from silo power. Then the silo doors would slide open, giving a "SILO SOFT" alarm inside the control room. The guidance system of the Titan II would then configure itself to take control of the missile and ingest data to guide the missile to the target. Subsequently, main engine ignition would occur. Thrust would be allowed to build for a few seconds, then the supports holding the missile in place inside the silo would be released using pyrotechnic bolts, allowing the missile to lift off. Development The Titan rocket family was established in October 1955, when the Air Force awarded the Glenn L. Martin Company a contract to build an intercontinental ballistic missile (ICBM). It became known as the Titan I, the nation's first two-stage ICBM and first underground silo-based ICBM. The Martin Company realized that the Titan I could be further improved and presented a proposal to the U.S. Air Force for an improved version. It would carry a larger warhead over a greater range with better accuracy