complexity of the RS-25 engines, following each flight they required removal for thorough inspection and meticulous maintenance. Prior to the Block II engines its primary engine component, the turbopump, had to be removed, disassembled, and overhauled after each use. The toxic propellants used for the OMS/RCS thrusters required special handling, during which time no other activities could be performed in areas sharing the same ventilation system. This increased turn-around time. The launch rate was significantly lower than initially expected. While not reducing absolute operating costs, more launches per year gives a lower cost per launch. Some early hypothetical studies examined 55 launches per year (see above), but the maximum possible launch rate was limited to 24 per year based on manufacturing capacity of the Michoud facility that constructs the external tank. Early in shuttle development, the expected launch rate was about 12 per year. Launch rates reached a peak of 9 per year in 1985 but averaged 4.5 for the entire program. When the decision was made on the main shuttle contractors in 1972, work was spread among companies to make the program more attractive to Congress, such as the contract for the Solid Rocket Boosters to Morton Thiokol in Utah. Over the course of the program, this raised operational costs, though the consolidation of the US aerospace industry in the 1990s meant the majority of the Shuttle was now with one company: the United Space Alliance, a joint venture of Boeing and Lockheed Martin. Cultural issues and problems Some researchers have criticized a pervasive shift in NASA culture away from safety in order to ensure that launches took place in a timely fashion, sometimes called "go fever". Allegedly, NASA upper-level management embraced this decreased safety focus in the 1980s while some engineers remained wary. According to sociologist Diane Vaughan, the aggressive launch schedules arose in the Reagan years as an attempt to rehabilitate America's post-Vietnam prestige. The physicist Richard Feynman, who was appointed to the official inquiry on the Challenger disaster, wrote in his report that working NASA engineers estimated the risk of mission failure to be "on the order of a percent", adding, "Official management, on the other hand, claims to believe the probability of failure is a thousand times less. One reason for this may be an attempt to assure the government of NASA perfection and success in order to ensure the supply of funds. The other