Shuttle was originally billed as a space vehicle that would be able to launch once a week and give low launch costs through amortization. Development costs were expected to be recouped through frequent access to space. These claims were made in an effort to obtain budgetary funding from the United States Congress. Beginning in 1981, the space shuttle began to be used for space travel. However, by the mid-1980s the concept of flying that many shuttle missions proved unrealistic and scheduled launch expectations were reduced 50%. Following the Challenger accident in 1986, missions were halted pending safety review. This hiatus became lengthy and ultimately lasted almost three years as arguments over funding and the safety of the program continued. Eventually the military resumed the use of expendable launch vehicles instead. Missions were put on hold again after the loss of Columbia in 2003. Overall, 135 missions were launched during the 30 years after the first orbital flight of Columbia, averaging approximately one every 3 months. Costs Some reasons for the higher-than-expected operational costs were: NASA secured funding from the US Air Force's budget in exchange for USAF input to the design process. In order to fulfill the USAF's mission to launch payloads into polar orbit, the USAF insisted on a very large cross-range requirement. This necessitated the Shuttle's huge delta wings, which are far larger than the stub wings of the original design. Besides adding drag and weight (almost 20 percent), the excessive number of heat tiles needed to protect the delta wings added greatly to maintenance costs, besides increasing operational risks such as resulted in the Columbia disaster. At Vandenberg Air Force Base the USAF duplicated the entire infrastructure needed to launch and service the Space Shuttle, at a cost of over 4 billion dollars. Following the Challenger explosion, the facility was dismantled after never having launched a single Shuttle mission. Aerospace engineer Robert Zubrin describes the Shuttle as having been designed "backwards" in that the Orbiter, the harder-to-recover portion, is made recoverable, while part of the booster (the liquid fuel tank) is thrown away even though it is easier to recover since it does not fly so high or fast. Maintenance of the thermal protection tiles was a very labor-intensive and costly process, with some 35,000 tiles needing to be inspected individually and with each tile specifically manufactured for one specific slot on the shuttle. Due to the