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The last remaining debate was over the nature of the boosters. NASA had been looking at no less than four solutions to this problem, one a development of the existing Saturn lower stage, another using "dumb" pressure-fed liquid fuel engines of a new design, and finally either a large single solid rocket, or two (or more) smaller ones. The decision was eventually made on the smaller solids due to their lower development costs (a decision that had been echoed throughout the whole Shuttle program). While the liquid fueled systems provided better performace and enhanced safety, delivery capability to orbit is much more a function of the upper-stage performance and weight than the lower. The money was simply better spent elsewhere.

With a baseline project now gelling, NASA started to work though the process of obtaining stable funding for the five years the project would take to develop. Here too they found themselves increasingly backed into a corner.

Instead they looked to reduce the year-to-year costs of development to a stable figure. That is, they wished to see the development budgets spread out over several more years. This is somewhat difficult to do--you can't build half a rocket. The result was another intense series of redesigns in which the re-usable booster was eventually abandoned as impossible to pay for. Instead a series of simpler rockets would launch the system, and then drop away for recovery. Another change was that the fuel for the shuttle itself was placed in an external tank instead of internal tanks from the previous designs. This allowed a larger payload bay in an otherwise much smaller craft, although it also meant throwing away the tankage after each launch.

Spacelab
Spacelab is a microgravity laboratory flown into space on the Space Shuttle. It consists of a large cylindrical main laboratory that is flown in the rear of the Space Shuttle cargo bay, connected to the crew compartment by a long tube. In April 1973 NASA and the ESA (then known as the ESRO) agreed to build a modular science package. Construction of Spacelab started in 1974 by the ERNO (represented by VFW-Fokker GmbH, later bought by MBB, and since 2003 part of EADS Space Transportation). The first lab, LM1, was given to NASA free of charge in exchange for flight opportunities for european astronauts. A second lab, LM2, was bought by NASA for its own needs. The system also included a system of external pallets for experiments in vacuum, built by

Space Shuttle Enterprise
Space Shuttle Enterprise The Shuttle Orbiter Enterprise (NASA Designation: OV-101) was the first Space Shuttle built for NASA. It was initially constructed without engines or a functional heat shield and was therefore not capable of space operations without a refit. It was intended to be the second space shuttle to fly after the Space Shuttle Columbia even though it was built first, however, it was found to be cheaper to refit a test article (STA-099) into the Space Shuttle Challenger. Originally planned to be called Constitution, the test vehicle was renamed following a write-in campaign after the starship featured on the television show Star Trek, which in turn was named for the various ships named USS Enterprise. Amusingly, in one of the subsequent Star Trek movies

The Air Force relucantly agreed, but only after demanding a large increase in capability to allow for launching their projected spy satellites (mirrors are heavy). These were quite large, weighing an estimated 40,000 lbs, and needed to be put into polar orbit, which requires more energy to get to than the more common LEO. And since the AF also wanted to be able to abort after a single orbit (as did NASA), and land at the launch site (unlike NASA), the spacecraft would also require the ability to manuver significantly to either side of its orbital track to adjust for the launching point rotating away from it while in polar orbit - in a 90 minute orbit Vandenberg would move over 1,000 miles, whereas in a "normal" equatorial orbit NASA needed the range would be less than 400. This large 'cross-range' capability meant the craft had to have a greater lift to drag ratio than originally planned. This required the addition of bigger, heavier wings. America's first space station, the 75 ton Skylab was launched May 14, 1973 by a two-stage version of the Saturn V booster (the SL-1 mission). Severe damage was sustained during launch, including the loss of the station's micrometeoroid shield/sun shade and one of its main solar panels. Debris from the lost micrometeroid shield further complicated matters by pinning the remaining solar panel to the side of the station, preventing its deployment and thus leaving the station with a huge power deficit. The station underwent a extensive repair by the first crew launched on May 25, 1973 (the SL-2 mission) atop a Saturn 1B. Two additional missions followed on July 28, 1973 (SL-3) and November 16, 1973 (SL-4) with stay times of 28, 59, and 84 days, respectively. The last Skylab crew returned to Earth on February 8, 1974.

Skylab was actually the refitted S-IVB third stage of a Saturn V booster, a leftover from the Apollo program originally intended for one of the canceled moon landing missions (Apollos 18-20). A product of the Apollo Applications program (a program tasked with finding long-term uses for Apollo program hardware), Skylab was originally planned as a minimially-altered S-IVB to be launched on a Saturn IB rocket. The small size of the IB would have required Skylab to double as a rocket stage during launch, only being retrofitted as a space station once on-orbit. With the cancellation of Apollo missions 18-20 a Saturn V was made available and thus the "Wet Workshop" concept, as it was called, was put aside and Skylab was launched dry and fully outfitted. Skylab's grid flooring system is a highly visable legacy of the wet workshop concept.

Following the last mission the Station was positioned in a parking orbit expected to last at least 8 years. Increased solar activity heating the outer layers of the earth's atmosphere and thereby increasing drag on the Station led to an early reentry on July 11, 1979. Skylab disintegrated over Western Australia and the Indian Ocean, casting large pieces of debris in populated areas (without injury). The reentry prevented any further use by the then unfinished Space Shuttle as was envisioned by some at NASA.

Two flight-quality Skylabs were built, the second, a backup, is on display at the National Air and Space Museum in Washington, DC.

NASA had conducted a series of paper-projects throughout the 1960s on the topic of reusable spacecraft to replace their expedient "one-off" systems like Mercury, Gemini, and Apollo.

Shuttle description

The Space Shuttle consists of four main components; the reuseable orbiter itself, a large expendable external fuel tank, and a pair of reusable solid-fuel booster rockets. The fuel tank and booster rockets are jettisoned during ascent. The longest the shuttle has stayed in orbit in a single mission is 17.5 days, on mission STS-80 in November 1996.

The Shuttle has a large payload bay taking up much of its length. The payload bay doors have heat radiators mounted on their inner surfaces, and so are kept open while the Shuttle is in orbit for thermal control. Thermal control is also maintained by adjusting the orientation of the Shuttle relative to Earth and Sun. Inside the payload bay is the Remote Manipulator System, also known as the Canadarm, a robot arm used to retrieve and deploy payloads. Until the loss of Columbia, the Canadarm has only been included on missions where it will be used. Since the arm is a crucial part of the Thermal Protection Inspection procedures now required for shuttle flights, it will likely be included on all future flights.

List of human spaceflights
3 Mercury Missions 4 Shenzhou Missions 5 Skylab Missions 6 Soyuz Missions 7 Space Shuttle Missions 8 Voskhod Missions 9 Vostok Missions 10 Space stations 11 Spacecraft not yet flown with crew Apollo Missions Apollo 1 - crew perished in fire during training Apollo 7 Apollo 8 - first human flight around the moon Apollo 9 Apollo 10 Apollo 11 - first human moon landing Apollo 12 Apollo 13 - explosion en route to Moon forced emergency return to Earth by using free return trajectory Apollo 14 Apollo 15 Apollo 16 Apollo 17 Apollo-Soyuz - first joint Soviet-US mission Gemini Missions Gemini 3 Gemini 4 Gemini 5 Gemini 6A Gemini 7 Gemini 8 first docking in space Gemini 9A Gemini 10 Gemini 11 Gemini 12 Mercury Missions Mercury 3 Mercury 4