NASA engineers began designing a spacecraft much different from those used during the Apollo program. Apollo capsules and command modules were launched inside long cylindrical rockets. The thrust needed to get these vehicles off the ground was through the center of gravity of each rocket. The rockets were fueled by liquids: kerosene and liquid hydrogen-oxygen.
The shuttle design was completely different. At first engineers hoped to develop a fully reusable vehicle. Budget constraints soon made it obvious that this was not going to be possible. NASA designed a three-part vehicle for the shuttle:
- A reusable space plane called an orbiter
- An expendable external liquid fuel tank for the orbiter's three main engines
- A reusable pair of external rocket boosters containing a powdered fuel.
Figure 4.1 shows the major components of the space shuttle design. The launch sequence called for the fuel tank and the rocket boosters to be jettisoned away from
FIGURE 4.1 Space shuttle components
The orbiter holds the crew compartment and payload bay. (See Figure 4.2 for a typical orbiter layout.) The payload bay measures sixty feet by fifteen feet. The shuttle was designed to transport the orbiter into space 115 to 690 miles above the Earth's surface. This is considered low-Earth orbit or LEO.
The orbiter had to be capable of maneuvering while in space and during landing. The early designs were based on the Air Force's X-series of high-performance aircraft. Unlike the Apollo capsules, the orbiter was intended to be reusable. It had to land on the ground, rather than splash down into the ocean. At first engineers included jet engines on the orbiter for use within Earth's atmosphere. These proved to be too expensive and too heavy for the structure and were eliminated. Instead, the orbiter was designed to glide through the air to its landing site.
The orbiter was built to carry a crew of seven under normal circumstances, for a typical mission time of seven days in space. The maximum mission time for this crew number is thirty days (assuming that adequate supplies have been packed). The orbiter was designed to hold up to ten people in an emergency.
FIGURE 4.2 Orbiter
One of the most difficult design problems for the orbiter was a thermal protection system that could be reused. Previous spacecraft had been well protected from the intense heat of reentry, but their thermal protection materials were rendered unusable after one reentry. At first designers hoped to cloak the orbiter in metal plates that could withstand high temperatures. This proved to be too heavy. So the orbiter was built out of light-weight aluminum, and its underside was covered with high-tech thermal blankets and tiles. More than 24,000 individual tiles had to be applied by hand. These light-weight tiles are made of sand silicate fibers mixed with a ceramic material.
The new spacecraft had to be light enough to get off the ground, but large enough to carry military payloads that weighed substantially more than what shuttle engineers had expected. The Department of Defense also wanted the shuttle to be able to fly polar orbits (that is, orbits crossing over the North and South Poles). This meant that a launching facility on the West Coast was required, so that the shuttle could launch in a southerly direction toward the South Pole. In April 1972 it was decided that the Air Force would build this facility at Vandenberg Air Force Base in California.
The primary launch facility was authorized at Kennedy Space Center (KSC) in Florida. The KSC location
FIGURE 4.3 Space shuttle launch sites
Figure 4.3 shows the original launch azimuths (angles) planned for Vandenberg and KSC. The azimuths were chosen so that launch trajectories did not cross over heavily populated areas or foreign soil. They also ensured that any parts jettisoned from the shuttle during ascent would fall harmlessly into the ocean. It was assumed that the shuttle would land at Vandenberg, KSC, or the Air Force's White Sands Testing Facility in New Mexico.
The Department of Defense insisted that the shuttle be designed to return to Vandenberg after only one polar orbit. This was a technological challenge because it meant that the shuttle had to fly more than 1,000 miles to the east during reentry. Engineers call this the "crossrange requirement." In order to meet this requirement the shuttle was given delta wings (symmetrical triangular wings designed for subsonic and supersonic flight) and an enhanced thermal protection system.
NASA had to meet the design demands of the military in order to keep the project alive. However, this added substantially to the development costs for the spacecraft.
TABLE 4.1 Space shuttle statistics
| TABLE 4.1 | ||
|---|---|---|
| Space shuttle statistics | ||
| Overall shuttle | Orbiter | |
| SOURCE: "Table," in Space Transportation System, National Aeronautics and Space Administration, Kennedy Space Center, August 31, 2000, http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts_overview.html#sts_overview (accessed January 31, 2006) | ||
| Length | 184.2 feet | 122.17 feet |
| Height | 76.6 feet | 56.67 feet |
| Wingspan | — | 78.06 feet |
| Approximate weight | ||
| • Gross lift-off, which will vary depending on payload weight and onboard consumables | 4.5 million pounds | — |
| • Nominal end of mission landing with payload, which will vary depending on payload return weight | — | 230,000 pounds |
| Thrust (sea level) | — | |
| • Solid rocket boosters | 3,300,000 pounds of thrust each in vacuum | — |
| • Orbiter main engines | — | 393,800 pounds of thrust each at sea level at 104 percent |
| Cargo bay | ||
| • Length | — | 60 feet |
| • Diameter | — | 15 feet |
The original date for the first space shuttle launch was to be March 1978. This date was postponed several times due to budget and equipment problems. The shuttle's main engines and thermal protection tiles proved to be particularly troublesome. In 1979 President Jimmy Carter reassessed the need for the space shuttle program and considered canceling it. Historians believe he decided to continue shuttle development because the United States wanted to launch intelligence satellites to monitor the Soviet Union's nuclear missile program (http://anon.nasa-global.speedera.net/anon.nasa-global/CAIB/CAIB_lowres_chapter1.pdf.) The White House and the Congress put their support behind the space shuttle. In early 1981 NASA declared that development was complete. The shuttle was "finished" and was only 15% over its original budget.
Table 4.1 lists major design parameters of the shuttle and the orbiter. Figure 4.4 shows various views of the space shuttle orbiter.
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