Various space shuttle designs had been evolving since the 1950s. The U.S. Air Force had examined several options based on a reusable manned space plane that could be maneuvered in flight and glided to a landing. The best-known program was called DynaSoar (short for Dynamic Soaring). The DynaSoar concept included an expendable launch vehicle to carry a space plane out of Earth's atmosphere.

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 the orbiter during the ascent phase. The rocket boosters were designed to be recovered and refilled with fuel for the next launch. The external tank was to be jettisoned above Earth's atmosphere and burn up during reentry.

The orbiter holds the crew compartment and payload bay. (See Figure 4.2 for a typical orbiter layout.) The pay-load bay measures sixty feet by fifteen feet. The shuttle was designed to transport the orbiter into space 115–690 miles above the Earth's surface. This is considered Low-Earth Orbit or LEO.

FIGURE 4.1
Space shuttle components

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 land, rather than splashdown 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. 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.

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 FIGURE 4.2
Orbite 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 was chosen to allow the shuttle to launch in an easterly direction over the Atlantic Ocean and assume an equatorial orbit (an orbit along lines of latitude near or at the equator.)

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.

FIGURE 4.3
Space shuttle launch sites

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 "cross-range 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. Most of the design work took place during the mid-1970s. This was a time of high inflation for the U.S. economy. High inflation means that the value of the dollar goes down. NASA would be designated funds in one year, but by the time those funds were received in the next year, their purchasing value had decreased.

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 U.S. wanted to launch intelligence satellites to monitor the Soviet Union's nuclear missile program. The White House and the Congress put their support behind the space shuttle. In FIGURE 4.4
Space shuttle orbiter early 1981 NASA declared that development was complete. The shuttle was "finished" and was only 15 percent 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.