The Space Shuttle concept had its genesis in the 1960s, when the Apollo lunar landing spacecraft was in full development but had not yet flown. From the earliest days of the space program, it seemed logical that the goal of frequent, economical access to space might best be served by a reusable launch system. In February, 1967, the President's Science Advisory Committee lent weight to the idea of a reusable spacecraft by recommending that studies be made "of more economical ferrying systems, presumably involving partial or total recovery and use."
In September, 1969, two months after the initial lunar landing, a Space Task Group chaired by the Vice President offered a choice of three long-range plans:
In March, 197O, President Nixon made it clear that, while he favored a continuing active space program, funding on the order of Apollo was not in the cards. He opted for the shuttle-tended space base as a long-range goal but deferred going ahead with the space station pending development of the shuttle vehicle. Thus the reusable Space Shuttle, earlier considered only the transport element of a broad, multi-objective space plan, became the focus of NASA's near-term future.
The Space Shuttle Design
The embryo Shuttle program faced a number of evolutionary design changes before it would become a system in being. The first design was based on a "fly back" concept in which two stages, each manned, would fly back to a horizontal, airplane-like landing. The first stage was a huge, winged, rocket-powered vehicle that would carry the smaller second stage piggyback; the carrier would provide the thrust for liftoff and flight through the atmosphere, then release its passenger-the orbiting vehicle-and return to Earth. The Orbiter, containing the crew and payload, would continue into space under its own rocket power, complete its mission and then fly back to Earth.
The second-stage craft, conceived prior to 1970 as a space station ferry, was a vehicle considerably larger than the later Space Shuttle Orbiter. It carried its rocket propellants internally, had a flight deck sufficiently large to seat 12 space station-bound passengers and a cargo bay big enough to accommodate space station modules. The Orbiter's size put enormous weight-lifting and thrust-generating demands on the first-stage design.
This two-stage, fully reusable design represented the optimum Space Shuttle in terms of «routine, economical access to space," the catchphrase that was becoming the primary guideline for development of Earth-to-orbit systems. It was,  however, less than optimum in terms of the development investment required: an estimated $10-13 billion, a figure that met with disfavor in both Congress and the Office of Management and Budget.
In 1971, NASA went back to the drawing board, aware that development cost rather than system capability would probably be the determining factor in getting a green light for Shuttle development. Government and industry studies sought developmental economies in the configuration. One proposal found acceptance: eliminate the Orbiter's internal tanks and carry the propellant in a single, disposable External Tank. It provided a smaller r, cheaper Orbiter r without substantial performance loss.
For the launch system, NASA examined a number of possibilities. One was a winged but unmanned recoverable liquid-fuel vehicle based on the eminently successful Saturn 5 rocket from the Apollo Program. Other plans envisioned simpler but also recoverable liquid-fuel systems, expendable solid rockets and the reusable Solid Rocket Booster. NASA had been using solid-fuel vehicles for launching some small unmanned spacecraft, but solids as boosters for manned flight was a technology new to the agency. Mercury, Gemini and Apollo astronauts had all been rocketed into space by liquid-fuel systems. Nonetheless, the recoverable Solid Rocket Booster won the nod, even though the liquid rocket offered potentially lower operating costs.
 The overriding reason was that pricing estimates indicated a lower cost of development for the solid booster.
Emerging from this round of design decision making was the Space Shuttle: a three-element system composed of the Orbiter, an expendable external fuel tank carrying liquid propellants for the Orbiter's engines, and two recoverable Solid Rocket Boosters. It would cost, NASA estimated early in 1972, $6.2 billion to develop and test a five-Orbiter Space Shuttle system, about half what the two-stage "fly back" design would have cost. To achieve that reduction, NASA had to accept somewhat higher system operating costs and sacrifice full reusability. The compromise design retained recoverability and reuse of two of the three elements and still promised to trim substantially the cost of delivering payloads to orbit.
The final configuration was selected in March, 1972.
The Space Shuttle Development
In August, 1972, NASA awarded a contract to Rockwell International Corporation's Space Transportation Systems Division for design and development of the Space Shuttle Orbiter. Martin Marietta Denver Aerospace was assigned development and fabrication of the External Tank, Morton Thiokol Corporation was awarded the contract for the Solid Rocket Boosters, and Rocketdyne, a division of Rockwell, was selected to develop the Orbiter main engines.
NASA divided managerial responsibility for the program among three of its field centers. Johnson Space Center, Houston, Texas, was assigned management of the Orbiter. Marshall Space Flight Center, Huntsville, Alabama, was made responsible for the Orbiter's main engines, the External Tank and the Solid Rocket Boosters. Kennedy Space Center, Merritt Island, Florida, was given the job of assembling the Space Shuttle components, checking them out and conducting launches. Because these three centers will be mentioned repeatedly in this report, they will hereafter be identified simply as Johnson, Marshall and Kennedy.
It was in an increasingly austere fiscal environment that NASA struggled through the Shuttle development years of the 1970s. The planned five-Orbiter fleet was reduced to four. Budgetary difficulties were compounded by engineering problems and, inevitably in a major new system whose development pushes the frontiers of technology, there was cost growth. This combination of factors induced schedule slippage. The initial orbital test flights were delayed by more than two years.
The first Shuttle test flights were conducted at Dryden Flight Research Facility, California, in 1977. The test craft was the Orbiter Enterprise, a full-size vehicle that lacked engines and other systems needed for orbital flight. The purpose of these tests was to check out the aerodynamic and flight control characteristics of the Orbiter in atmospheric flight. Mounted piggyback atop a modified Boeing 747, the Enterprise was carried to altitude and released for a gliding approach and landing at the Mojave Desert test center. Five such flights were made. They served to validate the Orbiter's computers and other systems. They also demonstrated the craft's subsonic handling qualities, in particular its performance in the precise unpowered landings that would be required on all Shuttle flights.
The Enterprise test flights were followed-in 1977-80-by extensive ground tests of Shuttle systems, including vibration tests of the entire assembly-Orbiter, External Tank and Solid Rocket Boosters-at Marshall. Main engine test firings were conducted at National Space Technology Laboratories at Bay St. Louis, Mississippi, and on the launch pad at Kennedy.
By early 1981, the Space Shuttle was ready for an orbital flight test program. This was carefully crafted to include more than 1,000 tests and data collection procedures. All flights were to be launched from Kennedy and terminate at Edwards Air Force Base, where the Dryden Flight Research Facility is located (actually the third flight landed at White Sands Test Facility, New Mexico, because the normally dry lakebed at Edwards was flooded). Originally intended as a six-mission program, the orbital test series was reduced to four flights:
With the landing of STS-4, the orbital flight test program came to an end with 95 percent of its objectives accomplished. The interval between flights had been trimmed from seven months to four, then three. NASA declared the Space Shuttle "operational," a term that has encountered some criticism because it erroneously suggests that the Shuttle had attained an airline-like degree of routine operation. In any event, NASA regarded all flights after STS-4 operational in the sense that payload requirements would take precedence over spacecraft testing, requiring larger crews.
After completing the orbital test in mid- 1982, NASA began the "operational phase" of the Space Shuttle program, beginning with STS-5. The STS -for Space Transportation System- sequential numbering was still in effect at that time; after STS-9 NASA changed the method of numbering missions. Thereafter each flight was designated by two numbers and a letter, such as 41-B. The first digit indicates the fiscal year of the scheduled launch (4 for 1984). The second digit identifies the launch site (1 is Kennedy, 2 Vandenberg Air Force Base, California). The letter corresponds to the alphabetical sequence for the fiscal year, B being the second mission scheduled. Here is a brief summary of the 21 missions launched from late 1982 to January, 1986:
Including the initial orbital tests, the Space Shuttle flew 24 successful missions over a 57-month period. Columbia made seven trips into space, Discovery six and Atlantis two. Challenger flew most frequently-nine times prior to its fateful last flight.
In those 24 flights, the Shuttle demonstrated its ability to deliver a wide variety of payloads; its ability to serve as an orbital laboratory; its utility as a platform for erection of large structures; and its use for retrieval and repair of orbiting satellites.
Elements of the Space Shuttle
The Space Shuttle is the principal component of a national Space Transportation System designed to accommodate not only NASA's predictable needs but also those of the Department of Defense and commercial payload sponsors. Technically speaking, transportation system hardware embraces not only the Shuttle but its Spacelab laboratory component, the upper stage propulsion units, contemplated heavy lift vehicles  and space tugs for moving payloads from one orbit to another. To provide for the broadest possible spectrum of civil/military missions, the Space Shuttle was designed to deliver 65,000 pounds of payload to an easterly low Earth orbit or 32,000 pounds to polar orbit. The following sections describe the main elements of the Shuttle system.
The Orbiter is as large as a mid-size airline transport and has a structure like that of an aircraft: an aluminum alloy skin stiffened with stringers to form a shell over frames and bulkheads of aluminum or aluminum alloy. The major structural sections of the Orbiter are the forward fuselage, which encompasses the pressurized crew compartment; the mid fuselage, which contains the payload bay; the payload bay doors; the aft fuselage, from which the main engine nozzles project; and the vertical tail, which splits open along the trailing edge to provide a speed brake used during entry and landing.
The crew compartment is divided into two levels-the flight deck on top and the middeck below. Besides working space, the crew compartment contains the systems needed to provide a habitable environment (atmosphere, temperature, food, water, the crew sleep facilities and waste management). It also houses the electronic, guidance and navigation systems.
The Orbiter crew may include as many as eight people, although generally the limit is seven. The crew consists of the commander, the captain of the ship; the pilot, second in command; and two or more mission specialists. One or more payload specialists can also be accommodated. A mission specialist coordinates activities of the Orbiter and crew in support of a given payload objective. A payload specialist may manage specific experiments. The commander, pilot and mission specialists are career astronauts assigned to the mission by NASA. Payload specialists do not come from the Astronaut Office. They are assigned, by payload sponsors in coordination with NASA.
Cargoes up to 24 tons have been carried in the payload bay. Clamshell doors on the top of the Orbiter meet along the craft's spine to enclose the bay, which is 15 feet wide and 60 feet long.
The payload bay is designed to hold securely a wide range of objects. They may include one or more communications satellites to be launched from orbit, an autonomous Spacelab for experiments in space, or cargo disposed on special pallets. To handle cargo in orbital flight, the payload bay has the 50-foot mechanical arm that is controlled from within the crew compartment. A television camera and lights mounted near the end of the arm enable the operator to see what the "hand" is doing.
Just as important as delivering cargo to orbit is recovering a satellite and bringing it back to Earth-retrieving a satellite in need of refurbishment, for example. The Orbiter can carry 16 tons of cargo back from space.
The feasibility of a reusable Space Shuttle hinges on a particularly vital requirement: protecting the Orbiter from the searing heat generated by friction with the atmosphere when the craft returns to Earth. Temperatures during entry may rise as high as 2,750 degrees Fahrenheit on the leading edge of the wing and 600 degrees on the upper fuselage, the "coolest" area. The thermal protection system devised for the Orbiter must prevent the temperature of the aluminum skin from rising above 350 degrees during either ascent or entry.
The Orbiter has four kinds of external insulation that are applied to various parts of the structure according to the temperature each is likely to experience. The craft's nose cap and the leading edges of the wings are protected with an all-carbon composite consisting of layers of graphite cloth in a carbon matrix. The outer layers are converted chemically to silicon carbide, the same material that has long been used as an abrasive in grindstones. Areas subjected to the next greatest heat are shielded with high-temperature ceramic tiles about six inches square and varying in thickness from one to five inches, depending on the protection needed. So-called "low temperature" tiles are of the same material- nearly pure glass, of which 90 percent of the volume is "air"-for use on areas requiring less protection. (Low-temperature is relative; tiles so designated can withstand a temperature of 1,200 degrees Fahrenheit.) About 3O,000 tiles, each different, are installed on each Orbiter.
Space Shuttle Main Engines
The three high-performance rocket engines in the aft section of the Orbiter fire for about the first 8 1/2 minutes of flight after liftoff. At sea level, each engine generates 375,000 pounds of thrust at 100 percent throttle.
The propellants for the engines are the fuel  (liquid hydrogen) and the oxidizer (liquid oxygen) carried in the External Tank. Combustion takes place in two stages. First, the propellants are mixed and partly burned in pre-burners. Hot gases from the pre-burners drive the high-pressure turbopumps which deliver propellants to the main injector. Combustion, once initiated by electrical igniters, is self-sustaining. Before firing, the very cold liquid propellant is allowed to flow into the system as far as the pre-burners and combustion chamber to cool the pumps and ducts so that the hydrogen and oxygen in the system will remain liquid when the engine is started.
The main engines have been throttled over a range of 65 to 104 percent of the thrust at sea level. At liftoff, they are thrusting at 100 percent. Computers command engine thrust to 104 percent as soon as the Shuttle clears the tower. They throttle to 65 percent to reduce the maximum aerodynamic loads that occur at an altitude of about 34,000 feet. Thereafter, the thrust is again increased to provide an acceleration of three times that of gravity in the last minute or so of powered flight.
The External Tank carries the propellants for the Orbiter's main engines-143,000 gallons of liquid oxygen and 383,000 gallons of liquid hydrogen, which is much lighter than a comparable volume of oxygen. Together, the propellants weigh a little more than 790 tons. Martin Marietta Denver Aerospace, Michoud, Louisiana, builds the tank, a welded aluminum alloy cylinder with an ogive nose and a hemispherical tail. It is 154 feet long and 27 1/2 feet in diameter.
Because the Orbiter and the two Solid Rocket Boosters are attached to it at liftoff, the External Tank absorbs the thrust of the combined propulsion system. It withstands complex load effects and pressures from the propellants.
The liquid oxygen tank forms the nose of the External Tank. It contains oxidizer kept liquid at a temperature of - 297 degrees Fahrenheit. A removable conical nose cap acts as an aerodynamic fairing. Inside the tank, baffles reduce sloshing and the associated control problems. The liquid hydrogen tank does not need baffles because the fuel is so light that sloshing does not induce significant forces. The liquid hydrogen tank accounts for the greater part of the External Tank. Its contents are even colder than the LOX: - 423 degrees Fahrenheit.
The intertank structure or "intertank" connects the two propellant tanks. It is a cylindrical structural section that houses instruments and receives and distributes most of the thrust load from the Solid Rocket Boosters. The front end of each booster is connected to the External Tank at the intertank midsection.
A multi-layered thermal coating covers the outside of the External Tank to protect it from extreme temperature variations during pre-launch, launch, and the first 8 1/2 minutes of flight. That insulation reduces the boil-off rate of the propellants, which must be kept at very low temperatures to remain liquid. It also is meant to minimize ice that might form from condensation on the outside of the propellant tanks.
In addition to the Solid Rocket Booster forward attachment points on either side of the intertank, three other attachment points link each booster to the aft major ring frame of the External Tank. The boosters are thus connected to the tank at four points, one forward and three aft.
Three structural elements link the Orbiter to the External Tank. A "wishbone" attachment beneath the crew compartment connects the forward end of the Orbiter to the tank. The two aft connections are tripods at the base of the External Tank.
A command from the Orbiter computer jettisons the External Tank 18 seconds after main engine cutoff, about 8 /2 minutes after liftoff. To ensure that it will travel a predictable path, a tumble system rotates the tank end-over-end at a minimum rate of two revolutions per minute. The tank breaks up upon atmospheric entry, falling into the planned area of the Indian or Pacific Ocean about an hour after liftoff. The External Tank is the only main component of the Space Shuttle that is not recovered and reused.
Solid Rocket Boosters
The two solid-propellant rocket boosters are almost as long as the External Tank and attached to each side of it. They contribute about 80 percent of the total thrust at liftoff; the rest comes from the Orbiter's three main engines. Roughly two minutes after liftoff and 24 miles down range, the solid rockets have exhausted their fuel. Explosives separate the boosters from the External Tank. Small rocket motors move them away from the External Tank and the Orbiter, which continue toward orbit under thrust of the Shuttle's main engines.
 The Solid Rocket Booster is made up of several subassemblies: the nose cone, Solid Rocket Motor and the nozzle assembly. Marshall is responsible for the Solid Rocket Booster; Morton Thiokol, Inc., Wasatch Division, Brigham City, Utah, is the contractor for the Solid Rocket Motors. Each Solid Rocket Motor case is made of 11 individual cylindrical weld free steel sections about 12 feet in diameter. When assembled, they form a tube almost 116 feet long. The 11 sections are the forward dome section, six cylindrical sections, the aft External Tank-attach ring section, two stiffener sections, and the aft dome section.
The 11 sections of the motor case are joined by tang-and-clevis joints held together by 177 steel pins around the circumference of each joint.
After the sections have been machined to fine tolerances and fitted, they are partly assembled at the factory into four casting segments. Those four cylindrical segments are the parts of the motor case into which the propellant is poured (or cast). They are shipped by rail in separate pieces to Kennedy.
Joints assembled before the booster is shipped are known as factory joints. Joints between the four casting segments are called field joints; they are connected at Kennedy when the booster segments are stacked for final assembly.
Orbital Maneuvering System
The two engine pods on the aft fuselage of the Orbiter contain maneuvering engines and their propellant-monomethyl hydrazine (the fuel) and nitrogen tetroxide (the oxidizer). Helium pressurizes the propellant tanks, and the fuel and the oxidizer ignite on contact.
Forty-four small rocket motors in the Orbiter's nose and aft section maneuvering system pods allow adjustments of the vehicle's attitude in pitch, yaw, and roll axes. They also may be used to make small changes of velocity along one of the Orbiter's three axes.
Flight of a Shuttle
Except for ascent and entry, all of the Shuttle's typical seven-day mission is in orbit. That is where the goals of a given mission are accomplished: scientific experiments carried out; satellites deployed into orbit, retrieved or repaired; observations made of the Earth and the solar system. The Shuttle makes one revolution of the Earth approximately every 90 minutes during the satellite mission.
When it comes out of orbit, the Shuttle is moving at about 17, 500 miles an hour. Reaction engines position the Orbiter nose forward again for entry into the atmosphere. Those thrusters continue to control the Orbiter's attitude until the atmosphere becomes dense enough for the aerodynamic surfaces to take effect.
The Shuttle enters the ever-thickening blanket of atmosphere at 400,000 feet of altitude and a speed of more than 17,000 miles an hour (about Mach 25). The Orbiter's nose is positioned 40 degrees above its flight path. That attitude increases aerodynamic drag, thus helping to dissipate the tremendous amount of energy that the spacecraft has when it enters the atmosphere. Friction heats the surface of the Orbiter, which is protected by thermal tiles, and ionizes the surrounding air, preventing radio communication with Earth for the next 13 minutes.
The flight control system's computer program allows use of the reaction thrusters and aerodynamic surfaces in combination to control the spacecraft. At Mach 4.2, the rudder is activated, and the last reaction thrusters are deactivated at Mach 1. Thereafter, the craft is entirely maneuvered like an airplane by movement of the aerodynamic control surfaces: elevons, rudder, speed brake, and body flap.
In the landing approach, the Orbiter has no propulsion. It has only its velocity and altitude. Its energy must be carefully managed to maneuver the Shuttle aerodynamically to a safe landing. Beginning this terminal phase, the glide slope is steep-19 degrees-as the Orbiter descends toward the runway. Half a minute before touchdown and two miles from the runway, the craft flares to a shallow, almost flat 1.5 degree glide slope. Touchdown occurs at 225 miles per hour. On the runway, the Orbiter rolls to a stop, and the mission is complete. .
1. Space Task Group Report to the President, "The Post-Apollo Space Program: Directions for the Future," September, 1969, pages 20 and 21.