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When NASA decided at the end of the Apollo Moon program that it needed something new for future space travel, various concepts were proposed and from those emerged the Space Shuttle. North American Rockwell was given the prime contract in July 1972 to build five Space Shuttle orbiters, the first one being a prototype to be used for glide and ground tests. Work to build the first shuttle orbiter, OV-101, began on June 4, 1974, at the Rockwell Air Force Plant 42, Site 1, in Palmdale, California. Final assembly of all the components that came from various subcontractors started in March 1975. On August 25, 1975, the final assembly was complete.

At Christmastime 1975, readers of the trade journal Astronautics & Aeronautics saw an arresting color photo on the cover of the January 1976 issue. It showed what was unmistakably an airplane in final assembly within a hangar â€“ and which equally unmistakably was a Space Shuttle orbiter. â€œSpace Shuttle 1976,â€ read the coverâ€™s caption; â€œInto Mainstream Development.â€ The wings and vertical fin were in place, along with much of the fuselage. This was OV-101, which later that year â€“ as a result of the mail campaign organized by fans of the popular Star Trek TV seriesâ€“ was to receive the name â€œEnterprise.â€

The Trekkers had only forgotten one thing â€“ Enterprise was not built to fly in space like the â€œrealâ€ starship. But nonetheless she had a useful career in flight and ground test. In September 1976, amid considerable ceremony, she was rolled out for public display, thus showing dramatically that the shuttle program indeed was building hardware.

During the summer of 1976, shortly before the rollout, OV-101 had served as the test vehicle for the Horizontal Ground Vibration Test, conducted in Palmdale. Earlier vibration testing had used an accurate structural model, at one-quarter scale, with water in its External Tank to simulate liquid oxygen and air replacing the very lightweight liquid hydrogen. The new tests gave engineers their first opportunity to verify their mathematical models by taking data on the structural dynamics of an actual flight orbiter.

Although OV-101 was not identical to the configuration planned for OV-102, the differences were well understood and accounted for in the model. For instance, Enterprise did not have provisions for mounting real OMS pods, but used structural boilerplate replicas, and the vertical stabilizer was built-up using skin and stringers as opposed to the integrally machined structure of OV-102. The payload installed in the orbiter during the HGVT was the 10,000-pound Development Flight Instrumentation package that would be used during the atmospheric flight tests.

There were two test configurations, one with the orbiter supported in a â€œfree-freeâ€ condition to simulate reentry and landing, and the other with the orbiter rigidly attached to the ground at its External Tank supports to represent the configuration during ascent. Tests were also conducted with the payload bay doors opened to simulate an on-orbit configuration. Ferry locks were used to secure the aerodynamic control surfaces during testing, mainly to prevent unexpected damage. The tests vibrated this vehicle at frequencies from 0.5 to 50 hertz, determining natural or resonant frequencies and their damping. Other measurements determined frequency response at the locations of sensors used for guidance and control. Following the completion of the tests, minor modifications were made to the vehicle prior to the public rollout.

The following year Enterprise would be used for Approach and Landing Tests at Edwards Air Force Base, California. It would be flown piggyback atop the SCA Boeing 747, later to be used to ferry orbiters from coast to coast, and released, gliding back to a landing as if returning from space. Following the ALT flights, OV-101 continued to find useful roles, first in structural tests and then in exercising the shuttleâ€™s launch facilities, and for a short while even in â€œdiplomaticâ€ service.

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The Space Shuttle system consists of the orbiter, a large liquid-fuel tank that will not be recoverable, and two reusable solid-fuel boosters. The shuttle is designed to be used as many as one hundred times on missions ranging from an average of seven days to as much as a month, with a two-week turnaround and preparation period for the next flight.

In the words of James Fletcher, â€œAny discussion of future space initiatives must start with the Space Shuttle, the key to opening up near space to quick, easy, and economical access. With the Space Shuttle, operations to and from low-altitude Earth orbit â€“ for both manned and unmanned exploration, science and applications â€“ will become routine and relatively inexpensive.â€

NASA and the Pentagon are already pushing Congress and the President for funds to construct two or three more shuttles, but both agencies are beginning to meet increased opposition, led by Walter Mondale. Opponents say that the shuttle will be used to â€œmake workâ€ and thus to spend increased money on space exploration. No small part of the questions are concerned with the shuttleâ€™s ability to be used militarily, examining up-close Soviet spy satellites and even (though the Pentagon denies this) as a nuclear bomber.

Regardless, the shuttle promises a tremendous diversity of missions. NASA has reportedly come up with over five hundred possibilities, including satellite placement, maintenance, repair, and retrieval; placement of scientific labs in orbit; establishing an optical telescopic observatory above the atmosphere will for the first time be feasible; and delivery and construction of powered space vehicles for missions to deep space.

The reported cost will be in the neighborhood of ten million dollars per mission, according to NASA estimates, even though the agency quoted a price of around twenty million to a European consortium which is working on a manned laboratory designed to be placed in orbit by the shuttle. This is still in marked contrast to the average of thirty million per throw-away rocket launch at present.

The Enterprise will not be the first shuttle to go into orbit, but will be used to make the necessary atmospheric flight tests. The second orbiter, OV-102, will make the first orbital flight in March 1979, if everything goes according to schedule. The Enterprise will probably make her first spaceflight sometime in 1983.

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At one point, it was considered feasible to fit turbofans to the orbiter to enable it to fly from place to place under its own power, but this idea was eventually dropped when it was calculated that the spacecraftâ€™s wings generated insufficient lift to provide the required range with the relatively small quantity of fuel it could carry.

In February 1974 NASA deleted its requirement for air-breathing jet engines on the orbiter. A new review of overall shuttle requirements, issued the following month, made no mention of such engines whatsoever. The orbiter now was a glider and would remain so. NASA was faced with the question of how to conduct the atmospheric flight tests, as well as how to ferry the orbiter from a remote landing location back to the launch site.

In anticipating the use of a carrier, NASA needed the largest airplane it could get, and its first thoughts were of the C-5A cargo aircraft. This appeared readily available because the Air Force might provide on from its existing fleet, as part of its cooperation with NASA on the Space Shuttle. By contrast, a Boeing 747 would have to be purchased, which would add cost. In August 1973 NASA awarded a $1 million study contract to Lockheed, builder of the C-5A. In October a similar contract went to Boeing, to examine the possible use of a 747.

Of particular concern was whether the orbiter could separate cleanly from the back of the carrier without striking its tail surfaces. John M. Conroy, who had developed the Boeing Stratocruiser-derived Pregnant Guppy and Super Guppy used to transport Saturn rocket stages during the Apollo program, stepped in with a similarly grandiose proposal for an entirely new shuttle carrier aircraft called â€œVirtus,â€ Latin for â€œcourage.â€

â€œVirtusâ€ was to carry the orbiter â€“ or alternately, other oversized payloads such as External Tanks â€“ between twin fuselages, beneath the wing. During flight testing the orbiter could be dropped in flight as if it were a bomb. Nowadays Richard Bransonâ€™s Virgin Galactic uses a similar design for launching the â€œVSS Enterprise.â€ With a wingspan of 450 feet and a length of 280 â€œVirtusâ€ would dwarf a 747, which had a span and length of 196 and 232 feet, respectively.

Conroy expected to use a cockpit and forward fuselage of a C-97, The Air Forceâ€™s version of his beloved Stratocruiser, to eliminate the high costs of developing a new design. He also expected to use landing gear taken from surplus B-52 bombers. Even so, this craft would take two years for construction and six months for flight test and certification. As an aircraft of entirely novel type, it carried the risk of cost overruns and schedule delays. By contrast, the C-5A and 747 were known quantities, and NASA preferred to choose one or the other.

By mid-April 1974 NASA had apparently decided on using the C-5A and, on April 24, George Low wrote a letter to the Secretary of the Air Force, John l. McLucas, outlining the plan. A week later the Air Force agreed in principle. But analysis showed that risk existed for flying the orbiter from the back of the C-5A. The T-shaped tail, with the horizontal stabilizer high atop the vertical fin, produced aerodynamic effects that inherently would cause the aircraft to pitch toward the orbiter during separation.

A C-5A pilot could prevent a collision with the tail with a large and carefully timed movement of the controls, but if this was not done properly, the collision indeed would take place â€“ and could shear off the horizontal stabilizer.

The 747 was much safer. It lacked a T-shaped tail; its tail showed a conventional configuration, with horizontal stabilizers mounted to the fuselage, below the orbiter, and a vertical fin standing alone. Its aerodynamics was far more favorable for air launch. The pilot would not need to take sudden evasive action to prevent collision. Yet if the orbiter did collide with the fin (the horizontal stabilizers being below and out of the way), the consequence would not be catastrophic. The 747 could lose a large portion of this fin and still return safely.

The 747 had other advantages. Carrying the orbiter, it had an estimated range of 2,320 nautical miles, enough for a nonstop transcontinental flight. Some hopeful souls even argued that it could reach the mainland from Hawaii with the orbiter, though this proved not to be true. Still, the C-5A had much less range and would require inflight refueling. The standard C-5A was equipped for this, but there was no experience in refueling it with an orbiter on its back. Hence it would be necessary to develop this experience by flying a C-5A with a dummy orbiter. The 747 avoided this problem.

The 747 had a further safety advantage in that the presence of the orbiter was most destabilizing while mated, allowing flight tests of this reduction in stability prior to flights with actual air launch. This was not possible with the C-5A, for its greatest destabilization occurred just after separation. Again, this difference resulted from the dissimilarity between the two airplanesâ€™ tail shape.

The 747 could use shorter runways than the C-5A, if an engine were to fail during takeoff. The 747 could mount engines of greater power to increase the air-launch attitude, for better realism of the orbiter flights that were to simulate return from space. Carrier aircraft were expected to see extensive use, at a time when people expected to fly the shuttle up to sixty times per year. This gave a further advantage to the 747, for it had a structural life of sixty thousand hours while the lifetime of the C-5A wing was no more than twelve thousand. This reflected the fact that commercial 747s flew every day, whereas the military C-5A flew less frequently.

Aside from all of that, it was the availability of low-cost used 747 aircraft, and the relative scarcity of C-5As, that finally drove the selection of the Being aircraft. After being informed of possible Air Force restrictions on the use of the C-5As, NASA decided that it was easier to have complete control over their destiny and own the SCA than it was to compete with military priorities.

The decision, the choice of carrier aircraft, came from within NASAâ€™s Johnson Space Center in Houston. In May 1974 the center director, Christopher Kraft, wrote to William Schneider in Washington, the Acting Associate Administrator for Manned Spaceflight: â€œDear Bill: This letter requests authorization for NASA Johnson Space Center to purchase a Boeing 747 aircraft.â€

On June 13 the Space Shuttle Program Office gave a briefing to the NASA/DOD Space Transportation System Committee, comparing the C-5A and 747 and recommending the latter. This committee concurred in the choice. On the next day, NASA Associate Administrator George M. Low saw this briefing, and he gave approval to Kraftâ€™s request of a month earlier. The procurement went through quickly, on July 18, 1974, with NASA paying $15.6 million for a used Boeing 747-123 of American Airlines (registration N9668, msn 20107). The aircraft was the 86th plane off the 747 production line, and had been delivered to American on October 29, 1970. It had logged 8,999 hours during 2,985 flights, mostly on long-haul flights between New York and Los Angeles. NASA did not name it but merely gave it a new civil registration number, N905NA. Its commercial markings remained plainly visible during subsequent years in the space program.

The technical community expressed calm assurance that a mated ferry flight program was a feasible undertaking. The separation of the two vehicles in flight did not produce the same response. The Space Shuttle already had two parallel separations to contend wit â€“ separating the SRBs from the ET, and jettisoning the ET from the orbiter. Both required knowledge of the aerodynamic effects when the vehicles were in proximity, and a great deal of wind tunnel time had been spent studying the separation maneuvers. Additional tests now had to be accomplished to understand the separation effects of the 747 and the orbiter, although the fact that this would happen at subsonic speeds greatly simplified matters.

Structural clearance was the initial concern, but computer simulations revealed that the vortex wake of the SCA might present a larger issue. Wake vortices are narrow zones of extremely severe turbulence that trail for miles behind an airplaneâ€™s wingtips. They are particularly strong when the airplane is large. Separation was to be accomplished by the mated vehicles entering a dive to increase airspeed, followed by the 747 reducing power and deploying spoilers to reduce lift and increase drag. Such a configuration was necessary to create the relative motion required to aerodynamically drive the two vehicles apart. Unfortunately, this also resulted in a near maximum vortex wake condition since the SCA was now configured, essentially, in a landing configuration.

Additional wind tunnel tests were scheduled at Langley, and during August 1974 this soon to be modified SCA 747 contributed to aviation safety by conducting thirty wake vortex research flights using various combinations of wing spoilers in an attempt to reduce wake vortices. Smoke generators, attached to the 747â€™s wingtips and aft fuselage, made vortices visible. A Lear Jet and an Air Force T-37 jet trainer, flying as chase aircraft, flew close to the danger zones.

Tests without the 747â€™s wing spoilers deployed produced violent â€œupsetâ€ problems for the T-37 at a distance of approximately three miles, and minor disturbances could be felt at distances as great as ten miles. With two spoilers deployed on the 747â€™s wing panels, the T-37 could fly at a distance of three miles and not experience difficulties. The results of the SCA tests, complementing a 1973/1974 joint NASA-FAA wake vortices study with a Boeing 727, led to shorter spacing between landings and take-offs, which, in turn, helped alleviate air-traffic congestion.

Subsequently, the 747 was used for additional Space Shuttle tests â€“ a Lockheed F-104 from Dryden was positioned near the 747 wing and both vehicles flew a simulated separation maneuver. When the 747 reached appropriate conditions for separation, the F-104 pulled away and replicated the planned orbiter maneuver following separation. The test confirmed that adequate clearance between the 747 vortex wake and the orbiter flight path would be maintained. The tests revealed that although the separation maneuver would need to be flown very precisely, there were no major technical reasons not to proceed with the atmospheric flight tests.

Because the orbiter was to be mounted in front of the vertical fin, it reduced that finâ€™s effectiveness. To restore the diminished stability of the 747, designers crafted rectangular fins, ten by twenty feet, to fit on the ends of the horizontal stabilizers. Called â€œvertical endplates,â€ these devices had a drag strut connecting to the upper surface of the horizontal stabilizers. Wind-tunnel tests at the University of Washington, close to Boeingâ€™s main plants, verified their usefulness and â€“ though developed as removable structures â€“ these vertical endplates were never removed from the SCA.

In addition, Boeing won a $30 million contract from Rockwell to carry through the 747â€™s physical modifications. The work, which took place at the 747â€™s production facilities near Everett, Washington, was started on August 2, 1976.

The commercial 747 had been built to carry passengers or air cargo on a strong deck within the fuselage. The fuselage proper withstood internal cabin pressure and external aerodynamic forces, but it most certainly had not been built to support the weight of a 150,000-pound orbiter. The fuselage of NASAâ€™s new jet therefore had additional bulkheads installed, for extra strength. In accordance with design principles dating to the 1930s, the 747 used â€œstressed skinâ€ construction. Its loads, weights, and stresses were not only borne by the internal framework but were carried in part by the aircraft skin, which served as a major structural element in its own right. This skin was reinforced in critical areas, with overlays of sheet aluminum being riveted into place.

The 747 longitudinal trim system was modified to permit two degrees more trim in order to counteract a nose-up tendency cause by the downwash off the orbiter wings onto the 747 horizontal stabilizers. Additionally, most of the lower (main) deck interior was stripped, although a number of seats were retained to transport support personnel on ferry missions. A slide escape system was also provided, so that the Boeingâ€™s flight crew could abandon ship if things went badly awry.

The cockpit received controls and displays needed for the air launch and ferry missions. These included a sideslip indicator because the 747 had a tendency to yaw when mated with the orbiter. A load measurement system was installed to record the attach forces between the two vehicles during the mated portion of each flight. Load cells instrumented to measure axial and shear forces were located on each of the three attach struts. This data was evaluated in real-time and used as part of the criteria for approving separation during the first tail-cone-off flight. Displays and controls for the Separation Monitoring and Control System (SMCS) were added at the pilot stations, and an orbiter-to-ground S-band relay link and SCA-to-orbiter intercom were also carried. Other new equipment included L-band telemetry and C-band transponders.

An additional modification took the form of a mass inertia damper installed in the 747â€™s forward fuselage. This apparatus consisted of a 993-pound mass that shifted position laterally by means of rollers set into the cabin floor. Shifting of then mass damped out oscillations caused by air flowing over the orbiter and inducing turbulence across the Boeingâ€™s tail.

The orbiter would be carried on top of the 231-foot long 747 fuselage, mounted to struts at three points â€“ one forward and two aft â€“ that matched the socket fittings intended for attaching the External Tank during ascent. The orbiter mated location on the SCA was selected based on static stability and control, required structural modification, weight, and mission performance. Ballast was carried by the SCA in standard 747 cargo containers in the forward cargo compartment to ensure center-of-gravity limits were not exceeded. This ballast was adjusted for each ferry flight based on which orbiter (each had a different empty weight) was being carried, and any payloads that were installed in the orbiter.

The forward struts, which had the shape of an inverted V, came in two varieties:

- a telescopic orbiter forward support assembly to be used during atmospheric flight tests. This was a bipod consisting of two tubes, each 13 feet long, with an adjustable drag strut on the aft-side of each tube to support the bipod after orbiter release. The orbiter would be mounted at a six degree angle-of-attack using this support, making separation a little easier.

- a fixed orbiter forward support assembly for use during ferry missions. This was also a bipod, consisting of two 8.5-foot long tubes, but without the drag struts. The orbiter would be mounted at a three degree angle-of-attack using this support, providing less drag during ferry flights.

Rear orbiter supports, called aft support assemblies, were mounted atop the aft fuselage. Each consisted of a drag strut twelve feet long and a vertical strut 4.5 feet long. The right-aft support was fitted with a non-adjustable 4.8-foot side strut, and the left-aft support was fitted with dual non-load-bearing adjustable side snubbers.

These structural modifications added some 11,500 pounds to the empty weight of NASAâ€™s 747. It therefore was given a stronger and more capable rudder control. In addition, the extra weight demanded more power from the planeâ€™s engines, which therefore went into the shop for their own improvements. The four Pratt & Whitney JT9D-3A engines were converted to the JT9D-7AHW configuration, increasing available power from 43.500 pound-force to 46,950 pound-force. They also were modified for water injection. This sprayed water into the enginesâ€™ hot internal airflow during takeoff, cooling the air and making it denser so as to burn more fuel. Water injection promised additional takeoff thrust for use in ferry missions, when the SCA would carry both the orbiter and a full load of fuel.

The maximum airspeed of an SCA was Mach 0.6 (250 knots), typical cruise altitude during a ferry mission was 13,000 to 15,000 feet, and the maximum ferry range was 1,150 miles. Without an orbiter attached, the aircraft was able to attain altitudes of 24,000 to 26,000 feet and had a range of 6,300 miles. The minimum crew carried by an SCA during ferry missions was two pilots and two flight engineers, although only one flight engineer was required when not carrying an orbiter. N905NA had an empty weight of 318,053 pounds. The maximum weight at takeoff was 710,000 pounds, and maximum landing weight was 600,000 pounds.

The work was completed early in December 1976. The 747, well modified, flew on a short hop from Everett to Seattleâ€™s Boeing Field, for a flight-test program that ran through the Christmas holidays. On January 14, 1977, Boeing turned the SCA over to Rockwell for acceptance testing. Upon completion of weight and balance checks and another limited flight test series, the aircraft was flown to Edwards Air Force Base and handed over to NASAâ€™s Dryden Flight Research Center.