Space Shuttle design process
Before the Apollo 11 Moon landing in 1969, NASA began studies of Space Shuttle designs as early as October 1968. These initial Phase A studies focused on supporting a future space station and ferrying a minimum crew of four with about 50,000 pounds of cargo. The goal was rapid turnaround for future flights while larger payloads like space station modules were lifted by the Saturn V rocket. By June 1970, Phase B studies became more detailed and specific regarding these capabilities.
Two primary designs emerged as front-runners during this period. Engineers at the Manned Spaceflight Center championed a two-stage system with delta-winged spacecraft that proved generally complex. Maxime Faget designed an attempt to re-simplify the concept into what he called the DC-3, utilizing conventional straight wings similar to his earlier Mercury capsule work. Numerous offerings from commercial companies also appeared but fell by the wayside as each NASA lab pushed for its own version.
All of this activity took place amidst other NASA teams proposing wide variety post-Apollo missions costing as much as Apollo or more. As these projects fought for funding, the NASA budget faced severe constraints. Three options were eventually presented to United States Vice President Spiro Agnew in 1969. The shuttle project rose to the top largely due to tireless campaigning by its supporters. By 1970 the shuttle had been selected as the one major project for the short-term post-Apollo time frame.
Initially, a fully reusable design was preferred because it involved a very large winged crewed booster carrying a smaller winged crewed orbiter. The booster vehicle would lift the orbiter to a certain altitude and speed before separating. The booster would return and land horizontally while the orbiter continued into low Earth orbit. After completing its mission, the winged orbiter would re-enter and land horizontally on a runway. The idea was that full reusability would promote lower operating costs.
However, further studies showed a huge booster was needed to lift an orbiter with the desired payload capability. In space and aviation systems, cost is closely related to mass so this meant the overall vehicle cost would be very high. Both booster and orbiter would have rocket engines plus jet engines for use within the atmosphere plus separate fuel and control systems for each propulsion mode. Concurrent discussions about how much funding would be available to develop the program added pressure.
The combined space station and Air Force payload requirements were not sufficient to reach desired shuttle launch rates. Therefore, the plan shifted to all future U.S. space launches using only the Space Shuttle. Most other expendable boosters would be phased out. The reusable booster was eventually abandoned due to several factors including high price, technical complexity, and development risk. Instead, a partially reusable design was selected where an external propellant tank was discarded for each launch.
During the mid-1960s the United States Air Force had both of its major piloted space projects canceled. This demonstrated its need to cooperate with NASA to place military astronauts and payloads in orbit. The Air Force launched more than 200 satellite reconnaissance missions between 1959 and 1970. The military's large volume of payloads would be valuable in making the shuttle more economical.
NASA sought Air Force support for the shuttle after the Six-Day War exposed limitations in the United States satellite reconnaissance network. Air Force involvement emphasized the ability to launch spy satellites southward into polar orbit from Vandenberg AFB. This required higher energies than for lower inclination orbits. To return to Earth after one orbit despite the Earth rotating 1,000 miles beneath the orbital track required a larger delta wing size than the earlier simple DC-3 shuttle.
Despite potential benefits for the Air Force, the military was satisfied with its expendable boosters and had less need for the shuttle than NASA. Because the space agency needed outside support, the Defense Department gained primary control over the design process. For example, NASA planned a 46-foot cargo bay but NRO specified a 50-foot bay because it expected future intelligence satellites to become larger. When Faget again proposed a 46-foot wide payload bay, the military almost immediately insisted on retaining the 50-foot width.
While NASA would likely have chosen liquid boosters had it complete control over the design, the Office of Management and Budget insisted on less expensive solid boosters due to their lower projected development costs. While a liquid-fueled booster design provided better performance, lower per-flight costs, less environmental impact and less developmental risk, solid boosters were seen as requiring less funding to develop at a time when the Shuttle program had many different elements competing for limited development funds.
Engineers at NASA's Marshall Space Flight Center were particularly concerned about solid rocket reliability for crewed missions. The final design which was selected included a winged orbiter with three liquid-fueled engines, a large expendable external tank holding liquid propellant for these engines, and two reusable solid rocket boosters. This configuration balanced cost constraints against technical risks while meeting Air Force requirements for polar orbit launches from Vandenberg AFB.
The decision to use solid rockets also meant throwing away the tank after each launch but this was a relatively small portion of operating costs. Earlier designs assumed the winged orbiter would also have jet engines to assist maneuvering in the atmosphere after re-entering. However NASA ultimately chose a gliding orbiter based partially on experience from previous rocket-then-glide vehicles such as the X-15 and lifting bodies.
In the spring of 1972 Lockheed Aircraft, McDonnell Douglas, Grumman, and North American Rockwell submitted proposals to build the shuttle. The NASA selection group thought that Lockheed's shuttle was too complex and too expensive since the company had no experience with building crewed spacecraft. McDonnell Douglas's proposal was too expensive and had technical issues. Grumman had an excellent design which also seemed too expensive.
North American's shuttle had the lowest cost and most realistic cost projections. Its design was the easiest for ongoing maintenance and the Apollo 13 accident involving North American's command and service module demonstrated its experience with electrical system failures. NASA announced its choice of North American on the 26th of July 1972. This selection process reflected the intense pressure to control development costs while meeting Air Force payload requirements.
The Air Force also gained the equivalent of the use of one of the shuttles for free despite not paying for the shuttle's development or construction. In exchange for the NASA concessions, the Air Force testified to the Senate Space Committee on the shuttle's behalf in March 1971. Congress reportedly told DoD that it would not pay for any satellites not designed to fit into the shuttle cargo bay.
The Space Shuttle program used the HAL/S programming language throughout its operational life. The first microprocessor used was the 8088 and later the 80386. The Space Shuttle orbiter avionics computer was the IBM AP-101. These specific hardware components formed the backbone of flight control and mission management systems during all shuttle missions.
NASA examined four solutions to the booster problem including development of existing Saturn lower stage, simple pressure-fed liquid-fuel engines of a new design, a large single solid rocket, or two smaller ones. Engineers at NASA's Marshall Space Flight Center were particularly concerned about solid rocket reliability for crewed missions. The final configuration balanced these technical choices with budget constraints imposed by the Office of Management and Budget.
The decision to omit jet engines from the orbiter reduced complexity and increased payload capacity. This choice aligned with experience from previous rocket-then-glide vehicles such as the X-15 and lifting bodies. The resulting system required sophisticated software to manage reentry and landing procedures without atmospheric propulsion assistance.
It was developed with the original development cost and time estimates given to President Richard M. Nixon in 1971 at a cost of $4.2 billion in 1971 dollars versus an original $5.15 billion estimate. The operational costs, flight rate, payload capacity, and reliability were different than anticipated. Maximum external tank production rate was limited to 24 tanks per year at NASA's Michoud Assembly Facility.
Potential military use of the shuttle probably caused President Jimmy Carter to not cancel the shuttle in 1979 and 1980 when the program was years behind schedule and hundreds of millions of dollars over budget. The Air Force planned on having its own fleet of shuttles and re-built a separate launch facility originally derived from the canceled Manned Orbiting Laboratory program at Vandenberg called Space Launch Complex Six.
For various reasons due in large part to the loss of Space Shuttle Challenger on the 28th of January 1986 work on SLC-6 was eventually discontinued and no shuttle launches from that location ever took place. SLC-6 was eventually used for launching the Lockheed Martin-built Athena expendable launch vehicles which included the successful IKONOS commercial Earth observation satellite in September 1999 before being reconfigured once again to handle the new generation of Boeing Delta IV's.
Common questions
When did NASA begin studies of Space Shuttle designs before the Apollo 11 Moon landing?
NASA began studies of Space Shuttle designs as early as October 1968. These initial Phase A studies focused on supporting a future space station and ferrying a minimum crew of four with about 50,000 pounds of cargo.
Which company won the contract to build the Space Shuttle orbiter in July 1972?
North American Rockwell won the contract to build the Space Shuttle orbiter. NASA announced its choice of North American on the 26th of July 1972 after evaluating proposals from Lockheed Aircraft, McDonnell Douglas, Grumman, and North American Rockwell.
Why was the fully reusable design abandoned for the Space Shuttle program?
The fully reusable design was abandoned due to high price, technical complexity, and development risk. Further studies showed that a huge booster was needed to lift an orbiter with the desired payload capability, which meant the overall vehicle cost would be very high.
What role did the United States Air Force play in the Space Shuttle design process?
The Defense Department gained primary control over the design process because NASA needed outside support. The military insisted on retaining a 50-foot wide payload bay and required higher energies for polar orbit launches from Vandenberg AFB.
When was work on the Space Launch Complex Six facility discontinued?
Work on SLC-6 was eventually discontinued following the loss of Space Shuttle Challenger on the 28th of January 1986. No shuttle launches from that location ever took place before it was reconfigured to handle Boeing Delta IV rockets.