Even before the Project Apollo moon landing in 1969, in October 1968, NASA began early studies of space shuttle designs. The early studies were denoted “Phase A”, and in June 1970, “Phase B”, which were more detailed and specific. The primary intended use of the space shuttle was supporting the future space station, ferrying a minimum crew of four and about 20,000 pounds (9,100 kg) of cargo, and able to be rapidly turned around for future flights.
Two designs emerged as front-runners. One was designed by engineers at the Manned Spaceflight Center, and championed especially byGeorge Mueller. This was a two-stage system with delta-winged spacecraft, and generally complex. An attempt to re-simplify was made in the form of the DC-3, designed by Maxime Faget, who had designed the Mercury capsule among other vehicles. Numerous offerings from a variety of commercial companies were also offered, but generally fell by the wayside as each NASA lab pushed for its own version.
All of this was taking place in the midst of other NASA teams proposing a wide variety of post-Apollo missions, a number of which would cost as much as Apollo or more. As each of these projects fought for funding, the NASA budget was at the same time being severely constrained. Three were eventually presented to Vice President 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.
When funding for even this came into question, there were concerns that the project might be cancelled. This led to an effort to interest the US Air Force in using the shuttle for their missions as well.
The Air Force was mildly interested, but demanded a much larger vehicle, far larger than the original concepts. To lower the development costs of the resulting designs, boosters were added, a throw-away fuel tank was adopted, and many other changes made that greatly lowered the reusability and greatly added to vehicle and operational costs. With the Air Force’s approval, the system emerged in its operational form.
In 1969, United States Vice President Agnew chaired the National Aeronautics and Space Council, which discussed post-Apollo options for manned space activities. The recommendations of the Council would heavily influence the decisions of the administration. The Council considered four major options:
– Discontinuing manned space activities
Based on the advice of the Space Council, President Nixon made the decision to pursue the low earth orbital infrastructure option. This program mainly consisted of construction of a space station, along with the development of a Space Shuttle. Funding restrictions precluded pursuing the development of both programs simultaneously, however. NASA chose to develop the Space Shuttle program first, and then planned to use the shuttle in order to construct and service a space station.
Shuttle design debate
During the early shuttle studies, there was a debate over the optimal shuttle design that best balanced capability, development cost, and operational cost. Initially a fully reusable design was preferred. This involved a very large winged manned booster which would carry a smaller winged manned orbiter. The booster vehicle would lift the orbiter to a certain altitude and speed, then separate. The booster would return and land horizontally, while the orbiter continued into low earth orbit. After completing its mission, the winged orbiter would reenter 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 weight, 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. In addition there were concurrent discussions about how much funding would be available to develop the program.
Another competing approach was maintaining the Saturn V production line and using its large payload capacity to launch a space station in a few payloads rather than many smaller shuttle payloads. A related concept was servicing the space station using the Air Force Titan III-M to launch a larger Gemini capsule, called “Big Gemini”, rather than using the shuttle.
The shuttle supporters answered that given enough launches, a reusable system would have lower overall costs than disposable rockets. If dividing total program costs over a given number of launches, a high shuttle launch rate would result in lower per-launch costs. This in turn would make the shuttle cost competitive with or superior to expendable launchers. Some theoretical studies mentioned 55 shuttle launches per year, however the final design chosen would not support that launch rate. In particular the maximum external tank production rate was limited to 24 tanks per year at NASA’sMichoud Assembly Facility.
The combined space station and Air Force payload requirements weren’t sufficient to reach desired shuttle launch rates. Therefore the plan was for all future U.S. space launches—space station, Air Force, commercial satellites, and scientific research—to use only the space shuttle. Most other expendable boosters would be phased out.
The reusable booster was eventually abandoned due to several factors: high price (combined with limited funding), technical complexity, and development risk. Instead, a partially (not fully) reusable design was selected, where an external propellent tank was discarded for each launch, and the booster rockets and shuttle orbiter were refurbished for reuse.
Initially the orbiter was to carry its own liquid propellant. However studies showed carrying the propellant in an external tank allowed a larger payload bay in an otherwise much smaller craft. It 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 reentering. 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. Omitting the jet engines and their fuel would reduce complexity and increase payload.