Beginning to Explore Alternative Shuttle Designs

NASA in mid-1970 had issued, along with the two Phase B preliminary design contracts to North American Rockwell and McDonnell Douglas, three smaller study contracts to examine alternative shuttle designs. While the Lockheed and Chrysler studies were managed by the Marshall Space Flight Center in Alabama, a Grumman/Boeing study was managed by the Manned Spacecraft Center (MSC) in Houston. Houston used this study contract as a means of getting industry analysis of various ideas with respect to shuttle design emerging both from within NASA and from the various study contractors. In particular, Grumman began in late 1970 to examine a shuttle orbiter design in which the fuel tanks holding the very low tempera­ture liquid hydrogen needed as fuel for the orbiter’s advanced shuttle engines were moved from inside the orbiter fuselage to the vehicle’s exterior and discarded when the fuel was expended. The idea of expendable fuel tanks was not new; several of the 1969 Phase A contractors had initially examined their use, but George Mueller in August 1969 had mandated that the studies from that point on would only consider fully reusable designs. Because liquid hydrogen is light in weight but accounted for three-fourths of the volume of shuttle propellant, the hydrogen tanks had to be large, and removing them from the vehicle’s internal structure made possible shrinking the size and weight of the orbiter by some 30 percent. Having expendable fuel tanks, Grumman suggested, would lower orbiter development costs by more than $1 billion while not adding significantly to per flight costs; in the budgetary context of 1970-1971, this was a very attractive prospect. NASA on April 1, 1971, added an additional task to the two more in-depth Phase B studies, asking North American Rockwell and McDonnell Douglas to examine an orbiter with two external hydrogen fuel tanks.

As industry studies continued in mid-1971 and NASA’s in-house engi­neering design team at MSC also focused on a smaller, lower cost, less complex orbiter, the idea of using a single large external and expendable propellant tank containing both hydrogen fuel and oxygen oxidizer gained increasing acceptance, and became a part of the consensus orbiter design that was emerging from Houston’s efforts. The cost of discarding the exter­nal tank on each flight was seen as acceptable in terms of the overall costs of both developing and operating the shuttle, given the savings in develop­ment costs resulting from designing a smaller orbiter and the relatively low increase in the cost of each flight associated with using an expendable pro­pellant tank.41