Credit: © Mark Wade
Status: Study 1976.
In January 1976 the Sigma Corporation proposed use of a single liquid rocket booster under the external tank to replace the two solid rocket boosters. The pod would be powered by 3 to 4 F-1 engines or by three high performance liquid oxygen/kerosene engines being designed by the System Development Corporation. These developed either 308,000 kgf or 363,000 kgf at sea level, depending on the version selected. Both parallel and serial burns of the LRB and the Shuttle/External tank combination were studied. The external tank would be stretched and the orbiter strengthened to handle the increased payload available. However use of the booster would require the construction of new launch facilities.