EDIN05 Credit - © Mark Wade

In February 1976 this version of the shuttle was proposed. A single liquid rocket booster under the external tank would replace the two solid rocket boosters.

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.

• Jenkins, Dennis R,, Space Shuttle: The History of the National Space Transportation System : The First 100 Missions, Third edition, Voyageur Press, 2001. ISBN: 0963397451. Excellent - the most comprehensive account of the design, development, and flights of the space shuttle.Takes the reader from the maze of designs during the first shuttle competition to future plans. More at amazon.com...