Parallel expendable solid rocket motors. Proposed concepts: [1] McDonnell-Douglas/Martin Marietta, [5] North American Rockwell/General Dynamics, [8] Grumman/Boeing.

Parallel recoverable pressure-fed liquid rocket boosters. [2] McDonnell-Douglas/Martin Marietta, [4] North American Rockwell/General Dynamics, [7] Grumman/Boeing.

A single recoverable pressure-fed liquid rocket booster. [3] McDonnell-Douglas/Martin Marietta, [6] North American Rockwell/General Dynamics.

A single recoverable liquid rocket booster derived from the Saturn S-IC stage. [9] Grumman/Boeing.

The contractors reported their findings and recommendations in February 1972. Boeing recommended its own S-IC stage with retrorocket/parachute recovery at sea and also urged that solid rocket motors should be eliminated from further consideration. Lockheed and McDonnell-Douglas (who initially was quite optimistic about the pressure-fed booster) again advocated solids, noting that expendable Thor, Delta, Titan & Scout solid rocket motors had a very good safety record. North American Rockwell again refused to pick a favored concept and only noted that a case could be made for either solid or liquid boosters depending on what cost goals were most important.