What kind of aircraft gave the space shuttle a piggy-back prior to 1981?
Boeing 747-48EF Boeing 747-121A Boeing 747SR-81 Boeing 747-368 747-100 Boeing 7474 Boeing 747-244B Brien Wygle Boeing 747-700X Boeing 747X Boeing 747-100 Boeing 747-3B5 Boeing 747-2R7F Boeing 747-2B5F 747-700X Boeing 747-4B5 Boeing 747-409 Boeing 747-200BM Boeing Model 747 B742 B747 BOEING 747 Boeing747 Boeing 747-SR46 Boeing 747SP-09 747adv Boeing 747-244B Combi Boeing 747-100SR 747-SR46 747-100B Boeing 747 Classic 747-200 Boeing 747-121SF B747-300B Boing 747 747-200F Jack Waddell Boeing 747-130 Boeing 747-357 Boeing 747-230B Boeing 747-283B Boeing B-747 200B Boeing B-747 200F Boeing 747-146SR Boeing 747-236B Boeing 747-300B Boeing 747SP-21 Boeing 747-200 Boeing 747-258F/SCD Boeing 747-428BCF 747-300 Boeing 747-121 Boeing 747-122 Boeing B747 Boeing 747-430 Boeing 747-209B Boeing 747-438 Boeing 747SR Boeing 747-600X Boeing 747-249F Boeing 747-100SR-46 Boeing 747-237 Boeing 747-230 Boeing 747SR-46 Boeing 747-200M Boeing 747-200B Boeing 747-200C Boeing 747-131 B747-100B Boeing 747-200F Boeing 747-136 B 747 Boeing 742 Boeing 747 Boieng 74 Boeing 747 B-18210 Boeing 747-258F Queen of the Skies Boeing 747-100B 747-600X Boeing 747-300 Boeing 747-246B Boeing 747-237B B74F Boeing 747-500X 747X Boeing C-19 747-300B Megatop 747-500X Boeing 747–44AF (SCD) Boeing 747-168B Boeing 747-206B B-747 Boeing 747-44AF (SCD) The Boeing 747 74M Boeing-747