<prompt> Bheka isigaba esingezansi bese uphendula umbuzo ngemuva kwalokho ngolimi olufanayo nesigaba: Isigaba: Ekuqaleni kwawo-1990, i-Gulfstream Aerospace kanye neSukhoi Design Bureau baqala umzamo ohlanganyelwe wokuthuthukisa indiza yebhizinisi encane ye-supersonic, ikhodi ebizwa ngokuthi i-S-21. Ngenxa yesidingo semakethe esiphikisanayo sokuhamba ngezindiza kwe-supersonic, ukuzibophezela kuphrojekthi kwaphela amandla futhi ukubambezeleka kwafakwa. IGulfstream yagcina ixazulule ubudlelwano, yize iSukhoi yaqhubeka nokusebenza ku-S-21. Umbuzo: Yini le nkampani ezimbili zezindiza ezakhiwe ndawonye?  <response>: I-S-21 