Tell me about the Sukhoi-Gulfstream 
 The Sukhoi-Gulfstream S-21 was a projected Russian-American supersonic business jet.

In the early 1990s, Gulfstream Aerospace and the Sukhoi Design Bureau began a joint effort to develop a supersonic small business jet, code named the S-21. Due to questionable market demand for commercial supersonic air travel, commitment to the project weakened and delays mounted.