Cryogenic rocket engines use liquid oxygen and hydrogen propellants which offer the highest energy efficiency for rockets requiring large thrust. The United States first developed these engines in the 1960s, while the Soviet Union did not succeed until 1987. India sought to import cryogenic engine technology in the 1990s but faced sanctions from the US and later Russia backed out of the deal. As a result, ISRO had to indigenously develop the technology which took 16 years to achieve success with the GSLV launch in 2010. Cryogenic engines provide clean, economical propulsion but also technical challenges like boil off and leakage of the extremely cold and reactive propellants.