Cryogenic rocket engines use cryogenic fuels like liquid oxygen and liquid hydrogen that are stored at very low temperatures below -150°C. The United States first developed cryogenic rocket engines in 1963. Key components include the combustion chamber, injectors, turbo pumps, and nozzle. Cryogenic engines offer high energy density but present challenges like leakage and embrittlement. India successfully launched its first indigenous cryogenic upper stage in 2014. Future engine technologies being researched include ion engines and nuclear thermal rockets.