A cryogenic engine is a type of rocket engine that uses very cold liquids as propellants. The word “cryogenic” comes from the Greek words “kyros” (cold) and “genes” (produced). A cryogenic engine usually uses liquid oxygen and liquid hydrogen as the oxidizer and the fuel, respectively. These liquids are stored at very low temperatures, below -183 °C for oxygen and below -253 °C for hydrogen. When these liquids are injected into the combustion chamber, they ignite and produce a high-pressure gas that exits through the nozzle, creating thrust.
Cryogenic engines have many advantages over other types of rocket engines, such as solid or storable liquid engines.
Some of these advantages are:
• Higher efficiency: Cryogenic engines have a higher specific impulse, which means they produce more thrust for every kilogram of propellant they consume. This reduces the mass and cost of the rocket and increases its payload capacity.
• Higher performance: Cryogenic engines have a higher thrust-to-weight ratio, which means they produce more thrust for every kilogram of engine mass. This improves the acceleration and maneuverability of the rocket and reduces the gravity losses.
• Cleaner and safer: Cryogenic engines use environmentally friendly and non-toxic propellants, unlike solid or storable liquid engines that use chemicals that can cause pollution or health hazards. Cryogenic engines also have lower risks of explosion or fire, as the propellants are stored separately and only mixed in the combustion chamber.
Cryogenic engines are widely used in space exploration, especially for launching heavy satellites or spacecraft to high orbits or beyond Earth. Some of the examples of rockets that use cryogenic engines are:
• Atlas V: A US rocket that uses an RL-10 cryogenic engine in its upper stage. It has launched various missions, such as Mars Reconnaissance Orbiter, New Horizons, and Perseverance rover.
• Ariane 5: A European rocket that uses a Vulcain cryogenic engine in its core stage and an HM7B cryogenic engine in its upper stage. It has launched various missions, such as Rosetta, Galileo, and James Webb Space Telescope.
• GSLV Mk III: An Indian rocket that uses a CE-20 cryogenic engine in its upper stage. It has launched various missions, such as Chandrayaan-2 and Gaganyaan.
Cryogenic engines are one of the most powerful technologies for space exploration. They offer high efficiency, performance, cleanliness, and safety for launching heavy payloads to high orbits or beyond Earth. They are also one of the most challenging technologies to develop, as they require complex engineering and materials to handle extreme temperatures and pressures. Only a few countries in the world have mastered this technology, such as the US, Russia, Japan, India, France, and China.